WSCouldStorage/WSCloudStorage/CloudStorage/Global/GTMBase64.m

//
//  GTMBase64.m
//
//  Copyright 2006-2008 Google Inc.
//
//  Licensed under the Apache License, Version 2.0 (the "License"); you may not
//  use this file except in compliance with the License.  You may obtain a copy
//  of the License at
// 
//  http://www.apache.org/licenses/LICENSE-2.0
// 
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
//  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
//  License for the specific language governing permissions and limitations under
//  the License.
//

#import "GTMBase64.h"
#import "GTMDefines.h"

static const char *kBase64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
static const char *kWebSafeBase64EncodeChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
static const char kBase64PaddingChar = '=';
static const char kBase64InvalidChar = 99;

static const char kBase64DecodeChars[] = {
    // This array was generated by the following code:
    // #include <sys/time.h>
    // #include <stdlib.h>
    // #include <string.h>
    // main()
    // {
    //   static const char Base64[] =
    //     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    //   char *pos;
    //   int idx, i, j;
    //   printf("    ");
    //   for (i = 0; i < 255; i += 8) {
    //     for (j = i; j < i + 8; j++) {
    //       pos = strchr(Base64, j);
    //       if ((pos == NULL) || (j == 0))
    //         idx = 99;
    //       else
    //         idx = pos - Base64;
    //       if (idx == 99)
    //         printf(" %2d,     ", idx);
    //       else
    //         printf(" %2d/*%c*/,", idx, j);
    //     }
    //     printf("\n    ");
    //   }
    // }
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      62/*+*/, 99,      99,      99,      63/*/ */,
    52/*0*/, 53/*1*/, 54/*2*/, 55/*3*/, 56/*4*/, 57/*5*/, 58/*6*/, 59/*7*/,
    60/*8*/, 61/*9*/, 99,      99,      99,      99,      99,      99,
    99,       0/*A*/,  1/*B*/,  2/*C*/,  3/*D*/,  4/*E*/,  5/*F*/,  6/*G*/,
    7/*H*/,  8/*I*/,  9/*J*/, 10/*K*/, 11/*L*/, 12/*M*/, 13/*N*/, 14/*O*/,
    15/*P*/, 16/*Q*/, 17/*R*/, 18/*S*/, 19/*T*/, 20/*U*/, 21/*V*/, 22/*W*/,
    23/*X*/, 24/*Y*/, 25/*Z*/, 99,      99,      99,      99,      99,
    99,      26/*a*/, 27/*b*/, 28/*c*/, 29/*d*/, 30/*e*/, 31/*f*/, 32/*g*/,
    33/*h*/, 34/*i*/, 35/*j*/, 36/*k*/, 37/*l*/, 38/*m*/, 39/*n*/, 40/*o*/,
    41/*p*/, 42/*q*/, 43/*r*/, 44/*s*/, 45/*t*/, 46/*u*/, 47/*v*/, 48/*w*/,
    49/*x*/, 50/*y*/, 51/*z*/, 99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99
};

static const char kWebSafeBase64DecodeChars[] = {
    // This array was generated by the following code:
    // #include <sys/time.h>
    // #include <stdlib.h>
    // #include <string.h>
    // main()
    // {
    //   static const char Base64[] =
    //     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
    //   char *pos;
    //   int idx, i, j;
    //   printf("    ");
    //   for (i = 0; i < 255; i += 8) {
    //     for (j = i; j < i + 8; j++) {
    //       pos = strchr(Base64, j);
    //       if ((pos == NULL) || (j == 0))
    //         idx = 99;
    //       else
    //         idx = pos - Base64;
    //       if (idx == 99)
    //         printf(" %2d,     ", idx);
    //       else
    //         printf(" %2d/*%c*/,", idx, j);
    //     }
    //     printf("\n    ");
    //   }
    // }
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      62/*-*/, 99,      99,
    52/*0*/, 53/*1*/, 54/*2*/, 55/*3*/, 56/*4*/, 57/*5*/, 58/*6*/, 59/*7*/,
    60/*8*/, 61/*9*/, 99,      99,      99,      99,      99,      99,
    99,       0/*A*/,  1/*B*/,  2/*C*/,  3/*D*/,  4/*E*/,  5/*F*/,  6/*G*/,
    7/*H*/,  8/*I*/,  9/*J*/, 10/*K*/, 11/*L*/, 12/*M*/, 13/*N*/, 14/*O*/,
    15/*P*/, 16/*Q*/, 17/*R*/, 18/*S*/, 19/*T*/, 20/*U*/, 21/*V*/, 22/*W*/,
    23/*X*/, 24/*Y*/, 25/*Z*/, 99,      99,      99,      99,      63/*_*/,
    99,      26/*a*/, 27/*b*/, 28/*c*/, 29/*d*/, 30/*e*/, 31/*f*/, 32/*g*/,
    33/*h*/, 34/*i*/, 35/*j*/, 36/*k*/, 37/*l*/, 38/*m*/, 39/*n*/, 40/*o*/,
    41/*p*/, 42/*q*/, 43/*r*/, 44/*s*/, 45/*t*/, 46/*u*/, 47/*v*/, 48/*w*/,
    49/*x*/, 50/*y*/, 51/*z*/, 99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99,
    99,      99,      99,      99,      99,      99,      99,      99
};


