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#include <assert.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

void
str_replace_inline(char* str, char const* old, char const* new) {
  assert(str);
  assert(new);
  assert(old);

  int const newlen = strlen(new);
  int const oldlen = strlen(old);

  assert(oldlen > 0);
  assert(newlen <= oldlen);

  char* next = str;
  while (*str) {
    if (strncmp(str, old, oldlen) == 0) {
      strncpy(next, new, newlen);
      next += newlen;
      str += oldlen;
    }
    else {
      *next++ = *str++;
    }
  }
  while (*str) {
    *next++ = *str++;
  }
  *next = '\0';
}

void
str_replace_malloc(char** str, int* str_size, char const* old, char const* new) {
  assert(str != NULL);
  assert(*str != NULL);
  assert(str_size);
  assert(new != NULL);
  assert(old != NULL);

  int const newlen = strlen(new);
  int const oldlen = strlen(old);

  assert(oldlen > 0);

  if (newlen <= oldlen) {
    str_replace_inline(*str, old, new);
  }
  else {
    for (char* i = *str; *i; ++i) {
      if (strncmp(i, old, oldlen) == 0) {
        int new_str_size = (i - *str) + newlen * 2 + 100;
        char* new_str = malloc(new_str_size);
        if (!new_str) {
          perror("malloc failed");
          abort();
          // or whatever you want
        }
        strncpy(new_str, *str, i - *str);
        char* next = new_str + (i - *str);
        strncpy(next, new, newlen);
        next += newlen;
        i += oldlen;

        while (*i) {
          if (strncmp(i, old, oldlen) == 0) {
            strncpy(next, new, newlen);
            next += newlen;
            i += oldlen;
          }
          else {
            *next++ = *i++;
          }
          if ((next - new_str) + newlen >= new_str_size) {
            new_str_size *= 2;
            new_str_size += newlen + 1;
            char* new_new_str = malloc(new_str_size);
            if (!new_new_str) {
              perror("malloc failed");
              abort();
              // or whatever you want
            }
            strncpy(new_new_str, new_str, next - new_str);
            next = new_new_str + (next - new_str);
            free(new_str);
            new_str = new_new_str;
          }
        }
        *next = '\0';
        *str = new_str;
        *str_size = new_str_size;
        return;
      }
    }
  }
}

typedef struct {
  int id;
  char const* old;
  char const* new;
  char const* initial;
  char const* expected;
} Test;

int main() {
  Test tests[] = {
    {__LINE__, "a", "b", "aaaa", "bbbb"},
    {__LINE__, "a", "b", "abca", "bbcb"},
    {__LINE__, "aa", "b", "aaaa", "bb"},
    {__LINE__, "aa", "b", "aaab", "bab"},
    {__LINE__, "a", "bb", "a", "bb"},
    {__LINE__, "a", "bb", "aa", "bbbb"},
    {__LINE__, "old", "newish", "blah blah old blah oldold blah", "blah blah newish blah newishnewish blah"},
    {__LINE__, "old", "newish", "old blah old", "newish blah newish"},
  };
  for (Test* i = tests; i != tests + sizeof tests / sizeof *tests; ++i) {
    int size = strlen(i->initial) + 1;
    char* buf = malloc(size);
    strcpy(buf, i->initial);

    str_replace_malloc(&buf, &size, i->old, i->new);

    if (strcmp(buf, i->expected) != 0) {
      printf("failed test %d, expected '%s', got '%s'\n", i->id, i->expected, buf);
    }
    free(buf);
  }
}