Generate Chess960 starting position

Generate Chess960 starting position
You are encouraged to solve this task according to the task description, using any language you may know.

Chess960 is a variant of chess created by world champion Bobby Fischer. Unlike other variants of the game, Chess960 does not require a different material, but instead relies on a random initial position, with a few constraints:

• as in the standard chess game, all eight white pawns must be placed on the second rank.
• White pieces must stand on the first rank as in the standard game, in random column order but with the two following constraints:
  • the bishops must be placed on opposite color squares (i.e. they must be an odd number of spaces apart or there must be an even number of spaces between them)
  • the King must be between two rooks (with any number of other pieces between them all)
• Black pawns and pieces must be placed respectively on the seventh and eighth ranks, mirroring the white pawns and pieces, just as in the standard game. (That is, their positions are not independently randomized.)

With those constraints there are 960 possible starting positions, thus the name of the variant.

Task

The purpose of this task is to write a program that can randomly generate any one of the 960 Chess960 initial positions. You will show the result as the first rank displayed using either the chess symbols in Unicode (♔♕♖♗♘), the letters King Queen Rook Bishop kNight, or the corresponding letters in a language other than English.

11l

Translation of: Python: Correct by construction

F random960()
   V start = [‘R’, ‘K’, ‘R’]

   L(piece) [‘Q’, ‘N’, ‘N’]
      start.insert(random:(start.len + 1), piece)

   V bishpos = random:(start.len + 1)
   start.insert(bishpos, Char(‘B’))
   start.insert(random:(bishpos + 1), Char(‘B’))
   R start

print(random960())

Output:

[Q, B, N, R, K, B, N, R]

Action!

DEFINE MAX_NUMBERS="200"
DEFINE MAX_LEN="20"
DEFINE MAX_FACTORS="5"
DEFINE PTR="CARD"

PROC PrintResult(BYTE max,n BYTE ARRAY factors PTR ARRAY texts)
  BYTE i,j,t
  BYTE ARRAY values(MAX_FACTORS)

  FOR j=0 TO n-1
  DO
    values(j)=1
  OD

  FOR i=1 TO max
  DO
    t=0
    FOR j=0 TO n-1
    DO
      IF values(j)=0 THEN
        t=1 Print(texts(j))
      FI
      values(j)==+1
      IF values(j)=factors(j) THEN
        values(j)=0
      FI
    OD
    IF t=0 THEN PrintB(i) FI
    Put(32)
  OD
RETURN

BYTE FUNC Find(CHAR ARRAY s CHAR c BYTE POINTER err)
  BYTE i

  FOR i=1 TO s(0)
  DO
    IF s(i)=c THEN
      err^=0 RETURN (i)
    FI
  OD
  err^=1
RETURN (0)

PROC Main()
  BYTE max,i,n,pos,err
  BYTE ARRAY factors(MAX_FACTORS)
  PTR ARRAY texts(MAX_FACTORS)
  CHAR ARRAY
    s(100),tmp(100),
    t0(MAX_LEN),t1(MAX_LEN),t2(MAX_LEN),
    t3(MAX_LEN),t4(MAX_LEN)
  
  texts(0)=t0 texts(1)=t1 texts(2)=t2
  texts(3)=t3 texts(4)=t4

  DO
    PrintF("Max number (1-%B): ",MAX_NUMBERS)
    max=InputB()
  UNTIL max>=1 AND max<=MAX_NUMBERS
  OD

  n=0
  DO
    PrintF("Number of rules (1-%B): ",MAX_FACTORS)
    n=InputB()
  UNTIL n>=1 AND n<=MAX_FACTORS
  OD

  FOR i=0 TO n-1
  DO
    DO
      PrintF("Rule #%B (number space text):",i+1)
      InputS(s)
      pos=Find(s,' ,@err)
      IF pos=1 OR pos=s(0) THEN
        err=1
      FI
      IF err=0 THEN
        SCopyS(tmp,s,1,pos-1)
        factors(i)=ValB(tmp)
        IF factors(i)<2 THEN
          err=1
        FI
        SCopyS(texts(i),s,pos+1,s(0))
      FI
    UNTIL err=0
    OD
  OD

  PutE()
  PrintResult(max,n,factors,texts)
RETURN

Output:

Screenshot from Atari 8-bit computer

NBBQRNKR
NRKBRQBN
BNRBKRNQ
QBRKRNBN
RBKNRQBN
BNNBRKQR
NQNBRKBR
BRNKRBQN
NRKRQNBB
RNQBKRBN

APL

This function accepts a SP-ID and generates the corresponding position; to generate a random one, just pass in roll 960 (?960). (It's written for index origin 1, but reduces the passed-in SP-ID modulo 960 so that works without having to subtract 1 from the roll result.)

Works with: Dyalog APL

Works with: GNU APL

⍝ Utility functions
divmod ← {(⌊⍺÷⍵),⍵|⍺}
indices ← {(⍺∊⍵)/⍳⍴⍺} 
∇result ← place placement; array; index; piece; result
  (array piece index) ← placement
  array[(array indices '-')[index]] ← piece
  result ← array
∇
∇result ← chess960 spid; array; n; b1; b2; n1; n2; q
  spid ← 960 | spid
  array ← 8/'-'
  (n b1) ← spid divmod 4
  array[2+2×b1] ← 'B'
  (n b2) ← n divmod 4
  array[1+2×b2] ← 'B'
  (n q) ← n divmod 6
  array ← place array 'Q' (1+q)
  n1 ← 1⍳⍨n<4 7 9 10
  array ← place array 'N' n1
  n2 ← (1 2 3 4 2 3 4 3 4 4)[n+1]
  array ← place array 'N' n2
  array ← place array 'R' 1
  array ← place array 'K' 1
  array ← place array 'R' 1
  result ← spid, array
∇

Output:

      chess960 518
518 RNBQKBNR
      chess960 ?960
377 NRKBBRNQ
      chess960 ?960
487 QRBNKNRB

Arturo

; Using Edward Collins' single-die method
; http://www.edcollins.com/chess/fischer-random.htm

chess960: function [][
    result: array.of: 8 ø
    vacant: @0..7  ; open squares available to put pieces
    result\[remove 'vacant <= 2 * random 0 3]: 'bishop      ; place on random black square
    result\[remove 'vacant <= 1 + 2 * random 0 3]: 'bishop  ; place on random white square
    loop ['queen 'knight 'knight] 'piece [
        result\[remove 'vacant <= sample vacant]: piece     ; place on random open square
    ]
    result\[vacant\0]: 'rook  ; place king between rooks on remaining open squares
    result\[vacant\1]: 'king
    result\[vacant\2]: 'rook
    result
]

do.times:5 -> print chess960

Output:

bishop knight rook queen king knight rook bishop
knight rook queen bishop bishop king knight rook
rook bishop bishop knight knight king rook queen
bishop knight rook queen king knight rook bishop
rook king knight knight bishop queen rook bishop

AutoHotkey

Works with: AutoHotkey 1.1

Loop, 5
	Out .= Chess960() "`n"
MsgBox, % RTrim(Out, "`n")

Chess960() {
	P := {}
	P[K := Rand(2, 7)] := Chr(0x2654)	; King
	P[Rand(1, K - 1)] := Chr(0x2656)	; Rook 1
	P[Rand(K + 1, 8)] := Chr(0x2656)	; Rook 2
	Loop, 8
		Remaining .= P[A_Index] ? "" : A_Index "`n"
	Sort, Remaining, Random N
	P[Bishop1 := SubStr(Remaining, 1, 1)] := Chr(0x2657)	; Bishop 1
	Remaining := SubStr(Remaining, 3)
	Loop, Parse, Remaining, `n
		if (Mod(Bishop1 - A_LoopField, 2))
			Odd .= A_LoopField "`n"
		else
			Even .= A_LoopField "`n"
	X := StrSplit(Odd Even, "`n")
	P[X.1] := Chr(0x2657)	; Bishop 2
	P[X.2] := Chr(0x2655)	; Queen
	P[X.3] := Chr(0x2658)	; Knight 1
	P[X.4] := Chr(0x2658)	; Knight 2
	for Key, Val in P
		Out .= Val
	return Out
}

Rand(Min, Max) {
	Random, n, Min, Max
	return n
}

Output:

♕♘♖♗♗♘♔♖
♗♖♔♕♘♖♘♗
♖♗♘♘♗♔♖♕
♗♗♘♖♔♕♘♖
♘♗♖♔♗♘♕♖

BASIC

BASIC256

Translation of: Yabasic

for i = 1 to 10
  inicio$ = "RKR"
  pieza$ = "QNN"

  for n = 1 to length(pieza$)
    posic = int(rand*(length(inicio$) + 1)) + 1
    inicio$ = left(inicio$, posic-1) + mid(pieza$, n, 1) + right(inicio$, length(inicio$) - posic + 1)
  next n
  posic = int(rand*(length(inicio$) + 1)) + 1
  inicio$ = left(inicio$, posic-1) + "B" + right(inicio$, length(inicio$) - posic + 1)
  posic += 1 + 2 * int(int(rand*(length(inicio$) - posic)) / 2)
  inicio$ = left(inicio$, posic-1) + "B" + right(inicio$, length(inicio$) - posic + 1)
  print inicio$
next i
end

BBC BASIC

Works with: BBC BASIC for Windows

      VDU 23, 22, 240; 360; 8, 16, 16, 136
      *FONT Arial, 20
      FOR I% = 1 TO 10
        Rank1$ = "202"
        FOR Piece = 1 TO 3
          P% = RND(LENRank1$ + 1)
          Rank1$ = LEFT$(Rank1$, P% - 1) + MID$("144", Piece, 1) + MID$(Rank1$, P%)
        NEXT
        P% = RND(7)
        Rank1$ = LEFT$(Rank1$, P% - 1) + "3" + MID$(Rank1$, P%)
        IF P% > 5 P% += 1 ELSE P% += RND(4 - (P% >> 1)) * 2 - 1
        Rank1$ = LEFT$(Rank1$, P% - 1) + "3" + MID$(Rank1$, P%)
        FOR Piece = 1 TO 8
          VDU &E2, &99, &94 + VALMID$(Rank1$, Piece, 1)
        NEXT
        PRINT
      NEXT

Output:

♘ ♖ ♗ ♔ ♖ ♘ ♕ ♗
♘ ♖ ♔ ♘ ♕ ♖ ♗ ♗
♕ ♗ ♖ ♔ ♗ ♘ ♖ ♘
♖ ♘ ♔ ♗ ♖ ♕ ♗ ♘
♖ ♗ ♗ ♘ ♔ ♖ ♘ ♕
♘ ♖ ♗ ♗ ♕ ♔ ♘ ♖
♖ ♔ ♕ ♘ ♗ ♘ ♖ ♗
♘ ♘ ♖ ♔ ♖ ♗ ♗ ♕
♖ ♕ ♔ ♘ ♗ ♗ ♖ ♘
♖ ♕ ♔ ♘ ♘ ♗ ♗ ♖

Commodore BASIC

Works with: Commodore BASIC version 3.5,7.0

Besides the admittedly-trivial use of DO/LOOP, this implementation also exploits the BASIC 3.5+ ability to use MID$ as an lvalue.

100 REM CHESS 960
110 PRINT "SPID (-1 FOR RANDOM):";
120 OPEN 1,0:INPUT#1, SP$:CLOSE 1
130 SP=VAL(SP$)
140 IF SP<0 THEN SP=INT(RND(.)*960)
150 PRINT
160 DO WHILE SP>959: SP=SP-960: LOOP
170 AR$="--------"
180 P=SP
190 N=P AND 3:P=INT(P/4)
200 MID$(AR$,2*N+2,1)="B"
210 N=P AND 3:P=INT(P/4)
220 MID$(AR$,2*N+1,1)="B"
230 N=P-6*INT(P/6):P=INT(P/6)
240 P$="Q":GOSUB 420
250 N=P-10*INT(P/10):P=INT(P/10)
260 FOR N1=0 TO 3
270 : FOR N2=N1+1 TO 4
280 :   IF N<>0 THEN 340
290 :     P$="N":N=N1:GOSUB 420
300 :     P$="N":N=N2-1:GOSUB 420
310 :     N1=3
320 :     N2=4
340 :   N=N-1
350 : NEXT N2
360 NEXT N1
370 P$="R":N=0:GOSUB 420
380 P$="K":N=0:GOSUB 420
390 P$="R":N=0:GOSUB 420
400 PRINT STR$(SP);":";AR$
410 END
420 FOR I=1 TO LEN(AR$)
430 : IF MID$(AR$,I,1)<>"-" THEN 510
440 : IF N<>0 THEN 480
450 : MID$(AR$,I,1)=P$
460 : I=LEN(AR$)
470 : GOTO 510
480 : N=N-1
510 NEXT I
520 RETURN

Here's a version that doesn't use the advanced features:

Works with: Commodore BASIC version 2.0

100 REM CHESS 960
110 PRINT "SPID (-1 FOR RANDOM):";
120 OPEN 1,0:INPUT#1, SP$:CLOSE 1
130 SP=VAL(SP$)
140 IF SP<0 THEN SP=INT(RND(.)*960)
150 PRINT
160 IF SP>959 THEN SP=SP-960: GOTO 160
170 AR$="--------"
180 P=SP
190 N=P AND 3:P=INT(P/4)
200 AR$=LEFT$(AR$,2*N+1)+"B"+MID$(AR$,2*N+3)
210 N=P AND 3:P=INT(P/4)
220 AR$=LEFT$(AR$,2*N)+"B"+MID$(AR$,2*N+2)
230 N=P-6*INT(P/6):P=INT(P/6)
240 P$="Q":GOSUB 420
250 N=P-10*INT(P/10):P=INT(P/10)
260 FOR N1=0 TO 3
270 : FOR N2=N1+1 TO 4
280 :   IF N<>0 THEN 340
290 :     P$="N":N=N1:GOSUB 420
300 :     P$="N":N=N2-1:GOSUB 420
310 :     N1=3
320 :     N2=4
340 :   N=N-1
350 : NEXT N2
360 NEXT N1
370 P$="R":N=0:GOSUB 420
380 P$="K":N=0:GOSUB 420
390 P$="R":N=0:GOSUB 420
400 PRINT STR$(SP);":";AR$
410 END
420 FOR I=1 TO LEN(AR$)
430 : IF MID$(AR$,I,1)<>"-" THEN 510
440 : IF N<>0 THEN 480
450 : AR$=LEFT$(AR$,I-1)+P$+MID$(AR$,I+1)
460 : I=LEN(AR$)
470 : GOTO 510
480 : N=N-1
510 NEXT I
520 RETURN

Output:

The output is the same for both versions:

READY.
RUN
SPID (-1 FOR RANDOM):518
518:RNBQKBNR

READY.
RUN
SPID (-1 FOR RANDOM):-1
926:RKRQNBBN

READY.

