iphone - iPhone 上的压缩 API

Question

是否有可在 iPhone 上使用的压缩 API？我们正在为我们的 iPhone 应用程序构建一些 RESTful Web 服务来进行对话，但我们希望至少压缩一些对话以提高效率。

我不在乎格式(ZIP、LHA，等等)是什么，也不需要是安全的。

一些受访者指出，服务器可以压缩其输出，iPhone 可以使用它。我们的情况恰恰相反。我们将把压缩内容发布到网络服务。我们不关心压缩的相反方向。

Answer 1

如果您将对话数据存储在 NSData 对象中，CocoaDev wiki 上的人员发布了一个 NSData category，它添加了 gzip 和 zlib 压缩/解压缩作为简单方法。这些在 my iPhone application 中对我来说效果很好。

由于在 CocoaDev wiki 迁移到新主机时上述链接已失效，因此我在下面完整地复制了此类别。

界面:

@interface NSData (NSDataExtension)

// Returns range [start, null byte), or (NSNotFound, 0).
- (NSRange) rangeOfNullTerminatedBytesFrom:(int)start;

// Canonical Base32 encoding/decoding.
+ (NSData *) dataWithBase32String:(NSString *)base32;
- (NSString *) base32String;

// COBS is an encoding that eliminates 0x00.
- (NSData *) encodeCOBS;
- (NSData *) decodeCOBS;

// ZLIB
- (NSData *) zlibInflate;
- (NSData *) zlibDeflate;

// GZIP
- (NSData *) gzipInflate;
- (NSData *) gzipDeflate;

//CRC32
- (unsigned int)crc32;

// Hash
- (NSData*) md5Digest;
- (NSString*) md5DigestString;
- (NSData*) sha1Digest;
- (NSString*) sha1DigestString;
- (NSData*) ripemd160Digest;
- (NSString*) ripemd160DigestString;

@end

实现:

#import "NSData+CocoaDevUsersAdditions.h"
#include <zlib.h>
#include <openssl/md5.h>
#include <openssl/sha.h>
#include <openssl/ripemd.h>


@implementation NSData (NSDataExtension)

// Returns range [start, null byte), or (NSNotFound, 0).
- (NSRange) rangeOfNullTerminatedBytesFrom:(int)start
{
    const Byte *pdata = [self bytes];
    int len = [self length];
    if (start < len)
    {
        const Byte *end = memchr (pdata + start, 0x00, len - start);
        if (end != NULL) return NSMakeRange (start, end - (pdata + start));
    }
    return NSMakeRange (NSNotFound, 0);
}

+ (NSData *) dataWithBase32String:(NSString *)encoded
{
    /* First valid character that can be indexed in decode lookup table */
    static int charDigitsBase = '2';

    /* Lookup table used to decode() characters in encoded strings */
    static int charDigits[] =
    {   26,27,28,29,30,31,-1,-1,-1,-1,-1,-1,-1,-1 //   23456789:;<=>?
        ,-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14 // @ABCDEFGHIJKLMNO
        ,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1 // PQRSTUVWXYZ[\]^_
        ,-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14 // `abcdefghijklmno
        ,15,16,17,18,19,20,21,22,23,24,25                // pqrstuvwxyz
    };

    if (! [encoded canBeConvertedToEncoding:NSASCIIStringEncoding]) return nil;
    const char *chars = [encoded cStringUsingEncoding:NSASCIIStringEncoding]; // avoids using characterAtIndex.
    int charsLen = [encoded lengthOfBytesUsingEncoding:NSASCIIStringEncoding];

