Finite state machine
A Finite state machine (FSM) is computational abstraction which maps a finite number of states to other states within the same set, via transitions. An FSM can only be in one state at any given moment. Transitions can either be explicit or implicit; explicit transitions are triggered by an input signal and implicit transitions by the internal state of the system (that is, the current state). Implicit transitions thus represent "automatic" or sequenced states that are generally processed between explicit transitions (although they can also be used to provide an optional path when no valid transition exists for a given input signal).
- Example
Consider the model of a simple vending machine. The machine is initially in the "ready" state, which maps to exactly two states in the following way:
- ready -> deposit -> waiting
- ready -> quit -> exit
The variables in bold-face represent transitions. Any input signal not corresponding to one of those transitions can either trigger an error or be ignored. Otherwise, the current state is updated and the process is repeated. If, for example, a deposit input signal is encountered, the FSM will move to the "waiting" state, which defines these transitions:
- waiting -> select -> dispense
- waiting -> refund -> refunding
The "dispense" state defines only one transition:
- dispense -> remove -> ready
Note, however, that in this example the "refunding" state doesn't actually require input in order to move to the "ready" state, so an implicit transition is defined as such:
- refunding -> ready
- Task
Implement a finite state machine which handles both explicit and implicit transitions. Then demonstrate an example which models some real-world process.
- See also
- Computers Without Memory (Finite State Automata), A Computerphile Video.
BASIC
Sinclair ZX81 BASIC
Works with 1k of RAM.
There doesn't seem much point, in BASIC, implementing a 'general' FSM that would accept a list of states and transition rules as parameters, because an unstructured BASIC program in essence already is that list.
Within each state, if the transition is implicit we just GOTO the next state. If it is explicit, we loop until the user presses a key corresponding to a valid transition. Invalid inputs are ignored.
The line 100 GOTO 110 is superfluous, because it would go there anyway; but it is worth including it in case we wanted to modify the program later and transition somewhere else out of the dispense state.
Note that the program uses no variables and makes no use of the return stack: all the state is expressed in the (so to speak) state.
<lang basic> 10 PRINT "PRESS D(EPOSIT) OR Q(UIT)"
20 IF INKEY$="D" THEN GOTO 50 30 IF INKEY$="Q" THEN STOP 40 GOTO 20 50 PRINT "PRESS S(ELECT) OR R(EFUND)" 60 IF INKEY$="S" THEN GOTO 90 70 IF INKEY$="R" THEN GOTO 140 80 GOTO 60 90 PRINT "DISPENSED"
100 GOTO 110 110 PRINT "PRESS R(EMOVE)" 120 IF INKEY$="R" THEN GOTO 10 130 GOTO 120 140 PRINT "REFUNDED" 150 GOTO 10</lang>
- Output:
It will be seen that the user has pressed, in order, D, R, D, S, R, and Q.
PRESS D(EPOSIT) OR Q(UIT) PRESS S(ELECT) OR R(EFUND) REFUNDED PRESS D(EPOSIT) OR Q(UIT) PRESS S(ELECT) OR R(EFUND) DISPENSED PRESS R(EMOVE) PRESS D(EPOSIT) OR Q(UIT)
C
This is an unapologetic implementation of goto. There have been a lot of curse words and obituaries written about it and the inventors of Java were glad to exclude it from the language, but to be fair, goto is one of the many things C inherited from languages such as Assembly or BASIC that make it truly awesome, especially when it comes to such requirements. After all, can there be a clearer and simpler implementation of a Finite State Machine (not counting BASIC ) ? <lang C>
- include<stdio.h>
int main() { char str[10];
ready: do{ printf("\nMachine is READY. (D)eposit or (Q)uit :"); scanf("%s",str); }while(!(str[0]!='D'||str[0]!='d'||str[0]!='q'||str[0]!='Q'));
if(str[0]=='q'||str[0]=='Q') goto quit; goto waiting;
waiting: do{ printf("(S)elect product or choose to (R)efund :"); scanf("%s",str); }while(!(str[0]!='s'||str[0]!='S'||str[0]!='r'||str[0]!='R'));
if(str[0]=='s'||str[0]=='S'){ printf("Dispensing product..."); goto dispense; }
else{ printf("Please collect refund."); goto ready; }
dispense: do{ printf("\nPlease (C)ollect product. :"); scanf("%s",str); }while(!(str[0]!='c'||str[0]!='C'));
goto ready;
quit: printf("Thank you, shutting down now.");
return 0; } </lang> Machine simulation :
C:\rosettaCode>fsm.exe Machine is READY. (D)eposit or (Q)uit :D (S)elect product or choose to (R)efund :S Dispensing product... Please (C)ollect product. :C Machine is READY. (D)eposit or (Q)uit :D (S)elect product or choose to (R)efund :R Please collect refund. Machine is READY. (D)eposit or (Q)uit :Q Thank you, shutting down now.