FreeBASIC

Translation of: Yabasic

Randomize Timer
For i As Byte = 1 To 10
    Dim As String inicio = "RKR", pieza = "QNN"
    Dim As Byte posic
    
    For n As Byte = 1 To Len(pieza)
        posic = Int(Rnd*(Len(inicio) + 1)) + 1
        inicio = Left(inicio, posic-1) + _
        Mid(pieza, n, 1) +_
        Right(inicio, Len(inicio) - posic + 1)
    Next n
    posic = Int(Rnd*(Len(inicio) + 1)) + 1
    inicio = Left(inicio, posic-1) + "B" + Right(inicio, Len(inicio) - posic + 1)
    posic = posic + 1 + 2 * Int(Int(Rnd*(Len(inicio) - posic)) / 2)
    inicio = Left(inicio, posic-1) + "B" + Right(inicio, Len(inicio) - posic + 1)
    Print inicio
Next i

QBasic

Translation of: Yabasic

RANDOMIZE TIMER

FOR i = 1 TO 10
    inicio$ = "RKR"
    pieza$ = "QNN"
    'Dim posic
    
    FOR n = 1 TO LEN(pieza$)
        posic = INT(RND * (LEN(inicio$) + 1)) + 1
        inicio$ = LEFT$(inicio$, posic - 1) + MID$(pieza$, n, 1) + RIGHT$(inicio$, LEN(inicio$) - posic + 1)
    NEXT n
    posic = INT(RND * (LEN(inicio$) + 1)) + 1
    inicio$ = LEFT$(inicio$, posic - 1) + "B" + RIGHT$(inicio$, LEN(inicio$) - posic + 1)
    posic = posic + 1 + 2 * INT(INT(RND * (LEN(inicio$) - posic)) / 2)
    inicio$ = LEFT$(inicio$, posic - 1) + "B" + RIGHT$(inicio$, LEN(inicio$) - posic + 1)
    PRINT inicio$
NEXT i
END

Yabasic

Translation of: Seed7

start$ = "RKR"
piece$ = "QNN"

for piece = 1 to len(piece$)
    pos = int(ran(len(start$) + 1)) + 1
    start$ = left$(start$, pos-1) + mid$(piece$, piece, 1) + right$(start$, len(start$) - pos + 1)
next
pos = int(ran(len(start$) + 1)) + 1
start$ = left$(start$, pos-1) + "B" + right$(start$, len(start$) - pos + 1)
pos = pos + 1 + 2 * int(int(ran(len(start$) - pos)) / 2)
start$ = left$(start$, pos-1) + "B" + right$(start$, len(start$) - pos + 1)
print start$

Befunge

Similar to the Ruby SP-ID solution, this generates the start position for a random number in the Chess960 numbering scheme.

#.#.#.#.065*0#v_1-\>>?1v
v,":".:%*8"x"$<^!:\*2<+<
>48*,:4%2*1#v+#02#\3#g<<
v"B"*2%4:/4p<vg0:+1<\-1<
>\0p4/:6%0:0g>68*`#^_\:|
v"RKRNN"p11/6$p0\ "Q" \<
>"NRNKRRNNKRNRKNRRNKNR"v
v"NRNKRNRKNRNRKRNRNNKR"<
>"RKRNN"11g:!#v_\$\$\$\v
v _v#!`*86:g0:<^!:-1$\$<
>$\>,1+ :7`#@_^> v960v <

Output:

856 : RBKNBRNQ

C

As noted in the C implementation for the Sparkline in unicode task, unicode output is reliable only on Linux/Unix systems. This implementation thus has compiler directives to check whether the underlying system is Windows or Linux, if Windows, only letters are printed, otherwise Unicode output is displayed. 9 rows are displayed.

#include<stdlib.h>
#include<locale.h>
#include<wchar.h>
#include<stdio.h>
#include<time.h>

char rank[9];

int pos[8];

void swap(int i,int j){
	int temp = pos[i];
	pos[i] = pos[j];
	pos[j] = temp;
}

void generateFirstRank(){
	 int kPos,qPos,bPos1,bPos2,rPos1,rPos2,nPos1,nPos2,i;
	 
	 for(i=0;i<8;i++){
		 rank[i] = 'e';
		 pos[i] = i;
	 }
		 
	 do{
		 kPos = rand()%8;
		 rPos1 = rand()%8;
		 rPos2 = rand()%8;
	 }while((rPos1-kPos<=0 && rPos2-kPos<=0)||(rPos1-kPos>=0 && rPos2-kPos>=0)||(rPos1==rPos2 || kPos==rPos1 || kPos==rPos2));

	 rank[pos[rPos1]] = 'R';
	 rank[pos[kPos]] = 'K';
	 rank[pos[rPos2]] = 'R';
	 
	 swap(rPos1,7);
	 swap(rPos2,6);
	 swap(kPos,5);
	 
	 do{
		 bPos1 = rand()%5;
		 bPos2 = rand()%5;
	 }while(((pos[bPos1]-pos[bPos2])%2==0)||(bPos1==bPos2));

	 rank[pos[bPos1]] = 'B';
	 rank[pos[bPos2]] = 'B';
	 
	 swap(bPos1,4);
	 swap(bPos2,3);
	 
	 do{
		 qPos = rand()%3;
		 nPos1 = rand()%3;
	 }while(qPos==nPos1);
	 
	 rank[pos[qPos]] = 'Q';
	 rank[pos[nPos1]] = 'N';
	 
	 for(i=0;i<8;i++)
		 if(rank[i]=='e'){
			 rank[i] = 'N';
			 break;
		 }		
}

void printRank(){
	int i;
	
	#ifdef _WIN32
		printf("%s\n",rank);
	#else
	{
		setlocale(LC_ALL,"");
		printf("\n");
		for(i=0;i<8;i++){
			if(rank[i]=='K')
				printf("%lc",(wint_t)9812);
			else if(rank[i]=='Q')
				printf("%lc",(wint_t)9813);
			else if(rank[i]=='R')
				printf("%lc",(wint_t)9814);
			else if(rank[i]=='B')
				printf("%lc",(wint_t)9815);
			if(rank[i]=='N')
				printf("%lc",(wint_t)9816);
		}
	}
	#endif
}

int main()
{
	int i;
	
	srand((unsigned)time(NULL));
	
	for(i=0;i<9;i++){
		generateFirstRank();
		printRank();
	}
	
	return 0;
}

Output on Linux :

♗♗♖♕♘♘♔♖
♘♖♕♔♗♗♖♘
♖♘♔♖♕♘♗♗
♘♘♖♗♗♔♖♕
♗♘♖♘♕♔♖♗
♕♗♗♘♖♔♘♖
♕♘♖♔♗♖♘♗
♗♘♘♕♖♔♖♗
♖♘♘♕♗♔♖♗

Output on Windows :

BRKNNQRB
RBNQNKBR
RNQKNBBR
RQNKNBBR
QBBNRKNR
BNRBQKNR
BRQKNNRB
RNKBNRBQ
QNRBBNKR

C#

Translation of: Go

using System;

class Program
{
    struct Symbols
    {
        public char K, Q, R, B, N;

        public Symbols(char k, char q, char r, char b, char n)
        {
            K = k; Q = q; R = r; B = b; N = n;
        }
    }

    private static Symbols A = new Symbols('K', 'Q', 'R', 'B', 'N');
    private static Symbols W = new Symbols('♔', '♕', '♖', '♗', '♘');
    private static Symbols B = new Symbols('♚', '♛', '♜', '♝', '♞');

    private static string[] krn = new string[]
    {
        "nnrkr", "nrnkr", "nrknr", "nrkrn",
        "rnnkr", "rnknr", "rnkrn",
        "rknnr", "rknrn",
        "rkrnn"
    };

    private static string Chess960(Symbols sym, int id)
    {
        char[] pos = new char[8];
        int q = id / 4, r = id % 4;
        pos[r * 2 + 1] = sym.B;
        r = q % 4; q /= 4; 
        pos[r * 2] = sym.B;
        r = q % 6; q /= 6; 
        int placementIndex = 0; // Adjusted variable name to prevent conflict
        for (int i = 0; ; i++)
        {
            if (pos[i] != '\0') continue;
            if (r == 0)
            {
                pos[i] = sym.Q;
                break;
            }
            r--;
        }
        while (pos[placementIndex] != '\0') placementIndex++; // Adjusted loop to prevent conflict
        foreach (char f in krn[q])
        {
            while (pos[placementIndex] != '\0') placementIndex++;
            switch (f)
            {
                case 'k':
                    pos[placementIndex] = sym.K;
                    break;
                case 'r':
                    pos[placementIndex] = sym.R;
                    break;
                case 'n':
                    pos[placementIndex] = sym.N;
                    break;
            }
        }
        return new string(pos);
    }

    static void Main(string[] args)
    {
        Console.WriteLine(" ID  Start position");
        foreach (int id in new int[] { 0, 518, 959 })
        {
            Console.WriteLine($"{id,3}  {Chess960(A, id)}");
        }
        Console.WriteLine("\nRandom");
        Random rand = new Random();
        for (int i = 0; i < 5; i++)
        {
            Console.WriteLine(Chess960(W, rand.Next(960)));
        }
    }
}

Output:

 ID  Start position
  0  BBQNNRKR
518  RNBQKBNR
959  RKRNNQBB

Random
♘♕♖♔♗♖♘♗
♗♗♖♘♔♖♕♘
♖♕♘♔♖♗♗♘
♘♖♗♔♘♗♖♕
♖♘♕♗♔♘♗♖

C++

#include <iostream>
#include <string>
#include <time.h>
using namespace std;

namespace
{
    void placeRandomly(char* p, char c)
    {
	int loc = rand() % 8;
	if (!p[loc])
	    p[loc] = c;
	else
	    placeRandomly(p, c);    // try again
    }
    int placeFirst(char* p, char c, int loc = 0)
    {
	while (p[loc]) ++loc;
	p[loc] = c;
        return loc;
    }

    string startPos()
    {
	char p[8]; memset( p, 0, 8 );

	// bishops on opposite color
	p[2 * (rand() % 4)] = 'B';
	p[2 * (rand() % 4) + 1] = 'B';

	// queen knight knight, anywhere
	for (char c : "QNN")
	    placeRandomly(p, c);

	// rook king rook, in that order
	placeFirst(p, 'R', placeFirst(p, 'K', placeFirst(p, 'R')));

	return string(p, 8);
    }
}   // leave local

namespace chess960
{
    void generate( int c )
    {
	for( int x = 0; x < c; x++ )
	    cout << startPos() << "\n";
    }
}

int main( int argc, char* argv[] )
{
    srand( time( NULL ) );
    chess960::generate( 10 );
    cout << "\n\n";
    return system( "pause" );
}

Output:

NQBRNBKR
RKBQNBNR
RKBRNNQB
QRBNNKRB
BRKNRBQN
QNRBBKNR
BQRBKNRN
RNBKQBNR
QRNKBBRN
QRBKNBRN

Clojure

(ns c960.core
  (:gen-class)
  (:require [clojure.string :as s]))

;; legal starting rank - unicode chars for rook, knight, bishop, queen, king, bishop, knight, rook
(def starting-rank [\♖ \♘ \♗ \♕ \♔ \♗ \♘ \♖])

(defn bishops-legal?
  "True if Bishops are odd number of indicies apart"
  [rank]
  (odd? (apply - (cons 0 (sort > (keep-indexed #(when (= \♗ %2) %1) rank))))))

(defn king-legal?
  "True if the king is between two rooks"
  [rank]
  (let [king-&-rooks (filter #{\♔ \♖} rank)]
    (and
     (= 3 (count king-&-rooks))
     (= \u2654 (second king-&-rooks)))))


