Talk:Closest-pair problem/C: Difference between revisions

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2. It's shorter and quite a bit faster;
2. It's shorter and quite a bit faster;
3. It's cleaner IMO.
3. It's cleaner IMO.
<snip: code removed from talk> --[[User:Ledrug|Ledrug]] 03:34, 18 June 2011 (UTC)
<lang C>#include <stdio.h>
#include <stdlib.h>
#include <values.h>
#include <math.h>


I am not in love with my own code, expecially when it does not work properly!:D Go replace it, if this one works (I am going to test it, but I trust it works)! (Note: there's no explicit "segfault with 200,000 points or more"... I'll keep my code so a day maybe I'll know why this happens:D) --[[User:ShinTakezou|ShinTakezou]] 14:54, 18 June 2011 (UTC)
typedef struct { double x, y; } point_t, *point;

/* note: even though l_list and r_list are used by each recursion of closest(), they
are always used in sequence, no need to allocate them repeatedly */
int *l_list, *r_list;

inline double dist(point a, point b)
{
double dx = a->x - b->x, dy = a->y - b->y;
return dx * dx + dy * dy;
}

inline int cmp_dbl(double a, double b)
{
return a < b ? -1 : a > b ? 1 : 0;
}


double brute_force(point pts, int max_n, int *a, int *b)
{
int i, j;
point pi;
double d, min_dist = MAXDOUBLE;

for (i = 0, pi = pts; i < max_n; pi++, i++) {
for (j = i + 1; j < max_n; j++) {
if ( (d = dist(pi, pts + j)) < min_dist ) {
*a = i;
*b = j;
min_dist = d;
}
}
}
return min_dist;
}

void sort_y(point pts, int n, int *list)
{
int cmp_y(const void *a, const void *b) {
return cmp_dbl(pts[*(int*)a].y, pts[*(int*)b].y);
}
qsort(list, n, sizeof(int), cmp_y);
}

double closest(point pts, int n, point *a, point *b)
{
int left, right, lsize, rsize, i;
point pl, a1, b1;
double min_d, d, dsqrt, median;

/* don't divide if there aren't enough points to divide */
if (n == 2) {
*a = pts;
*b = pts + 1;
return dist(pts, pts + 1);
}

if (n == 3) {
min_d = dist(pts, pts + 1);
*a = pts; *b = pts + 1;

if (min_d > (d = dist(pts, pts + 2))) {
min_d = d;
*b = pts + 2;
}
if (min_d > (d = dist(pts + 1, pts + 2))) {
min_d = d;
*a = pts + 1;
*b = pts + 2;
}
return min_d;
}

/* get left and right results */
left = right = n / 2;
if ((min_d = closest(pts, left, a, b)) > (d = closest(pts + left, n - left, &a1, &b1))) {
min_d = d;
*a = a1;
*b = b1;
}

median = pts[left].x;
dsqrt = sqrt(min_d);

/* find points within +- min distance on X from the center line, list their indices */
for ( lsize = 0, left--;
median - pts[left].x < dsqrt && left;
l_list[lsize++] = left--);

for ( rsize = 0;
pts[right].x - median < dsqrt && right < n;
r_list[rsize++] = right++);

/* sort the indices by y: don't touch the point data which is sorted by x */
sort_y(pts, lsize, l_list);
sort_y(pts, rsize, r_list);

/* climb up left and right list and compare distance */
for (left = right = 0; left < lsize; left ++) {
/* next point in left list */
pl = pts + l_list[left];
median = pl->y;

/* climb up right list until the y is not too low */
while (median > dsqrt + pts[r_list[right]].y && right < rsize)
right++;

if (right >= rsize) break;

for (i = right; i < rsize; i++) {
/* right y is too high, break */
if (pts[ r_list[i] ].y > median + dsqrt) break;

if (min_d > (d = dist(pts + r_list[i], pl))) {
*a = pl;
*b = pts + r_list[i];
min_d = d;
dsqrt = sqrt(min_d);
}
}
}

return min_d;
}

#define NP 1000000
int main(int argc, char **argv)
{
int i, j;
double d;
point a, b;

point pts = malloc(sizeof(point_t) * NP);
l_list = malloc(sizeof(int) * NP);
r_list = malloc(sizeof(int) * NP);

for(i = 0; i < NP; i++) {
pts[i].x = 20 * (double) rand()/RAND_MAX;
pts[i].y = 20 * (double) rand()/RAND_MAX;
}

d = brute_force(pts, NP, &i, &j);
printf("brute force: %g, between (%f,%f) and (%f,%f)\n",
d, pts[i].x, pts[i].y, pts[j].x, pts[j].y);


int cmp_x(const void *a, const void *b) {
return cmp_dbl( ((point)a)->x, ((point)b)->x );
}
qsort(pts, NP, sizeof(point_t), cmp_x);

printf("min: %g; ", sqrt(closest(pts, NP, &a, &b)));
printf("point (%f,%f) and (%f,%f)\n", a->x, a->y, b->x, b->y);

return 0;
}</lang>
--[[User:Ledrug|Ledrug]] 21:34, 17 June 2011 (UTC)

Latest revision as of 03:35, 20 June 2011

your code does NOT RUN when i compile with devc or Turbo C???? (unsigned comment added by 113.22.126.190 at 21:06, 24 October 2010)

Correct it? Try setting compiler flags for compatibility with a specific C standard? --Michael Mol 13:11, 25 October 2010 (UTC)
"The code does not run"... is like pretending to derive solar physics by the statement "the sun shines"... By the way "devc" (Dev C++ IDE?) usually is used with (an old version of) gcc. On my test machine (GNU/Linux) it runs, except for some "evil dataset" that someone gave me once upon a time... I've inspected the code with valgrind, debugged it, ... but I was not able to unwind the flow that gives the problem... I know this code hides some oddity somewhere. Likely the better thing is to rewrite it from scratch, but I've not the courage yet! :) — ShinTakezou 17:52, 30 May 2011 (UTC)

Propose replacing code

I suggest replace current code sample with rewritten code below. Reasons: 1. It doesn't segfault with 200,000 points or more; 2. It's shorter and quite a bit faster; 3. It's cleaner IMO. <snip: code removed from talk> --Ledrug 03:34, 18 June 2011 (UTC)

I am not in love with my own code, expecially when it does not work properly!:D Go replace it, if this one works (I am going to test it, but I trust it works)! (Note: there's no explicit "segfault with 200,000 points or more"... I'll keep my code so a day maybe I'll know why this happens:D) --ShinTakezou 14:54, 18 June 2011 (UTC)

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