C++
<lang C>
- include <map>
template <typename State, typename Transition = State> class finite_state_machine { protected: State current; std::map<State, std::map<Transition, State>> database; public: finite_state_machine() { set(State()); } void set(State const& state) { current = state; } State get() const { return current; } void clear() { database.clear(); } void add(State const& state, Transition const& transition, State const& next) { database[state][transition] = next; } /* Add a state which is also it's own transition (and thus a link in a chain of sequences)
- /
void add(State const& state_and_transition, State const& next) { add(state_and_transition, state_and_transition, next); } bool process(Transition const& transition) { auto const& transitions = database[current], found = transitions.find(transition); if(found == transitions.end()) return false; auto const& next = found->second; set(next); return true; } /* Process so-called "automatic transitions" (ie: sequencing)
- /
bool process() { return process(get()); } /* A set of utility functions that may be helpful for displaying valid transitions to the user, etc...
- /
template <typename PushBackContainer> bool get_valid_transitions(State const& state, PushBackContainer& container) { container.clear(); auto const& found = database.find(state); if(found == database.end()) return false; auto const& transitions = found->second; if(transitions.size() == 0) return false; for(auto const& iterator : transitions) { auto const& transition = iterator.first; container.push_back(transition); } return true; } template <typename Container> bool get_valid_transitions(Container& container) { return get_valid_transitions(get(), container); } };
/* Example usage: a simple vending machine
- /
- include <string>
- include <vector>
- include <iostream>
using namespace std; void print(string const& message) { cout << message << endl; } int main() { finite_state_machine<string> machine; machine.add("ready", "quit", "exit"); machine.add("ready", "deposit", "waiting"); machine.add("waiting", "select", "dispense"); machine.add("waiting", "refund", "refunding"); machine.add("dispense", "remove", "ready"); machine.add("refunding", "ready"); machine.set("ready"); for(;;) { string state = machine.get(); if(state == "ready") print("Please deposit coins."); else if(state == "waiting") print("Please select a product."); else if(state == "dispense") print("Dispensed...please remove product from tray."); else if(state == "refunding") print("Refunding money..."); else if(state == "exit") break; else print("Internal error: unaccounted state '" + state + "'!"); /* Handle "automatic" transitions */ if(machine.process()) continue; vector<string> transitions; machine.get_valid_transitions(transitions); string options; for(auto const& transition : transitions) { if(!options.empty()) options += ", "; options += transition; } print("[" + state + "] Input the next transition (" + options + "): "); string transition; cout << " > "; cin >> transition; if(!machine.process(transition)) print( "Error: invalid transition!"); } } </lang>
- Output:
Please deposit coins. [ready] Enter the next transition (deposit, quit): > deposit Please select a product. [waiting] Enter the next transition (refund, select): > refund Refunding money... Please deposit coins. [ready] Enter the next transition (deposit, quit): > deposit Please select a product. [waiting] Enter the next transition (refund, select): > select Dispensed...please remove product from tray. [dispense] Enter the next transition (remove): > remove Please deposit coins. [ready] Enter the next transition (deposit, quit): > quit
Go
<lang go>package main
import (
"bufio" "fmt" "log" "os" "strings"
)
type state int
const (
ready state = iota waiting exit dispense refunding
)
func check(err error) {
