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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 sha256 implements the SHA224 and SHA256 hash algorithms as defined
// in FIPS 180-2.
package sha256

import (
        "crypto"
        "hash"
)

func init() {
        crypto.RegisterHash(crypto.SHA224, New224)
        crypto.RegisterHash(crypto.SHA256, New)
}

// The size of a SHA256 checksum in bytes.
const Size = 32

// The size of a SHA224 checksum in bytes.
const Size224 = 28

// The blocksize of SHA256 and SHA224 in bytes.
const BlockSize = 64

const (
        _Chunk     = 64
        _Init0     = 0x6A09E667
        _Init1     = 0xBB67AE85
        _Init2     = 0x3C6EF372
        _Init3     = 0xA54FF53A
        _Init4     = 0x510E527F
        _Init5     = 0x9B05688C
        _Init6     = 0x1F83D9AB
        _Init7     = 0x5BE0CD19
        _Init0_224 = 0xC1059ED8
        _Init1_224 = 0x367CD507
        _Init2_224 = 0x3070DD17
        _Init3_224 = 0xF70E5939
        _Init4_224 = 0xFFC00B31
        _Init5_224 = 0x68581511
        _Init6_224 = 0x64F98FA7
        _Init7_224 = 0xBEFA4FA4
)

// digest represents the partial evaluation of a checksum.
type digest struct {
        h     [8]uint32
        x     [_Chunk]byte
        nx    int
        len   uint64
        is224 bool // mark if this digest is SHA-224
}

func (d *digest) Reset() {
        if !d.is224 {
                d.h[0] = _Init0
                d.h[1] = _Init1
                d.h[2] = _Init2
                d.h[3] = _Init3
                d.h[4] = _Init4
                d.h[5] = _Init5
                d.h[6] = _Init6
                d.h[7] = _Init7
        } else {
                d.h[0] = _Init0_224
                d.h[1] = _Init1_224
                d.h[2] = _Init2_224
                d.h[3] = _Init3_224
                d.h[4] = _Init4_224
                d.h[5] = _Init5_224
                d.h[6] = _Init6_224
                d.h[7] = _Init7_224
        }
        d.nx = 0
        d.len = 0
}

// New returns a new hash.Hash computing the SHA256 checksum.
func New() hash.Hash {
        d := new(digest)
        d.Reset()
        return d
}

// New224 returns a new hash.Hash computing the SHA224 checksum.
func New224() hash.Hash {
        d := new(digest)
        d.is224 = true
        d.Reset()
        return d
}

func (d *digest) Size() int {
        if !d.is224 {
                return Size
        }
        return Size224
}

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 >> (56 - 8*i))
        }
        d.Write(tmp[0:8])

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

        h := d.h[:]
        size := Size
        if d.is224 {
                h = d.h[:7]
                size = Size224
        }

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

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