    // Note that the code below could detect non canonical Base32 length within the loop. However canonical Base32 length can be tested before entering the loop.
    // A canonical Base32 length modulo 8 cannot be:
    // 1 (aborts discarding 5 bits at STEP n=0 which produces no byte),
    // 3 (aborts discarding 7 bits at STEP n=2 which produces no byte),
    // 6 (aborts discarding 6 bits at STEP n=1 which produces no byte).
    switch (charsLen & 7) { // test the length of last subblock
        case 1: //  5 bits in subblock:  0 useful bits but 5 discarded
        case 3: // 15 bits in subblock:  8 useful bits but 7 discarded
        case 6: // 30 bits in subblock: 24 useful bits but 6 discarded
            return nil; // non-canonical length
    }
    int charDigitsLen = sizeof(charDigits);
    int bytesLen = (charsLen * 5) >> 3;
    Byte bytes[bytesLen];
    int bytesOffset = 0, charsOffset = 0;
    // Also the code below does test that other discarded bits
    // (1 to 4 bits at end) are effectively 0.
    while (charsLen > 0)
    {
        int digit, lastDigit;
        // STEP n = 0: Read the 1st Char in a 8-Chars subblock
        // Leave 5 bits, asserting there's another encoding Char
        if ((digit = (int)chars[charsOffset] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        lastDigit = digit << 3;
        // STEP n = 5: Read the 2nd Char in a 8-Chars subblock
        // Insert 3 bits, leave 2 bits, possibly trailing if no more Char
        if ((digit = (int)chars[charsOffset + 1] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        bytes[bytesOffset] = (Byte)((digit >> 2) | lastDigit);
        lastDigit = (digit & 3) << 6;
        if (charsLen == 2) {
            if (lastDigit != 0) return nil; // non-canonical end
            break; // discard the 2 trailing null bits
        }
        // STEP n = 2: Read the 3rd Char in a 8-Chars subblock
        // Leave 7 bits, asserting there's another encoding Char
        if ((digit = (int)chars[charsOffset + 2] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        lastDigit |= (Byte)(digit << 1);
        // STEP n = 7: Read the 4th Char in a 8-chars Subblock
        // Insert 1 bit, leave 4 bits, possibly trailing if no more Char
        if ((digit = (int)chars[charsOffset + 3] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        bytes[bytesOffset + 1] = (Byte)((digit >> 4) | lastDigit);
        lastDigit = (Byte)((digit & 15) << 4);
        if (charsLen == 4) {
            if (lastDigit != 0) return nil; // non-canonical end
            break; // discard the 4 trailing null bits
        }
        // STEP n = 4: Read the 5th Char in a 8-Chars subblock
        // Insert 4 bits, leave 1 bit, possibly trailing if no more Char
        if ((digit = (int)chars[charsOffset + 4] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        bytes[bytesOffset + 2] = (Byte)((digit >> 1) | lastDigit);
        lastDigit = (Byte)((digit & 1) << 7);
        if (charsLen == 5) {
            if (lastDigit != 0) return nil; // non-canonical end
            break; // discard the 1 trailing null bit
        }
        // STEP n = 1: Read the 6th Char in a 8-Chars subblock
        // Leave 6 bits, asserting there's another encoding Char
        if ((digit = (int)chars[charsOffset + 5] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        lastDigit |= (Byte)(digit << 2);
        // STEP n = 6: Read the 7th Char in a 8-Chars subblock
        // Insert 2 bits, leave 3 bits, possibly trailing if no more Char
        if ((digit = (int)chars[charsOffset + 6] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        bytes[bytesOffset + 3] = (Byte)((digit >> 3) | lastDigit);
        lastDigit = (Byte)((digit & 7) << 5);
        if (charsLen == 7) {
            if (lastDigit != 0) return nil; // non-canonical end
            break; // discard the 3 trailing null bits
        }
        // STEP n = 3: Read the 8th Char in a 8-Chars subblock
        // Insert 5 bits, leave 0 bit, next encoding Char may not exist
        if ((digit = (int)chars[charsOffset + 7] - charDigitsBase) < 0 || digit >= charDigitsLen || (digit = charDigits[digit]) == -1)
            return nil; // invalid character
        bytes[bytesOffset + 4] = (Byte)(digit | lastDigit);
        //// This point is always reached for chars.length multiple of 8
        charsOffset += 8;
        bytesOffset += 5;
        charsLen -= 8;
    }
    // On loop exit, discard the n trailing null bits
    return [NSData dataWithBytes:bytes length:sizeof(bytes)];
}

