/* 
 * Knuth-Morris-Pratt string matcher (C)
 * 
 * Copyright (c) 2017 Project Nayuki. (MIT License)
 * https://www.nayuki.io/page/knuth-morris-pratt-string-matching
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy of
 * this software and associated documentation files (the "Software"), to deal in
 * the Software without restriction, including without limitation the rights to
 * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
 * the Software, and to permit persons to whom the Software is furnished to do so,
 * subject to the following conditions:
 * - The above copyright notice and this permission notice shall be included in
 *   all copies or substantial portions of the Software.
 * - The Software is provided "as is", without warranty of any kind, express or
 *   implied, including but not limited to the warranties of merchantability,
 *   fitness for a particular purpose and noninfringement. In no event shall the
 *   authors or copyright holders be liable for any claim, damages or other
 *   liability, whether in an action of contract, tort or otherwise, arising from,
 *   out of or in connection with the Software or the use or other dealings in the
 *   Software.
 */

#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>


// Searches for the given pattern string in the given text string using the Knuth-Morris-Pratt string matching algorithm.
// If the pattern is found, a pointer to the start of the earliest match in 'text' is returned. Otherwise NULL is returned.
// However in the case that malloc() fails, NULL is also returned.
const char *kmp_search(const char *pattern, const char *text) {
	if (pattern[0] == '\0')
		return text;  // Immediate match
	
	// Allocate memory for LSP table
	size_t pattern_len = strlen(pattern);
	if (SIZE_MAX / sizeof(size_t) < pattern_len)
		return NULL;
	size_t *lsp = malloc(pattern_len * sizeof(size_t));
	if (lsp == NULL)
		return NULL;
	
	// Compute longest suffix-prefix table
	lsp[0] = 0;  // Base case
	for (size_t i = 1; i < pattern_len; i++) {
		size_t j = lsp[i - 1];  // Start by assuming we're extending the previous LSP
		while (j > 0 && pattern[i] != pattern[j])
			j = lsp[j - 1];
		if (pattern[i] == pattern[j])
			j++;
		lsp[i] = j;
	}
	
	// Walk through text string
	for (size_t j = 0; *text != '\0'; text++) {  // j is the number of chars matched in pattern
		while (j > 0 && *text != pattern[j])
			j = lsp[j - 1];  // Fall back in the pattern
		if (*text == pattern[j]) {
			j++;  // Next char matched, increment position
			if (j == pattern_len) {
				free(lsp);
				return text - (j - 1);
			}
		}
	}
	
	// Not found
	free(lsp);
	return NULL;
}