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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) }