- (NSString *) base32String
{
    /* Lookup table used to canonically encode() groups of data bits */
    static char canonicalChars[] =
    {   'A','B','C','D','E','F','G','H','I','J','K','L','M' // 00..12
        ,'N','O','P','Q','R','S','T','U','V','W','X','Y','Z' // 13..25
        ,'2','3','4','5','6','7'                             // 26..31
    };
    const Byte *bytes = [self bytes];
    int bytesOffset = 0, bytesLen = [self length];
    int charsOffset = 0, charsLen = ((bytesLen << 3) + 4) / 5;
    char chars[charsLen];
    while (bytesLen != 0) {
        int digit, lastDigit;
        // INVARIANTS FOR EACH STEP n in [0..5[; digit in [0..31[;
        // The remaining n bits are already aligned on top positions
        // of the 5 least bits of digit, the other bits are 0.
        ////// STEP n = 0: insert new 5 bits, leave 3 bits
        digit = bytes[bytesOffset] & 255;
        chars[charsOffset] = canonicalChars[digit >> 3];
        lastDigit = (digit & 7) << 2;
        if (bytesLen == 1) { // put the last 3 bits
            chars[charsOffset + 1] = canonicalChars[lastDigit];
            break;
        }
        ////// STEP n = 3: insert 2 new bits, then 5 bits, leave 1 bit
        digit = bytes[bytesOffset + 1] & 255;
        chars[charsOffset + 1] = canonicalChars[(digit >> 6) | lastDigit];
        chars[charsOffset + 2] = canonicalChars[(digit >> 1) & 31];
        lastDigit = (digit & 1) << 4;
        if (bytesLen == 2) { // put the last 1 bit
            chars[charsOffset + 3] = canonicalChars[lastDigit];
            break;
        }
        ////// STEP n = 1: insert 4 new bits, leave 4 bit
        digit = bytes[bytesOffset + 2] & 255;
        chars[charsOffset + 3] = canonicalChars[(digit >> 4) | lastDigit];
        lastDigit = (digit & 15) << 1;
        if (bytesLen == 3) { // put the last 1 bits
            chars[charsOffset + 4] = canonicalChars[lastDigit];
            break;
        }
        ////// STEP n = 4: insert 1 new bit, then 5 bits, leave 2 bits
        digit = bytes[bytesOffset + 3] & 255;
        chars[charsOffset + 4] = canonicalChars[(digit >> 7) | lastDigit];
        chars[charsOffset + 5] = canonicalChars[(digit >> 2) & 31];
        lastDigit = (digit & 3) << 3;
        if (bytesLen == 4) { // put the last 2 bits
            chars[charsOffset + 6] = canonicalChars[lastDigit];
            break;
        }
        ////// STEP n = 2: insert 3 new bits, then 5 bits, leave 0 bit
        digit = bytes[bytesOffset + 4] & 255;
        chars[charsOffset + 6] = canonicalChars[(digit >> 5) | lastDigit];
        chars[charsOffset + 7] = canonicalChars[digit & 31];
        //// This point is always reached for bytes.length multiple of 5
        bytesOffset += 5;
        charsOffset += 8;
        bytesLen -= 5;
    }
    return [NSString stringWithCString:chars length:sizeof(chars)];
}

#define FinishBlock(X) \
(*code_ptr = (X), \
code_ptr = dst++, \
code = 0x01)

- (NSData *) encodeCOBS
{
    if ([self length] == 0) return self;

    NSMutableData *encoded = [NSMutableData dataWithLength:([self length] + [self length] / 254 + 1)];
    unsigned char *dst = [encoded mutableBytes];
    const unsigned char *ptr = [self bytes];
    unsigned long length = [self length];
    const unsigned char *end = ptr + length;
    unsigned char *code_ptr = dst++;
    unsigned char code = 0x01;
    while (ptr < end)
    {
        if (*ptr == 0) FinishBlock(code);
        else
        {
            *dst++ = *ptr;
            code++;
            if (code == 0xFF) FinishBlock(code);
        }
        ptr++;
    }
    FinishBlock(code);

    [encoded setLength:((Byte *)dst - (Byte *)[encoded mutableBytes])];
    return [NSData dataWithData:encoded];
}