(defn c960
  "Return a legal rank for c960 chess"
  ([] (c960 1))
  ([n]
   (->> #(shuffle starting-rank)
        repeatedly
        (filter #(and (king-legal? %) (bishops-legal? %)))
        (take n)
        (map #(s/join ", " %)))))


(c960)
;; => "♗, ♖, ♔, ♕, ♘, ♘, ♖, ♗"
(c960)
;; => "♖, ♕, ♘, ♔, ♗, ♗, ♘, ♖"
(c960 4)
;; => ("♘, ♖, ♔, ♘, ♗, ♗, ♖, ♕" "♗, ♖, ♔, ♘, ♘, ♕, ♖, ♗" "♘, ♕, ♗, ♖, ♔, ♗, ♘, ♖" "♖, ♔, ♘, ♘, ♕, ♖, ♗, ♗")

Common Lisp

Common Lisp: generate from SP-ID

Translation of: Raku

(defun chess960-from-sp-id
  (&optional (sp-id (random 360 (make-random-state t))))
    (labels
      ((combinations (lst r)
          (cond
            ((numberp lst)
              (combinations (loop for i from 0 while (< i lst) collect i) r))
            ((= r 1)
              (mapcar #'list lst))
            (t
              (loop for i in lst append
                (let ((left (loop for j in lst if (< i j) collect j)))
                  (mapcar (lambda (c) (cons i c))
                          (combinations left (1- r))))))))

       (enumerate (ary)
          (loop for item across ary for index from 0
                collect (list index item))))

       (let* 
         ((first-bishop -1)
          (knight-combo '())
          (placements (list
            ;divisor  function to get position                              piece symbol
            (list  4  (lambda (n) (setq first-bishop n)
                                  (1+ (* 2 n)))                             '♝)
            (list  4  (lambda (n) ( - (* 2 n) (if (> n first-bishop) 1 0))) '♝)
            (list  6  #'identity                                            '♛)
            (list 10  (lambda (n)
                        (setq knight-combo (nth n (combinations 5 2)))
                        (car knight-combo))                                 '♞)
            (list  1  (lambda (n) (1- (cadr knight-combo)))                 '♞)
            (list  1  (lambda (n) 0)                                        '♜)
            (list  1  (lambda (n) 0)                                        '♚)
            (list  1  (lambda (n) 0)                                        '♜)))
          (p sp-id)
          (ary (make-array 8 :initial-element '-)))

         (loop for (divisor func piece) in placements doing
           (let* ((n (mod p divisor))
             (square (funcall func n)))
              (setq p (floor p divisor))
              (setq index 
                (car (nth square (remove-if-not (lambda (p) (eq (cadr p) '-))
                                                (enumerate ary)))))
              (setf (aref ary index) piece)))

      (list sp-id ary))))

;; demo 

(format t "~a~%" (chess960-from-sp-id 518))
(format t "~a~%" (chess960-from-sp-id))

Output:

(518 #(♜ ♞ ♝ ♛ ♚ ♝ ♞ ♜))
(246 #(♞ ♜ ♝ ♚ ♛ ♝ ♞ ♜))

D

Translation of: Python

D: Indexing

void main() {
    import std.stdio, std.range, std.algorithm, std.string, permutations2;

    const pieces = "KQRrBbNN";
    alias I = indexOf;
    auto starts = pieces.dup.permutations.filter!(p =>
            I(p, 'B') % 2 != I(p, 'b') % 2 && // Bishop constraint.
            // King constraint.
            ((I(p, 'r') < I(p, 'K') && I(p, 'K') < I(p, 'R')) ||
             (I(p, 'R') < I(p, 'K') && I(p, 'K') < I(p, 'r'))))
        .map!toUpper.array.sort().uniq;
    writeln(starts.walkLength, "\n", starts.front);
}

Output:

960
BBNNQRKR

D: Regexp

void main() {
    import std.stdio, std.regex, std.range, std.algorithm, permutations2;

    immutable pieces = "KQRRBBNN";
    immutable bish = r"B(|..|....|......)B";
    immutable king = r"R.*K.*R";
    auto starts3 = permutations(pieces.dup)
                   .filter!(p => p.match(bish) && p.match(king))
                   .array.sort().uniq;
    writeln(starts3.walkLength, "\n", starts3.front);
}

The output is the same.

D: Correct by construction

void main() {
    import std.stdio, std.random, std.array, std.range;

    // Subsequent order unchanged by insertions.
    auto start = "RKR".dup;
    foreach (immutable piece; "QNN")
        start.insertInPlace(uniform(0, start.length), piece);

    immutable bishpos = uniform(0, start.length);
    start.insertInPlace(bishpos, 'B');
    start.insertInPlace(iota(bishpos % 2, start.length, 2)[uniform(0,$)], 'B');
    start.writeln;
}

Output:

QBNNBRKR

EasyLang

Translation of: Lua

len t$[] 8
proc randins c$ l r . pos .
   repeat
      pos = randint (r - l + 1) + l - 1
      until t$[pos] = ""
   .
   t$[pos] = c$
.
randins "K" 2 7 king
randins "R" 1 (king - 1) h
randins "R" (king + 1) 8 h
randins "B" 1 8 b1
repeat
   randins "B" 1 8 b2
   until (b2 - b1) mod 2 <> 0
   t$[b2] = ""
.
randins "Q" 1 8 b1
randins "N" 1 8 b1
randins "N" 1 8 b1
print strjoin t$[]

Output:

RBBNQNKR

EchoLisp

(define-values (K Q R B N) (iota 5))
(define *pos* (list R N B Q K B N R)) ;; standard starter

;; check opposite color bishops, and King between rooks
(define (legal-pos p)
    (and
            (> (list-index K p) (list-index R p))
            (> (list-index K (reverse p)) (list-index R (reverse p)))
            (even? (+ (list-index B p) (list-index B (reverse p))))))
            
;; random shuffle current position until a legal one is found
(define (c960) 
	(set! *pos* (shuffle *pos*)) 
	(if (legal-pos *pos*) 
		(map unicode-piece *pos*)  (c960)))

Output:

(define (unicode-piece i) (unicode->string (+ 0x2654 i)))

(legal-pos *pos*) → #t ;; starter is OK
(c960)
 (♗ ♖ ♔ ♗ ♕ ♘ ♘ ♖)
(c960)
 (♘ ♗ ♗ ♕ ♖ ♘ ♔ ♖)
(c960)
 (♖ ♘ ♗ ♘ ♔ ♕ ♖ ♗)
;; etc.

Elixir

Translation of: Ruby

Works with: Elixir version 1.1

Elixir: shuffle pieces until all regexes match

defmodule Chess960 do
  @pieces   ~w(♔ ♕ ♘ ♘ ♗ ♗ ♖ ♖)             # ~w(K Q N N B B R R)
  @regexes  [~r/♗(..)*♗/, ~r/♖.*♔.*♖/]        # [~r/B(..)*B/, ~r/R.*K.*R/]
  
  def shuffle do
    row = Enum.shuffle(@pieces) |> Enum.join
    if Enum.all?(@regexes, &Regex.match?(&1, row)), do: row, else: shuffle
  end
end

Enum.each(1..5, fn _ -> IO.puts Chess960.shuffle end)

Output:

♘♗♘♖♗♔♕♖
♗♖♔♗♕♘♘♖
♗♗♕♖♔♖♘♘
♘♗♖♔♗♘♕♖
♖♕♘♘♗♗♔♖

Elixir: Construct

defmodule Chess960 do
  def construct do
    row = Enum.reduce(~w[♕ ♘ ♘], ~w[♖ ♔ ♖], fn piece,acc ->
            List.insert_at(acc, :rand.uniform(length(acc)+1)-1, piece)
          end)
    [Enum.random([0, 2, 4, 6]), Enum.random([1, 3, 5, 7])]
    |> Enum.sort
    |> Enum.reduce(row, fn pos,acc -> List.insert_at(acc, pos, "♗") end)
    |> Enum.join
  end
end

Enum.each(1..5, fn _ -> IO.puts Chess960.construct end)

Output:

♖♔♗♘♖♕♘♗
♘♗♘♕♖♔♗♖
♗♖♔♘♘♗♖♕
♖♗♘♘♕♔♗♖
♖♕♗♘♘♗♔♖

Elixir: Generate from SP-ID

defmodule Chess960 do
  @krn  ~w(NNRKR NRNKR NRKNR NRKRN RNNKR RNKNR RNKRN RKNNR RKNRN RKRNN)
  
  def start_position, do: start_position(:rand.uniform(960)-1)
  
  def start_position(id) do
    pos = List.duplicate(nil, 8)
    q = div(id, 4)
    r = rem(id, 4)
    pos = List.replace_at(pos, r * 2 + 1, "B")
    q = div(q, 4)
    r = rem(q, 4)
    pos = List.replace_at(pos, r * 2, "B")
    q = div(q, 6)
    r = rem(q, 6)
    i = Enum.reject(0..7, &Enum.at(pos,&1)) |> Enum.at(r)
    pos = List.replace_at(pos, i, "Q")
    krn = Enum.at(@krn, q) |> String.codepoints
    Enum.reject(0..7, &Enum.at(pos,&1))
    |> Enum.zip(krn)
    |> Enum.reduce(pos, fn {i,x},acc -> List.replace_at(acc,i,x) end)
    |> Enum.join
  end
end

IO.puts "Generate Start Position from ID number"
Enum.each([0,518,959], fn id ->
  :io.format "~3w : ~s~n", [id, Chess960.start_position(id)]
end)
IO.puts "\nGenerate random Start Position"
Enum.each(1..5, fn _ -> IO.puts Chess960.start_position end)

Output:

Generate Start Position from ID number
  0 : BBQNNRKR
518 : BRNKNBRQ
959 : RKRQNNBB

Generate random Start Position
RQKBBNNR
RBBQKNNR
RQKNNRBB
RKRQBBNN
RNBNKQRB

Factor

Single die method

Using the single die method: https://en.wikipedia.org/wiki/Chess960_starting_position#Single_die_method

USING: io kernel math random sequences ;
IN: rosetta-code.chess960

: empty ( seq -- n ) 32 swap indices random ;           ! return a random empty index (i.e. equal to 32) of seq
: next ( seq -- n ) 32 swap index ;                     ! return the leftmost empty index of seq
: place ( seq elt n -- seq' ) rot [ set-nth ] keep ;    ! set nth member of seq to elt, keeping seq on the stack

: white-bishop ( -- elt n ) CHAR: ♗ 4 random 2 * ;
: black-bishop ( -- elt n ) white-bishop 1 + ;
: queen ( seq -- seq elt n ) CHAR: ♕ over empty ;
: knight ( seq -- seq elt n ) CHAR: ♘ over empty ;
: rook ( seq -- seq elt n ) CHAR: ♖ over next ;
: king ( seq -- seq elt n ) CHAR: ♔ over next ;

: chess960 ( -- str )
    "        " clone
    black-bishop place
    white-bishop place
    queen place
    knight place
    knight place
    rook place
    king place
    rook place ;

: chess960-demo ( -- ) 5 [ chess960 print ] times ;

MAIN: chess960-demo

Output:

♕♖♗♘♔♘♖♗
♕♗♖♘♗♘♔♖
♗♘♕♖♔♗♖♘
♘♖♗♕♔♗♘♖
♗♗♘♖♘♔♕♖

Built-in

Factor comes with a chess960 position generator:

USING: chess960 prettyprint ;

chess960-position .

Output:

{ rook bishop king knight bishop queen rook knight }

Forth

\ make starting position for Chess960, constructive

\             0    1    2    3    4    5    6    7    8    9
create krn S" NNRKRNRNKRNRKNRNRKRNRNNKRRNKNRRNKRNRKNNRRKNRNRKRNN" mem,

create pieces 8 allot

: chess960 ( n -- )
  pieces 8 erase
  4 /mod swap  2* 1+ pieces + 'B swap c!
  4 /mod swap  2*    pieces + 'B swap c!
  6 /mod swap  pieces swap bounds begin dup c@ if swap 1+ swap then 2dup > while 1+ repeat drop 'Q swap c!
  5 * krn +  pieces 8 bounds do i c@ 0= if dup c@ i c! 1+ then loop drop
  cr pieces 8 type ;

0   chess960   \ BBQNNRKR ok
518 chess960   \ RNBQKBNR ok
959 chess960   \ RKRNNQBB ok

960 choose chess960    \ random position

Fortran

This implementation simply iterates through all 960 positions.

program chess960
    implicit none
    
    integer, pointer  :: a,b,c,d,e,f,g,h
    integer, target   :: p(8)
    a => p(1)
    b => p(2)
    c => p(3)
    d => p(4)
    e => p(5)
    f => p(6)
    g => p(7)
    h => p(8)

    king: do a=2,7                                        ! King on an internal square
        r1: do b=1,a-1                                    ! R1 left of the King
            r2: do c=a+1,8                                ! R2 right of the King
                b1: do d=1,7,2                            ! B1 on an odd square
                    if (skip_pos(d,4)) cycle
                    b2: do e=2,8,2                        ! B2 on an even square
                        if (skip_pos(e,5)) cycle
                        queen: do f=1,8                   ! Queen anywhere else
                            if (skip_pos(f,6)) cycle
                            n1: do g=1,7                  ! First knight
                                if (skip_pos(g,7)) cycle
                                n2: do h=g+1,8            ! Second knight (indistinguishable from first)
                                    if (skip_pos(h,8)) cycle
                                    if (sum(p) /= 36) stop 'Loop error'  ! Sanity check
                                    call write_position
                                end do n2
                            end do n1
                        end do queen
                    end do b2
                end do b1
            end do r2
        end do r1
    end do king
    
contains

    logical function skip_pos(i, n)
        integer, intent(in) :: i, n
        skip_pos = any(p(1:n-1) == i)
    end function skip_pos

    subroutine write_position
        integer           :: i, j
        character(len=15) :: position = ' '
        character(len=1), parameter  :: names(8) = ['K','R','R','B','B','Q','N','N']
        do i=1,8
            j = 2*p(i)-1
            position(j:j) = names(i)
        end do
        write(*,'(a)') position
    end subroutine write_position
    
end program chess960

Output:

