/*
NAME
  crc.go
DESCRIPTION
  See Readme.md

AUTHOR
  Saxon Milton <saxon@ausocean.org>

LICENSE
  crc.go is Copyright (C) 2018 the Australian Ocean Lab (AusOcean)

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
 for more details.

  You should have received a copy of the GNU General Public License
  along with revid in gpl.txt.  If not, see http://www.gnu.org/licenses.
*/

package psi

import (
	"encoding/binary"
	"hash/crc32"
	"math/bits"
)

// addCrc appends a crc table to a given psi table in bytes
func addCrc(out []byte) []byte {
	out = append(out, make([]byte, 4)...)
	out = updateCrc(out)
	return out
}

// updateCrc updates the crc of bytes slice, writing the checksum into the last four bytes.
func updateCrc(b []byte) []byte {
	crc32 := crc32_Update(0xffffffff, crc32_MakeTable(bits.Reverse32(crc32.IEEE)), b[1:len(b)-4])
	binary.BigEndian.PutUint32(b[len(b)-4:], crc32)
	return b
}

func crc32_MakeTable(poly uint32) *crc32.Table {
	var t crc32.Table
	for i := range t {
		crc := uint32(i) << 24
		for j := 0; j < 8; j++ {
			if crc&0x80000000 != 0 {
				crc = (crc << 1) ^ poly
			} else {
				crc <<= 1
			}
		}
		t[i] = crc
	}
	return &t
}

func crc32_Update(crc uint32, tab *crc32.Table, p []byte) uint32 {
	for _, v := range p {
		crc = tab[byte(crc>>24)^v] ^ (crc << 8)
	}
	return crc
}