if err != nil {
log.Fatal(err)
}
}
func fsm() {
fmt.Println("Please enter your option when prompted")
fmt.Println("(any characters after the first will be ignored)")
state := ready
var trans string
scanner := bufio.NewScanner(os.Stdin)
for {
switch state {
case ready:
for {
fmt.Print("\n(D)ispense or (Q)uit : ")
scanner.Scan()
trans = scanner.Text()
check(scanner.Err())
if len(trans) == 0 {
continue
}
option := strings.ToLower(trans)[0]
if option == 'd' {
state = waiting
break
} else if option == 'q' {
state = exit
break
}
}
case waiting:
fmt.Println("OK, put your money in the slot")
for {
fmt.Print("(S)elect product or choose a (R)efund : ")
scanner.Scan()
trans = scanner.Text()
check(scanner.Err())
if len(trans) == 0 {
continue
}
option := strings.ToLower(trans)[0]
if option == 's' {
state = dispense
break
} else if option == 'r' {
state = refunding
break
}
}
case dispense:
for {
fmt.Print("(R)emove product : ")
scanner.Scan()
trans = scanner.Text()
check(scanner.Err())
if len(trans) == 0 {
continue
}
option := strings.ToLower(trans)[0]
if option == 'r' {
state = ready
break
}
}
case refunding:
// no transitions defined
fmt.Println("OK, refunding your money")
state = ready
case exit:
fmt.Println("OK, quitting")
return
}
}
}
func main() {
fsm()
}</lang>
- Output:
Sample input/output:
Please enter your option when prompted (any characters after the first will be ignored) (D)ispense or (Q)uit : d OK, put your money in the slot (S)elect product or choose a (R)efund : s (R)emove product : r (D)ispense or (Q)uit : d OK, put your money in the slot (S)elect product or choose a (R)efund : r OK, refunding your money (D)ispense or (Q)uit : q OK, quitting
Java
<lang java>import java.util.*;
public class FiniteStateMachine {
private enum State {
Ready(true, "Deposit", "Quit"),
Waiting(true, "Select", "Refund"),
Dispensing(true, "Remove"),
Refunding(false, "Refunding"),
Exiting(false, "Quiting");
State(boolean exp, String... in) {
inputs = Arrays.asList(in);
explicit = exp;
}
State nextState(String input, State current) {
if (inputs.contains(input)) {
return map.getOrDefault(input, current);
}
return current;
}
final List<String> inputs;
final static Map<String, State> map = new HashMap<>();
final boolean explicit;
static {
map.put("Deposit", State.Waiting);
map.put("Quit", State.Exiting);
map.put("Select", State.Dispensing);
map.put("Refund", State.Refunding);
map.put("Remove", State.Ready);
map.put("Refunding", State.Ready);
}
}
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
State state = State.Ready;
while (state != State.Exiting) {
System.out.println(state.inputs);
if (state.explicit){
System.out.print("> ");
state = state.nextState(sc.nextLine().trim(), state);
} else {
state = state.nextState(state.inputs.get(0), state);
}
}
}
}</lang>
[Deposit, Quit] > Deposit [Select, Refund] > Refund [Refunding] [Deposit, Quit] > Deposit [Select, Refund] > Quit [Select, Refund] > Select [Remove] > Remove [Deposit, Quit] > Quit
JavaScript
On browser using Blocking
<lang java>//States var states = [{
'name': 'Ready',
'initial': true,
'events': {
'Deposit': 'Waiting',
'Quit': 'Exiting',
}
}, {
'name': 'Waiting',
'events': {
'Select': 'Dispensing',
'Refund': 'Refunding'
}
}, {
'name': 'Dispensing',
'events': {
'Remove': 'Ready'
}
}, {
'name': 'Refunding',
'events': {
getReady: 'Ready'
}
}, {
'name': 'Exiting',
'events': {}
}];
function StateMachine(states) {
this.states = states;
this.indexes = {};
for (var i = 0; i < this.states.length; i++) {
this.indexes[this.states[i].name] = i;
if (this.states[i].initial) {
this.currentState = this.states[i];
}
}
}; StateMachine.prototype.consumeEvent = function(e) {
if (this.currentState.events[e]) {
this.currentState = this.states[this.indexes[this.currentState.events[e]]];
}
} StateMachine.prototype.getStatus = function() {
