/* Replace strings in possibly piped input to possibly piped output.
 * 
 * Written to not use string functions.
 *
 * By Kragen Javier Sitaker, 02022.  Public domain, CC0.
 */
#include <stdio.h>              /* fgets(), fputs(), fprintf() */
#include <stdlib.h>             /* exit() */


char *argv0;

void usage()
{
  fprintf(stderr, "%s: Usage: %s pattern replacement <infile >outfile\n",
                   argv0,     argv0);
  exit(-1);
}


void fail(char *why)
{
  fprintf(stderr, "%s: %s\n", argv0, why);
  exit(1);
}


/* like strstr, but returns pointer to end of string on failure */
char *find(char *haystack, char *needle)
{
  for (; *haystack; haystack++) {
    for (size_t i = 0; needle[i]; i++) {
      if (needle[i] != haystack[i]) goto not_found;
    }
    return haystack; /* found: fell off end of needle without mismatch */
  not_found: ;
  }
  return haystack;
}


/* basically memcpy, except it aborts if the buffer is too small */
char *cmove(char *dest, char *src, size_t n, char *dest_end)
{
  size_t m = dest_end - dest;
  if (n > m) fail("string too long");
  for (size_t i = 0; i < n; i++) dest[i] = src[i];
  return dest + n;
}


size_t length(char *s)
{
  size_t n = 0;
  while (s[n]) n++;
  return n;
}


enum { bufsiz = 2048 };
char buf1[bufsiz], buf2[bufsiz];

int main(int argc, char **argv)
{
  argv0 = argv[0];
  if (argc != 3) usage();
  char *pattern = argv[1], *replacement = argv[2], *dest_end = buf2 + bufsiz;

  size_t n = length(replacement), pattern_len = length(pattern);

  for (;;) {
    if (!fgets(buf1, bufsiz, stdin)) return 0; /* ambiguous whether EOF or error */
    if (length(buf1) == bufsiz - 1) fail("input line too long");

    char *si = buf1, *di = buf2; /* source index, destination index */
    for (;;) {
      char *found = find(si, pattern);
      di = cmove(di, si, found - si, dest_end);
      if (!*found) break;
      di = cmove(di, replacement, n, dest_end);
      si = found + pattern_len;
    }
    di = cmove(di, "", 1, dest_end); /* terminating NUL for fputs */

    if (fputs(buf2, stdout) < 0) fail("fputs");
  }
}