// 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.

// This package implements the 32-bit cyclic redundancy check, or CRC-32, checksum.
// See http://en.wikipedia.org/wiki/Cyclic_redundancy_check for information.
package crc32

import (
	"hash"
	"os"
)

// The size of a CRC-32 checksum in bytes.
const Size = 4

// Predefined polynomials.
const (
	// Far and away the most common CRC-32 polynomial.
	// Used by ethernet (IEEE 802.3), v.42, fddi, gzip, zip, png, mpeg-2, ...
	IEEE = 0xedb88320

	// Castagnoli's polynomial, used in iSCSI.
	// Has better error detection characteristics than IEEE.
	// http://dx.doi.org/10.1109/26.231911
	Castagnoli = 0x82f63b78

	// Koopman's polynomial.
	// Also has better error detection characteristics than IEEE.
	// http://dx.doi.org/10.1109/DSN.2002.1028931
	Koopman = 0xeb31d82e
)

// Table is a 256-word table representing the polynomial for efficient processing.
type Table [256]uint32

// MakeTable returns the Table constructed from the specified polynomial.
func MakeTable(poly uint32) *Table {
	t := new(Table)
	for i := 0; i < 256; i++ {
		crc := uint32(i)
		for j := 0; j < 8; j++ {
			if crc&1 == 1 {
				crc = (crc >> 1) ^ poly
			} else {
				crc >>= 1
			}
		}
		t[i] = crc
	}
	return t
}

// IEEETable is the table for the IEEE polynomial.
var IEEETable = MakeTable(IEEE)

// digest represents the partial evaluation of a checksum.
type digest struct {
	crc uint32
	tab *Table
}

// New creates a new hash.Hash32 computing the CRC-32 checksum
// using the polynomial represented by the Table.
func New(tab *Table) hash.Hash32 { return &digest{0, tab} }

// NewIEEE creates a new hash.Hash32 computing the CRC-32 checksum
// using the IEEE polynomial.
func NewIEEE() hash.Hash32 { return New(IEEETable) }

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

func (d *digest) Reset() { d.crc = 0 }

func update(crc uint32, tab *Table, p []byte) uint32 {
	crc = ^crc
	for _, v := range p {
		crc = tab[byte(crc)^v] ^ (crc >> 8)
	}
	return ^crc
}

// Update returns the result of adding the bytes in p to the crc.
func Update(crc uint32, tab *Table, p []byte) uint32 {
	return update(crc, tab, p)
}

func (d *digest) Write(p []byte) (n int, err os.Error) {
	d.crc = update(d.crc, d.tab, p)
	return len(p), nil
}

func (d *digest) Sum32() uint32 { return d.crc }

func (d *digest) Sum() []byte {
	p := make([]byte, 4)
	s := d.Sum32()
	p[0] = byte(s >> 24)
	p[1] = byte(s >> 16)
	p[2] = byte(s >> 8)
	p[3] = byte(s)
	return p
}

// Checksum returns the CRC-32 checksum of data
// using the polynomial represented by the Table.
func Checksum(data []byte, tab *Table) uint32 { return update(0, tab, data) }

// ChecksumIEEE returns the CRC-32 checksum of data
// using the IEEE polynomial.
func ChecksumIEEE(data []byte) uint32 { return update(0, IEEETable, data) }