return this.currentState.name;
} var fsm = new StateMachine(states); var s, currentButtons, answer; while ((s = fsm.getStatus()) !== "Exiting") {
switch (s) {
case "Refunding":
window.lert('Refunding');
fsm.consumeEvent("getReady")
break;
case "Dispensing":
case "Waiting":
case "Ready":
currentButtons = Object.keys(fsm.states[fsm.indexes[s]].events)
answer = window.prompt(currentButtons.join(' ') + '?');
answer = currentButtons.find(function(key) {
return key.match(new RegExp('^' + answer, 'i'))
});
if (answer) {
fsm.consumeEvent(answer);
}
}
} </lang>
Kotlin
<lang scala>// version 1.1.51
enum class State { READY, WAITING, EXIT, DISPENSE, REFUNDING }
fun fsm() {
println("Please enter your option when prompted")
println("(any characters after the first will be ignored)")
var state = State.READY
var trans: String
while (true) {
when (state) {
State.READY -> {
do {
print("\n(D)ispense or (Q)uit : ")
trans = readLine()!!.toLowerCase().take(1)
}
while (trans != "d" && trans != "q")
state = if (trans == "d") State.WAITING else State.EXIT
}
State.WAITING -> {
println("OK, put your money in the slot")
do {
print("(S)elect product or choose a (R)efund : ")
trans = readLine()!!.toLowerCase().take(1)
}
while (trans != "s" && trans != "r")
state = if (trans == "s") State.DISPENSE else State.REFUNDING
}
State.DISPENSE -> {
do {
print("(R)emove product : ")
trans = readLine()!!.toLowerCase().take(1)
}
while (trans != "r")
state = State.READY
}
State.REFUNDING -> {
// no transitions defined
println("OK, refunding your money")
state = State.READY
}
State.EXIT -> {
println("OK, quitting")
return
}
}
}
}
fun main(args: Array<String>) {
fsm()
}</lang>
Sample input/output:
Please enter your option when prompted (any characters after the first will be ignored) (D)ispense or (Q)uit : d OK, put your money in the slot (S)elect product or choose a (R)efund : s (R)emove product : r (D)ispense or (Q)uit : d OK, put your money in the slot (S)elect product or choose a (R)efund : r OK, refunding your money (D)ispense or (Q)uit : q OK, quitting
Pascal
(Free Pascal 3.0.0)
This version uses fairly vanilla pascal to implement the task. I have added some confections to vend and some simple money handeling. It uses the table method to implement a FSM which is an explicit table with a dispatch loop.
<lang Pascal> {
fsm1.pas Copyright 2018 Trevor Pearson <trevor @ nb-LadyNada co uk > This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
Implementing a simulation of a vending machine using a finite state
machine. I have used the classic table based method and added a
* little extra stuff to give the routines something to do.
}
program fsm1.pas;
uses sysutils;
type
state = (Null,Ready,Waiting,Refund,Dispense,Stop); event = (Epsilon := 1,Deposit,Select,Cancel,Remove,Quit,Error);
Item = record Name : string[12]; Stock: shortint; price: currency; end;
var
amountPaid, itemPrice , changeDue: currency; I,J : integer;
machineState: state; newState: state; machineEvent: event; entry:string; vend : array[0..4,0..4] of Item; machine : array[1..7,1..7] of state;
{ The following routines implement the transitions }
procedure TOready();
var
i,j : integer;
begin
{ Always set the state of a state machine as the first thing you do
We also set the event to epsiion we can allways set it to error if there is a problem}
machineState := Ready; machineEvent := Epsilon;
{ Now do whatever we need to to transition into this state and check for errors}
Writeln(' Trevors vending machine');
Writeln();
WriteLn (' A B C D' );
for i:=1 to 2 do begin
write(i,' ');
for j:=1 to 4 do begin
write(vend[j,i].Name,' ':(12-length(vend[j,i].Name)));
end;
WriteLn(); Write(' '); for j:=1 to 4 do begin write('£',vend[j,i].price:4:2,' ');
end;
Writeln();
end;
{ We should have delt with money } if (amountPaid > 0) then machineEvent := Error; if (changeDue > 0) then machineEvent := Error;
