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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)) }