// Tests a character to see if it's a whitespace character.
//
// Returns:
//   YES if the character is a whitespace character.
//   NO if the character is not a whitespace character.
//
GTM_INLINE BOOL IsSpace(unsigned char c) {
    // we use our own mapping here because we don't want anything w/ locale
    // support.
    static BOOL kSpaces[256] = {
        0, 0, 0, 0, 0, 0, 0, 0, 0, 1,  // 0-9
        1, 1, 1, 1, 0, 0, 0, 0, 0, 0,  // 10-19
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 20-29
        0, 0, 1, 0, 0, 0, 0, 0, 0, 0,  // 30-39
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 40-49
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 50-59
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 60-69
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 70-79
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 80-89
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 90-99
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 100-109
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 110-119
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 120-129
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 130-139
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 140-149
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 150-159
        1, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 160-169
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 170-179
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 180-189
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 190-199
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 200-209
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 210-219
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 220-229
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 230-239
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  // 240-249
        0, 0, 0, 0, 0, 1,              // 250-255
    };
    return kSpaces[c];
}

// Calculate how long the data will be once it's base64 encoded.
//
// Returns:
//   The guessed encoded length for a source length
//
GTM_INLINE NSUInteger CalcEncodedLength(NSUInteger srcLen, BOOL padded) {
    NSUInteger intermediate_result = 8 * srcLen + 5;
    NSUInteger len = intermediate_result / 6;
    if (padded) {
        len = ((len + 3) / 4) * 4;
    }
    return len;
}

// Tries to calculate how long the data will be once it's base64 decoded.
// Unlike the above, this is always an upperbound, since the source data
// could have spaces and might end with the padding characters on them.
//
// Returns:
//   The guessed decoded length for a source length
//
GTM_INLINE NSUInteger GuessDecodedLength(NSUInteger srcLen) {
    return (srcLen + 3) / 4 * 3;
}


@interface GTMBase64 (PrivateMethods)

+(NSData *)baseEncode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char *)charset
               padded:(BOOL)padded;

+(NSData *)baseDecode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char*)charset
       requirePadding:(BOOL)requirePadding;

+(NSUInteger)baseEncode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
                 padded:(BOOL)padded;

+(NSUInteger)baseDecode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
         requirePadding:(BOOL)requirePadding;

@end


@implementation GTMBase64

//
// Standard Base64 (RFC) handling
//

+(NSData *)encodeData:(NSData *)data {
    return [self baseEncode:[data bytes]
                     length:[data length]
                    charset:kBase64EncodeChars
                     padded:YES];
}

+(NSData *)decodeData:(NSData *)data {
    return [self baseDecode:[data bytes]
                     length:[data length]
                    charset:kBase64DecodeChars
             requirePadding:YES];
}

+(NSData *)encodeBytes:(const void *)bytes length:(NSUInteger)length {
    return [self baseEncode:bytes
                     length:length
                    charset:kBase64EncodeChars
                     padded:YES];
}

+(NSData *)decodeBytes:(const void *)bytes length:(NSUInteger)length {
    return [self baseDecode:bytes
                     length:length
                    charset:kBase64DecodeChars
             requirePadding:YES];
}

+(NSString *)stringByEncodingData:(NSData *)data {
    NSString *result = nil;
    NSData *converted = [self baseEncode:[data bytes]
                                  length:[data length]
                                 charset:kBase64EncodeChars
                                  padded:YES];
    if (converted) {
        result = [[NSString alloc] initWithData:converted
                                        encoding:NSASCIIStringEncoding];
    }
    return result;
}