end;
procedure TOwaiting(); begin machineState := Waiting; if ((machineEvent = Select) and (amountPaid >= itemPrice)) then machineEvent := Epsilon; if ((machineEvent = Deposit) and (amountPaid >= itemPrice)) then machineEvent := Epsilon;
end;
procedure TOrefund();
begin machineState := Refund;
machineEvent := Epsilon;
if (amountPaid > 0) then changeDue := amountPaid;
WriteLn('REFUNDING >> £' , changeDue:2:2);
changeDue := 0;
amountPaid := 0;
end;
procedure TOdispense(); begin
machineState := Dispense;
if (amountPaid >= vend[I,J].price) then begin
machineEvent := Remove;
changeDue := amountPaid - vend[I,J].price ;
amountPaid := 0;
vend[I,J].Stock := vend[I,J].Stock - 1;
WriteLn('Vending >>',vend[I,J].Name);
end
else machineState := Waiting;
end;
procedure TOstop(); begin machineState := Stop; machineEvent := Epsilon; { There should not be any transaction in process } if ((amountPaid >0) or (changeDue >0)) then machineEvent := Error;
end;
procedure Init; var k,l: integer; begin
{ Lets pretend we have some stuff in this machine }
vend[0,0].Name := 'Dummy';
vend[0,0].Stock := 0;
vend[0,0].price := 9999;
vend[1,1].Name := 'Snickers';
vend[1,1].Stock := 12; vend[1,1].price := 0.50;
vend[2,1].Name := 'Aero';
vend[2,1].Stock := 12; vend[2,1].price := 0.50;
vend[3,1].Name := 'Bounty'; vend[3,1].Stock := 10; vend[3,1].price := 0.75;
vend[4,1].Name := 'Creme egg'; vend[4,1].Stock := 15; vend[4,1].price := 0.60;
vend[1,2].Name := 'Coke-Cola'; vend[1,2].Stock := 6; vend[1,2].price := 1.10;
vend[2,2].Name := 'Pepsi'; vend[2,2].Stock := 6; vend[2,2].price := 1.25;
vend[3,2].Name := '7 up'; vend[3,2].Stock := 6; vend[3,2].price := 1.15;
vend[4,2].Name := 'Dr Pepper'; vend[4,2].Stock := 6; vend[4,2].price := 1.99;
{ Set up the state table }
for k :=1 to 7 do begin
for l :=1 to 6 do machine[k,l] := Null;
end;
machine[ord(Ready),ord(Deposit)] := Waiting; machine[ord(Waiting),ord(Deposit)] := Dispense; machine[ord(Waiting),ord(Select)] := Dispense; machine[ord(Waiting),ord(Cancel)] := Refund; machine[ord(Dispense),ord(Remove)] := Refund; machine[ord(Dispense),ord(Error)] := Refund; machine[ord(Refund),ord(epsilon)] := Ready; machine[ord(Ready),ord(Select)] := Waiting; machine[ord(Ready),ord(Quit)] := Stop;
{ There should be no money entered so no change is due
* set itemPrice to a huge dummy amount}
amountPaid := 0; changeDue := 0; itemPrice := 999; I:= 0; J:=0;
end;
begin
Init;
TOready;
{ Here comes the magic bit ... We check for events and if an event
* occurs we look up on the table to see if we need to transition to
* another state. If we do we call the TO_xxxxx procedure. BUT we do
* this in the other order to check for machine generated events like
* Error and Epsilon. }
repeat
newState := machine[ord(machineState),ord(machineEvent)];
case (newState) of Ready : TOready; Waiting : TOwaiting; Dispense : Todispense; Refund: Torefund; Stop: TOStop; end;
{ We get some user input and assign an event to it
- If the user enters a number we convert it to currency and set a
- deposit event If we have a letter we are making a selection }
if (machineState = Ready) or (machineState = Waiting) then begin
WriteLn;
Writeln('Enter Selectian A1..D4'); Writeln('or deposit amount e.g, 0.20 -- 20p piece.'); Write('Or X to cancel, Q to stop this machine :'); ReadLn (entry); if ((entry = 'q') or (entry = 'Q')) then machineEvent := Quit; if ((entry = 'x') or (entry = 'X')) then machineEvent := Cancel; if ((entry[1] in ['a'..'d']) or (entry[1] in ['A'..'D'])) then machineEvent:= Select; if (entry[1] in ['0'..'9']) then begin machineEvent := Deposit; amountPaid := StrToCurr(entry); end; if (machineEvent = Select) then begin I := ord(entry[1]) - 64; if (I > 5) then I := I - 32; J := ord(entry[2]) - ord('0'); end;
end; until machineEvent = Quit;
end.
