C＃实现Base64编码与解码及规则

/// <summary>
    /// Base64编码类。
    /// 将byte[]类型转换成Base64编码的string类型。
    /// </summary>
    public class Base64Encoder
    {
        byte[] source;
        int length, length2;
        int blockCount;
        int paddinGCount;
        public static Base64Encoder Encoder = new Base64Encoder();

        public Base64Encoder()
        {
        }

        private void init(byte[] input)
        {
            source = input;
            length = input.Length;
            if ((length % 3) == 0)
            {
                paddingCount = 0;
                blockCount = length / 3;
            }
            else
            {
                paddingCount = 3 - (length % 3);
                blockCount = (length + paddingCount) / 3;
            }
            length2 = length + paddingCount;
        }

        public string GetEncoded(byte[] input)
        {
            //初始化
            init(input);

            byte[] source2;
            source2 = new byte[length2];

            for (int x = 0; x < length2; x++)
            {
                if (x < length)
                {
                    source2[x] = source[x];
                }
                else
                {
                    source2[x] = 0;
                }
            }

            byte b1, b2, b3;
            byte temp, temp1, temp2, temp3, temp4;
            byte[] buffer = new byte[blockCount * 4];
            char[] result = new char[blockCount * 4];
            for (int x = 0; x < blockCount; x++)
            {
                b1 = source2[x * 3];
                b2 = source2[x * 3 + 1];
                b3 = source2[x * 3 + 2];

                temp1 = (byte)((b1 & 252) >> 2);

                temp = (byte)((b1 & 3) << 4);
                temp2 = (byte)((b2 & 240) >> 4);
                temp2 += temp;

                temp = (byte)((b2 & 15) << 2);
                temp3 = (byte)((b3 & 192) >> 6);
                temp3 += temp;

                temp4 = (byte)(b3 & 63);

                buffer[x * 4] = temp1;
                buffer[x * 4 + 1] = temp2;
                buffer[x * 4 + 2] = temp3;
                buffer[x * 4 + 3] = temp4;

            }

            for (int x = 0; x < blockCount * 4; x++)
            {
                result[x] = sixbit2char(buffer[x]);
            }


            switch (paddingCount)
            {
                case 0: break;
                case 1: result[blockCount * 4 - 1] = '='; break;
                case 2: result[blockCount * 4 - 1] = '=';
                    result[blockCount * 4 - 2] = '=';
                    break;
                default: break;
            }
            return new string(result);
        }
        private char sixbit2char(byte b)
        {
            char[] lookupTable = new char[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','+','/'};

            if ((b >= 0) && (b <= 63))
            {
                return lookupTable[(int)b];
            }
            else
            {

                return ' ';
            }
        }

    }

///<summary>
///Base64解码类
///将Base64编码的string类型转换成byte[]类型
///</summary>
publicclassBase64Decoder
{
char[]source;
intlength,length2,length3;
intblockCount;
intpaddingCount;
publicstaticBase64DecoderDecoder=newBase64Decoder();

publicBase64Decoder()
{
}

privatevoidinit(char[]input)
{
inttemp=0;
source=input;
length=input.Length;

for(intx=0;x<2;x++)
{
if(input[length-x-1]=='=')
temp++;
}
paddingCount=temp;

blockCount=length/4;
length2=blockCount*3;
}

publicbyte[]GetDecoded(stringstrInput)
{
//初始化
init(strInput.ToCharArray());

byte[]buffer=newbyte[length];
byte[]buffer2=newbyte[length2];

for(intx=0;x<length;x++)
{
buffer[x]=char2sixbit(source[x]);
}

byteb,b1,b2,b3;
bytetemp1,temp2,temp3,temp4;

for(intx=0;x<blockCount;x++)
{
temp1=buffer[x*4];
temp2=buffer[x*4+1];
temp3=buffer[x*4+2];
temp4=buffer[x*4+3];

b=(byte)(temp1<<2);
b1=(byte)((temp2&48)>>4);
b1+=b;

b=(byte)((temp2&15)<<4);
b2=(byte)((temp3&60)>>2);
b2+=b;

b=(byte)((temp3&3)<<6);
b3=temp4;
b3+=b;

buffer2[x*3]=b1;
buffer2[x*3+1]=b2;
buffer2[x*3+2]=b3;
}

length3=length2-paddingCount;
byte[]result=newbyte[length3];

for(intx=0;x<length3;x++)
{
result[x]=buffer2[x];
}

returnresult;
}

privatebytechar2sixbit(charc)
{
char[]lookupTable=newchar[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','+','/'};
if(c=='=')
return0;
else
{
for(intx=0;x<64;x++)
{
if(lookupTable[x]==c)
return(byte)x;
}

return0;
}

}
}
//解码类结束