The first ten positions:

R K R B B Q N N
R K R B B N Q N
R K R B B N N Q
R K R Q B B N N
R K R N B B Q N
R K R N B B N Q
R K R Q B N N B
R K R N B Q N B
R K R N B N Q B
R K R B Q N B N

Go

Translation of: Ruby

package main

import (
    "fmt"
    "math/rand"
)

type symbols struct{ k, q, r, b, n rune }

var A = symbols{'K', 'Q', 'R', 'B', 'N'}
var W = symbols{'♔', '♕', '♖', '♗', '♘'}
var B = symbols{'♚', '♛', '♜', '♝', '♞'}

var krn = []string{
    "nnrkr", "nrnkr", "nrknr", "nrkrn",
    "rnnkr", "rnknr", "rnkrn",
    "rknnr", "rknrn",
    "rkrnn"}

func (sym symbols) chess960(id int) string {
    var pos [8]rune
    q, r := id/4, id%4
    pos[r*2+1] = sym.b
    q, r = q/4, q%4
    pos[r*2] = sym.b
    q, r = q/6, q%6
    for i := 0; ; i++ {
        if pos[i] != 0 {
            continue
        }
        if r == 0 {
            pos[i] = sym.q
            break
        }
        r--
    }
    i := 0
    for _, f := range krn[q] {
        for pos[i] != 0 {
            i++
        }
        switch f {
        case 'k':
            pos[i] = sym.k
        case 'r':
            pos[i] = sym.r
        case 'n':
            pos[i] = sym.n
        }
    }
    return string(pos[:])
}

func main() {
    fmt.Println(" ID  Start position")
    for _, id := range []int{0, 518, 959} {
        fmt.Printf("%3d  %s\n", id, A.chess960(id))
    }
    fmt.Println("\nRandom")
    for i := 0; i < 5; i++ {
        fmt.Println(W.chess960(rand.Intn(960)))
    }
}

Output:

 ID  Start position
  0  BBQNNRKR
518  RNBQKBNR
959  RKRNNQBB

Random
♗♘♖♗♘♔♕♖
♕♘♖♔♘♖♗♗
♖♘♗♔♖♕♘♗
♘♘♖♕♗♔♖♗
♗♕♘♗♘♖♔♖

Haskell

import Data.List
import qualified Data.Set as Set

data Piece = K | Q | R | B | N deriving (Eq, Ord, Show)

isChess960 :: [Piece] -> Bool
isChess960 rank =
  (odd . sum $ findIndices (== B) rank) && king > rookA && king < rookB
  where
    Just king      = findIndex (== K) rank
    [rookA, rookB] = findIndices (== R) rank

main :: IO ()
main = mapM_ (putStrLn . concatMap show) . Set.toList . Set.fromList
       . filter isChess960 $ permutations [R,N,B,Q,K,B,N,R]

Output:

QRKRBBNN
QRKRBNNB
QRKRNBBN
QRKRNNBB
QRKBRNBN
...

J

The simplest J approach for this task is to initially build a table of the starting positions. Then, for the task, we pick one position from the table at random. There are 40320 distinct permutations of 8 items and 5040 distinct permutations of these chess pieces and (as the task name points out) only 960 permutations which also satisfy the constraints on bishop and rook position, so little memory is needed to generate the table. Also, since the table is built at "compile time", execution is fast (though "compilation" is reasonably fast also).

row0=: u: 9812+2}.5|i.10
king=: u:9812
rook=: u:9814
bish=: u:9815
pos=: I.@e.
bishok=: 1=2+/ .| pos&bish
rookok=: pos&rook -: (<./,>./)@pos&(rook,king)
ok=: bishok*rookok
perm=: A.&i.~ !
valid=: (#~ ok"1) ~.row0{"1~perm 8
gen=: valid {~ ? bind 960

Example use:

   gen''
♘♗♖♔♗♕♖♘
   gen''
♗♘♘♗♖♔♖♕
   gen''
♖♗♔♘♘♕♗♖
   gen''
♖♔♕♗♗♘♖♘

Java

Works with: Java version 1.5+

Regex inspired by (original) Python Regexp, prints ten examples.

import java.util.Arrays;
import java.util.Collections;
import java.util.List;

public class Chess960{
	private static List<Character> pieces = Arrays.asList('R','B','N','Q','K','N','B','R');

	public static List<Character> generateFirstRank(){
		do{
			Collections.shuffle(pieces);
		}while(!check(pieces.toString().replaceAll("[^\\p{Upper}]", ""))); //List.toString adds some human stuff, remove that 
		
		return pieces;
	}

	private static boolean check(String rank){
		if(!rank.matches(".*R.*K.*R.*")) return false;			//king between rooks
		if(!rank.matches(".*B(..|....|......|)B.*")) return false;	//all possible ways bishops can be placed
		return true;
	}

	public static void main(String[] args){
		for(int i = 0; i < 10; i++){
			System.out.println(generateFirstRank());
		}
	}
}

Output:

[R, N, K, N, R, B, B, Q]
[B, B, Q, R, N, K, N, R]
[R, K, Q, N, N, R, B, B]
[N, B, B, N, R, K, Q, R]
[R, Q, B, B, K, N, N, R]
[R, K, B, Q, N, B, N, R]
[N, N, R, K, Q, B, B, R]
[R, N, K, Q, N, B, B, R]
[N, R, B, K, Q, B, N, R]
[N, Q, N, R, K, B, B, R]

JavaScript

This conforms to Altendörfer's single die method[1], though the die will give no "needless" numbers.

function ch960startPos() {
  var rank = new Array(8),
      // randomizer (our die)
      d = function(num) { return Math.floor(Math.random() * ++num) },
      emptySquares = function() {
        var arr = [];
        for (var i = 0; i < 8; i++) if (rank[i] == undefined) arr.push(i);
        return arr;
      };
  // place one bishop on any black square
  rank[d(2) * 2] = "♗";
  // place the other bishop on any white square
  rank[d(2) * 2 + 1] = "♗";
  // place the queen on any empty square
  rank[emptySquares()[d(5)]] = "♕";
  // place one knight on any empty square
  rank[emptySquares()[d(4)]] = "♘";
  // place the other knight on any empty square
  rank[emptySquares()[d(3)]] = "♘";
  // place the rooks and the king on the squares left, king in the middle
  for (var x = 1; x <= 3; x++) rank[emptySquares()[0]] = x==2 ? "♔" : "♖";
  return rank;
}

// testing (10 times)
for (var x = 1; x <= 10; x++) console.log(ch960startPos().join(" | "));

Output:

The test-output (exemplary each):

♖ | ♗ | ♗ | ♔ | ♘ | ♖ | ♘ | ♕
♗ | ♗ | ♕ | ♖ | ♔ | ♘ | ♘ | ♖
♖ | ♕ | ♘ | ♗ | ♗ | ♔ | ♘ | ♖
♖ | ♗ | ♔ | ♘ | ♗ | ♕ | ♘ | ♖
♗ | ♖ | ♕ | ♔ | ♘ | ♗ | ♘ | ♖
♖ | ♗ | ♗ | ♕ | ♔ | ♘ | ♖ | ♘
♗ | ♘ | ♖ | ♗ | ♔ | ♘ | ♕ | ♖
♕ | ♘ | ♗ | ♖ | ♔ | ♗ | ♖ | ♘
♗ | ♘ | ♖ | ♘ | ♕ | ♗ | ♔ | ♖
♘ | ♗ | ♖ | ♔ | ♗ | ♘ | ♖ | ♕

jq

Adapted from Wren

Works with: jq

Also works with gojq, the Go implementation of jq

Also works with fq, a Go implementation of a large subset of jq

### Utilities
# The glyphs in .
def chars: explode[] | [.] | implode;

# input: an array
# $keys : an array of strings
def objectify($keys):
  with_entries(.key = $keys[.key]) ;

def lpad($len): tostring | ($len - length) as $l | (" " * $l)[:$l] + .;


def Symbols: ["k", "q", "r", "b", "n"];

def A: ["K", "Q", "R", "B", "N"]     | objectify(Symbols);
def W: ["♔", "♕", "♖", "♗", "♘"]  | objectify(Symbols);
def B: ["♚", "♛", "♜", "♝", "♞"]  | objectify(Symbols);

def krn: [
    "nnrkr", "nrnkr", "nrknr", "nrkrn",
    "rnnkr", "rnknr", "rnkrn",
    "rknnr", "rknrn",
    "rkrnn"
];

# $sym specifies the Symbols
# $id specifies the position
def chess960($sym):
  . as $id
  | { q:   (($id/4)|floor),
      r:   ($id % 4)
    }
  | .pos[.r*2+1] = $sym.b
  | .t = .q
  | .q |= ((./4)|floor)
  | .r = (.t % 4)
  | .pos[.r*2] = $sym.b
  | .t = .q
  | .q |= ((./6)|floor)
  | .r = .t % 6
  | .i = 0
  | .break = false
  | until( .break; 
      if .pos[.i] == null
      then if .r == 0
           then .pos[.i] = $sym.q
           | .break = true
           else .r += -1
           end
      else .
      end
      | .i += 1 )
  | .i = 0      
  | reduce (krn[.q]|chars) as $f (.;
      # find next insertion point
      until(.pos[.i] == null; .i += 1)
      | if  $f | IN("k", "r", "n")
        then .pos[.i] = $sym[$f]
        else .
        end )
  | .pos
  | join(" ") ;

def display960($sym):
 "\(lpad(3))  \(chess960($sym))";
 
" ID  Start position",
( 0, 518, 959 |  display960(A) ),
"\nPseudo-random starting positions:",
(699, 889, 757, 645, 754 | display960(W))

Output:

 ID  Start position
  0  B B Q N N R K R
518  R N B Q K B N R
959  R K R N N Q B B

Pseudo-random starting positions:
699  ♖ ♕ ♔ ♘ ♗ ♘ ♖ ♗
889  ♖ ♕ ♔ ♗ ♗ ♖ ♘ ♘
757  ♖ ♔ ♗ ♗ ♘ ♘ ♖ ♕
645  ♖ ♘ ♗ ♗ ♔ ♖ ♕ ♘
754  ♗ ♖ ♔ ♘ ♘ ♗ ♖ ♕

Julia

Works with: Julia version 0.6

function generateposition()
    # Placeholder knights
    rank = ['♘', '♘', '♘', '♘', '♘', '♘', '♘', '♘']
    lrank = length(rank)

    # Check if a space is available
    isfree(x::Int) = rank[x] == '♘'

    # Place the King
    rank[indking = rand(2:lrank-1)] = '♔'

    # Place rooks
    rank[indrook = rand(filter(isfree, 1:lrank))] = '♖'
    if indrook > indking
        rank[rand(filter(isfree, 1:indking-1))] = '♖'
    else
        rank[rand(filter(isfree, indking+1:lrank))] = '♖'
    end

    # Place bishops
    rank[indbish = rand(filter(isfree, 1:8))] = '♗'
    pbish = filter(iseven(indbish) ? isodd : iseven, 1:lrank)
    rank[rand(filter(isfree, pbish))] = '♗'

    # Place queen
    rank[rand(filter(isfree, 1:lrank))] = '♕'
    return rank
end

@show generateposition()

Output:

generateposition() = ['♘', '♗', '♗', '♖', '♕', '♔', '♘', '♖']

Kotlin

object Chess960 : Iterable<String> {
    override fun iterator() = patterns.iterator()

    private operator fun invoke(b: String, e: String) {
        if (e.length <= 1) {
            val s = b + e
            if (s.is_valid()) patterns += s
        } else {
            for (i in 0 until e.length) {
                invoke(b + e[i], e.substring(0, i) + e.substring(i + 1))
            }
        }
    }

    private fun String.is_valid(): Boolean {
        val k = indexOf('K')
        return indexOf('R') < k && k < lastIndexOf('R') &&
            indexOf('B') % 2 != lastIndexOf('B') % 2
    }

    private val patterns = sortedSetOf<String>()

    init {
        invoke("", "KQRRNNBB")
    }
}

fun main(args: Array<String>) {
    Chess960.forEachIndexed { i, s -> println("$i: $s") }
}

Output:

0: BBNNQRKR
1: BBNNRKQR
2: BBNNRKRQ
...
957: RQNNBKRB
958: RQNNKBBR
959: RQNNKRBB

Lua

-- Insert 'str' into 't' at a random position from 'left' to 'right'
function randomInsert (t, str, left, right)
    local pos
    repeat pos = math.random(left, right) until not t[pos]
    t[pos] = str
    return pos
end