</lang>
OUTPUT:
*** Selection First ****
Trevors vending machine
A B C D
1 Snickers Aero Bounty Creme egg
£0.50 £0.50 £0.75 £0.60
2 Coke-Cola Pepsi 7 up Dr Pepper
£1.10 £1.25 £1.15 £1.99
Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :d1
Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :0.99
Vending >>Creme egg
REFUNDING >> £0.39
Trevors vending machine
A B C D
1 Snickers Aero Bounty Creme egg
£0.50 £0.50 £0.75 £0.60
2 Coke-Cola Pepsi 7 up Dr Pepper
£1.10 £1.25 £1.15 £1.99
Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :q
*** Deposit First ***
Trevors vending machine
A B C D
1 Snickers Aero Bounty Creme egg
£0.50 £0.50 £0.75 £0.60
2 Coke-Cola Pepsi 7 up Dr Pepper
£1.10 £1.25 £1.15 £1.99
Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :2.00
Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :b2
Vending >>Pepsi
REFUNDING >> £0.75
Trevors vending machine
A B C D
1 Snickers Aero Bounty Creme egg
£0.50 £0.50 £0.75 £0.60
2 Coke-Cola Pepsi 7 up Dr Pepper
£1.10 £1.25 £1.15 £1.99
Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :q
Perl 6
<lang perl6>#===== The state machine =====#
class StateMachine {
class State {...}
class Transition {...}
has State %!state; has &.choose-transition is rw;
method add-state(Str $id, &action)
{
%!state{$id} = State.new(:$id, :&action);
}
multi method add-transition(Str $from, Str $to)
{
%!state{$from}.implicit-next = %!state{$to};
}
multi method add-transition(Str $from, $id, Str $to)
{
%!state{$from}.explicit-next.push: Transition.new(:$id, to => %!state{$to});
}
method run(Str $initial-state)
{
my $state = %!state{$initial-state};
loop {
$state.action.();
if $state.implicit-next -> $_ { $state = $_; }
elsif $state.explicit-next -> $_ { $state = &.choose-transition.(|$_).to; }
else { last; }
}
}
class Transition {
has $.id;
has State $.to;
}
class State {
has $.id;
has &.action;
has State $.implicit-next is rw;
has Transition @.explicit-next;
}
}
- ===== Usage example: Console-based vending machine =====#
my StateMachine $machine .= new;
$machine.choose-transition = sub (*@transitions) {
say "[{.key + 1}] {.value.id}" for @transitions.pairs;
loop {
my $n = val get;
return @transitions[$n - 1] if $n ~~ Int && $n ~~ 1..@transitions;
say "Invalid input; try again.";
}
}
$machine.add-state("ready", { say "Please deposit coins."; }); $machine.add-state("waiting", { say "Please select a product."; }); $machine.add-state("dispense", { sleep 2; say "Please remove product from tray."; }); $machine.add-state("refunding", { sleep 1; say "Refunding money..."; }); $machine.add-state("exit", { say "Shutting down..."; });
$machine.add-transition("ready", "quit", "exit"); $machine.add-transition("ready", "deposit", "waiting"); $machine.add-transition("waiting", "select", "dispense"); $machine.add-transition("waiting", "refund", "refunding"); $machine.add-transition("dispense", "remove", "ready"); $machine.add-transition("refunding", "ready");
$machine.run("ready");</lang>
Phix
<lang Phix>enum READY, WAITING, DISPENSE, REFUND, QUIT -- (or just use strings if you prefer)
integer state = READY, ch = ' ' while true do
printf(1,"%c\n",ch)
switch state do
case READY: printf(1,"Machine is READY. (D)eposit or (Q)uit :")
while true do
ch = upper(wait_key())
if ch='D' then state = WAITING exit end if
if ch='Q' then state = QUIT exit end if
end while
case WAITING: printf(1,"(S)elect product or choose to (R)efund :")
while true do
ch = upper(wait_key())
if ch='S' then state = DISPENSE exit end if
if ch='R' then state = REFUND exit end if
end while
case DISPENSE: printf(1,"Dispensing product...")
