bsdiff 算法具体实现讲解

bsdiff 算法是一种用于生成两个二进制文件差异的算法，常用于软件更新中。它的实现基于三个步骤：生成差异文件、合并差异文件和生成新文件。

1. 生成差异文件

生成差异文件的过程是将旧版本文件和新版本文件进行对比，找出两个文件之间的差异，并将差异写入到一个新的文件中，这个新文件就是差异文件。bsdiff 算法生成差异文件的方法是将旧版本文件分成若干个块，然后对每个块进行编码，得到一个编码表。然后将新版本文件分成同样大小的块，对每个块进行哈希匹配，找到与旧版本文件中相同内容的块，并在编码表中查找相应的编码。如果找到了，则将编码写入到差异文件中；如果没有找到，则将原始块写入到差异文件中。

2. 合并差异文件

合并差异文件的过程是将旧版本文件和差异文件进行合并，得到新版本文件。bsdiff 算法合并差异文件的方法是将差异文件分成若干个块，然后对每个块进行解码，得到一个解码表。然后将旧版本文件分成同样大小的块，对每个块进行哈希匹配，找到与差异文件中相同内容的块，并在解码表中查找相应的解码。如果找到了，则将解码得到的新块写入到新版本文件中；如果没有找到，则将原始块写入到新版本文件中。

3. 生成新文件

生成新文件的过程是将合并后的文件与旧版本文件进行比较，确保新文件与旧文件一致。bsdiff 算法生成新文件的方法是对新版本文件和旧版本文件进行哈希匹配，确保两个文件的哈希值相同。如果哈希值相同，则说明新文件与旧文件一致；如果哈希值不同，则说明生成新文件出现了错误。

相关问题

C语言实现bsdiff算法

bsdiff算法是一种用于生成增量补丁的算法，它可以快速地生成一个旧版本和新版本之间的差异文件。下面是一个简单的C语言实现示例：

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

#define MIN(x, y) ((x) < (y) ? (x) : (y))

// 定义一个结构体，用于保存文件的内容和长度
typedef struct {
    char *data;
    off_t length;
} file_data;

// 读取文件内容到file_data结构体中
file_data read_file(const char *filename) {
    file_data fd = {NULL, 0};
    FILE *fp = fopen(filename, "rb");
    if (fp == NULL) {
        fprintf(stderr, "Unable to open file: %s\n", filename);
        return fd;
    }
    fseek(fp, 0, SEEK_END);
    fd.length = ftell(fp);
    rewind(fp);
    fd.data = (char *)malloc(fd.length);
    if (fd.data == NULL) {
        fclose(fp);
        return fd;
    }
    fread(fd.data, fd.length, 1, fp);
    fclose(fp);
    return fd;
}

// 计算文件的差异
int bsdiff(const char *old_filename, const char *new_filename, const char *patch_filename) {
    file_data old_file = read_file(old_filename);
    if (old_file.data == NULL || old_file.length == 0) {
        fprintf(stderr, "Unable to read old file\n");
        return -1;
    }
    file_data new_file = read_file(new_filename);
    if (new_file.data == NULL || new_file.length == 0) {
        free(old_file.data);
        fprintf(stderr, "Unable to read new file\n");
        return -1;
    }

    FILE *fp = fopen(patch_filename, "wb");
    if (fp == NULL) {
        free(old_file.data);
        free(new_file.data);
        fprintf(stderr, "Unable to create patch file\n");
        return -1;
    }

    // 写入文件头
    fprintf(fp, "BSDIFF40");
    off_t newsize = new_file.length;
    fwrite(&newsize, sizeof(off_t), 1, fp);

    // 分配内存
    char *I = (char *)malloc((old_file.length + 1) * sizeof(char));
    if (I == NULL) {
        fclose(fp);
        free(old_file.data);
        free(new_file.data);
        fprintf(stderr, "Memory allocation error\n");
        return -1;
    }
    char *V = (char *)malloc((old_file.length + 1) * sizeof(char));
    if (V == NULL) {
        fclose(fp);
        free(old_file.data);
        free(new_file.data);
        free(I);
        fprintf(stderr, "Memory allocation error\n");
        return -1;
    }

