//Copyright (c) Microsoft Corporation.  All rights reserved.
using System;
using System.Text;

// **************************************************************
// * Raw implementation of the MD5 hash algorithm
// * from RFC 1321.
// *
// * Written By: Reid Borsuk and Jenny Zheng
// * Copyright (c) Microsoft Corporation.  All rights reserved.
// **************************************************************

// Simple struct for the (a,b,c,d) which is used to compute the mesage digest.    
struct ABCDStruct 
{
    public uint A;
    public uint B;
    public uint C;
    public uint D;
}

public sealed class MD5Core
{    
    //Prevent CSC from adding a default public constructor
    private MD5Core() {}

    public static byte[] GetHash(string input, Encoding encoding)
    {
        if (null == input)
            throw new System.ArgumentNullException("input", "Unable to calculate hash over null input data");
        if (null == encoding)
            throw new System.ArgumentNullException("encoding", "Unable to calculate hash over a string without a default encoding. Consider using the GetHash(string) overload to use UTF8 Encoding");

        byte[] target = encoding.GetBytes(input);

        return GetHash(target);
    }

    public static byte[] GetHash(string input)
    {
        return GetHash(input, new UTF8Encoding());
    }

    public static string GetHashString(byte[] input)
    {
        if (null == input)
            throw new System.ArgumentNullException("input", "Unable to calculate hash over null input data");

        string retval = BitConverter.ToString(GetHash(input));
        retval = retval.Replace("-", "");

        return retval;
    }

    public static string GetHashString(string input, Encoding encoding)
    {
        if (null == input)
            throw new System.ArgumentNullException("input", "Unable to calculate hash over null input data");
        if (null == encoding)
            throw new System.ArgumentNullException("encoding", "Unable to calculate hash over a string without a default encoding. Consider using the GetHashString(string) overload to use UTF8 Encoding");
        
        byte[] target = encoding.GetBytes(input);

        return GetHashString(target);
    }

    public static string GetHashString(string input)
    {
        return GetHashString(input, new UTF8Encoding());
    }

    public static byte[] GetHash(byte[] input)
    {
        if (null == input)
            throw new System.ArgumentNullException("input", "Unable to calculate hash over null input data");

        //Intitial values defined in RFC 1321
        ABCDStruct abcd = new ABCDStruct();
        abcd.A = 0x67452301;
        abcd.B = 0xefcdab89;
        abcd.C = 0x98badcfe;
        abcd.D = 0x10325476;

        //We pass in the input array by block, the final block of data must be handled specialy for padding & length embeding
        int startIndex = 0;
        while (startIndex <= input.Length - 64)
        {
            MD5Core.GetHashBlock(input, ref abcd, startIndex);
            startIndex += 64;
        }
        // The final data block. 
        return MD5Core.GetHashFinalBlock(input, startIndex, input.Length - startIndex, abcd, (Int64)input.Length * 8);
    }

    internal static byte[] GetHashFinalBlock(byte[] input, int ibStart, int cbSize, ABCDStruct ABCD, Int64 len)
    {
        byte[] working = new byte[64];  
        byte[] length = BitConverter.GetBytes(len);

        //Padding is a single bit 1, followed by the number of 0s required to make size congruent to 448 modulo 512. Step 1 of RFC 1321  
        //The CLR ensures that our buffer is 0-assigned, we don't need to explicitly set it. This is why it ends up being quicker to just
        //use a temporary array rather then doing in-place assignment (5% for small inputs)
        Array.Copy(input, ibStart, working, 0, cbSize);
        working[cbSize] = 0x80;

        //We have enough room to store the length in this chunk
        if (cbSize < 56) 
        {
            Array.Copy(length, 0, working, 56, 8);
            GetHashBlock(working, ref ABCD, 0);
        }
        else  //We need an aditional chunk to store the length
        {
            GetHashBlock(working, ref ABCD, 0);
            //Create an entirely new chunk due to the 0-assigned trick mentioned above, to avoid an extra function call clearing the array
            working = new byte[64];
            Array.Copy(length, 0, working, 56, 8);
            GetHashBlock(working, ref ABCD, 0);
        }
        byte[] output = new byte[16];
        Array.Copy(BitConverter.GetBytes(ABCD.A), 0, output, 0, 4);
        Array.Copy(BitConverter.GetBytes(ABCD.B), 0, output, 4, 4);
        Array.Copy(BitConverter.GetBytes(ABCD.C), 0, output, 8, 4);
        Array.Copy(BitConverter.GetBytes(ABCD.D), 0, output, 12, 4);
        return output;
    }

