C語言字符串中的AC自動機實現

AC自動機（Aho-Corasick算法）是一種用于在文本中查找多個模式字符串的高效算法。它基于Trie樹（字典樹）和有限狀態自動機實現。以下是一個簡單的C語言實現：

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

#define ALPHABET_SIZE 256

typedef struct TrieNode {
    struct TrieNode *children[ALPHABET_SIZE];
    int isEndOfPattern;
    int patternIndex;
    struct TrieNode *fail;
} TrieNode;

TrieNode *createTrieNode() {
    TrieNode *node = (TrieNode *)malloc(sizeof(TrieNode));
    node->isEndOfPattern = 0;
    node->patternIndex = -1;
    node->fail = NULL;
    for (int i = 0; i < ALPHABET_SIZE; i++) {
        node->children[i] = NULL;
    }
    return node;
}

void insertPattern(TrieNode *root, const char *pattern, int patternIndex) {
    TrieNode *node = root;
    for (int i = 0; pattern[i] != '\0'; i++) {
        if (node->children[(int)pattern[i]] == NULL) {
            node->children[(int)pattern[i]] = createTrieNode();
        }
        node = node->children[(int)pattern[i]];
    }
    node->isEndOfPattern = 1;
    node->patternIndex = patternIndex;
}

void buildFailureLinks(TrieNode *root) {
    TrieNode *queue[1000];
    int front = 0, rear = 0;
    for (int i = 0; i < ALPHABET_SIZE; i++) {
        if (root->children[i]) {
            root->children[i]->fail = root;
            queue[rear++] = root->children[i];
        }
    }
    while (front< rear) {
        TrieNode *currentNode = queue[front++];
        for (int i = 0; i < ALPHABET_SIZE; i++) {
            if (currentNode->children[i]) {
                TrieNode *childNode = currentNode->children[i];
                queue[rear++] = childNode;
                TrieNode *failNode = currentNode->fail;
                while (failNode && !failNode->children[i]) {
                    failNode = failNode->fail;
                }
                if (!failNode) {
                    childNode->fail = root;
                } else {
                    childNode->fail = failNode->children[i];
                    if (childNode->fail->isEndOfPattern) {
                        childNode->isEndOfPattern = 1;
                        childNode->patternIndex = childNode->fail->patternIndex;
                    }
                }
            }
        }
    }
}

void searchPatterns(const char *text, TrieNode *root) {
    TrieNode *node = root;
    for (int i = 0; text[i] != '\0'; i++) {
        while (node && !node->children[(int)text[i]]) {
            node = node->fail;
        }
        node = node->children[(int)text[i]];
        if (!node) {
            node = root;
        }
        TrieNode *temp = node;
        while (temp && temp->isEndOfPattern) {
            printf("Pattern %d found at index %d\n", temp->patternIndex, i - strlen(patterns[temp->patternIndex]) + 1);
            temp = temp->fail;
        }
    }
}

int main() {
    const char *patterns[] = {"he", "she", "his", "hers"};
    int numPatterns = sizeof(patterns) / sizeof(patterns[0]);

    TrieNode *root = createTrieNode();
    for (int i = 0; i < numPatterns; i++) {
        insertPattern(root, patterns[i], i);
    }
    buildFailureLinks(root);

    const char *text = "ushers";
    searchPatterns(text, root);

    return 0;
}

這個程序首先創建一個Trie樹，然后將要搜索的模式字符串插入到Trie樹中。接下來，它構建失敗鏈接，這是AC自動機的關鍵部分。最后，它在給定的文本中搜索模式字符串。當找到匹配時，它會輸出模式字符串的索引和在文本中的位置。