1/*
2 * An implementation of replacing substrings in a string.
3 *
4 * Source: http://creativeandcritical.net/str-replace-c
5 * Author: Laird Shaw
6 * License: Public domain
7 *
8 * Brought on 2016-02-24
9 */
10
11#include <string.h>
12#include <stdlib.h>
13#include <stddef.h>
14
15#include "repl_str.h"
16
17
18/* An optimised string replacement function that caches string positions so that
19 * strstr() doesn't need to be called twice for each position.
20 *
21 * This code is entirely by me, and released into the public domain, so have no
22 * doubts about your right to use it.
23 */
24char *replace_substr(const char *str, const char *old, const char *new) {
25
26	/* Adjust each of the below values to suit your needs. */
27
28	/* Increment positions cache size initially by this number. */
29	size_t cache_sz_inc = 16;
30	/* Thereafter, each time capacity needs to be increased,
31	 * multiply the increment by this factor. */
32	const size_t cache_sz_inc_factor = 3;
33	/* But never increment capacity by more than this number. */
34	const size_t cache_sz_inc_max = 1048576;
35
36	char *pret, *ret = NULL;
37	const char *pstr2, *pstr = str;
38	size_t i, count = 0;
39	ptrdiff_t *pos_cache = NULL;
40	size_t cache_sz = 0;
41	size_t cpylen, orglen, retlen, newlen, oldlen = strlen(old);
42
43	/* Find all matches and cache their positions. */
44	while ((pstr2 = strstr(pstr, old)) != NULL) {
45		count++;
46
47		/* Increase the cache size when necessary. */
48		if (cache_sz < count) {
49			cache_sz += cache_sz_inc;
50			pos_cache = realloc(pos_cache, sizeof(*pos_cache) * cache_sz);
51			if (pos_cache == NULL) {
52				goto end_repl_str;
53			}
54			cache_sz_inc *= cache_sz_inc_factor;
55			if (cache_sz_inc > cache_sz_inc_max) {
56				cache_sz_inc = cache_sz_inc_max;
57			}
58		}
59
60		pos_cache[count-1] = pstr2 - str;
61		pstr = pstr2 + oldlen;
62	}
63
64	orglen = pstr - str + strlen(pstr);
65
66	/* Allocate memory for the post-replacement string. */
67	if (count > 0) {
68		newlen = strlen(new);
69		retlen = orglen + (newlen - oldlen) * count;
70	} else	retlen = orglen;
71	ret = malloc(retlen + 1);
72	if (ret == NULL) {
73		goto end_repl_str;
74	}
75
76	if (count == 0) {
77		/* If no matches, then just duplicate the string. */
78		strcpy(ret, str);
79	} else {
80		/* Otherwise, duplicate the string whilst performing
81		 * the replacements using the position cache. */
82		pret = ret;
83		memcpy(pret, str, pos_cache[0]);
84		pret += pos_cache[0];
85		for (i = 0; i < count; i++) {
86			memcpy(pret, new, newlen);
87			pret += newlen;
88			pstr = str + pos_cache[i] + oldlen;
89			cpylen = (i == count-1 ? orglen : pos_cache[i+1]) - pos_cache[i] - oldlen;
90			memcpy(pret, pstr, cpylen);
91			pret += cpylen;
92		}
93		ret[retlen] = '\0';
94	}
95
96end_repl_str:
97	/* Free the cache and return the post-replacement string,
98	 * which will be NULL in the event of an error. */
99	free(pos_cache);
100	return ret;
101}
102