对20byte数据进行霍夫曼无损编码的c实现

下面是一个简单的C语言实现，包括霍夫曼编码树的构建和编码过程：

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

#define MAX_TREE_HT 100

// 霍夫曼编码树节点结构体
struct MinHeapNode {
    char data;
    unsigned freq;
    struct MinHeapNode *left, *right;
};

// 霍夫曼编码树结构体
struct MinHeap {
    unsigned size;
    unsigned capacity;
    struct MinHeapNode **array;
};

// 创建一个新的霍夫曼编码树节点
struct MinHeapNode* newNode(char data, unsigned freq) {
    struct MinHeapNode* node = (struct MinHeapNode*) malloc(sizeof(struct MinHeapNode));
    node->left = node->right = NULL;
    node->data = data;
    node->freq = freq;
    return node;
}

// 创建一个新的霍夫曼编码树
struct MinHeap* createMinHeap(unsigned capacity) {
    struct MinHeap* minHeap = (struct MinHeap*) malloc(sizeof(struct MinHeap));
    minHeap->size = 0;
    minHeap->capacity = capacity;
    minHeap->array = (struct MinHeapNode**) malloc(minHeap->capacity * sizeof(struct MinHeapNode*));
    return minHeap;
}

// 交换两个霍夫曼编码树节点
void swapMinHeapNode(struct MinHeapNode** a, struct MinHeapNode** b) {
    struct MinHeapNode* t = *a;
    *a = *b;
    *b = t;
}

// 维护霍夫曼编码树的最小堆
void minHeapify(struct MinHeap* minHeap, int idx) {
    int smallest = idx;
    int left = 2 * idx + 1;
    int right = 2 * idx + 2;
    if (left < minHeap->size && minHeap->array[left]->freq < minHeap->array[smallest]->freq)
        smallest = left;
    if (right < minHeap->size && minHeap->array[right]->freq < minHeap->array[smallest]->freq)
        smallest = right;
    if (smallest != idx) {
        swapMinHeapNode(&minHeap->array[smallest], &minHeap->array[idx]);
        minHeapify(minHeap, smallest);
    }
}

// 判断最小堆是否只有一个节点
int isSizeOne(struct MinHeap* minHeap) {
    return minHeap->size == 1;
}

// 从最小堆中提取最小值节点
struct MinHeapNode* extractMin(struct MinHeap* minHeap) {
    struct MinHeapNode* temp = minHeap->array[0];
    minHeap->array[0] = minHeap->array[minHeap->size - 1];
    --minHeap->size;
    minHeapify(minHeap, 0);
    return temp;
}

// 将新的节点插入到最小堆中
void insertMinHeap(struct MinHeap* minHeap, struct MinHeapNode* minHeapNode) {
    ++minHeap->size;
    int i = minHeap->size - 1;
    while (i && minHeapNode->freq < minHeap->array[(i - 1) / 2]->freq) {
        minHeap->array[i] = minHeap->array[(i - 1) / 2];
        i = (i - 1) / 2;
    }
    minHeap->array[i] = minHeapNode;
}

// 判断一个节点是否为叶子节点
int isLeaf(struct MinHeapNode* root) {
    return !(root->left) && !(root->right);
}

// 创建一个新的霍夫曼编码树，并构建编码表
struct MinHeapNode* buildHuffmanTree(char* data, int* freq, int size) {
    struct MinHeapNode *left, *right, *top;
    struct MinHeap* minHeap = createMinHeap(size);
    for (int i = 0; i < size; ++i)
        insertMinHeap(minHeap, newNode(data[i], freq[i]));
    while (!isSizeOne(minHeap)) {
        left = extractMin(minHeap);
        right = extractMin(minHeap);
        top = newNode('$', left->freq + right->freq);
        top->left = left;
        top->right = right;
        insertMinHeap(minHeap, top);
    }
    return extractMin(minHeap);
}

// 将每个字符的编码存储到哈希表中
void storeCodes(struct MinHeapNode* root, char* codes[], int top) {
    if (root->left) {
        codes[top] = (char*) malloc(sizeof(char) * 2);
        codes[top][0] = '0';
        storeCodes(root->left, codes, top + 1);
    }
    if (root->right) {
        codes[top] = (char*) malloc(sizeof(char) * 2);
        codes[top][0] = '1';
        storeCodes(root->right, codes, top + 1);
    }
    if (isLeaf(root)) {
        codes[top] = (char*) malloc(sizeof(char) * 2);
        codes[top][0] = '\0';
        codes[top][1] = '\0';
    }
}

// 压缩数据
void compressData(char* input, char* output) {
    int freq[256] = { 0 };
    int size = strlen(input);
    for (int i = 0; i < size; ++i)
        ++freq[input[i]];
    char data[size];
    int freq2[size];
    int index = 0;
    for (int i = 0; i < 256; ++i)
        if (freq[i] != 0) {
            data[index] = (char) i;
            freq2[index] = freq[i];
            ++index;
        }
    struct MinHeapNode* root = buildHuffmanTree(data, freq2, index);
    char* codes[MAX_TREE_HT];
    storeCodes(root, codes, 0);
    int len = 0;
    for (int i = 0; i < size; ++i) {
        for (int j = 0; j < index; ++j) {
            if (input[i] == data[j]) {
                strcat(output, codes[j]);
                len += strlen(codes[j]);
            }
        }
    }
}

int main() {
    char input[] = "Hello, world!";
    char output[100] = "";
    compressData(input, output);
    printf("Original data: %s\n", input);
    printf("Compressed data: %s\n", output);
    return 0;
}

需要注意的是，上述代码只是一个简单的实现，不考虑各种边界情况和错误处理。在实际使用中，需要根据具体需求进行改进和完善。