iPhone开发笔记——iOS的Base64编码

+ (NSData*) base64Decode:(NSString *)string
{
    unsigned long ixtext, lentext;
    unsigned char ch, inbuf[4], outbuf[4];
    short i, ixinbuf;
    Boolean flignore, flendtext = false;
    const unsigned char *tempcstring;
    NSMutableData *theData;
 
    if (string == nil) {
        return [NSData data];
    }
 
    ixtext = 0;
 
    tempcstring = (const unsigned char *)[string UTF8String];
 
    lentext = [string length];
 
    theData = [NSMutableData dataWithCapacity: lentext];
 
    ixinbuf = 0;
 
    while (true) {
        if (ixtext >= lentext){
            break;
        }
 
        ch = tempcstring [ixtext++];
 
        flignore = false;
 
        if ((ch >= 'A') && (ch <= 'Z')) {
            ch = ch - 'A';
        } else if ((ch >= 'a') && (ch <= 'z')) {
            ch = ch - 'a' + 26;
        } else if ((ch >= '0') && (ch <= '9')) {
            ch = ch - '0' + 52;
        } else if (ch == '+') {
            ch = 62;
        } else if (ch == '=') {
            flendtext = true;
        } else if (ch == '/') {
            ch = 63;
        } else {
            flignore = true;
        }
 
        if (!flignore) {
            short ctcharsinbuf = 3;
            Boolean flbreak = false;
 
            if (flendtext) {
                if (ixinbuf == 0) {
                    break;
                }
 
                if ((ixinbuf == 1) || (ixinbuf == 2)) {
                    ctcharsinbuf = 1;
                } else {
                    ctcharsinbuf = 2;
                }
 
                ixinbuf = 3;
 
                flbreak = true;
            }
 
            inbuf [ixinbuf++] = ch;
 
            if (ixinbuf == 4) {
                ixinbuf = 0;
 
                outbuf[0] = (inbuf[0] << 2) | ((inbuf[1] & 0x30) >> 4);
                outbuf[1] = ((inbuf[1] & 0x0F) << 4) | ((inbuf[2] & 0x3C) >> 2);
                outbuf[2] = ((inbuf[2] & 0x03) << 6) | (inbuf[3] & 0x3F);
 
                for (i = 0; i < ctcharsinbuf; i++) {
                    [theData appendBytes: &outbuf[i] length: 1];
                }
            }
 
            if (flbreak) {
                break;
            }
        }
    }
 
    return theData;
}
 
+ (NSString*) base64Encode:(NSData *)data
{
    static char base64EncodingTable[64] = {
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
        'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
        'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
        'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/'
    };
    int length = [data length];
    unsigned long ixtext, lentext;
    long ctremaining;
    unsigned char input[3], output[4];
    short i, charsonline = 0, ctcopy;
    const unsigned char *raw;
    NSMutableString *result;
 
    lentext = [data length];
    if (lentext < 1)
        return @"";
    result = [NSMutableString stringWithCapacity: lentext];
    raw = [data bytes];
    ixtext = 0;
 
    while (true) {
        ctremaining = lentext - ixtext;
        if (ctremaining <= 0)
            break;       
        for (i = 0; i < 3; i++) {
            unsigned long ix = ixtext + i;
            if (ix < lentext)
                input[i] = raw[ix];
            else
                input[i] = 0;
        }
        output[0] = (input[0] & 0xFC) >> 2;
        output[1] = ((input[0] & 0x03) << 4) | ((input[1] & 0xF0) >> 4);
        output[2] = ((input[1] & 0x0F) << 2) | ((input[2] & 0xC0) >> 6);
        output[3] = input[2] & 0x3F;
        ctcopy = 4;
        switch (ctremaining) {
            case 1:
                ctcopy = 2;
                break;
            case 2:
                ctcopy = 3;
                break;
        }
 
        for (i = 0; i < ctcopy; i++)
            [result appendString: [NSString stringWithFormat: @"%c", base64EncodingTable[output[i]]]];
 
        for (i = ctcopy; i < 4; i++)
            [result appendString: @"="];
 
        ixtext += 3;
        charsonline += 4;
 
        if ((length > 0) && (charsonline >= length))
            charsonline = 0;
    }    
    return result;
}