    // 生成差异
    off_t scan = 0;
    off_t len = 0;
    off_t lastscan = 0;
    off_t lastpos = 0;
    off_t oldsize = old_file.length;
    off_t scsc = 0;
    off_t overlap = 0;
    off_t Sf, lenf, Sb, lenb;
    off_t *pos = (off_t *)malloc((newsize + 1) * sizeof(off_t));
    if (pos == NULL) {
        fclose(fp);
        free(old_file.data);
        free(new_file.data);
        free(I);
        free(V);
        fprintf(stderr, "Memory allocation error\n");
        return -1;
    }
    off_t i;
    for (i = 0; i < newsize; i++) {
        pos[i] = -1;
    }

    // 计算V和I数组
    for (scan = 0; scan < newsize; scan++) {
        char c = new_file.data[scan];
        len = 0;
        for (i = 0; scan + i < newsize; i++) {
            if (new_file.data[scan + i] == c) {
                len++;
            } else {
                break;
            }
        }
        if (len >= 8 && scan + len < newsize) {
            // 计算hash值
            unsigned int h = 0;
            for (i = 0; i < len; i++) {
                h = h * 31 + new_file.data[scan + i];
            }
            // 将hash值添加到pos数组中
            for (i = MIN(oldsize - 1, h % (oldsize - 1));; i--) {
                if (pos[i] == -1) {
                    pos[i] = h % (oldsize - 1);
                    break;
                }
                if (i == 0) {
                    i = oldsize;
                }
            }
        }
    }
    // 计算V和I数组
    i = 0;
    // V[0] = 0;
    for (i = 0; i < oldsize; i++) {
        V[i] = 0;
    }
    for (i = 0; i < newsize; i++) {
        char c = new_file.data[i];
        len = 0;
        for (off_t j = i; j < newsize; j++) {
            if (new_file.data[j] == c) {
                len++;
            } else {
                break;
            }
        }
        if (len >= 8 && i + len < newsize) {
            unsigned int h = 0;
            for (off_t j = 0; j < len; j++) {
                h = h * 31 + new_file.data[i + j];
            }
            off_t posn = pos[h % (oldsize - 1)];
            if (posn != -1) {
                off_t delta = i - posn;
                off_t j = 0;
                while (i + j < newsize && posn + j < oldsize && new_file.data[i + j] == old_file.data[posn + j]) {
                    j++;
                }
                if (j > overlap) {
                    Sf = i;
                    lenf = j - overlap;
                    Sb = posn + j;
                    lenb = j - overlap;
                    overlap = j;
                }
                if (j == overlap && i - posn < delta) {
                    Sf = i;
                    lenf = j - overlap;
                    Sb = posn + j;
                    lenb = delta - overlap;
                    overlap = j;
                }
            }
        }
        V[i] = overlap;
        I[i] = Sf;
    }
    free(pos);

    // 写入差异
    off_t scanpos = 0;
    off_t lenpos = 0;
    fwrite(&lenf, sizeof(off_t), 1, fp);
    fwrite(&lenb, sizeof(off_t), 1, fp);
    fwrite(&Sf, sizeof(off_t), 1, fp);
    fwrite(&Sb, sizeof(off_t), 1, fp);
    fwrite(&new_file.data[0], lenf, 1, fp);
    for (scanpos = lenf, lenpos = lenf; scanpos < newsize;) {
        off_t i = 0, j = 0;
        for (i = 0; scanpos + i < newsize; i++) {
            j = I[scanpos + i] + lenf;
            if (scanpos + i + lenpos < newsize && j + lenpos < oldsize && new_file.data[scanpos + i + lenpos] == old_file.data[j + lenpos]) {
                continue;
            }
            break;
        }
        if (i != 0) {
            fwrite(&i, sizeof(off_t), 1, fp);
        } else {
            j = scanpos + lenpos;
            for (i = lenpos; i < newsize - scanpos; i++) {
                if (V[scanpos + i] >= j - scanpos) {
                    j = i + scanpos;
                }
            }
            i = j - scanpos;
            fwrite(&i, sizeof(off_t), 1, fp);
        }
        fwrite(&new_file.data[scanpos], i, 1, fp);
        for (off_t k = 0; k < i; k++) {
            if (scanpos + k < newsize) {
                V[scanpos + k] = V[scanpos + k] - i;
            }
        }
        scanpos += i;
        lenpos = i;
    }

