package store

import (
	"encoding/binary"
	"hash/crc32"
	"io"
	"os"
)

// The checkpoint names the WAL position up to which every index has been made
// durable, plus the next pubkey serial. Recovery replays the WAL from pos.
//
//	"TCK1" | pos uint64 | nextPubSer uint64 | crc32(first 20) uint32
const (
	ckptMagic  = "TCK1"
	ckptSize   = 24
	ckptCRCOff = 20
)

type checkpoint struct {
	f       *os.File
	pos     uint64
	nextPub uint64
}

func openCheckpoint(path string) (*checkpoint, error) {
	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0o644)
	if err != nil {
		return nil, err
	}
	c := &checkpoint{f: f}
	buf := make([]byte, ckptSize)
	n, err := f.ReadAt(buf, 0)
	if err != nil && err != io.EOF {
		f.Close()
		return nil, err
	}
	if n == ckptSize && string(buf[0:4]) == ckptMagic &&
		crc32.ChecksumIEEE(buf[:ckptCRCOff]) == binary.BigEndian.Uint32(buf[ckptCRCOff:]) {
		c.pos = binary.BigEndian.Uint64(buf[4:12])
		c.nextPub = binary.BigEndian.Uint64(buf[12:20])
	}
	return c, nil
}

func (c *checkpoint) Get() (pos, nextPub uint64) { return c.pos, c.nextPub }

func (c *checkpoint) Set(pos, nextPub uint64) error {
	buf := make([]byte, ckptSize)
	copy(buf, ckptMagic)
	binary.BigEndian.PutUint64(buf[4:12], pos)
	binary.BigEndian.PutUint64(buf[12:20], nextPub)
	binary.BigEndian.PutUint32(buf[ckptCRCOff:], crc32.ChecksumIEEE(buf[:ckptCRCOff]))
	if _, err := c.f.WriteAt(buf, 0); err != nil {
		return err
	}
	c.pos = pos
	c.nextPub = nextPub
	return nil
}

func (c *checkpoint) Sync() error { return c.f.Sync() }

func (c *checkpoint) Close() error { return c.f.Close() }