-- Generate a random Chess960 start position for white major pieces
function chess960 ()
    local t, b1, b2 = {}
    local kingPos = randomInsert(t, "K", 2, 7)
    randomInsert(t, "R", 1, kingPos - 1)
    randomInsert(t, "R", kingPos + 1, 8)
    b1 = randomInsert(t, "B", 1, 8)
    b2 = randomInsert(t, "B", 1, 8)
    while (b2 - b1) % 2 == 0 do
        t[b2] = false
        b2 = randomInsert(t, "B", 1, 8)
    end
    randomInsert(t, "Q", 1, 8)
    randomInsert(t, "N", 1, 8)
    randomInsert(t, "N", 1, 8)
    return t
end

-- Main procedure
math.randomseed(os.time())
print(table.concat(chess960()))

Output:

NNRQBBKR

M2000 Interpreter

Module Chess960 {
	function OneFrom960$ {
		def q$="♕", h$="♘", t$="♖", b$="♗", k$="♔"
		def integer b1, b2, t1, t2, k, q
		buffer p as integer *8
		return p, 0:=string$(h$, 8)
		k=random(1, 6)
		t1=random(0,k-1)
		t2=random(k+1, 7)
		used=list:=k, t1, t2
		do : b1=random(0,7): until not exist(used, b1)
		append used, b1
		n=b1 mod 2
		do : b2=random(0,7): until not exist(used, b2) and b2 mod 2 <> n 	
		append used, b2
		do : q=random(0,7): until not exist(used, q)	
		// place pawns to positions
		return p, k:=k$, t1:=t$, t2:=t$, b1:=b$, b2:=b$, q:=q$
		=Eval$(p)+{
		}  // append  new line to every solution
	}
	document doc$
	For i=0 to 7:doc$+=OneFrom960$(): next
	clipboard doc$
	report doc$
}
Chess960

Output:

♘♗♗♘♖♕♔♖
♗♕♖♘♘♔♖♗
♗♗♕♖♔♘♖♘
♘♖♕♗♗♔♖♘
♖♔♘♗♖♕♗♘
♖♔♖♗♘♘♗♕
♘♘♗♕♖♗♔♖
♖♔♕♘♘♗♗♖

Mathematica / Wolfram Language

This example does not show the output mentioned in the task description on this page (or a page linked to from here). Please ensure that it meets all task requirements and remove this message. Note that phrases in task descriptions such as "print and display" and "print and show" for example, indicate that (reasonable length) output be a part of a language's solution.

Generates all possible initial conditions, filters for validity, and chooses a random element.

Print[StringJoin[
   RandomChoice[
    Select[Union[
      Permutations[{"\[WhiteKing]", "\[WhiteQueen]", "\[WhiteRook]", 
        "\[WhiteRook]", "\[WhiteBishop]", "\[WhiteBishop]", 
        "\[WhiteKnight]", "\[WhiteKnight]"}]], 
     MatchQ[#, {___, "\[WhiteRook]", ___, "\[WhiteKing]", ___, 
         "\[WhiteRook]", ___}] && 
       OddQ[Subtract @@ Flatten[Position[#, "\[WhiteBishop]"]]] &]]]];

MiniScript

This version uses the Unicode piece characters. If running in Mini Micro — which supports Unicode but does not have these characters in its font — just replace the the piece characters with letters.

// placeholder knights
rank = ["♘"] * 8

// function to get a random free space from a to b, inclusive
randFree = function(a, b)
    free = []
    for i in range(a, b)
        if rank[i] == "♘" then free.push i
    end for
    return free[rnd * free.len]
end function

// place the king
kingIdx = randFree(1, 6)
rank[kingIdx] = "♔"

// place rooks
rank[randFree(0, kingIdx - 1)] = "♖"
rank[randFree(kingIdx + 1, 7)] = "♖"

// place bishops
bishIdx = randFree(0, 7)
rank[bishIdx] = "♗"
while true
    i = randFree(0, 7)
    if i % 2 != bishIdx % 2 then break
end while
rank[i] = "♗"

// place queen
rank[randFree(0, 7)] = "♕"

print join(rank, " ")

Output:

♘ ♖ ♕ ♔ ♖ ♘ ♗ ♗

Nim

import random, strutils

type

  # Chess pieces on first row.
  Pieces {.pure.} = enum
    King = "♔",
    Queen = "♕",
    Rook1 = "♖",
    Rook2 = "♖",
    Bishop1 = "♗",
    Bishop2 = "♗",
    Knight1 = "♘",
    Knight2 = "♘"

  # Position counted from 0.
  Position = range[0..7]

  # Position of pieces.
  Positions = array[Pieces, Position]


func pop[T](s: var set[T]): T =
  ## Remove and return the first element of a set.
  for val in s:
    result = val
    break
  s.excl(result)


proc choose[T](s: var set[T]): T =
  ## Choose randomly a value from a set and remove it from the set.
  result = sample(s)
  s.excl(result)


proc positions(): Positions =
  ## Return a randomly chosen list of piece positions for the first row.

  var pos = {Position.low..Position.high}

  # Set bishops.
  result[Bishop1] = sample([0, 2, 4, 6])    # Black squares.
  result[Bishop2] = sample([1, 3, 5, 7])    # White squares.
  pos = pos - {result[Bishop1], result[Bishop2]}

  # Set queen.
  result[Queen] = pos.choose()

  # Set knights.
  result[Knight1] = pos.choose()
  result[Knight2] = pos.choose()

  # In the remaining three pieces, the king must be between the two rooks.
  result[Rook1] = pos.pop()
  result[King] = pos.pop()
  result[Rook2] = pos.pop()


#———————————————————————————————————————————————————————————————————————————————————————————————————

randomize()

for _ in 1..10:
  var row: array[8, string]
  let pos = positions()
  for piece in Pieces:
    row[pos[piece]] = $piece
  echo row.join("  ")

Output:

♘  ♘  ♕  ♖  ♔  ♗  ♗  ♖
♖  ♗  ♗  ♔  ♕  ♘  ♘  ♖
♕  ♗  ♖  ♔  ♘  ♘  ♗  ♖
♖  ♘  ♗  ♕  ♔  ♘  ♖  ♗
♖  ♘  ♗  ♗  ♘  ♕  ♔  ♖
♕  ♘  ♖  ♗  ♗  ♔  ♘  ♖
♗  ♘  ♖  ♔  ♖  ♗  ♘  ♕
♖  ♔  ♕  ♘  ♗  ♖  ♘  ♗
♘  ♖  ♗  ♔  ♕  ♗  ♘  ♖
♗  ♕  ♖  ♗  ♘  ♔  ♖  ♘

Objeck

Translation of: C++

class Chess960 {
  function : Main(args : String[]) ~ Nil {
    Generate(10);
  }
  
  function : Generate(c : Int) ~ Nil {
    for(x := 0; x < c; x += 1;) {
      StartPos()->PrintLine();
    };
  }
  
  function : StartPos() ~ String {
    p := Char->New[8];
    
    # bishops
    b1 : Int; b2 : Int;
    while(true) {
      b1 := GetPosition(); b2 := GetPosition(); 
      
      b1c := b1 and 1; b2c := b2 and 1;
      c := b1c = 0 & b2c <> 0;
      if(c) {
        break;
      };
    };
    p[b1] := 0x2657; p[b2] := 0x2657;

    # queen, knight, knight
    q := false;
    for(x := 0; x < 3; x += 1;) {
      do { 
        b1 := GetPosition(); 
      } while( p[b1] <> '\0');
      
      if(<>q) { 
        p[b1] := 0x2655; q := true; 
      }
      else { 
        p[b1] := 0x2658;
      };
    };

    # rook king rook
    q := false;
    for(x := 0; x < 3; x += 1;) {
      a := 0;
      while(a < 8) {
        if(p[a] = '\0') {
          break;
        };
        a += 1;
      };

      if(<>q) { 
        p[a] := 0x2656; q := true; 
      }
      else { 
        p[a] := 0x2654; q := false; 
      };
    };

    s := "";
    for(x := 0; x < 8; x += 1;) { s->Append(p[x]); };
    return s;
  }

  function : GetPosition() ~ Int {
    return (Float->Random() * 1000)->As(Int) % 8;
  }
}

Output:

♗♖♕♔♖♗♘♘
♕♗♖♔♗♘♖♘
♖♘♔♘♕♖♗♗
♖♗♔♘♖♘♗♕
♖♔♖♘♕♗♗♘
♗♘♖♕♔♘♖♗
♗♖♔♕♖♘♘♗
♗♖♔♘♘♖♕♗
♖♕♔♖♘♘♗♗
♗♖♘♔♘♖♕♗

PARI/GP

chess960() =
{
  my (C = vector(8), i, j, r);

  C[random(4) * 2 + 1] = C[random(4) * 2 + 2] = "B";
  for (i = 1, 3, while (C[r = random(8) + 1],); C[r] = Vec("NNQ")[i]);
  for (i = 1, 8, if (!C[i], C[i] = Vec("RKR")[j++]));
  C
}

Output:

gp > for(i=1, 10, print(chess960()));
["N", "R", "Q", "K", "N", "R", "B", "B"]
["R", "K", "N", "B", "N", "R", "B", "Q"]
["B", "R", "K", "N", "R", "B", "Q", "N"]
["R", "B", "Q", "K", "B", "N", "R", "N"]
["R", "B", "K", "N", "N", "Q", "B", "R"]
["N", "Q", "N", "R", "B", "K", "R", "B"]
["N", "Q", "R", "B", "K", "R", "B", "N"]
["N", "R", "B", "K", "R", "N", "Q", "B"]
["R", "K", "Q", "N", "B", "N", "R", "B"]
["B", "B", "R", "N", "K", "R", "N", "Q"]

Alternatively with recent version of PARI/GP >= 2.9:

gp > M=Map(["B","♗";"K","♔";"N","♘";"Q","♕";"R","♖"]);
gp > for(i=1,10,print(concat(apply((c)->mapget(M,c),chess960()))));
♗♖♘♔♕♗♖♘
♕♖♘♔♖♗♗♘
♖♕♘♔♘♖♗♗
♘♘♗♖♕♗♔♖
♘♘♖♗♗♔♖♕
♗♖♘♔♘♕♖♗
♖♔♘♗♗♕♘♖
♘♖♗♘♔♗♕♖
♖♔♕♗♗♘♖♘
♕♗♖♔♗♘♘♖

Perl

Directly generates a configuration by inserting pieces at random appropriate places. Each config has an equal chance of being produced.

sub rnd($) { int(rand(shift)) }

sub empties { grep !$_[0][$_], 0 .. 7 }

sub chess960 {
	my @s = (undef) x 8;
	@s[2*rnd(4), 1 + 2*rnd(4)] = qw/B B/;

	for (qw/Q N N/) {
		my @idx = empties \@s;
		$s[$idx[rnd(@idx)]] = $_;
	}

	@s[empties \@s] = qw/R K R/;
	@s
}
print "@{[chess960]}\n" for 0 .. 10;

Output:

R N B K R N Q B
N N R K B R Q B
N N Q R K R B B
Q R N K B N R B
R K R B N Q B N
B R K B Q N R N
B R N B Q K N R
R B Q N N K B R
N R N Q K R B B
R Q N K R B B N
R K N Q B B R N

Phix

Examines all 40320 permutations for validity and saves them in a list, which is easy to pick random entries from.
Using a dictionary (as commented out) is a little faster, but harder to extract random entries from.
For something faster, and truer to the task description, just use the commented out permute(rand(factorial(8) line, and quit as soon as you find a valid one (but I wanted to check that I had found exactly 960).

with javascript_semantics
sequence solutions = {}
--integer d = new_dict()
 
for i=1 to factorial(8) do
    sequence s = permute(i,"RNBQKBNR")
--  sequence s = permute(rand(factorial(8),"RNBQKBNR")
    integer b1 = find('B',s),
            b2 = find('B',s,b1+1)
    if and_bits(b2-b1,1)=1 then
        integer k = find('K',s)
        integer r1 = find('R',s)
        integer r2 = find('R',s,r1+1)
        if r1<k and k<r2 then
            if find(s,solutions)=0 then
--          if getd_index(s,d)=0 then
--              setd(s,0,d)
                solutions = append(solutions,s)
            end if
        end if
    end if
end for
printf(1,"Found %d solutions\n",{length(solutions)})
for i=1 to 5 do
    ?solutions[rand(length(solutions))]
end for

Output:

Found 960 solutions
"QRNNKRBB"
"BQRNKBNR"
"BRQBNNKR"
"QBNRBKRN"
"RNKBBQRN"

PicoLisp

This example does not show the output mentioned in the task description on this page (or a page linked to from here). Please ensure that it meets all task requirements and remove this message. Note that phrases in task descriptions such as "print and display" and "print and show" for example, indicate that (reasonable length) output be a part of a language's solution.

