/*
 * Copyright (c) 2024 The GoPlus Authors (goplus.org). All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package crc32

// llgo:skipall
import (
	"hash"

	"github.com/goplus/llgo/c"
	"github.com/goplus/llgo/c/zlib"
)

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

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

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

	// Koopman's polynomial.
	// Also has better error detection characteristics than IEEE.
	// https://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

// IEEETable is the table for the IEEE polynomial.
var IEEETable *Table = new(Table)

// MakeTable returns a Table constructed from the specified polynomial.
// The contents of this Table must not be modified.
func MakeTable(poly uint32) *Table {
	if poly == IEEE {
		return IEEETable
	}
	panic("todo: hash/crc32.MakeTable")
}

type digest uint32

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

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

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

func (d *digest) Write(p []byte) (n int, err error) {
	*d = digest(zlib.Crc32ZBytes(c.Ulong(*d), p))
	return len(p), nil
}

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

func (d *digest) Sum(in []byte) []byte {
	s := d.Sum32()
	return append(in, byte(s>>24), byte(s>>16), byte(s>>8), byte(s))
}

func New(tab *Table) hash.Hash32 {
	if tab == IEEETable {
		return new(digest)
	}
	panic("todo: hash/crc32.New")
}

// NewIEEE creates a new hash.Hash32 computing the CRC-32 checksum using
// the IEEE polynomial. Its Sum method will lay the value out in
// big-endian byte order. The returned Hash32 also implements
// encoding.BinaryMarshaler and encoding.BinaryUnmarshaler to marshal
// and unmarshal the internal state of the hash.
func NewIEEE() hash.Hash32 { return New(IEEETable) }

// Update returns the result of adding the bytes in p to the crc.
func Update(crc uint32, tab *Table, p []byte) uint32 {
	if tab == IEEETable {
		return uint32(zlib.Crc32ZBytes(c.Ulong(crc), p))
	}
	panic("todo: hash/crc32.Update")
}

// 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 uint32(zlib.Crc32ZBytes(0, data))
}