用C编写一个简单的压缩壳

以下是一个简单的用 C 语言编写的压缩壳示例，使用了LZ77压缩算法：

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

#define MAX_WINDOW_SIZE 4096
#define MAX_LOOKAHEAD_SIZE 18
#define MIN_MATCH_LENGTH 3

typedef struct {
    int position;
    int length;
    char next_char;
} Match;

void compress(FILE *input_file, FILE *output_file) {
    char input_buffer[MAX_WINDOW_SIZE + MAX_LOOKAHEAD_SIZE];
    char *lookahead_buffer = input_buffer + MAX_WINDOW_SIZE;
    int window_size = 0;
    int lookahead_size = 0;
    int current_position = 0;
    int i, j;
    Match longest_match;
    int bytes_written = 0;

    while (1) {
        // 读取输入文件到输入缓冲区
        int bytes_read = fread(lookahead_buffer, 1, MAX_LOOKAHEAD_SIZE, input_file);
        if (bytes_read == 0) {
            break;
        }

        // 更新窗口和前瞻缓冲区的大小
        window_size = (current_position > MAX_WINDOW_SIZE) ? MAX_WINDOW_SIZE : current_position;
        lookahead_size = bytes_read;

        // 查找最长匹配
        longest_match.length = 0;
        for (i = 0; i < lookahead_size - MIN_MATCH_LENGTH + 1; i++) {
            for (j = 0; j < window_size - MIN_MATCH_LENGTH + 1; j++) {
                if (lookahead_buffer[i + j] != input_buffer[j]) {
                    break;
                }
            }
            if (j >= longest_match.length) {
                longest_match.position = current_position - window_size + j;
                longest_match.length = j;
                longest_match.next_char = lookahead_buffer[i + j];
            }
        }

        // 写入压缩数据
        if (longest_match.length >= MIN_MATCH_LENGTH) {
            fwrite(&longest_match, sizeof(Match), 1, output_file);
            current_position += longest_match.length + 1;
        } else {
            longest_match.length = 0;
            longest_match.next_char = lookahead_buffer[0];
            fwrite(&longest_match, sizeof(Match), 1, output_file);
            current_position++;
        }

        // 将输入缓冲区往前移动
        for (i = 0; i < window_size - longest_match.length; i++) {
            input_buffer[i] = input_buffer[i + longest_match.length];
        }
        for (j = 0; j < longest_match.length; j++) {
            input_buffer[i + j] = lookahead_buffer[j];
        }
    }
}

void decompress(FILE *input_file, FILE *output_file) {
    char output_buffer[MAX_WINDOW_SIZE + MAX_LOOKAHEAD_SIZE];
    char *lookahead_buffer = output_buffer + MAX_WINDOW_SIZE;
    int window_size = 0;
    int lookahead_size = 0;
    int current_position = 0;
    int i;
    Match match;

    while (1) {
        // 读取压缩数据
        int bytes_read = fread(&match, sizeof(Match), 1, input_file);
        if (bytes_read == 0) {
            break;
        }

        // 更新窗口和前瞻缓冲区的大小
        window_size = (current_position > MAX_WINDOW_SIZE) ? MAX_WINDOW_SIZE : current_position;
        lookahead_size = match.length + 1;

        // 处理匹配和非匹配情况
        if (match.length >= MIN_MATCH_LENGTH) {
            for (i = 0; i < match.length; i++) {
                output_buffer[i] = output_buffer[current_position - window_size + i];
            }
            output_buffer[i] = match.next_char;
            fwrite(output_buffer, 1, lookahead_size, output_file);
            current_position += lookahead_size;
        } else {
            output_buffer[0] = match.next_char;
            fwrite(output_buffer, 1, 1, output_file);
            current_position++;
        }

        // 将输出缓冲区往前移动
        for (i = 0; i < window_size - lookahead_size; i++) {
            output_buffer[i] = output_buffer[i + lookahead_size];
        }
        for (i = 0; i < lookahead_size; i++) {
            lookahead_buffer[i] = output_buffer[i + window_size - lookahead_size];
        }
    }
}

int main(int argc, char *argv[]) {
    FILE *input_file;
    FILE *output_file;
    char *input_filename;
    char *output_filename;
    char *option;

    if (argc != 4) {
        printf("Usage: compress [-c|-d] input_file output_file\n");
        return 1;
    }

    option = argv[1];
    input_filename = argv[2];
    output_filename = argv[3];

    input_file = fopen(input_filename, "rb");
    if (input_file == NULL) {
        printf("Error: Unable to open input file %s\n", input_filename);
        return 1;
    }

    output_file = fopen(output_filename, "wb");
    if (output_file == NULL) {
        printf("Error: Unable to open output file %s\n", output_filename);
        return 1;
    }

    if (strcmp(option, "-c") == 0) {
        compress(input_file, output_file);
    } else if (strcmp(option, "-d") == 0) {
        decompress(input_file, output_file);
    } else {
        printf("Error: Invalid option %s\n", option);
        return 1;
    }

    fclose(input_file);
    fclose(output_file);

    return 0;
}

这个压缩壳包含了压缩和解压缩两个函数，根据命令行参数指定压缩模式或解压缩模式。压缩函数使用LZ77算法对输入文件进行压缩，解压缩函数则将压缩后的数据解压缩成原始文件。压缩数据存储在输出文件中。