Permutations and Combinations

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
void perm_show(int *p, int n) {
  for (int i = 0; i < n; i++) {
    printf("%d", p[i]);
    if (i < n - 1) printf(" ");
  }
  puts("");
}
void perm_put(int *p, bool *ok, int n, int pos, int k) {
  p[pos] = k;
  if (pos == n - 1) perm_show(p, n);
  else {
    ok[k] = false;
    for (int i = 1; i <= n; i++) {
      if (ok[i]) perm_put(p, ok, n, pos + 1, i);
    }
    ok[k] = true;
  }
}
void generate_permutation(void) {
  const int n = 4;
  int p[n];
  bool ok[n + 1];
  for (int i = 1; i <= n; i++) ok[i] = true;
  for (int i = 1; i <= n; i++) perm_put(p, ok, n, 0, i);
  return;
}
unsigned int first(unsigned int n) {
  return (unsigned int)((1U << n) - 1U);
}
unsigned int next_comb(unsigned int x) {
  unsigned int smallest = x & -x;
  unsigned int ripple = x + smallest;
  unsigned int new_smallest = ripple & -ripple;
  unsigned int ones = ((new_smallest / smallest) >> 1U) - 1U;
  return ripple | ones;
}
void print_comb(unsigned int c, int n) {
  for (int i = 0; i < n; i++) {
    if (c & 1U) {
      printf("%d", i + 1U);
      if (i < n - 1) printf(" ");
    }
    c >>= 1U;
  }
  puts("");
}
void generate_combination(void) {
  const unsigned int n = 8;
  const unsigned int k = 4;
  unsigned int x = first(k);
  for (int i = 1; !(x & ~first(n)); i++, x = next_comb(x)) print_comb(x, n);
  return;
}
int main(void) {
  puts("Permutation");
  generate_permutation();
  puts("");
  puts("Combination");
  generate_combination();
  return EXIT_SUCCESS;
}