printf(1,"Please (C)ollect product. :")
while true do
ch = upper(wait_key())
if ch='C' then state = READY exit end if
end while
case REFUND: printf(1,"Please collect refund.")
state = READY
ch = ' '
case QUIT: printf(1,"Thank you, shutting down now.\n")
exit
end switch
end while</lang>
- Output:
Machine is READY. (D)eposit or (Q)uit :D (S)elect product or choose to (R)efund :S Dispensing product...Please (C)ollect product. :C Machine is READY. (D)eposit or (Q)uit :D (S)elect product or choose to (R)efund :R Please collect refund. Machine is READY. (D)eposit or (Q)uit :Q Thank you, shutting down now.
Racket
<lang racket>#lang racket
(define states
'((ready (deposit . waiting)
(quit . exit))
(waiting (select . dispense)
(refund . refunding))
(dispense (remove . ready))
(refunding . ready)))
(define (machine states prompt get-action quit)
(let recur ((state (caar states)))
(printf "CURRENT STATE: ~a~%" state)
(if (eq? state 'exit)
(quit)
(recur (match (cdr (assoc state states))
[(list (and transitions (cons actions _)) ...)
(prompt "next action (from: ~a): " actions)
(match (assoc (get-action) transitions)
[(cons action new-state)
(printf "~a -> ~a -> ~a~%" state action new-state)
new-state]
[#f (printf "invalid action for~%") state])]
[auto-state
(printf "~a -> ~a~%" state auto-state)
auto-state])))))
(module+ main
(let/ec quit
(with-input-from-string "deposit select remove deposit refund quit"
(λ () (machine states void read quit)))))</lang>
- Output:
CURRENT STATE: ready ready -> deposit -> waiting CURRENT STATE: waiting waiting -> select -> dispense CURRENT STATE: dispense dispense -> remove -> ready CURRENT STATE: ready ready -> deposit -> waiting CURRENT STATE: waiting waiting -> refund -> refunding CURRENT STATE: refunding refunding -> ready CURRENT STATE: ready ready -> quit -> exit CURRENT STATE: exit
REXX
version 1
This version only works with:
- Personal REXX --or--
- PC/REXX
This is essentially a one-for-one translation of the BASIC program, with the following minor differences:
- the input allowed is either the uppercase or lowercase version of the letter(s)
- a mixture of uppercase and lowercase text is used for the output messages
- messages have extra blanks for readability (and options are spelled out)
<lang rexx>/*REXX pgm simulates a FSM (Finite State Machine), input is recognized by pressing keys.*/
10: say "Press D (deposit) or Q (quit)" /*display a prompt (message) to term. */
20: $=inkey(); upper $ /*since this a terminal, uppercase KEY.*/
if $=="D" then signal 50 /*Is response a "D" ? Process deposit.*/
if $=="Q" then exit /*Is response a "Q" ? Then exit pgm. */
signal 20 /*Response not recognized, re-issue msg*/
50: say "Press S (select) or R (refund)" /*display a prompt (message) to term. */
60: $=inkey(); upper $ /*since this a terminal, uppercase KEY.*/
if $=="S" then signal 90 /*Is response a "S" ? Then dispense it*/
if $=="R" then signal 140 /*Is response a "R" ? Then refund it. */
signal 60 /*Response not recognized? Re-issue msg*/
90: say "Dispensed" /*display what action just happened. */
signal 110 /*go and process another option. */
/* [↑] above statement isn't needed. */
110: say "Press R (remove)" /*display a prompt (message) to term. */ 120: $=inkey(); upper $ /*since this a terminal, uppercase KEY.*/
if $=="R" then signal 10 /*Is response a "R" ? Then remove it. */
signal 120 /*Response not recognized, re-issue msg*/
140: say "Refunded" /*display what action just happened. */
signal 10 /*go & re-start process (ready state). */</lang>
- output when using (pressing) the exact same input(s) as the BASIC entry: D R D S R Q
press D (deposit) or Q (quit) d ◄■■■■■■■■■■ user pressed this key. Press S (select) or R (refund) r ◄■■■■■■■■■■ user pressed this key. Refunded press D (deposit) or Q (quit) d ◄■■■■■■■■■■ user pressed this key. Press S (select) or R (refund) s ◄■■■■■■■■■■ user pressed this key. Dispensed Press R (remove) r ◄■■■■■■■■■■ user pressed this key. press D (deposit) or Q (quit) q ◄■■■■■■■■■■ user pressed this key.
