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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 adler32 implements the Adler-32 checksum.

// Defined in RFC 1950:

//      Adler-32 is composed of two sums accumulated per byte: s1 is

//      the sum of all bytes, s2 is the sum of all s1 values. Both sums

//      are done modulo 65521. s1 is initialized to 1, s2 to zero.  The

//      Adler-32 checksum is stored as s2*65536 + s1 in most-

//      significant-byte first (network) order.

package adler32

import "hash"

const (

        mod = 65521

)

// The size of an Adler-32 checksum in bytes.

const Size = 4

// digest represents the partial evaluation of a checksum.

type digest struct {

        // invariant: (a < mod && b < mod) || a <= b

        // invariant: a + b + 255 <= 0xffffffff

        a, b uint32

}

func (d *digest) Reset() { d.a, d.b = 1, 0 }

// New returns a new hash.Hash32 computing the Adler-32 checksum.

func New() hash.Hash32 {

        d := new(digest)

        d.Reset()

        return d

}

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

func (d *digest) BlockSize() int { return 1 }

// Add p to the running checksum a, b.

func update(a, b uint32, p []byte) (aa, bb uint32) {

        for _, pi := range p {

                a += uint32(pi)

                b += a

                // invariant: a <= b

                if b > (0xffffffff-255)/2 {

                        a %= mod

                        b %= mod

                        // invariant: a < mod && b < mod

                } else {

                        // invariant: a + b + 255 <= 2 * b + 255 <= 0xffffffff

                }

        }

        return a, b

}

// Return the 32-bit checksum corresponding to a, b.

func finish(a, b uint32) uint32 {

        if b >= mod {

                a %= mod

                b %= mod

        }

        return b<<16 | a

}

func (d *digest) Write(p []byte) (nn int, err error) {

        d.a, d.b = update(d.a, d.b, p)

        return len(p), nil

}

func (d *digest) Sum32() uint32 { return finish(d.a, d.b) }

func (d *digest) Sum(in []byte) []byte {

        s := d.Sum32()

        in = append(in, byte(s>>24))

        in = append(in, byte(s>>16))

        in = append(in, byte(s>>8))

        in = append(in, byte(s))

        return in

}

// Checksum returns the Adler-32 checksum of data.

func Checksum(data []byte) uint32 { return finish(update(1, 0, data)) }