- (NSData *) decodeCOBS
{
    if ([self length] == 0) return self;

    const Byte *ptr = [self bytes];
    unsigned length = [self length];
    NSMutableData *decoded = [NSMutableData dataWithLength:length];
    Byte *dst = [decoded mutableBytes];
    Byte *basedst = dst;

    const unsigned char *end = ptr + length;
    while (ptr < end)
    {
        int i, code = *ptr++;
        for (i=1; i<code; i++) *dst++ = *ptr++;
        if (code < 0xFF) *dst++ = 0;
    }

    [decoded setLength:(dst - basedst)];
    return [NSData dataWithData:decoded];
}

- (NSData *)zlibInflate
{
    if ([self length] == 0) return self;

    unsigned full_length = [self length];
    unsigned half_length = [self length] / 2;

    NSMutableData *decompressed = [NSMutableData dataWithLength: full_length + half_length];
    BOOL done = NO;
    int status;

    z_stream strm;
    strm.next_in = (Bytef *)[self bytes];
    strm.avail_in = [self length];
    strm.total_out = 0;
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;

    if (inflateInit (&strm) != Z_OK) return nil;

    while (!done)
    {
        // Make sure we have enough room and reset the lengths.
        if (strm.total_out >= [decompressed length])
            [decompressed increaseLengthBy: half_length];
        strm.next_out = [decompressed mutableBytes] + strm.total_out;
        strm.avail_out = [decompressed length] - strm.total_out;

        // Inflate another chunk.
        status = inflate (&strm, Z_SYNC_FLUSH);
        if (status == Z_STREAM_END) done = YES;
        else if (status != Z_OK) break;
    }
    if (inflateEnd (&strm) != Z_OK) return nil;

    // Set real length.
    if (done)
    {
        [decompressed setLength: strm.total_out];
        return [NSData dataWithData: decompressed];
    }
    else return nil;
}

- (NSData *)zlibDeflate
{
    if ([self length] == 0) return self;

    z_stream strm;

    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    strm.opaque = Z_NULL;
    strm.total_out = 0;
    strm.next_in=(Bytef *)[self bytes];
    strm.avail_in = [self length];

    // Compresssion Levels:
    //   Z_NO_COMPRESSION
    //   Z_BEST_SPEED
    //   Z_BEST_COMPRESSION
    //   Z_DEFAULT_COMPRESSION

    if (deflateInit(&strm, Z_DEFAULT_COMPRESSION) != Z_OK) return nil;

    NSMutableData *compressed = [NSMutableData dataWithLength:16384];  // 16K chuncks for expansion

    do {

        if (strm.total_out >= [compressed length])
            [compressed increaseLengthBy: 16384];

        strm.next_out = [compressed mutableBytes] + strm.total_out;
        strm.avail_out = [compressed length] - strm.total_out;

        deflate(&strm, Z_FINISH);

    } while (strm.avail_out == 0);

    deflateEnd(&strm);

    [compressed setLength: strm.total_out];
    return [NSData dataWithData: compressed];
}

- (NSData *)gzipInflate
{
    if ([self length] == 0) return self;

    unsigned full_length = [self length];
    unsigned half_length = [self length] / 2;

    NSMutableData *decompressed = [NSMutableData dataWithLength: full_length + half_length];
    BOOL done = NO;
    int status;

    z_stream strm;
    strm.next_in = (Bytef *)[self bytes];
    strm.avail_in = [self length];
    strm.total_out = 0;
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;

    if (inflateInit2(&strm, (15+32)) != Z_OK) return nil;
    while (!done)
    {
        // Make sure we have enough room and reset the lengths.
        if (strm.total_out >= [decompressed length])
            [decompressed increaseLengthBy: half_length];
        strm.next_out = [decompressed mutableBytes] + strm.total_out;
        strm.avail_out = [decompressed length] - strm.total_out;

        // Inflate another chunk.
        status = inflate (&strm, Z_SYNC_FLUSH);
        if (status == Z_STREAM_END) done = YES;
        else if (status != Z_OK) break;
    }
    if (inflateEnd (&strm) != Z_OK) return nil;

    // Set real length.
    if (done)
    {
        [decompressed setLength: strm.total_out];
        return [NSData dataWithData: decompressed];
    }
    else return nil;
}