(load "@lib/simul.l")

(seed (in "/dev/urandom" (rd 8)))

(loop
   (match
      '(@A B @B B @C)
      (shuffle '(Q B B N N 0 0 0)) )
   (NIL (bit? 1 (length @B))) )

(let Rkr '(R K R)
   (for I (append @A '(B) @B '(B) @C)
      (prin (if (=0 I) (pop 'Rkr) I)) )
   (prinl) )

(bye)

PowerShell

Works with: powershell version 2

function Get-RandomChess960Start
    {
    $Starts = @()

    ForEach ( $Q  in       0..3 ) {
    ForEach ( $N1 in       0..4 ) {
    ForEach ( $N2 in ($N1+1)..5 ) {
    ForEach ( $B1 in       0..3 ) {
    ForEach ( $B2 in       0..3 ) {
        $BB = $B1 * 2 + ( $B1 -lt $B2 )
        $BW = $B2 * 2 
        $Start = [System.Collections.ArrayList]( '♖', '♔', '♖' )
        $Start.Insert( $Q , '♕' )
        $Start.Insert( $N1, '♘' )
        $Start.Insert( $N2, '♘' )
        $Start.Insert( $BB, '♗' )
        $Start.Insert( $BW, '♗' )
        $Starts += ,$Start
        }}}}}

    $Index = Get-Random 960
    $StartString = $Starts[$Index] -join ''
    return $StartString
    }

Get-RandomChess960Start
Get-RandomChess960Start
Get-RandomChess960Start
Get-RandomChess960Start

Output:

♘♕♖♔♖♘♗♗
♗♕♘♖♔♗♖♘
♖♗♔♕♗♖♘♘
♘♖♔♖♕♗♗♘

Prolog

Uses random_permutation/2 defined in SWI-Prolog.

check(Row) :-
    nth1(King, Row, ♔),
    nth1(Rook1, Row, ♖),
    nth1(Rook2, Row, ♖),
    nth1(Bishop1, Row, ♗),
    nth1(Bishop2, Row, ♗),
    Rook1 < King, King < Rook2,
    (Bishop1 + Bishop2) mod 2 =:= 1.

generate(Row) :-
    random_permutation([♖,♘,♗,♕,♔,♗,♘,♖], Row),
    check(Row) ; generate(Row).

Example run:

?- generate(X).
X = [♘, ♗, ♖, ♕, ♘, ♔, ♗, ♖] ;
X = [♘, ♗, ♖, ♕, ♘, ♔, ♗, ♖] ;
X = [♘, ♖, ♘, ♕, ♔, ♖, ♗, ♗] ;
X = [♘, ♖, ♘, ♕, ♔, ♖, ♗, ♗]

Python

Python: Indexing

This uses indexing rather than regexps. Rooks and bishops are in upper and lower case to start with so they can be individually indexed to apply the constraints. This would lead to some duplication of start positions if not for the use of a set comprehension to uniquify the, (upper-cased), start positions.

>>> from itertools import permutations
>>> pieces = 'KQRrBbNN'
>>> starts = {''.join(p).upper() for p in permutations(pieces)
                     if p.index('B') % 2 != p.index('b') % 2 		# Bishop constraint
                     and ( p.index('r') < p.index('K') < p.index('R')	# King constraint	
                           or p.index('R') < p.index('K') < p.index('r') ) }
>>> len(starts)
960
>>> starts.pop()
'QNBRNKRB'
>>>

Python: Regexp

This uses regexps to filter permutations of the start position pieces rather than indexing.

>>> import re
>>> pieces = 'KQRRBBNN'
>>> bish = re.compile(r'B(|..|....|......)B').search
>>> king = re.compile(r'R.*K.*R').search
>>> starts3 = {p for p in (''.join(q) for q in permutations(pieces))
            if bish(p) and king(p)}
>>> len(starts3)
960
>>> starts3.pop()
'QRNKBNRB'
>>>

Python: Correct by construction

Follows Perl algorithm of constructing one start position randomly, according to the rules. (See talk page for tests).

from random import choice

def random960():
    start = ['R', 'K', 'R']         # Subsequent order unchanged by insertions.
    #
    for piece in ['Q', 'N', 'N']:
        start.insert(choice(range(len(start)+1)), piece)
    #
    bishpos = choice(range(len(start)+1))
    start.insert(bishpos, 'B')
    start.insert(choice(range(bishpos + 1, len(start) + 1, 2)), 'B')
    return start
    return ''.join(start).upper()

print(random960())

Output:

['N', 'R', 'K', 'N', 'B', 'Q', 'R', 'B']

Python: Generate all positions then choose one randomly

from random import choice

def generate960():
    start = ('R', 'K', 'R')         # Subsequent order unchanged by insertions.

    # Insert QNN in all combinations of places
    starts = {start}
    for piece in ['Q', 'N', 'N']:
        starts2 = set()
        for s in starts:
            for pos in range(len(s)+1):
                s2 = list(s)
                s2.insert(pos, piece)
                starts2.add(tuple(s2))
        starts = starts2
    
    # For each of the previous starting positions insert the bishops in their 16 positions
    starts2 = set()
    for s in starts:
        for bishpos in range(len(s)+1):
            s2 = list(s)
            s2.insert(bishpos, 'B')
            for bishpos2 in range(bishpos+1, len(s)+2, 2):
                s3 = s2[::]
                s3.insert(bishpos2, 'B')
                starts2.add(tuple(s3))
                
    return  list(starts2)

gen = generate960()
print(''.join(choice(gen)))

Output:

NRBQNKRB

R

pieces <- c("R","B","N","Q","K","N","B","R")

generateFirstRank <- function() {
  attempt <- paste0(sample(pieces), collapse = "")
  while (!check_position(attempt)) {
    attempt <- paste0(sample(pieces), collapse = "")
  }
  return(attempt)
}

check_position <- function(position) {
  if (regexpr('.*R.*K.*R.*', position) == -1) return(FALSE)
  if (regexpr('.*B(..|....|......|)B.*', position) == -1) return(FALSE)
  TRUE
}

convert_to_unicode <- function(s) {
  s <- sub("K","\u2654", s)
  s <- sub("Q","\u2655", s)
  s <- gsub("R","\u2656", s)
  s <- gsub("B","\u2657", s)
  s <- gsub("N","\u2658", s)
}

cat(convert_to_unicode(generateFirstRank()), "\n")

Output:

♘♗♘♖♗♕♔♖

Racket

Constructive:

#lang racket
(define white (match-lambda ['P #\♙] ['R #\♖] ['B #\♗] ['N #\♘] ['Q #\♕] ['K #\♔]))
(define black (match-lambda ['P #\♟] ['R #\♜] ['B #\♝] ['N #\♞] ['Q #\♛] ['K #\♚]))

(define (piece->unicode piece colour)
  (match colour ('w white) ('b black)) piece)

(define (find/set!-random-slot vec val k (f values))
  (define r (f (random k)))
  (cond
    [(vector-ref vec r)
     (find/set!-random-slot vec val k f)]
    [else
     (vector-set! vec r val)
     r]))

(define (chess960-start-position)
  (define v (make-vector 8 #f))  
  ;; Kings and Rooks
  (let ((k (find/set!-random-slot v (white 'K) 6 add1)))
    (find/set!-random-slot v (white 'R) k)
    (find/set!-random-slot v (white 'R) (- 7 k) (curry + k 1)))
  ;; Bishops -- so far only three squares allocated, so there is at least one of each colour left
  (find/set!-random-slot v (white 'B) 4 (curry * 2))
  (find/set!-random-slot v (white 'B) 4 (compose add1 (curry * 2)))
  ;; Everyone else
  (find/set!-random-slot v (white 'Q) 8)
  (find/set!-random-slot v (white 'N) 8)
  (find/set!-random-slot v (white 'N) 8)
  (list->string (vector->list v)))

(chess960-start-position)

Output:

"♖♘♗♕♔♗♘♖"

Well that's embarassing... the stupid thing has only gone and randomly generated a classic chess starting position.

Try again:

"♘♖♔♕♗♗♖♘"

Raku

(formerly Perl 6) First, using a list with three rooks and no king, we keep generating a random piece order until the two bishops are on opposite colors. Then we sneakily promote the second of the three rooks to a king.

repeat until m/ '♗' [..]* '♗' / { $_ = < ♖ ♖ ♖ ♕ ♗ ♗ ♘ ♘ >.pick(*).join }
s:2nd['♖'] = '♔';
say .comb;

Output:

♕ ♗ ♖ ♘ ♔ ♖ ♗ ♘

Here's a more "functional" solution that avoids side effects

sub chess960 {
    .subst(:nth(2), /'♜'/, '♚') given
    first rx/ '♝' [..]* '♝' /,
    < ♛ ♜ ♜ ♜ ♝ ♝ ♞ ♞ >.pick(*).join xx *;
}

say chess960;

Output:

♛♝♜♚♝♞♞♜

We can also pregenerate the list of 960 positions, though the method we use below is a bit wasteful, since it generates 40320 candidates only to throw most of them away. This is essentially the same filtering algorithm but written in the form of a list comprehension rather than nested map and grep. (The list comprehension is actually faster currently.) Note that the constant is calculated at compile time, because, well, it's a constant. Just a big fancy one.

constant chess960 =
   < ♛ ♜ ♜ ♜ ♝ ♝ ♞ ♞ >.permutations».join.unique.grep( / '♝' [..]* '♝' / )».subst(:nth(2), /'♜'/, '♚');

.say for chess960;

Here's a much faster way (about 30x) to generate all 960 variants by construction. No need to filter for uniqueness, since it produces exactly 960 entries.

constant chess960 = gather for 0..3 -> $q {
    (my @q = <♜ ♚ ♜>).splice($q, 0, '♛');
    for 0 .. @q -> $n1 {
        (my @n1 = @q).splice($n1, 0, '♞');
        for $n1 ^.. @n1 -> $n2 {
            (my @n2 = @n1).splice($n2, 0, '♞');
            for 0 .. @n2 -> $b1 {
                (my @b1 = @n2).splice($b1, 0, '♝');
                for $b1+1, $b1+3 ...^ * > @b1 -> $b2 {
                    (my @b2 = @b1).splice($b2, 0, '♝');
                    take @b2.join;
                }
            }
        }
    }
}

CHECK { note "done compiling" }
note +chess960;
say chess960.pick;

Output:

done compiling
960
♜♚♝♜♞♛♞♝

If you run this you'll see that most of the time is spent in compilation, so in the case of separate precompilation the table of 960 entries merely needs to be deserialized back into memory. Picking from those entries guarantees uniform distribution over all possible boards.

♛♝♜♚♝♞♞♜

Raku: Generate from SP-ID

There is a standard numbering scheme for Chess960 positions, assigning each an index in the range 0..959. This function will generate the corresponding position from a given index number (or fall back to a random one if no index is specified, making it yet another solution to the general problem).

subset Pos960 of Int where { $_ ~~ ^960 };
sub chess960(Pos960 $position = (^960).pick) {

  # We remember the remainder used to place first bishop in order to place the
  # second
  my $b1;
  
  # And likewise remember the chosen combination for knights between those
  # placements
  my @n;
  
  # Piece symbols and positioning rules in order.  Start with the position
  # number. At each step, divide by the divisor; the quotient becomes the
  # dividend for the next step.  Feed the remainder into the specified code block
  # to get a space number N, then place the piece in the Nth empty space left in
  # the array.
  my @rules = (
    #divisor, mapping function,                      piece
    ( 4,      { $b1 = $_; 2 * $_ + 1 },              '♝'  ),
    ( 4,      { 2 * $_ - ($_ > $b1 ?? 1 !! 0) },     '♝'  ),
    ( 6,      { $_ },                                '♛'  ),
    (10,      { @n = combinations(5,2)[$_]; @n[0] }, '♞'  ),
    ( 1,      { @n[1]-1 },                           '♞'  ),
    ( 1,      { 0 },                                 '♜'  ),
    ( 1,      { 0 },                                 '♚'  ),
    ( 1,      { 0 },                                 '♜'  )
  
  );
  
  # Initial array, using '-' to represent empty spaces
  my @array = «-» xx 8;
  
  # Working value that starts as the position number but is divided by the
  # divisor at each placement step.
  my $p = $position;
  
  # Loop over the placement rules
  for @rules -> ($divisor, $block, $piece) {
  
    # get remainder when divided by divisor
    (my $remainder, $p) = $p.polymod($divisor);
  
    # apply mapping function
    my $space = $block($remainder);
  
    # find index of the $space'th element of the array that's still empty
    my $index = @array.kv.grep(-> $i,$v { $v eq '-' })[$space][0];
  
    # and place the piece
    @array[$index] = $piece;
  }
  return @array;
}

# demo code
say chess960(518); #standard optning position
say chess960; # (it happened to pick #300)

Output:

♜ ♞ ♝ ♛ ♚ ♝ ♞ ♜
♛ ♝ ♞ ♜ ♚ ♜ ♝ ♞

REXX

Random starting position is correct by construction   (both REXX entries).

