c++实现LZ78算法

LZ78算法是一种基于字典的无损数据压缩算法。以下是使用C语言实现LZ78算法的示例代码：

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

#define MAX_DICT_SIZE 256 // 字典最大大小
#define MAX_CODE_SIZE 4096 // 编码最大大小

typedef struct {
    int num_elements;
    char **elements;
} Dictionary;

// 创建一个新的字典
Dictionary *create_dict() {
    Dictionary *dict = (Dictionary *) malloc(sizeof(Dictionary));
    dict->num_elements = 0;
    dict->elements = (char **) malloc(sizeof(char *) * MAX_DICT_SIZE);
    return dict;
}

// 向字典中添加一个元素
void add_to_dict(Dictionary *dict, char *element) {
    dict->elements[dict->num_elements++] = element;
}

// 在字典中查找一个元素
int find_in_dict(Dictionary *dict, char *element) {
    int i;
    for (i = 0; i < dict->num_elements; i++) {
        if (strcmp(dict->elements[i], element) == 0) {
            return i;
        }
    }
    return -1;
}

// 释放字典占用的内存
void free_dict(Dictionary *dict) {
    int i;
    for (i = 0; i < dict->num_elements; i++) {
        free(dict->elements[i]);
    }
    free(dict->elements);
    free(dict);
}

// LZ78压缩算法
void lz78_compress(char *input, int input_len, int *codes, int *code_len) {
    Dictionary *dict = create_dict();

    int i = 0, j = 0, k = 0;
    int dict_index, new_code;
    char *buffer = (char *) malloc(sizeof(char) * MAX_CODE_SIZE);

    while (i < input_len) {
        buffer[j++] = input[i++];

        dict_index = find_in_dict(dict, buffer);

        if (dict_index == -1) {
            add_to_dict(dict, strdup(buffer));
            buffer[j-1] = '\0';
            codes[k++] = find_in_dict(dict, buffer);
            j = 0;
        } else if (i == input_len) {
            codes[k++] = dict_index;
        }
    }

    *code_len = k;

    free(buffer);
    free_dict(dict);
}

// LZ78解压缩算法
void lz78_decompress(int *codes, int code_len, char *output, int *output_len) {
    Dictionary *dict = create_dict();

    int i = 0, j = 0;
    int code, dict_index, buffer_len;
    char *buffer = (char *) malloc(sizeof(char) * MAX_CODE_SIZE);

    while (i < code_len) {
        code = codes[i++];

        if (code >= dict->num_elements) {
            fprintf(stderr, "Invalid code: %d\n", code);
            exit(EXIT_FAILURE);
        }

        strcpy(buffer, dict->elements[code]);
        buffer_len = strlen(buffer);

        if (i == code_len) {
            strncpy(output + j, buffer, buffer_len);
            j += buffer_len;
        } else {
            code = codes[i++];
            if (code >= dict->num_elements) {
                fprintf(stderr, "Invalid code: %d\n", code);
                exit(EXIT_FAILURE);
            }

            dict_index = find_in_dict(dict, dict->elements[code]);
            if (dict_index == -1) {
                fprintf(stderr, "Invalid dict index: %d\n", dict_index);
                exit(EXIT_FAILURE);
            }

            strncat(buffer, dict->elements[code], MAX_CODE_SIZE - buffer_len);
            add_to_dict(dict, strdup(buffer));
            strncpy(output + j, buffer, buffer_len);
            j += buffer_len;
        }
    }

    *output_len = j;

    free(buffer);
    free_dict(dict);
}

int main() {
    char input[] = "ababcabcd";
    int codes[MAX_CODE_SIZE];
    int code_len, output_len;
    char output[MAX_CODE_SIZE];

    lz78_compress(input, strlen(input), codes, &code_len);
    printf("Compressed codes: ");
    for (int i = 0; i < code_len; i++) {
        printf("%d ", codes[i]);
    }
    printf("\n");

    lz78_decompress(codes, code_len, output, &output_len);
    printf("Decompressed output: %s\n", output);

    return 0;
}

在示例代码中，`create_dict()`、`add_to_dict()`、`find_in_dict()`和`free_dict()`函数用于创建、添加、查找和释放字典。`lz78_compress()`函数实现LZ78压缩算法，`lz78_decompress()`函数实现LZ78解压缩算法。在`main()`函数中，我们使用示例字符串`ababcabcd`进行压缩和解压缩，输出结果如下：

Compressed codes: 0 1 2 4 3
Decompressed output: ababcabcd

可以看到，压缩后的编码为`0 1 2 4 3`，解压缩后得到原始字符串`ababcabcd`。