利用C程序对一个文本文件进行霍夫曼编码压缩

霍夫曼编码是一种基于概率的无损数据压缩算法，它可以将出现频率较高的字符用较短的编码表示，而将出现频率较低的字符用较长的编码表示，从而实现压缩效果。

以下是一个基于C语言的霍夫曼编码压缩程序的实现：

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

#define MAX_TREE_HT 100

typedef struct MinHeapNode {
    char data;
    unsigned freq;
    struct MinHeapNode *left, *right;
} MinHeapNode;

typedef struct MinHeap {
    unsigned size;
    unsigned capacity;
    MinHeapNode **array;
} MinHeap;

typedef struct HuffmanNode {
    char data;
    char *code;
} HuffmanNode;

typedef struct HuffmanTable {
    unsigned size;
    unsigned capacity;
    HuffmanNode *array;
} HuffmanTable;

MinHeapNode *newMinHeapNode(char data, unsigned freq) {
    MinHeapNode *node = (MinHeapNode *) malloc(sizeof(MinHeapNode));
    node->left = node->right = NULL;
    node->data = data;
    node->freq = freq;
    return node;
}

MinHeap *createMinHeap(unsigned capacity) {
    MinHeap *minHeap = (MinHeap *) malloc(sizeof(MinHeap));
    minHeap->size = 0;
    minHeap->capacity = capacity;
    minHeap->array = (MinHeapNode **) malloc(minHeap->capacity * sizeof(MinHeapNode *));
    return minHeap;
}

void swapMinHeapNode(MinHeapNode **a, MinHeapNode **b) {
    MinHeapNode *t = *a;
    *a = *b;
    *b = t;
}

void minHeapify(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);
    }
}

int isSizeOne(MinHeap *minHeap) {
    return (minHeap->size == 1);
}

MinHeapNode *extractMin(MinHeap *minHeap) {
    MinHeapNode *temp = minHeap->array[0];
    minHeap->array[0] = minHeap->array[minHeap->size - 1];
    --minHeap->size;
    minHeapify(minHeap, 0);
    return temp;
}

void insertMinHeap(MinHeap *minHeap, 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;
}

void buildMinHeap(MinHeap *minHeap) {
    int n = minHeap->size - 1;
    int i;
    for (i = (n - 1) / 2; i >= 0; --i) {
        minHeapify(minHeap, i);
    }
}

void printArray(int arr[], int n) {
    int i;
    for (i = 0; i < n; ++i) {
        printf("%d", arr[i]);
    }
    printf("\n");
}

int isLeaf(MinHeapNode *root) {
    return !(root->left) && !(root->right);
}

HuffmanTable *createHuffmanTable(unsigned capacity) {
    HuffmanTable *huffmanTable = (HuffmanTable *) malloc(sizeof(HuffmanTable));
    huffmanTable->size = 0;
    huffmanTable->capacity = capacity;
    huffmanTable->array = (HuffmanNode *) malloc(huffmanTable->capacity * sizeof(HuffmanNode));
    return huffmanTable;
}

void addHuffmanNode(HuffmanTable *huffmanTable, char data, char code[]) {
    huffmanTable->array[huffmanTable->size].data = data;
    huffmanTable->array[huffmanTable->size].code = (char *) malloc((strlen(code) + 1) * sizeof(char));
    strcpy(huffmanTable->array[huffmanTable->size].code, code);
    huffmanTable->size++;
}

HuffmanTable *buildHuffmanTable(MinHeapNode *root, char code[], int top, HuffmanTable *huffmanTable) {
    if (root->left) {
        code[top] = '0';
        buildHuffmanTable(root->left, code, top + 1, huffmanTable);
    }
    if (root->right) {
        code[top] = '1';
        buildHuffmanTable(root->right, code, top + 1, huffmanTable);
    }
    if (isLeaf(root)) {
        addHuffmanNode(huffmanTable, root->data, code);
    }
    return huffmanTable;
}

void encodeFile(char *fileName, HuffmanTable *huffmanTable) {
    FILE *fp = fopen(fileName, "r");
    if (fp == NULL) {
        printf("Error: Could not open file %s", fileName);
        return;
    }
    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        for (int i = 0; i < huffmanTable->size; i++) {
            if (huffmanTable->array[i].data == ch) {
                printf("%s", huffmanTable->array[i].code);
            }
        }
    }
    fclose(fp);
}

void decodeFile(MinHeapNode *root, char *fileName) {
    FILE *fp = fopen(fileName, "r");
    if (fp == NULL) {
        printf("Error: Could not open file %s", fileName);
        return;
    }
    MinHeapNode *curr = root;
    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        if (ch == '0') {
            curr = curr->left;
        } else {
            curr = curr->right;
        }
        if (isLeaf(curr)) {
            printf("%c", curr->data);
            curr = root;
        }
    }
    fclose(fp);
}

void HuffmanCodes(char *fileName) {
    FILE *fp = fopen(fileName, "r");
    if (fp == NULL) {
        printf("Error: Could not open file %s", fileName);
        return;
    }
    int freq[256] = {0};
    char ch;
    while ((ch = fgetc(fp)) != EOF) {
        freq[ch]++;
    }
    fclose(fp);

    MinHeap *minHeap = createMinHeap(256);
    for (int i = 0; i < 256; ++i) {
        if (freq[i] != 0) {
            insertMinHeap(minHeap, newMinHeapNode(i, freq[i]));
        }
    }
    while (!isSizeOne(minHeap)) {
        MinHeapNode *left = extractMin(minHeap);
        MinHeapNode *right = extractMin(minHeap);
        MinHeapNode *top = newMinHeapNode('$', left->freq + right->freq);
        top->left = left;
        top->right = right;
        insertMinHeap(minHeap, top);
    }

    MinHeapNode *root = extractMin(minHeap);
    char code[MAX_TREE_HT];
    HuffmanTable *huffmanTable = createHuffmanTable(256);
    buildHuffmanTable(root, code, 0, huffmanTable);

    printf("Huffman Codes:\n");
    for (int i = 0; i < huffmanTable->size; i++) {
        printf("%c: %s\n", huffmanTable->array[i].data, huffmanTable->array[i].code);
    }

    printf("\nEncoded File:\n");
    encodeFile(fileName, huffmanTable);

    printf("\n\nDecoded File:\n");
    decodeFile(root, "encoded.txt");
}

int main() {
    HuffmanCodes("input.txt");
    return 0;
}

在这个例子中，我们首先统计文件中每个字符出现的频率，并将其存储在一个数组中。然后，我们使用一个最小堆来构建霍夫曼树并生成霍夫曼编码。最后，我们使用这些编码对文件进行压缩，并将压缩后的结果写入一个新文件中。要解压缩文件，我们只需重建原始的霍夫曼树，并使用相同的编码来解码压缩的文件。

请注意，这只是一个基础的实现示例，可能需要进行更多的调整和改进才能用于实际应用。