    // 释放内存
    free(I);
    free(V);
    free(old_file.data);
    free(new_file.data);
    fclose(fp);

    return 0;
}

int main(int argc, char *argv[]) {
    if (argc != 4) {
        fprintf(stderr, "Usage: %s oldfile newfile patchfile\n", argv[0]);
        return -1;
    }
    if (bsdiff(argv[1], argv[2], argv[3]) != 0) {
        return -1;
    }
    return 0;
}

需要注意的是，这个实现只是一个简单的示例，可能不适用于所有情况。在实际应用中，还需要进行更多的测试和优化。

用C++实现bsdiff算法。

bsdiff算法是一种用于生成两个二进文件之间差异的算法，常用于软件升级和增量更新。下面是一个C++实现的示例代码：

#include <fstream>
#include <vector>

typedef unsigned char uint8_t;
typedef unsigned int uint32_t;
typedef unsigned long long uint64_t;

const uint64_t MAX_LEN = 0xFFFFFFFFFFFFFFFF;

class bsdiff {
public:
    static bool diff(const char* old_file, const char* new_file, const char* patch_file);
    static bool patch(const char* old_file, const char* new_file, const char* patch_file);
private:
    static void split(uint64_t* sa, uint8_t* old_data, uint64_t old_size);
    static uint64_t search(uint8_t* new_data, uint64_t new_size, uint8_t* old_data, uint64_t old_size, uint64_t pos, uint64_t len, uint64_t* offset);
    static void encode(std::ofstream& fout, uint64_t x);
    static void copy(std::ofstream& fout, uint8_t* data, uint64_t size);
};

bool bsdiff::diff(const char* old_file, const char* new_file, const char* patch_file) {
    std::ifstream old_f(old_file, std::ios::binary);
    if (!old_f) return false;
    old_f.seekg(0, std::ios::end);
    uint64_t old_size = old_f.tellg();
    old_f.seekg(0, std::ios::beg);
    std::vector<uint8_t> old_data(old_size);
    old_f.read(reinterpret_cast<char*>(&old_data[0]), old_size);
    old_f.close();

    std::ifstream new_f(new_file, std::ios::binary);
    if (!new_f) return false;
    new_f.seekg(0, std::ios::end);
    uint64_t new_size = new_f.tellg();
    new_f.seekg(0, std::ios::beg);
    std::vector<uint8_t> new_data(new_size);
    new_f.read(reinterpret_cast<char*>(&new_data[0]), new_size);
    new_f.close();

    std::ofstream patch_f(patch_file, std::ios::binary);
    if (!patch_f) return false;

    uint64_t* sa = new uint64_t[(old_size + 1) / 2];
    split(sa, &old_data[0], old_size);

    uint64_t i = 0;
    uint64_t len = 0;
    uint64_t pos = 0;
    uint64_t last_offset = 0;
    while (i < new_size) {
        uint64_t offset = 0;
        pos = search(&new_data[0], new_size, &old_data[0], old_size, sa[i], old_size - sa[i], &offset);

        if (i + pos - last_offset >= MAX_LEN || pos == old_size) {
            encode(patch_f, i - last_offset);
            encode(patch_f, pos - last_offset);
            copy(patch_f, &new_data[i], pos - last_offset);
            last_offset = pos;
        }

        i += pos - sa[i];
    }

    encode(patch_f, i - last_offset);
    encode(patch_f, new_size - last_offset);
    copy(patch_f, &new_data[i], new_size - last_offset);

    delete[] sa;
    patch_f.close();
    return true;
}

bool bsdiff::patch(const char* old_file, const char* new_file, const char* patch_file) {
    std::ifstream old_f(old_file, std::ios::binary);
    if (!old_f) return false;
    old_f.seekg(0, std::ios::end);
    uint64_t old_size = old_f.tellg();
    old_f.seekg(0, std::ios::beg);
    std::vector<uint8_t> old_data(old_size);
    old_f.read(reinterpret_cast<char*>(&old_data[0]), old_size);
    old_f.close();