generates one random position

/*REXX program generates a random starting position  for the  Chess960  game. */
parse arg seed .                       /*allow for (RANDOM BIF) repeatability.*/
if seed\==''  then call random ,,seed  /*if SEED was specified,  use the seed.*/
@.=.                                   /*define the first rank as being empty.*/
r1=random(1,6)                         /*generate the first rook:  rank 1.    */
@.r1='R'                               /*place the  first rook  on  rank1.    */
          do  until  r2\==r1  &  r2\==r1-1  &  r2\==r1+1
          r2=random(1,8)               /*find placement for the 2nd rook.     */
          end   /*forever*/
@.r2='r'                               /*place the second rook  on  rank 1.   */
k=random(min(r1, r2)+1, max(r1, r2)-1) /*find a random position for the king. */
@.k='K'                                /*place king between the two rooks.    */
          do _=0      ; b1=random(1,8);  if @.b1\==.  then iterate;  c=b1//2
            do forever; b2=random(1,8)       /* c=color of bishop ►──┘        */
            if @.b2\==. | b2==b1 | b2//2==c  then iterate /*is a bad position?*/
            leave _                    /*found position for the 2 clergy*/
            end   /*forever*/          /* [↑]  find a place for the 1st bishop*/
          end     /* _ */              /* [↑]    "  "   "    "   "  2nd    "  */
@.b1='B'                               /*place the  1st  bishop on  rank 1.   */
@.b2='b'                               /*  "    "   2nd     "    "    "  "    */
                                       /*place the two knights on rank 1.     */
   do  until @._='N';  _=random(1,8);   if @._\==.  then iterate; @._='N';   end
   do  until @.!='n';  !=random(1,8);   if @.!\==.  then iterate; @.!='n';   end
_=                                     /*only the queen is left to be placed. */
   do i=1  for 8;  _=_ || @.i;   end   /*construct the output: first rank only*/
say translate(translate(_, 'q', .))    /*stick a fork in it,  we're all done. */

output

NRQKBRNB

generates all 960 positions randomly

/*REXX program generates all random starting positions for the Chess960 game. */
parse arg seed .                       /*allow for (RANDOM BIF) repeatability.*/
if seed\==''  then call random ,,seed  /*if SEED was specified,  use the seed.*/
x.=0;  #=0;  rg='random generations: ' /*initialize game placeholder; # games.*/
       /*▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒*/
do t=1                                 /* [↓]  display every 1,000 generations*/   /*▒*/
if t//1000==0  then say  right(t,9)   rg     #     " unique starting positions."   /*▒*/
@.=.                                   /*define the first rank as being empty.*/   /*▒*/
r1=random(1,6)                         /*generate the first rook:  rank 1.    */   /*▒*/
@.r1='R'                               /*place the  first rook  on  rank1.    */   /*▒*/
          do  until  r2\==r1  &  r2\==r1-1  &  r2\==r1+1                           /*▒*/
          r2=random(1,8)               /*find placement for the 2nd rook.     */   /*▒*/
          end   /*forever*/                                                        /*▒*/
@.r2='r'                               /*place the second rook  on  rank 1.   */   /*▒*/
k=random(min(r1, r2)+1, max(r1, r2)-1) /*find a random position for the king. */   /*▒*/
@.k='K'                                /*place king between the two rooks.    */   /*▒*/
          do _=0      ; b1=random(1,8);  if @.b1\==.  then iterate;  c=b1//2       /*▒*/
            do forever; b2=random(1,8)       /* c=color of bishop ►──┘        */   /*▒*/
            if @.b2\==. | b2==b1 | b2//2==c  then iterate /*is a bad position?*/   /*▒*/
            leave _                    /*found position for the 2 clergy*/         /*▒*/
            end   /*forever*/          /* [↑]  find a place for the 1st bishop*/   /*▒*/
          end     /* _ */              /* [↑]    "  "   "    "   "  2nd    "  */   /*▒*/
@.b1='B'                               /*place the  1st  bishop on  rank 1.   */   /*▒*/
@.b2='b'                               /*  "    "   2nd     "    "    "  "    */   /*▒*/
                                       /*place the two knights on rank 1.     */   /*▒*/
   do  until @._='N';  _=random(1,8);   if @._\==.  then iterate; @._='N';   end   /*▒*/
   do  until @.!='n';  !=random(1,8);   if @.!\==.  then iterate; @.!='n';   end   /*▒*/
_=                                     /*only the queen is left to be placed. */   /*▒*/
   do i=1  for 8;  _=_ || @.i;   end   /*construct the output: first rank only*/   /*▒*/
upper _                                /*uppercase all the chess pieces.      */   /*▒*/
if x._  then iterate                   /*This position found before?  Skip it.*/   /*▒*/
x._=1                                  /*define this position as being found. */   /*▒*/
#=#+1                                  /*bump the # of unique positions found,*/   /*▒*/
if #==960  then leave                                                              /*▒*/
end   /*t ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒*/
                                                                                   
say # 'unique starting positions found after '   t   "generations."                
                                       /*stick a fork in it,  we're all done. */         /**/

output

     1000 random generations:  515  unique starting positions.
     2000 random generations:  707  unique starting positions.
     3000 random generations:  796  unique starting positions.
     4000 random generations:  849  unique starting positions.
     5000 random generations:  883  unique starting positions.
     6000 random generations:  900  unique starting positions.
     7000 random generations:  922  unique starting positions.
     8000 random generations:  935  unique starting positions.
     9000 random generations:  942  unique starting positions.
    10000 random generations:  946  unique starting positions.
    11000 random generations:  953  unique starting positions.
    12000 random generations:  957  unique starting positions.
    13000 random generations:  959  unique starting positions.
    14000 random generations:  959  unique starting positions.
960 unique starting positions found after  14639 generations.

version 3 COMPUTE all possibilities

/*---------------------------------------------------------------
* Compute the 960 possible solutions
* There must be at least one field between the rooks
* The king is positioned on any field between the rooks
* The queen is placed on any unoccupied field
* bishops are placed so that they are on different colored fields
* what remains are the kNights...
*--------------------------------------------------------------*/
cnt.=0
Call time 'R'
Do r1=1 To 6
  Do r2=r1+1 To 8
    Do kk=r1+1 To r2-1
      poss=space(translate('12345678',' ',r1||kk||r2),0)
      Call rest
      End
   
 End
  End
say cnt.1 'solutions'
Say time('E')
Exit

rest:
Do i=1 To 5
  q=substr(poss,i,1)
  br=space(translate(poss,' ',q),0)
  Do b1i=1 To 3
    Do b2i=b1i+1 To 4
      Call finish
      End
    End
  End
Return

finish:
  b1=substr(br,b1i,1)
  b2=substr(br,b2i,1)
  If (b1+b2)//2>0 Then
    Call out
  Return

out:
  pos.='N'
  pos.r1='R'
  pos.r2='R'
  pos.kk='K'
  pos.q='Q'
  pos.b1='B'
  pos.b2='B'
  ol=''
  Do k=1 To 8
    ol=ol||pos.k
    End
  cnt.1=cnt.1+1
  If cnt.1<4 |,
     cnt.1>957 Then
    Say format(cnt.1,3) poss r1 kk r2  ol
  If cnt.1=4 Then
    Say '    ...'
  Return

Output:

  1 45678 1 2 3 RKRQBBNN
  2 45678 1 2 3 RKRQBNNB
  3 45678 1 2 3 RKRQNBBN
    ...
958 12345 6 7 8 BNNBQRKR
959 12345 6 7 8 NBBNQRKR
960 12345 6 7 8 NNBBQRKR
960 solutions

RPL

We use here the single die method, starting with a list of rooks that are gradually replaced by other pieces.

Works with: HP version 48G

≪ SWAP 1 ≪ 'R' SAME ≫ DOLIST 0 SWAP
   1 OVER SIZE FOR j 
      DUP j GET ROT + SWAP
      j 3 PICK PUT 
   NEXT
   SWAP DROP SWAP POS
≫ 'ROOKPOS' STO          

≪ { } 1 8 START 'R' + NEXT
   RAND 4 * FLOOR 2 * 1 + 'B' PUT
   RAND 4 * CEIL 2 * 'B' PUT
   DUP RAND 6 * CEIL ROOKPOS 'Q' PUT
   DUP RAND 5 * CEIL ROOKPOS 'N' PUT
   DUP RAND 4 * CEIL ROOKPOS 'N' PUT
   DUP 2 ROOKPOS 'K' PUT
≫ '→CH360' STO

Output:

3: { N Q R K B B R N }
2: { R K B B R Q N N }
1: { Q N R B K N B R }

Ruby

Ruby: shuffle pieces until all regexes match

Translation of Tcl.

pieces = %i(♔ ♕ ♘ ♘ ♗ ♗ ♖ ♖)
regexes = [/♗(..)*♗/, /♖.*♔.*♖/]
row = pieces.shuffle.join until regexes.all?{|re| re.match(row)} 
puts row

Output:

♕♖♗♘♔♖♘♗

Ruby: Construct

Uses the Perl idea of starting with [R,K,R] and inserting the rest:

row = [:♖, :♔, :♖]
[:♕, :♘, :♘].each{|piece| row.insert(rand(row.size+1), piece)}
[[0, 2, 4, 6].sample, [1, 3, 5, 7].sample].sort.each{|pos| row.insert(pos, :♗)}

puts row

Output:

♗♘♕♘♖♗♔♖

Ruby: Generate from SP-ID

Chess960 numbering scheme

KRN = %w(NNRKR NRNKR NRKNR NRKRN RNNKR RNKNR RNKRN RKNNR RKNRN RKRNN)

def chess960(id=rand(960))
  pos = Array.new(8)
  q, r = id.divmod(4)
  pos[r * 2 + 1] = "B"
  q, r = q.divmod(4)
  pos[r * 2] = "B"
  q, r = q.divmod(6)
  pos[pos.each_index.reject{|i| pos[i]}[r]] = "Q"
  krn = KRN[q].each_char
  pos.each_index {|i| pos[i] ||= krn.next}
  pos.join
end

puts "Generate Start Position from id number"
[0,518,959].each do |id|
  puts "%3d : %s" % [id, chess960(id)]
end

puts "\nGenerate random Start Position"
5.times {puts chess960}

Output:

Generate Start Position from id number
  0 : BBQNNRKR
518 : RNBQKBNR
959 : RKRNNQBB

Generate random Start Position
RNBNKBRQ
RKRNBBNQ
BBRNQKNR
NBRKNRBQ
BRKQNNRB

Rust

This example does not show the output mentioned in the task description on this page (or a page linked to from here). Please ensure that it meets all task requirements and remove this message. Note that phrases in task descriptions such as "print and display" and "print and show" for example, indicate that (reasonable length) output be a part of a language's solution.

Translation of: Kotlin

use std::collections::BTreeSet;

struct Chess960 ( BTreeSet<String> );

impl Chess960 {
    fn invoke(&mut self, b: &str, e: &str) {
        if e.len() <= 1 {
            let s = b.to_string() + e;
            if Chess960::is_valid(&s) { self.0.insert(s); }
        } else {
            for (i, c) in e.char_indices() {
                let mut b = b.to_string();
                b.push(c);
                let mut e = e.to_string();
                e.remove(i);
                self.invoke(&b, &e);
            }
        }
    }

    fn is_valid(s: &str) -> bool {
        let k = s.find('K').unwrap();
        k > s.find('R').unwrap() && k < s.rfind('R').unwrap() && s.find('B').unwrap() % 2 != s.rfind('B').unwrap() % 2
    }
}

// Program entry point.
fn main() {
    let mut chess960 = Chess960(BTreeSet::new());
    chess960.invoke("", "KQRRNNBB");

    for (i, p) in chess960.0.iter().enumerate() {
        println!("{}: {}", i, p);
    }
}

Rust 1.57 nightly

// Chess960: regex and unicode version, create 5 valid random positions.

use rand::{seq::SliceRandom, thread_rng};
use regex::Regex;

fn vec_to_string(v: Vec<&str>) -> String {
    let mut is_string = String::new();
    for ele in v {
        is_string.push_str(ele)
    }
    is_string
}
fn is_rook_king_ok(str_to_check: Vec<&str>) -> bool {
    Regex::new(r"(.*♖.*♔.*♖.*)")
        .unwrap()
        .is_match(vec_to_string(str_to_check.clone()).as_str())
}
fn is_two_bishops_ok(str_to_check: Vec<&str>) -> bool {
    Regex::new(r"(.*♗.{0}♗.*|.*♗.{2}♗.*|.*♗.{4}♗.*|.*♗.{6}♗.*)")
        .unwrap()
        .is_match(vec_to_string(str_to_check.clone()).as_str())
}
fn create_rnd_candidate() -> [&'static str; 8] {
    let mut rng = thread_rng();
    let mut chaine = ["♖", "♘", "♗", "♔", "♕", "♗", "♘", "♖"];

    loop {
        chaine.shuffle(&mut rng);
        if is_candidate_valide(chaine) {
            break chaine;
        }
    }
}
fn is_candidate_valide(s: [&str; 8]) -> bool {
    is_rook_king_ok(s.to_vec()) && is_two_bishops_ok(s.to_vec())
}
fn main() {
    for _ in 0..5 {
        println!("{:?}", create_rnd_candidate());
    }
}

Output:

["♕", "♘", "♗", "♗", "♖", "♘", "♔", "♖"]
["♖", "♔", "♗", "♕", "♘", "♗", "♖", "♘"]
["♗", "♖", "♕", "♔", "♘", "♗", "♖", "♘"]
["♘", "♖", "♘", "♕", "♔", "♖", "♗", "♗"]
["♖", "♗", "♕", "♔", "♗", "♘", "♖", "♘"]

Scala

Functional Programming, tail recursive, Unicode, RegEx

import scala.annotation.tailrec

object Chess960 extends App {

  private val pieces = List('♖', '♗', '♘', '♕', '♔', '♘', '♗', '♖')

  @tailrec
  private def generateFirstRank(pieces: List[Char]): List[Char] = {
    def check(rank: String) =
      rank.matches(".*♖.*♔.*♖.*") && rank.matches(".*♗(..|....|......|)♗.*")

    val p = scala.util.Random.shuffle(pieces)
    if (check(p.toString.replaceAll("[^\\p{Upper}]", "")))
      generateFirstRank(pieces)
    else p
  }

  loop(10)

  @tailrec
  private def loop(n: Int): Unit = {
    println(generateFirstRank(pieces))
    if (n <= 0) () else loop(n - 1)
  }
}

Output:

See it running in your browser by ScalaFiddle (JavaScript, non JVM) or by Scastie (JVM).

