package sarama

import (
	"encoding/binary"
	"fmt"
	"hash/crc32"
)

// crc32Field implements the pushEncoder and pushDecoder interfaces for calculating CRC32s.
type crc32Field struct {
	startOffset int
}

func (c *crc32Field) saveOffset(in int) {
	c.startOffset = in
}

func (c *crc32Field) reserveLength() int {
	return 4
}

func (c *crc32Field) run(curOffset int, buf []byte) error {
	crc := crc32.ChecksumIEEE(buf[c.startOffset+4 : curOffset])
	binary.BigEndian.PutUint32(buf[c.startOffset:], crc)
	return nil
}

func (c *crc32Field) check(curOffset int, buf []byte) error {
	crc := crc32.ChecksumIEEE(buf[c.startOffset+4 : curOffset])

	expected := binary.BigEndian.Uint32(buf[c.startOffset:])
	if crc != expected {
		return PacketDecodingError{fmt.Sprintf("CRC didn't match expected %#x got %#x", expected, crc)}
	}

	return nil
}