    std::ifstream patch_f(patch_file, std::ios::binary);
    if (!patch_f) return false;

    std::ofstream new_f(new_file, std::ios::binary);
    if (!new_f) return false;

    uint64_t old_pos = 0;
    uint64_t new_pos = 0;
    uint64_t cmd = 0;
    uint64_t len = 0;

    while (patch_f) {
        cmd = 0;
        len = 0;
        patch_f.read(reinterpret_cast<char*>(&cmd), sizeof(uint64_t));
        patch_f.read(reinterpret_cast<char*>(&len), sizeof(uint64_t));

        if (patch_f.eof()) break;

        if (cmd > 0) {
            std::vector<uint8_t> diff_data(len);
            patch_f.read(reinterpret_cast<char*>(&diff_data[0]), len);

            for (uint64_t i = 0; i < len; i++) {
                new_f.put(old_data[old_pos + i] + diff_data[i]);
            }
            old_pos += len;
            new_pos += len;
        } else {
            new_pos += len;
        }
    }

    patch_f.close();
    new_f.close();
    return true;
}

void bsdiff::split(uint64_t* sa, uint8_t* old_data, uint64_t old_size) {
    uint64_t i = 0;
    uint64_t j = 0;
    uint64_t k = 0;
    uint64_t x = 0;
    uint64_t y = 0;
    uint64_t tmp = 0;
    uint64_t* v = new uint64_t[old_size];

    for (i = 0; i < old_size; i++) {
        v[i] = i;
    }

    for (i = 0; i < old_size; i++) {
        sa[i] = v[0];
        v[0] = v[1];
        x = v[1];

        for (j = 1; j < old_size - i - 1; j++) {
            y = v[j + 1];
            if (old_data[sa[i] + j] > old_data[x + j]) {
                v[j] = x;
                x = y;
            } else if (old_data[sa[i] + j] > old_data[y + j]) {
                v[j] = sa[i] + j;
                x = y;
            } else {
                v[j] = y;
            }
        }

        v[old_size - i - 2] = x;
    }

    for (i = 0; i < old_size; i++) {
        sa[v[i]] = i;
    }

    delete[] v;
}

uint64_t bsdiff::search(uint8_t* new_data, uint64_t new_size, uint8_t* old_data, uint64_t old_size, uint64_t pos, uint64_t len, uint64_t* offset) {
    uint64_t start = 0;
    uint64_t end = old_size;
    uint64_t x = 0;
    uint64_t y = 0;
    uint64_t last = 0;

    while (start < end) {
        x = start + (end - start) / 2;
        last = (x > *offset) ? x - *offset : *offset - x;
        y = 0;
        while (y < len && x + y < old_size && new_data[pos + y] == old_data[x + y]) {
            y++;
        }

        if (y == len) {
            *offset = x;
            return x - *offset + len;
        }

        if (y > last) {
            end = x;
        } else {
            start = x + 1;
        }
    }

    return 0;
}

void bsdiff::encode(std::ofstream& fout, uint64_t x) {
    uint8_t buf[10];
    uint32_t i = 0;
    while (x >= 0x80) {
        buf[i++] = (x & 0x7F) | 0x80;
        x >>= 7;
    }
    buf[i++] = x & 0x7F;
    for (uint32_t j = 0; j < i; j++) {
        fout.put(buf[j]);
    }
}

void bsdiff::copy(std::ofstream& fout, uint8_t* data, uint64_t size) {
    for (uint64_t i = 0; i < size; i++) {
        fout.put(data[i]);
    }
}

该实现使用了STL中的vector和fstream库，代码较为简洁易读。其中，split函数用于计算old_data数组的后缀数组，search函数用于在old_data数组中查找与new_data数组匹配的最长子串。encode和copy函数用于将数据写入输出流中。diff函数用于生成差异文件，patch函数用于根据差异文件生成新文件。