Imperative Programming

Translation of: Kotlin

object Chess960 extends App {
    private def apply(b: String, e: String) {
        if (e.length <= 1) {
            val s = b + e
            if (is_valid(s)) patterns += s
        } else
            for (i <- 0 until e.length)
                apply(b + e(i), e.substring(0, i) + e.substring(i + 1))
    }

    private def is_valid(s: String) = {
        val k = s.indexOf('K')
        if (k < s.indexOf('R')) false
        else k < s.lastIndexOf('R') && s.indexOf('B') % 2 != s.lastIndexOf('B') % 2
    }

    private val patterns = scala.collection.mutable.SortedSet[String]()

    apply("", "KQRRNNBB")
    for ((s, i) <- patterns.zipWithIndex) println(s"$i: $s")
}

Scheme

Library: Scheme/SRFIs

(import (scheme base) (scheme write)
        (srfi 1)    ; list library
        (srfi 27))  ; random numbers

(random-source-randomize! default-random-source)

;; Random integer in [start, end)
(define (random-between start end)
  (let ((len (- end start 1)))
    (if (< len 2)
      start
      (+ start (random-integer len)))))

;; Random item in list
(define (random-pick lst)
  (if (= 1 (length lst))
    (car lst)
    (list-ref lst (random-integer (length lst)))))

;; Construct a random piece placement for Chess960
(define (random-piece-positions)
  (define (free-indices positions) ; return list of empty slot indices
    (let loop ((i 0)
               (free '()))
      (if (= 8 i)
        free
        (loop (+ 1 i)
              (if (string=? "." (vector-ref positions i))
                (cons i free)
                free)))))
  ;
  (define (place-king+rooks positions)
    (let ((king-posn (random-between 1 8)))
      (vector-set! positions king-posn "K")
      ; left-rook is between left-edge and king
      (vector-set! positions (random-between 0 king-posn) "R")
      ; right-rook is between right-edge and king
      (vector-set! positions (random-between (+ 1 king-posn) 8) "R")))
  ;
  (define (place-bishops positions)
    (let-values (((evens odds) (partition even? (free-indices positions))))
                (vector-set! positions (random-pick evens) "B")
                (vector-set! positions (random-pick odds) "B")))
  ;
  (let ((positions (make-vector 8 ".")))
    (place-king+rooks positions)
    (place-bishops positions)
    ;; place the queen in a random remaining slot
    (vector-set! positions (random-pick (free-indices positions)) "Q")
    ;; place the two knights in the remaining slots
    (for-each (lambda (idx) (vector-set! positions idx "N"))
              (free-indices positions))

    positions))

(display "First rank: ") (display (random-piece-positions)) (newline)

Output:

Ten sample runs:

First rank: #(R N N Q K R B B)
First rank: #(R K N N Q R B B)
First rank: #(Q R B N N B K R)
First rank: #(Q R B N N K R B)
First rank: #(R K N Q R B B N)
First rank: #(R K N B Q R B N)
First rank: #(R N K N B B R Q)
First rank: #(R B K Q B N R N)
First rank: #(B Q R N K N R B)
First rank: #(R B B Q N N K R)

Seed7

$ include "seed7_05.s7i";

const proc: main is func
  local
    var string: start is "RKR";
    var char: piece is ' ';
    var integer: pos is 0;
  begin
    for piece range "QNN" do
      pos := rand(1, succ(length(start)));
      start := start[.. pred(pos)] & str(piece) & start[pos ..];
    end for;
    pos := rand(1, succ(length(start)));
    start := start[.. pred(pos)] & "B" & start[pos ..];
    pos := succ(pos) + 2 * rand(0, (length(start) - pos) div 2);
    start := start[.. pred(pos)] & "B" & start[pos ..];
    writeln(start);
  end func;

Output:

NQBNRBKR

Sidef

func is_valid_960 (backrank) {
    var king = backrank.index('♚')
    var (rook1, rook2) = backrank.indices_of('♜')...
    king.is_between(rook1, rook2) || return false
    var (bishop1, bishop2) = backrank.indices_of('♝')...
    bishop1+bishop2 -> is_odd
}

func random_960_position(pieces = <♛ ♚ ♜ ♜ ♝ ♝ ♞ ♞>) {
    pieces.shuffle.permutations {|*a|
        return a if is_valid_960(a)
    }
}

say random_960_position().join(' ')

Output:

♝ ♝ ♜ ♚ ♞ ♛ ♜ ♞

Swift

func isValid960Position(_ firstRank: String) -> Bool {
  var rooksPlaced = 0
  var bishopColor = -1

  for (i, piece) in firstRank.enumerated() {
    switch piece {
    case "♚" where rooksPlaced != 1:
      return false
    case "♜":
      rooksPlaced += 1
    case "♝" where bishopColor == -1:
      bishopColor = i & 1
    case "♝" where bishopColor == i & 1:
      return false
    case _:
      continue
    }
  }

  return true
}

struct Chess960Counts {
  var king = 0, queen = 0, rook = 0, bishop = 0, knight = 0

  subscript(_ piece: String) -> Int {
    get {
      switch piece {
      case "♚": return king
      case "♛": return queen
      case "♜": return rook
      case "♝": return bishop
      case "♞": return knight
      case _:   fatalError()
      }
    }

    set {
      switch piece {
      case "♚": king = newValue
      case "♛": queen = newValue
      case "♜": rook = newValue
      case "♝": bishop = newValue
      case "♞": knight = newValue
      case _:   fatalError()
      }
    }
  }
}

func get960Position() -> String {
  var counts = Chess960Counts()
  var bishopColor = -1 // 0 - white 1 - black
  var output = ""

  for i in 1...8 {
    let validPieces = [
      counts["♜"] == 1 && counts["♚"] == 0 ? "♚" : nil, // king
      i == 1 || (counts["♛"] == 0) ? "♛" : nil, // queen
      i == 1 || (counts["♜"] == 0 || counts["♜"] < 2 && counts["♚"] == 1) ? "♜" : nil, // rook
      i == 1 || (counts["♝"] < 2 && bishopColor == -1 || bishopColor != i & 1) ? "♝" : nil, // bishop
      i == 1 || (counts["♞"] < 2) ? "♞" : nil // knight
    ].lazy.compactMap({ $0 })

    guard let chosenPiece = validPieces.randomElement() else {
      // Need to swap last piece with a bishop
      output.insert("♝", at: output.index(before: output.endIndex))

      break
    }

    counts[chosenPiece] += 1
    output += chosenPiece

    if bishopColor == -1 && chosenPiece == "♝" {
      bishopColor = i & 1
    }
  }

  assert(isValid960Position(output), "invalid 960 position \(output)")

  return output
}

var positions = Set<String>()

while positions.count != 960 {
  positions.insert(get960Position())
}

print(positions.count, positions.randomElement()!)

Output:

960 ♞♛♜♞♚♝♝♜

Tcl

Using regular expressions to filter a random permutation.

Library: Tcllib (Package: struct::list)

package require struct::list

proc chess960 {} {
    while true {
	set pos [join [struct::list shuffle {N N B B R R Q K}] ""]
	if {[regexp {R.*K.*R} $pos] && [regexp {B(..)*B} $pos]} {
	    return $pos
	}
    }
}

# A simple renderer
proc chessRender {position} {
    string map {P ♙ N ♘ B ♗ R ♖ Q ♕ K ♔} $position
}

# Output multiple times just to show scope of positions
foreach - {1 2 3 4 5} {puts [chessRender [chess960]]}

Output:

♕♖♘♔♗♗♘♖
♖♔♘♘♗♕♖♗
♘♖♗♗♕♔♘♖
♘♕♗♖♔♖♘♗
♘♘♖♔♗♗♕♖

UNIX Shell

Translation of: raku

Works with: bash

declare -a pieces=(♖ ♖ ♖ ♕ ♗ ♗ ♘ ♘)
declare -i i pick index
declare -ai picking_history
declare attempt

until [[ "$attempt" =~ ♗(..)*♗ ]]
do
  attempt=''
  picking_history=()
  for _ in {1..8}
  do
    while ((picking_history[pick=RANDOM%8]++))
    do :
    done
    attempt+="${pieces[pick]}"
  done
done
for i in {0..7}
do
  if [[ "${attempt:i:1}" = ♖ ]] && ((index++))
  then echo "${attempt:0:i}♔${attempt:i+1}"; break;
  fi
done

Wren

Translation of: Go

Library: Wren-dynamic

Library: Wren-fmt

import "random" for Random
import "./dynamic" for Tuple
import "./fmt" for Fmt

var Symbols = Tuple.create("Symbols", ["k", "q", "r", "b", "n"])

var A = Symbols.new("K", "Q", "R", "B", "N")
var W = Symbols.new("♔", "♕", "♖", "♗", "♘")
var B = Symbols.new("♚", "♛", "♜", "♝", "♞")

var krn = [
    "nnrkr", "nrnkr", "nrknr", "nrkrn",
    "rnnkr", "rnknr", "rnkrn",
    "rknnr", "rknrn",
    "rkrnn"
]

var NUL = "\0"

var chess960 = Fn.new { |sym, id|
    var pos = List.filled(8, NUL)
    var q = (id/4).floor
    var r = id % 4
    pos[r*2+1]= sym.b
    var t = q
    q = (q/4).floor
    r = t % 4
    pos[r*2] = sym.b
    t = q
    q = (q/6).floor
    r = t % 6
    var i = 0
    while (true) {
        if (pos[i] == NUL) {
            if (r == 0) {
                pos[i] = sym.q
                break
            }
            r = r - 1
        }
        i = i + 1
    }
    i = 0
    for (f in krn[q]) {
        while (pos[i] != NUL) i = i + 1
        pos[i] = (f == "k") ? sym.k :
                 (f == "r") ? sym.r :
                 (f == "n") ? sym.n : pos[i]
    }
    return pos.join(" ")
}

System.print(" ID  Start position")
for (id in [0, 518, 959]) Fmt.print("$3d  $s", id, chess960.call(A, id))
System.print("\nRandom")
var rand = Random.new()
for (i in 0..4) System.print(chess960.call(W, rand.int(960)))

Output:

 ID  Start position
  0  B B Q N N R K R
518  R N B Q K B N R
959  R K R N N Q B B

Random
♘ ♗ ♖ ♘ ♔ ♕ ♗ ♖
♖ ♘ ♗ ♔ ♕ ♘ ♖ ♗
♕ ♗ ♗ ♖ ♘ ♔ ♘ ♖
♖ ♔ ♗ ♕ ♘ ♗ ♘ ♖
♖ ♔ ♘ ♗ ♕ ♘ ♗ ♖

XPL0

char Col;

func ColNum(Start, Piece); \Return column number of Piece
int  Start, Piece, I;
[for I:= Start to 7 do
        if Col(I) = Piece then return I;
return -1;
];

proc Shuffle;           \Randomly rearrange pieces in columns
int  I, J, T;
[for I:= 8-1 downto 1 do
    [J:= Ran(I);        \range [0..I-1] (Sattolo cycle)
    T:= Col(I);  Col(I):= Col(J);  Col(J):= T;
    ];
];

int N, B1, B2, BOK, R1, R2, K, KOK;
[for N:= 1 to 5 do
        [Col:= "RNBQKBNR ";
        repeat  Shuffle;
                B1:= ColNum(0, ^B);
                B2:= ColNum(B1+1, ^B);
                BOK:= ((B1 xor B2) and 1) # 0;
                R1:= ColNum(0, ^R);
                R2:= ColNum(R1+1, ^R);
                K:= ColNum(0, ^K);
                KOK:= R1<K and K<R2;
        until   BOK and KOK;
        Text(0, Col);  CrLf(0);
        ];
]

Output:

BNRBQKRN 
RBKNNQBR 
BQRBNKNR 
NRBBQKNR 
RNKNBQRB 

zkl

Translation of: D

const pieces="KQRrBbNN";
starts:=pieces:Utils.Helpers.permuteW(_).filter(fcn(p){
   I:=p.index;
   I("B") % 2 != I("b") % 2 and // Bishop constraint.
   // King constraint.
   ((I("r") < I("K") and I("K") < I("R")) or
    (I("R") < I("K") and I("K") < I("r")))
}).pump(List,"concat","toUpper"):Utils.Helpers.listUnique(_);

N:=starts.len(); println(N);
glyphs:=Dictionary("K","\u2654", "Q","\u2655", "R","\u2656", "B","\u2657", "N","\u2658");
// pick some random starts and transform BBNRKQRN to glyphs
do(10){ starts[(0).random(N)].apply(glyphs.find).println() }

Output:

960
♗♕♘♖♘♔♖♗
♖♘♗♔♖♗♘♕
♖♗♘♔♗♕♖♘
♘♖♘♗♗♔♕♖
♘♘♗♖♕♔♖♗
♘♖♕♔♗♖♘♗
♘♖♗♘♕♔♖♗
♖♘♗♔♕♘♖♗
♖♔♖♕♘♘♗♗
♕♗♖♘♗♔♘♖