replace str static

#include <stdio.h>
#include <stdint.h>
#include <string.h>

void insert(int pos, char *s, const char *add) {
  int n = strlen(s), lenAdd = strlen(add);
  for (int i = n - 1; i >= pos; i--) {
    s[i + lenAdd] = s[i];
  }
  for (int i = 0; i < lenAdd; i++) {
    s[i + pos] = add[i];
  }
}

void del(int pos, int howMuch, char *s) {
  int n = strlen(s);
  for (int i = pos; i < n; i++) {
    s[i] = s[i + howMuch];
  }
}

uint32_t string_replace(char *where, const char *what, const char *replace) {
  uint32_t cnt = 0;

  char *f = strstr(where, what);
  const int lenRep = strlen(replace);
  const int lenWhat = strlen(what);
  while (f) {
    int pos = f - where;
    insert(pos, where, replace);
    del(pos + lenRep, lenWhat, where);
    cnt++;
    f = strstr(f + lenRep, what);
  }
  return cnt;
}

int main(void) {
  char s1[10003] = "", s2[31] = "", s3[31] = "";
  fgets(s1, 900, stdin);
  fgets(s2, 30, stdin);
  fgets(s3, 30, stdin);
  s1[strlen(s1) - 1] = 0;
  s2[strlen(s2) - 1] = 0;
  s3[strlen(s3) - 1] = 0;//*/
  uint32_t cnt = string_replace(s1, s2, s3);
  printf("found: %d\n", cnt);
  puts(s1);
  return 0;
}