C++写利用Huffman编码实现文件的压缩与解压缩，要求能够测试压缩率和解压缩速度

好的，我可以帮你回答这个问题。首先，Huffman编码是一种无损数据压缩算法，它可以将一些出现频率较高的字符用较短的编码表示，从而达到压缩数据的目的。下面是一些实现Huffman编码的基本步骤：

1. 统计字符出现频率，生成Huffman树。

2. 根据Huffman树生成编码表。

3. 将编码表写入压缩文件头部。

4. 读取原文件，将每个字符转换为对应的Huffman编码，并写入输出文件。

5. 解压缩时，读取压缩文件头部的编码表，根据编码表将Huffman编码转换为字符。

下面是一个用C++实现Huffman编码压缩和解压缩的示例代码：

#include <iostream>
#include <fstream>
#include <queue>
#include <unordered_map>
#include <vector>
#include <bitset>
#include <ctime>

using namespace std;

// Huffman树节点
struct HuffmanNode
{
    char ch;
    int freq;
    HuffmanNode *left, *right;
    HuffmanNode(char ch, int freq) : ch(ch), freq(freq), left(nullptr), right(nullptr) {}
};

// 比较函数，用于构建Huffman树
struct Compare
{
    bool operator()(const HuffmanNode* a, const HuffmanNode* b) const
    {
        return a->freq > b->freq;
    }
};

// 统计字符出现频率
unordered_map<char, int> getCharFreq(const string& input)
{
    unordered_map<char, int> freq;
    for (char ch : input)
    {
        ++freq[ch];
    }
    return freq;
}

// 构建Huffman树
HuffmanNode* buildHuffmanTree(const unordered_map<char, int>& freqMap)
{
    priority_queue<HuffmanNode*, vector<HuffmanNode*>, Compare> pq;
    for (auto& item : freqMap)
    {
        pq.push(new HuffmanNode(item.first, item.second));
    }
    while (pq.size() > 1)
    {
        HuffmanNode* left = pq.top();
        pq.pop();
        HuffmanNode* right = pq.top();
        pq.pop();
        HuffmanNode* parent = new HuffmanNode('\0', left->freq + right->freq);
        parent->left = left;
        parent->right = right;
        pq.push(parent);
    }
    return pq.top();
}

// 生成编码表
void generateEncodingTable(HuffmanNode* root, unordered_map<char, string>& encodingTable, string code)
{
    if (!root) return;
    if (root->ch != '\0')
    {
        encodingTable[root->ch] = code;
    }
    generateEncodingTable(root->left, encodingTable, code + "0");
    generateEncodingTable(root->right, encodingTable, code + "1");
}

// 编码
string encode(const string& input, const unordered_map<char, string>& encodingTable)
{
    string encoded;
    for (char ch : input)
    {
        encoded += encodingTable.at(ch);
    }
    return encoded;
}

// 解码
string decode(const string& encoded, HuffmanNode* root)
{
    string decoded;
    HuffmanNode* node = root;
    for (char bit : encoded)
    {
        if (bit == '0')
        {
            node = node->left;
        }
        else
        {
            node = node->right;
        }
        if (node->ch != '\0')
        {
            decoded += node->ch;
            node = root;
        }
    }
    return decoded;
}

// 将编码表写入文件头部
void writeEncodingTable(const unordered_map<char, string>& encodingTable, ofstream& outfile)
{
    for (auto& item : encodingTable)
    {
        outfile << item.first << item.second << endl;
    }
}

// 从文件头部读取编码表
unordered_map<string, char> readEncodingTable(ifstream& infile)
{
    unordered_map<string, char> encodingTable;
    string line;
    while (getline(infile, line))
    {
        char ch = line[0];
        string code = line.substr(1);
        encodingTable[code] = ch;
    }
    return encodingTable;
}

// 压缩文件
void compressFile(const string& inputFilename, const string& outputFilename)
{
    // 读取原文件
    ifstream infile(inputFilename, ios::in | ios::binary);
    if (!infile)
    {
        cerr << "Failed to open input file " << inputFilename << endl;
        return;
    }
    string input((istreambuf_iterator<char>(infile)), istreambuf_iterator<char>());
    infile.close();

    // 统计字符出现频率
    unordered_map<char, int> freqMap = getCharFreq(input);

    // 构建Huffman树
    HuffmanNode* root = buildHuffmanTree(freqMap);

    // 生成编码表
    unordered_map<char, string> encodingTable;
    generateEncodingTable(root, encodingTable, "");

    // 将编码表写入文件头部
    ofstream outfile(outputFilename, ios::out | ios::binary);
    if (!outfile)
    {
        cerr << "Failed to open output file " << outputFilename << endl;
        return;
    }
    writeEncodingTable(encodingTable, outfile);

    // 编码并写入输出文件
    string encoded = encode(input, encodingTable);
    int padding = 8 - (encoded.size() % 8);
    string paddingStr(padding, '0');
    outfile << paddingStr << encoded;
    outfile.close();
}

// 解压缩文件
void decompressFile(const string& inputFilename, const string& outputFilename)
{
    // 读取压缩文件头部的编码表
    ifstream infile(inputFilename, ios::in | ios::binary);
    if (!infile)
    {
        cerr << "Failed to open input file " << inputFilename << endl;
        return;
    }
    unordered_map<string, char> encodingTable = readEncodingTable(infile);

    // 读取压缩文件内容
    string encoded((istreambuf_iterator<char>(infile)), istreambuf_iterator<char>());
    infile.close();

    // 解码
    int padding = stoi(encoded.substr(0, 8), nullptr, 2);
    string decoded = decode(encoded.substr(8), buildHuffmanTree(encodingTable), padding);

    // 写入输出文件
    ofstream outfile(outputFilename, ios::out | ios::binary);
    if (!outfile)
    {
        cerr << "Failed to open output file " << outputFilename << endl;
        return;
    }
    outfile << decoded;
    outfile.close();
}

int main()
{
    string inputFilename = "input.txt";
    string compressedFilename = "compressed.bin";
    string decompressedFilename = "decompressed.txt";

    // 压缩文件
    clock_t start = clock();
    compressFile(inputFilename, compressedFilename);
    clock_t end = clock();
    cout << "Compressed file created." << endl;
    cout << "Compression time: " << (double)(end - start) / CLOCKS_PER_SEC << " seconds." << endl;

    // 解压缩文件
    start = clock();
    decompressFile(compressedFilename, decompressedFilename);
    end = clock();
    cout << "Decompressed file created." << endl;
    cout << "Decompression time: " << (double)(end - start) / CLOCKS_PER_SEC << " seconds." << endl;

    // 计算压缩率
    ifstream infile(inputFilename, ios::in | ios::binary);
    long long inputFileSize = infile.seekg(0, ios::end).tellg();
    infile.close();

    ifstream compressedFile(compressedFilename, ios::in | ios::binary);
    long long compressedFileSize = compressedFile.seekg(0, ios::end).tellg();
    compressedFile.close();

    double compressionRatio = (double)compressedFileSize / inputFileSize;
    cout << "Compression ratio: " << compressionRatio * 100 << "%" << endl;

    return 0;
}

在上面的示例代码中，我们用了一个简单的文本文件作为输入文件，使用了`ifstream`和`ofstream`来读写文件。我们还用了`clock`函数来计算压缩和解压缩文件所需的时间，以及计算压缩率。