Enumerate every permutation

next_permutation.c

#include <assert.h>
#include <stdbool.h>
#include <stdio.h>

bool next_permutation(unsigned a[], unsigned n)
{
    if (n <= 1)
        return false;
    unsigned i = n-2;
    while (!(a[i] < a[i+1]))
        if (i-- == 0)
            return false;
    assert(a[i] < a[i+1]);
    unsigned j = n-1;
    while (!(a[j] > a[i]))
        --j;
    assert(j > i);
    assert(a[j] > a[i]);
    unsigned t = a[i]; a[i] = a[j]; a[j] = t;
    for (++i, j=n-1; i < j; ++i, --j) {
        t = a[i]; a[i] = a[j]; a[j] = t;
    }
    return true;
}

int main()
{
    unsigned a[10];
    const unsigned n = sizeof(a) / sizeof(a[0]);
    for (unsigned i=0; i<n; ++i)
	a[i] = i+1;
    unsigned p = 0;
    do {
	printf("%u:", ++p);
	for (unsigned i=0; i<n; ++i)
	    printf(" %u", a[i]);
	printf("\n");
    } while (next_permutation(a, n));
    return 0;
}