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

void findreplace(char *str, const char *oldword, const char *newword);

int main(int argc, char *argv[]) {

  // ./a.out filename oldWord newWord
  if (argc != 4) {
    printf("usage: %s filename oldword newword\n", argv[0]);
    return 1;
  }

  char *filename = argv[1];
  char *oldWord  = argv[2];
  char *newWord  = argv[3];

  if (!strcmp(oldWord, newWord)) {
    printf("error: words are the same\n");
    return 1;
  }

  FILE *file = fopen(filename, "r");
  FILE *tmpf = fopen("tmp.txt", "w");
  if (!file || !tmpf) {
    printf("could not open files\n");
    return 1;
  }

  size_t size = 256;
  char buf[size];
  // loop
  while ((fgets(buf, size, file)) != NULL) {
    //printf("%s", buf);
    findreplace(buf, oldWord, newWord);

    fputs(buf, tmpf);
  }

  fclose(file);
  fclose(tmpf);

  remove(filename);
  rename("tmp.txt", filename);


  return 0;
}


void findreplace(char *str, const char *oldword, const char *newword) {
  char *pos, tmp[256];
  int index = 0;
  int oldLen = strlen(oldword);
  while ((pos = strstr(str, oldword)) != NULL) {
    strcpy(tmp, str);
    index = pos - str;
    str[index] = '\0';
    strcat(str, newword);
    strcat(str, tmp + index + oldLen);
  }
}