+(NSString *)stringByEncodingBytes:(const void *)bytes length:(NSUInteger)length {
    NSString *result = nil;
    NSData *converted = [self baseEncode:bytes
                                  length:length
                                 charset:kBase64EncodeChars
                                  padded:YES];
    if (converted) {
        result = [[NSString alloc] initWithData:converted
                                        encoding:NSASCIIStringEncoding];
    }
    return result;
}

+(NSData *)decodeString:(NSString *)string {
    NSData *result = nil;
    NSData *data = [string dataUsingEncoding:NSASCIIStringEncoding];
    if (data) {
        result = [self baseDecode:[data bytes]
                           length:[data length]
                          charset:kBase64DecodeChars
                   requirePadding:YES];
    }
    return result;
}

//
// Modified Base64 encoding so the results can go onto urls.
//
// The changes are in the characters generated and also the result isn't
// padded to a multiple of 4.
// Must use the matching call to encode/decode, won't interop with the
// RFC versions.
//

+(NSData *)webSafeEncodeData:(NSData *)data
                      padded:(BOOL)padded {
    return [self baseEncode:[data bytes]
                     length:[data length]
                    charset:kWebSafeBase64EncodeChars
                     padded:padded];
}

+(NSData *)webSafeDecodeData:(NSData *)data {
    return [self baseDecode:[data bytes]
                     length:[data length]
                    charset:kWebSafeBase64DecodeChars
             requirePadding:NO];
}

+(NSData *)webSafeEncodeBytes:(const void *)bytes
                       length:(NSUInteger)length
                       padded:(BOOL)padded {
    return [self baseEncode:bytes
                     length:length
                    charset:kWebSafeBase64EncodeChars
                     padded:padded];
}

+(NSData *)webSafeDecodeBytes:(const void *)bytes length:(NSUInteger)length {
    return [self baseDecode:bytes
                     length:length
                    charset:kWebSafeBase64DecodeChars
             requirePadding:NO];
}

+(NSString *)stringByWebSafeEncodingData:(NSData *)data
                                  padded:(BOOL)padded {
    NSString *result = nil;
    NSData *converted = [self baseEncode:[data bytes]
                                  length:[data length]
                                 charset:kWebSafeBase64EncodeChars
                                  padded:padded];
    if (converted) {
        result = [[NSString alloc] initWithData:converted
                                        encoding:NSASCIIStringEncoding];
    }
    return result;
}

+(NSString *)stringByWebSafeEncodingBytes:(const void *)bytes
                                   length:(NSUInteger)length
                                   padded:(BOOL)padded {
    NSString *result = nil;
    NSData *converted = [self baseEncode:bytes
                                  length:length
                                 charset:kWebSafeBase64EncodeChars
                                  padded:padded];
    if (converted) {
        result = [[NSString alloc] initWithData:converted
                                        encoding:NSASCIIStringEncoding];
    }
    return result;
}

+(NSData *)webSafeDecodeString:(NSString *)string {
    NSData *result = nil;
    NSData *data = [string dataUsingEncoding:NSASCIIStringEncoding];
    if (data) {
        result = [self baseDecode:[data bytes]
                           length:[data length]
                          charset:kWebSafeBase64DecodeChars
                   requirePadding:NO];
    }
    return result;
}

@end

@implementation GTMBase64 (PrivateMethods)

//
// baseEncode:length:charset:padded:
//
// Does the common lifting of creating the dest NSData.  it creates & sizes the
// data for the results.  |charset| is the characters to use for the encoding
// of the data.  |padding| controls if the encoded data should be padded to a
// multiple of 4.
//
// Returns:
//   an autorelease NSData with the encoded data, nil if any error.
//
+(NSData *)baseEncode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char *)charset
               padded:(BOOL)padded {
    // how big could it be?
    NSUInteger maxLength = CalcEncodedLength(length, padded);
    // make space
    NSMutableData *result = [NSMutableData data];
    [result setLength:maxLength];
    // do it
    NSUInteger finalLength = [self baseEncode:bytes
                                       srcLen:length
                                    destBytes:[result mutableBytes]
                                      destLen:[result length]
                                      charset:charset
                                       padded:padded];
    if (finalLength) {
        _GTMDevAssert(finalLength == maxLength, @"how did we calc the length wrong?");
    } else {
        // shouldn't happen, this means we ran out of space
        result = nil;
    }
    return result;
}

