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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 crc32 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"

        "sync"

)

// 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

// castagnoliTable points to a lazily initialized Table for the Castagnoli

// polynomial. MakeTable will always return this value when asked to make a

// Castagnoli table so we can compare against it to find when the caller is

// using this polynomial.

var castagnoliTable *Table

var castagnoliOnce sync.Once

func castagnoliInit() {

        castagnoliTable = makeTable(Castagnoli)

}

// IEEETable is the table for the IEEE polynomial.

var IEEETable = makeTable(IEEE)

// MakeTable returns the Table constructed from the specified polynomial.

func MakeTable(poly uint32) *Table {

        switch poly {

        case IEEE:

                return IEEETable

        case Castagnoli:

                castagnoliOnce.Do(castagnoliInit)

                return castagnoliTable

        }

        return makeTable(poly)

}

// 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

}

// 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) BlockSize() int { return 1 }

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 {

        if tab == castagnoliTable {

                return updateCastagnoli(crc, p)

        }

        return update(crc, tab, p)

}

func (d *digest) Write(p []byte) (n int, err 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(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 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) }