    // Performs a single block transform of MD5 for a given set of ABCD inputs
    /* If implementing your own hashing framework, be sure to set the initial ABCD correctly according to RFC 1321:
    //    A = 0x67452301;
    //    B = 0xefcdab89;
    //    C = 0x98badcfe;
    //    D = 0x10325476;
    */
    internal static void GetHashBlock(byte[] input, ref ABCDStruct ABCDValue, int ibStart)
    {
        uint[] temp = Converter(input, ibStart);
        uint a = ABCDValue.A; 
        uint b = ABCDValue.B;
        uint c = ABCDValue.C;
        uint d = ABCDValue.D;

        a = r1(a, b, c, d, temp[0 ], 7,  0xd76aa478);
        d = r1(d, a, b, c, temp[1 ], 12, 0xe8c7b756);
        c = r1(c, d, a, b, temp[2 ], 17, 0x242070db);
        b = r1(b, c, d, a, temp[3 ], 22, 0xc1bdceee);
        a = r1(a, b, c, d, temp[4 ], 7,  0xf57c0faf);
        d = r1(d, a, b, c, temp[5 ], 12, 0x4787c62a);
        c = r1(c, d, a, b, temp[6 ], 17, 0xa8304613);
        b = r1(b, c, d, a, temp[7 ], 22, 0xfd469501);
        a = r1(a, b, c, d, temp[8 ], 7,  0x698098d8);
        d = r1(d, a, b, c, temp[9 ], 12, 0x8b44f7af);
        c = r1(c, d, a, b, temp[10], 17, 0xffff5bb1);
        b = r1(b, c, d, a, temp[11], 22, 0x895cd7be);
        a = r1(a, b, c, d, temp[12], 7,  0x6b901122);
        d = r1(d, a, b, c, temp[13], 12, 0xfd987193);
        c = r1(c, d, a, b, temp[14], 17, 0xa679438e);
        b = r1(b, c, d, a, temp[15], 22, 0x49b40821);

        a = r2(a, b, c, d, temp[1 ], 5,  0xf61e2562);
        d = r2(d, a, b, c, temp[6 ], 9,  0xc040b340);
        c = r2(c, d, a, b, temp[11], 14, 0x265e5a51);
        b = r2(b, c, d, a, temp[0 ], 20, 0xe9b6c7aa);
        a = r2(a, b, c, d, temp[5 ], 5,  0xd62f105d);
        d = r2(d, a, b, c, temp[10], 9,  0x02441453);
        c = r2(c, d, a, b, temp[15], 14, 0xd8a1e681);
        b = r2(b, c, d, a, temp[4 ], 20, 0xe7d3fbc8);
        a = r2(a, b, c, d, temp[9 ], 5,  0x21e1cde6);
        d = r2(d, a, b, c, temp[14], 9,  0xc33707d6);
        c = r2(c, d, a, b, temp[3 ], 14, 0xf4d50d87);
        b = r2(b, c, d, a, temp[8 ], 20, 0x455a14ed);
        a = r2(a, b, c, d, temp[13], 5,  0xa9e3e905);
        d = r2(d, a, b, c, temp[2 ], 9,  0xfcefa3f8);
        c = r2(c, d, a, b, temp[7 ], 14, 0x676f02d9);
        b = r2(b, c, d, a, temp[12], 20, 0x8d2a4c8a);

        a = r3(a, b, c, d, temp[5 ], 4,  0xfffa3942);
        d = r3(d, a, b, c, temp[8 ], 11, 0x8771f681);
        c = r3(c, d, a, b, temp[11], 16, 0x6d9d6122);
        b = r3(b, c, d, a, temp[14], 23, 0xfde5380c);
        a = r3(a, b, c, d, temp[1 ], 4,  0xa4beea44);
        d = r3(d, a, b, c, temp[4 ], 11, 0x4bdecfa9);
        c = r3(c, d, a, b, temp[7 ], 16, 0xf6bb4b60);
        b = r3(b, c, d, a, temp[10], 23, 0xbebfbc70);
        a = r3(a, b, c, d, temp[13], 4,  0x289b7ec6);
        d = r3(d, a, b, c, temp[0 ], 11, 0xeaa127fa);
        c = r3(c, d, a, b, temp[3 ], 16, 0xd4ef3085);
        b = r3(b, c, d, a, temp[6 ], 23, 0x04881d05);
        a = r3(a, b, c, d, temp[9 ], 4,  0xd9d4d039);
        d = r3(d, a, b, c, temp[12], 11, 0xe6db99e5);
        c = r3(c, d, a, b, temp[15], 16, 0x1fa27cf8);
        b = r3(b, c, d, a, temp[2 ], 23, 0xc4ac5665);

        a = r4(a, b, c, d, temp[0 ], 6,  0xf4292244);
        d = r4(d, a, b, c, temp[7 ], 10, 0x432aff97);
        c = r4(c, d, a, b, temp[14], 15, 0xab9423a7);
        b = r4(b, c, d, a, temp[5 ], 21, 0xfc93a039);
        a = r4(a, b, c, d, temp[12], 6,  0x655b59c3);
        d = r4(d, a, b, c, temp[3 ], 10, 0x8f0ccc92);
        c = r4(c, d, a, b, temp[10], 15, 0xffeff47d);
        b = r4(b, c, d, a, temp[1 ], 21, 0x85845dd1);
        a = r4(a, b, c, d, temp[8 ], 6,  0x6fa87e4f);
        d = r4(d, a, b, c, temp[15], 10, 0xfe2ce6e0);
        c = r4(c, d, a, b, temp[6 ], 15, 0xa3014314);
        b = r4(b, c, d, a, temp[13], 21, 0x4e0811a1);
        a = r4(a, b, c, d, temp[4 ], 6,  0xf7537e82);
        d = r4(d, a, b, c, temp[11], 10, 0xbd3af235);
        c = r4(c, d, a, b, temp[2 ], 15, 0x2ad7d2bb);
        b = r4(b, c, d, a, temp[9 ], 21, 0xeb86d391);

        ABCDValue.A = unchecked(a + ABCDValue.A);
        ABCDValue.B = unchecked(b + ABCDValue.B);
        ABCDValue.C = unchecked(c + ABCDValue.C);
        ABCDValue.D = unchecked(d + ABCDValue.D);
        return; 
    }

    //Manually unrolling these equations nets us a 20% performance improvement
    private static uint r1(uint a, uint b, uint c, uint d, uint x, int s, uint t)
    {
        //                  (b + LSR((a + F(b, c, d) + x + t), s))
        //F(x, y, z)        ((x & y) | ((x ^ 0xFFFFFFFF) & z))
        return unchecked(b + LSR((a + ((b & c) | ((b ^ 0xFFFFFFFF) & d)) + x + t), s));
    }

    private static uint r2(uint a, uint b, uint c, uint d, uint x, int s, uint t)
    {
        //                  (b + LSR((a + G(b, c, d) + x + t), s))
        //G(x, y, z)        ((x & z) | (y & (z ^ 0xFFFFFFFF)))
        return unchecked(b + LSR((a + ((b & d) | (c & (d ^ 0xFFFFFFFF))) + x + t), s));
    }

    private static uint r3(uint a, uint b, uint c, uint d, uint x, int s, uint t)
    {
        //                  (b + LSR((a + H(b, c, d) + k + i), s))
        //H(x, y, z)        (x ^ y ^ z)
        return unchecked(b + LSR((a + (b ^ c ^ d) + x + t), s));
    }

    private static uint r4(uint a, uint b, uint c, uint d, uint x, int s, uint t)
    {
        //                  (b + LSR((a + I(b, c, d) + k + i), s))
        //I(x, y, z)        (y ^ (x | (z ^ 0xFFFFFFFF)))
        return unchecked(b + LSR((a + (c ^ (b | (d ^ 0xFFFFFFFF))) + x + t), s));
    }

    // Implementation of left rotate
    // s is an int instead of a uint becuase the CLR requires the argument passed to >>/<< is of 
    // type int. Doing the demoting inside this function would add overhead.
    private static uint LSR(uint i, int s)
    {
        return ((i << s) | (i >> (32-s)));
    }

    //Convert input array into array of UInts
    private static uint[] Converter(byte[] input, int ibStart)
    {
        if(null == input)
            throw new System.ArgumentNullException("input", "Unable convert null array to array of uInts");
        
        uint[] result = new uint[16];
        
        for (int i = 0; i < 16; i++)
        {
            result[i] = (uint)input[ibStart + i * 4];
            result[i] += (uint)input[ibStart + i * 4 + 1] << 8;
            result[i] += (uint)input[ibStart + i * 4 + 2] << 16;
            result[i] += (uint)input[ibStart + i * 4 + 3] << 24;
        }
       
        return result;
    }
}