//
// baseDecode:length:charset:requirePadding:
//
// Does the common lifting of creating the dest NSData.  it creates & sizes the
// data for the results.  |charset| is the characters to use for the decoding
// of the data.
//
// Returns:
//   an autorelease NSData with the decoded data, nil if any error.
//
//
+(NSData *)baseDecode:(const void *)bytes
               length:(NSUInteger)length
              charset:(const char *)charset
       requirePadding:(BOOL)requirePadding {
    // could try to calculate what it will end up as
    NSUInteger maxLength = GuessDecodedLength(length);
    // make space
    NSMutableData *result = [NSMutableData data];
    [result setLength:maxLength];
    // do it
    NSUInteger finalLength = [self baseDecode:bytes
                                       srcLen:length
                                    destBytes:[result mutableBytes]
                                      destLen:[result length]
                                      charset:charset
                               requirePadding:requirePadding];
    if (finalLength) {
        if (finalLength != maxLength) {
            // resize down to how big it was
            [result setLength:finalLength];
        }
    } else {
        // either an error in the args, or we ran out of space
        result = nil;
    }
    return result;
}

//
// baseEncode:srcLen:destBytes:destLen:charset:padded:
//
// Encodes the buffer into the larger.  returns the length of the encoded
// data, or zero for an error.
// |charset| is the characters to use for the encoding
// |padded| tells if the result should be padded to a multiple of 4.
//
// Returns:
//   the length of the encoded data.  zero if any error.
//
+(NSUInteger)baseEncode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
                 padded:(BOOL)padded {
    if (!srcLen || !destLen || !srcBytes || !destBytes) {
        return 0;
    }
    
    char *curDest = destBytes;
    const unsigned char *curSrc = (const unsigned char *)(srcBytes);
    
    // Three bytes of data encodes to four characters of cyphertext.
    // So we can pump through three-byte chunks atomically.
    while (srcLen > 2) {
        // space?
        _GTMDevAssert(destLen >= 4, @"our calc for encoded length was wrong");
        curDest[0] = charset[curSrc[0] >> 2];
        curDest[1] = charset[((curSrc[0] & 0x03) << 4) + (curSrc[1] >> 4)];
        curDest[2] = charset[((curSrc[1] & 0x0f) << 2) + (curSrc[2] >> 6)];
        curDest[3] = charset[curSrc[2] & 0x3f];
        
        curDest += 4;
        curSrc += 3;
        srcLen -= 3;
        destLen -= 4;
    }
    
    // now deal with the tail (<=2 bytes)
    switch (srcLen) {
        case 0:
            // Nothing left; nothing more to do.
            break;
        case 1:
            // One byte left: this encodes to two characters, and (optionally)
            // two pad characters to round out the four-character cypherblock.
            _GTMDevAssert(destLen >= 2, @"our calc for encoded length was wrong");
            curDest[0] = charset[curSrc[0] >> 2];
            curDest[1] = charset[(curSrc[0] & 0x03) << 4];
            curDest += 2;
            if (padded) {
                _GTMDevAssert(destLen >= 4, @"our calc for encoded length was wrong");
                curDest[0] = kBase64PaddingChar;
                curDest[1] = kBase64PaddingChar;
                curDest += 2;
            }
            break;
        case 2:
            // Two bytes left: this encodes to three characters, and (optionally)
            // one pad character to round out the four-character cypherblock.
            _GTMDevAssert(destLen >= 3, @"our calc for encoded length was wrong");
            curDest[0] = charset[curSrc[0] >> 2];
            curDest[1] = charset[((curSrc[0] & 0x03) << 4) + (curSrc[1] >> 4)];
            curDest[2] = charset[(curSrc[1] & 0x0f) << 2];
            curDest += 3;
            if (padded) {
                _GTMDevAssert(destLen >= 4, @"our calc for encoded length was wrong");
                curDest[0] = kBase64PaddingChar;
                curDest += 1;
            }
            break;
    }
    // return the length
    return (curDest - destBytes);
}