version 2
works withooRexx (and any other REXX). key and Enter must be pressed- <lang rexx>/*REXX pgm simulates a FSM (Finite State Machine), input is recognized by pressing keys.*/
10: k=inkey('D (deposit) or Q (quit)','DQ')
if k=="D" then signal 50 /*Is response a "D" ? Process deposit.*/
if k=="Q" then exit /*Is response a "Q" ? Then exit pgm. */
50: k=inkey('S (select) or R (refund)','SR');
if k=="S" then signal 90 /*Is response a "S" ? Then dispense it*/
if k=="R" then signal 140 /*Is response a "R" ? Then refund it. */
90: say "Dispensed" /*display what action just happened. */
signal 110 /*go and process another option. */
110: k=inkey('R (remove)','R');
if k=="R" then signal 10 /*Is response a "R" ? Then remove it. */
140: say "Refunded" /*display what action just happened. */
signal 10 /*go & re-start process (ready state). */
inkey: Parse Arg prompt,valid Do Forever
Say 'Press' prompt 'and Enter' Parse Upper Pull key k=left(key,1) If pos(k,valid)>0 Then Leave Else Say 'Invalid key, try again.' End
Return k</lang>
- Output:
Press D (deposit) or Q (quit) and Enter c Invalid key, try again. Press D (deposit) or Q (quit) and Enter d Press S (select) or R (refund) and Enter g Invalid key, try again. Press S (select) or R (refund) and Enter r Refunded Press D (deposit) or Q (quit) and Enter
zkl
A lame FSM, we just convert text to a [hopefully valid] zkl program, compile and run it.
If we need true state to state hops, we could use tail recursion (another name for goto). <lang zkl>class FSM{ // our Finite State Machine
var bank=0, item=Void;
fcn deposit(coin){ bank=coin }
fcn select(item){
if(bank){ bank=0; self.item=item; }
else print("Depost coin, then select ")
}
fcn take { if(item) item=Void; else print("Select first "); }
fcn refund { coin:=bank; bank=0; return(coin) }
// couple of wrappers to state changes
fcn state{ println("Bank(%4d), Item(%s)".fmt(bank,item)) }
fcn act(f){ print("%-10s-->".fmt(f.name)); f(); state(); }
}
Vault.add(FSM); // put class FSM where I can find it</lang> <lang zkl>fcn run(program){ // convert text to FSM instructions and run them
program=program.replace("(",".fp("); // deposit(10)-->deposit.fp(10)
a,b,p := 0,0,Sink("class P(FSM){ state(); ");
while(Void!=(b=program.find(";",a)))
{ p.write("act(",program[a,b-a],");"); a=b + 1; }
program=p.write(program[a,*],"}").close();
// println(program); // WTH did I just do?
Compiler.Compiler.compileText(program)(); // compile and run our little FSM
}</lang> <lang zkl>run("select(); take(); deposit(10); select(\"snickers\"); take();");</lang> The above is converted to: <lang zkl>class P(FSM){
state();
act(select.fp());
act( take.fp());
act( deposit.fp(10));
act( select.fp("snickers"));
act( take.fp());
}</lang> The .fp() is function application (ie deferred execution) so I can extract the function name and print it.
- Output:
Bank( 0), Item(Void) select -->Depost coin, then select Bank( 0), Item(Void) take -->Select first Bank( 0), Item(Void) deposit -->Bank( 10), Item(Void) select -->Bank( 0), Item(snickers) take -->Bank( 0), Item(Void)