- (NSData *)gzipDeflate
{
    if ([self length] == 0) return self;

    z_stream strm;

    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    strm.opaque = Z_NULL;
    strm.total_out = 0;
    strm.next_in=(Bytef *)[self bytes];
    strm.avail_in = [self length];

    // Compresssion Levels:
    //   Z_NO_COMPRESSION
    //   Z_BEST_SPEED
    //   Z_BEST_COMPRESSION
    //   Z_DEFAULT_COMPRESSION

    if (deflateInit2(&strm, Z_DEFAULT_COMPRESSION, Z_DEFLATED, (15+16), 8, Z_DEFAULT_STRATEGY) != Z_OK) return nil;

    NSMutableData *compressed = [NSMutableData dataWithLength:16384];  // 16K chunks for expansion

    do {

        if (strm.total_out >= [compressed length])
            [compressed increaseLengthBy: 16384];

        strm.next_out = [compressed mutableBytes] + strm.total_out;
        strm.avail_out = [compressed length] - strm.total_out;

        deflate(&strm, Z_FINISH);

    } while (strm.avail_out == 0);

    deflateEnd(&strm);

    [compressed setLength: strm.total_out];
    return [NSData dataWithData:compressed];
}

// --------------------------------CRC32-------------------------------
static const unsigned long crc32table[] =
{
    0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
    0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988, 0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
    0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
    0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
    0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172, 0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
    0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
    0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
    0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924, 0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
    0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
    0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
    0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e, 0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
    0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
    0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
    0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0, 0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
    0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
    0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
    0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a, 0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
    0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
    0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
    0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc, 0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
    0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
    0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
    0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236, 0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
    0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
    0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
    0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38, 0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
    0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
    0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
    0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2, 0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
    0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
    0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
    0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94, 0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
};

- (unsigned int)crc32
{
    unsigned int    crcval;
    unsigned int    x, y;
    const void      *bytes;
    unsigned int    max;

    bytes = [self bytes];
    max = [self length];
    crcval = 0xffffffff;
    for (x = 0, y = max; x < y; x++) {
        crcval = ((crcval >> 8) & 0x00ffffff) ^ crc32table[(crcval ^ (*((unsigned char *)bytes + x))) & 0xff];
    }

    return crcval ^ 0xffffffff;
}

// Hash function, by [[DamienBob]]

#define HEComputeDigest(method)                     \
method##_CTX ctx;                               \
unsigned char digest[method##_DIGEST_LENGTH];       \
method##_Init(&ctx);                            \
method##_Update(&ctx, [self bytes], [self length]);     \
method##_Final(digest, &ctx);

#define HEComputeDigestNSData(method)               \
HEComputeDigest(method)                     \
return [NSData dataWithBytes:digest length:method##_DIGEST_LENGTH];

#define HEComputeDigestNSString(method)             \
static char __HEHexDigits[] = "0123456789abcdef";       \
unsigned char digestString[2*method##_DIGEST_LENGTH];\
unsigned int i;                                 \
HEComputeDigest(method)                     \
for(i=0; i<method##_DIGEST_LENGTH; i++) {               \
    digestString[2*i]   = __HEHexDigits[digest[i] >> 4];    \
    digestString[2*i+1] = __HEHexDigits[digest[i] & 0x0f];\
}                                           \
return [NSString stringWithCString:(char *)digestString length:2*method##_DIGEST_LENGTH];

#define SHA1_CTX                SHA_CTX
#define SHA1_DIGEST_LENGTH      SHA_DIGEST_LENGTH

- (NSData*) md5Digest
{
    HEComputeDigestNSData(MD5);
}

- (NSString*) md5DigestString
{
    HEComputeDigestNSString(MD5);
}

- (NSData*) sha1Digest
{
    HEComputeDigestNSData(SHA1);
}

- (NSString*) sha1DigestString
{
    HEComputeDigestNSString(SHA1);
}

- (NSData*) ripemd160Digest
{
    HEComputeDigestNSData(RIPEMD160);
}

- (NSString*) ripemd160DigestString
{
    HEComputeDigestNSString(RIPEMD160);
}

@end