//
// baseDecode:srcLen:destBytes:destLen:charset:requirePadding:
//
// Decodes the buffer into the larger.  returns the length of the decoded
// data, or zero for an error.
// |charset| is the character decoding buffer to use
//
// Returns:
//   the length of the encoded data.  zero if any error.
//
+(NSUInteger)baseDecode:(const char *)srcBytes
                 srcLen:(NSUInteger)srcLen
              destBytes:(char *)destBytes
                destLen:(NSUInteger)destLen
                charset:(const char *)charset
         requirePadding:(BOOL)requirePadding {
    if (!srcLen || !destLen || !srcBytes || !destBytes) {
        return 0;
    }
    
    int decode;
    NSUInteger destIndex = 0;
    int state = 0;
    char ch = 0;
    while (srcLen-- && (ch = *srcBytes++) != 0)  {
        if (IsSpace(ch))  // Skip whitespace
            continue;
        
        if (ch == kBase64PaddingChar)
            break;
        
        decode = charset[(unsigned int)ch];
        if (decode == kBase64InvalidChar)
            return 0;
        
        // Four cyphertext characters decode to three bytes.
        // Therefore we can be in one of four states.
        switch (state) {
            case 0:
                // We're at the beginning of a four-character cyphertext block.
                // This sets the high six bits of the first byte of the
                // plaintext block.
                _GTMDevAssert(destIndex < destLen, @"our calc for decoded length was wrong");
                destBytes[destIndex] = decode << 2;
                state = 1;
                break;
            case 1:
                // We're one character into a four-character cyphertext block.
                // This sets the low two bits of the first plaintext byte,
                // and the high four bits of the second plaintext byte.
                _GTMDevAssert((destIndex+1) < destLen, @"our calc for decoded length was wrong");
                destBytes[destIndex] |= decode >> 4;
                destBytes[destIndex+1] = (decode & 0x0f) << 4;
                destIndex++;
                state = 2;
                break;
            case 2:
                // We're two characters into a four-character cyphertext block.
                // This sets the low four bits of the second plaintext
                // byte, and the high two bits of the third plaintext byte.
                // However, if this is the end of data, and those two
                // bits are zero, it could be that those two bits are
                // leftovers from the encoding of data that had a length
                // of two mod three.
                _GTMDevAssert((destIndex+1) < destLen, @"our calc for decoded length was wrong");
                destBytes[destIndex] |= decode >> 2;
                destBytes[destIndex+1] = (decode & 0x03) << 6;
                destIndex++;
                state = 3;
                break;
            case 3:
                // We're at the last character of a four-character cyphertext block.
                // This sets the low six bits of the third plaintext byte.
                _GTMDevAssert(destIndex < destLen, @"our calc for decoded length was wrong");
                destBytes[destIndex] |= decode;
                destIndex++;
                state = 0;
                break;
        }
    }
    
    // We are done decoding Base-64 chars.  Let's see if we ended
    //      on a byte boundary, and/or with erroneous trailing characters.
    if (ch == kBase64PaddingChar) {               // We got a pad char
        if ((state == 0) || (state == 1)) {
            return 0;  // Invalid '=' in first or second position
        }
        if (srcLen == 0) {
            if (state == 2) { // We run out of input but we still need another '='
                return 0;
            }
            // Otherwise, we are in state 3 and only need this '='
        } else {
            if (state == 2) {  // need another '='
                while ((ch = *srcBytes++) && (srcLen-- > 0)) {
                    if (!IsSpace(ch))
                        break;
                }
                if (ch != kBase64PaddingChar) {
                    return 0;
                }
            }
            // state = 1 or 2, check if all remain padding is space
            while ((ch = *srcBytes++) && (srcLen-- > 0)) {
                if (!IsSpace(ch)) {
                    return 0;
                }
            }
        }
    } else {
        // We ended by seeing the end of the string.
        
        if (requirePadding) {
            // If we require padding, then anything but state 0 is an error.
            if (state != 0) {
                return 0;
            }
        } else {
            // Make sure we have no partial bytes lying around.  Note that we do not
            // require trailing '=', so states 2 and 3 are okay too.
            if (state == 1) {
                return 0;
            }
        }
    }
    
    // If then next piece of output was valid and got written to it means we got a
    // very carefully crafted input that appeared valid but contains some trailing
    // bits past the real length, so just toss the thing.
    if ((destIndex < destLen) &&
        (destBytes[destIndex] != 0)) {
        return 0;
    }
    
    return destIndex;
}

@end