laneser

先看看隨便寫的 code:

        public static class WebSafeBase64
        {
            public static string ToWebSafeBase64String(byte[] input)
            {
                var result = new StringBuilder(Convert.ToBase64String(input).TrimEnd('='));
                result.Replace('+', '-');
                result.Replace('/', '_');
                return result.ToString();
            }

            public static byte[] FromWebSafeBase64String(string base64ForUrlInput)
            {
                int padChars = (base64ForUrlInput.Length % 4) == 0 ? 0 : (4 - (base64ForUrlInput.Length % 4));
                var result = new StringBuilder(base64ForUrlInput, base64ForUrlInput.Length + padChars);
                result.Append(String.Empty.PadRight(padChars, '='));
                result.Replace('-', '+');
                result.Replace('_', '/');
                return Convert.FromBase64String(result.ToString());
            }
        }

然後看看正經八百寫的 code...

    public static class Base64
    {
        public static char[] websavebase64Chars = new char[]
{ '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','-','_' };

        public static byte[] CharToByteMapping = GenCharToByteMapp();

        public static byte[] GenCharToByteMapp()
        {
            var r = new byte[256];
            for (int i = 0; i < websavebase64Chars.Length; i++)
            {
                r[(byte)websavebase64Chars[i]] = (byte)i;
            }
            return r;
        }

        public static char[] websavebase64Charsx4 = Genwebsavebase64Charsx4();

        public static char[] Genwebsavebase64Charsx4()
        {
            var r = new char[websavebase64Chars.Length*4];
            Array.Copy(websavebase64Chars, 0,  r, 0, websavebase64Chars.Length);
            Array.Copy(websavebase64Chars, 0,  r, websavebase64Chars.Length, websavebase64Chars.Length);
            Array.Copy(websavebase64Chars, 0,  r, websavebase64Chars.Length*2, websavebase64Chars.Length);
            Array.Copy(websavebase64Chars, 0,  r, websavebase64Chars.Length*3, websavebase64Chars.Length);
            return r;
        }

        public static IEnumerable ToSixBitsChar(IEnumerable bytes)
        {
            int r = 0;
            int len = 0;
            foreach (var b in bytes)
            {
                r = r << 8 | b;
                len++;
                if (len == 3)
                {
                    var m4 = websavebase64Charsx4[(byte)r];
                    r >>= 6;
                    var m3 = websavebase64Charsx4[(byte)r];
                    r >>= 6;
                    var m2 = websavebase64Charsx4[(byte)r];
                    r >>= 6;
                    var m1 = websavebase64Charsx4[(byte)r];
                    yield return m1;
                    yield return m2;
                    yield return m3;
                    yield return m4;
                    r = 0;
                    len = 0;
                }
            }
            switch (len)
            {
                case 1:
                    r <<= 4;
                    var m2 = websavebase64Charsx4[(byte)r];
                    r >>= 6;
                    var m1 = websavebase64Charsx4[(byte)r];
                    yield return m1;
                    yield return m2;
                    break;
                case 2:
                    r <<= 2;
                    var mm3 = websavebase64Charsx4[(byte)r];
                    r >>= 6;
                    var mm2 = websavebase64Charsx4[(byte)r];
                    r >>= 6;
                    var mm1 = websavebase64Charsx4[(byte)r];
                    yield return mm1;
                    yield return mm2;
                    yield return mm3;
                    break;
            }
        }

        private static IEnumerable FromSixBitsChar(IEnumerable chars)
        {
            var CharInput = chars.GetEnumerator();
            while (CharInput.MoveNext())
            {
                var b1 = CharToByteMapping[CharInput.Current] << 2;
                if (CharInput.MoveNext() == false)
                {
                    // this should not happen, but just give up if this happend.
                    yield return (byte)b1;
                    yield break;
                }

                var b2 = CharToByteMapping[CharInput.Current];
                var b2part1 = (b2 & 48) >> 4;
                var b2part2 = (b2 & 15) << 4;
                var r1 = b1 | b2part1;
                yield return (byte)r1;

                if (CharInput.MoveNext() == false)
                    yield break;

                var b3 = CharToByteMapping[CharInput.Current];
                var b3part1 = (b3 & 60) >> 2;
                var b3part2 = (b3 & 3) << 6;

                var r2 = b2part2 | b3part1;
                yield return (byte)r2;

                if (CharInput.MoveNext() == false)
                    yield break;

                var b4 = CharToByteMapping[CharInput.Current];
                var r3 = b3part2 | b4;
                yield return (byte)r3;
            }
        }

        public static string Encode(byte[] data)
        {
            return new String(ToSixBitsChar(data).ToArray());
        }

        public static byte[] Decode(string data)
        {
            return FromSixBitsChar(data).ToArray();
        }
    }

最後...

比賽結果: