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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package md5 implements the MD5 hash algorithm as defined in RFC 1321.
package md5

import (
        "crypto"
        "hash"
)

func init() {
        crypto.RegisterHash(crypto.MD5, New)
}

// The size of an MD5 checksum in bytes.
const Size = 16

// The blocksize of MD5 in bytes.
const BlockSize = 64

const (
        _Chunk = 64
        _Init0 = 0x67452301
        _Init1 = 0xEFCDAB89
        _Init2 = 0x98BADCFE
        _Init3 = 0x10325476
)

// digest represents the partial evaluation of a checksum.
type digest struct {
        s   [4]uint32
        x   [_Chunk]byte
        nx  int
        len uint64
}

func (d *digest) Reset() {
        d.s[0] = _Init0
        d.s[1] = _Init1
        d.s[2] = _Init2
        d.s[3] = _Init3
        d.nx = 0
        d.len = 0
}

// New returns a new hash.Hash computing the MD5 checksum.
func New() hash.Hash {
        d := new(digest)
        d.Reset()
        return d
}

func (d *digest) Size() int { return Size }

func (d *digest) BlockSize() int { return BlockSize }

func (d *digest) Write(p []byte) (nn int, err error) {
        nn = len(p)
        d.len += uint64(nn)
        if d.nx > 0 {
                n := len(p)
                if n > _Chunk-d.nx {
                        n = _Chunk - d.nx
                }
                for i := 0; i < n; i++ {
                        d.x[d.nx+i] = p[i]
                }
                d.nx += n
                if d.nx == _Chunk {
                        _Block(d, d.x[0:])
                        d.nx = 0
                }
                p = p[n:]
        }
        n := _Block(d, p)
        p = p[n:]
        if len(p) > 0 {
                d.nx = copy(d.x[:], p)
        }
        return
}

func (d0 *digest) Sum(in []byte) []byte {
        // Make a copy of d0 so that caller can keep writing and summing.
        d := *d0

        // Padding.  Add a 1 bit and 0 bits until 56 bytes mod 64.
        len := d.len
        var tmp [64]byte
        tmp[0] = 0x80
        if len%64 < 56 {
                d.Write(tmp[0 : 56-len%64])
        } else {
                d.Write(tmp[0 : 64+56-len%64])
        }

        // Length in bits.
        len <<= 3
        for i := uint(0); i < 8; i++ {
                tmp[i] = byte(len >> (8 * i))
        }
        d.Write(tmp[0:8])

        if d.nx != 0 {
                panic("d.nx != 0")
        }

        var digest [Size]byte
        for i, s := range d.s {
                digest[i*4] = byte(s)
                digest[i*4+1] = byte(s >> 8)
                digest[i*4+2] = byte(s >> 16)
                digest[i*4+3] = byte(s >> 24)
        }

        return append(in, digest[:]...)
}