gists/c-programming/strings/str_replace.c

/*
 * str_replace.c
 *
 * Author: Intel A80486DX2-66
 * License: Creative Commons Zero 1.0 Universal
 *
 * To-Do:
 * - add ability to replace from the end (negative value of max_count)
 */

#include "str_replace.h"

char* str_replace(
  const char* str,
  const char* substr,
  const char* replacement,
  ssize_t max_count
) {
	/*
	 * input arguments:
	 * 	max_count: N <= 0 for all replacements, N > 0 for N replacement
	 *
	 * the output string should be freed!
	 */
	size_t substr_len = strlen(substr),
	       replacement_len = strlen(replacement),
	       count = 0;
	const char* p = str;

	// count the number of occurrences of the substring
	for (; (p = strstr(p, substr)) != NULL; count++) {
		if (max_count >= 0 && count >= (size_t) max_count)
			break;
		p += substr_len;
	}

	// allocate a buffer to hold the modified string
	size_t new_len = strlen(str) + count * (replacement_len - substr_len);

	char* result = malloc((new_len + 1) * sizeof(char));
	if (result == NULL) {
		perror("calloc");
		exit(EXIT_FAILURE);
	}

	const char* q = str;
	char* r = result;
	for (; (p = strstr(q, substr)) != NULL && count > 0; count--) {
		size_t len = p - q;
		memcpy(r, q, len);
		r += len;
		memcpy(r, replacement, replacement_len);
		r += replacement_len;
		q = p + substr_len;
	}
	strcpy(r, q);

	return result;
}

#ifdef TEST
#include <stdio.h>

int main(void) {
	const char* str = "Hello; peace! This is a peaceful test.",
	          * substr = "peace",
	          * replacement1 = "universe",
	          * replacement2 = "_____";

	char* result1 = str_replace(str, substr, replacement1, -1),
	    * result2 = str_replace(str, substr, replacement2, 1);

	puts(result1); free(result1);
	puts(result2); free(result2);

	return 0;
}
#endif