package litestream

import (
	"encoding/binary"
	"io"
)

// Magic number specified at the beginning of WAL files.
const (
	MagicLittleEndian = 0x377f0682
	MagicBigEndian    = 0x377f0683
)

const (
	WriteVersionOffset = 18
	ReadVersionOffset  = 19
)

// ReadVersion returns the SQLite write & read version.
// Returns 1 for legacy & 2 for WAL.
func ReadVersion(b []byte) (writeVersion, readVersion uint8, err error) {
	if len(b) < ReadVersionOffset {
		return 0, 0, io.ErrUnexpectedEOF
	}
	return b[WriteVersionOffset], b[ReadVersionOffset], nil
}

// Checksum computes a running checksum over a byte slice.
func Checksum(bo binary.ByteOrder, s uint64, b []byte) (_ uint64, err error) {
	// Ensure byte slice length is divisible by 8.
	if len(b)%8 != 0 {
		return 0, ErrChecksumMisaligned
	}

	// Iterate over 8-byte units and compute checksum.
	s0, s1 := uint32(s>>32), uint32(s&0xFFFFFFFF)
	for i := 0; i < len(b); i += 8 {
		s0 += bo.Uint32(b[i:]) + s1
		s1 += bo.Uint32(b[i+4:]) + s0
	}
	return uint64(s0)<<32 | uint64(s1), nil
}