// Package checkpoint provides an atomic checksummed checkpoint file.
// Format: 8 bytes uint64 serial (BE) + 4 bytes CRC32-IEEE checksum (BE).
package checkpoint

import (
	"encoding/binary"
	"fmt"
	"os"
)

const fileSize = 12

type Checkpoint struct {
	path string
	ser  uint64
}

func Open(path string) (*Checkpoint, error) {
	c := &Checkpoint{path: path}
	data, err := os.ReadFile(path)
	if err != nil {
		if os.IsNotExist(err) {
			return c, nil
		}
		return nil, err
	}
	if len(data) != fileSize {
		fmt.Fprintf(os.Stderr, "checkpoint: invalid size %d, treating as zero\n", len(data))
		return c, nil
	}
	ser := binary.BigEndian.Uint64(data[:8])
	stored := binary.BigEndian.Uint32(data[8:12])
	if crc32ieee(data[:8]) != stored {
		fmt.Fprintf(os.Stderr, "checkpoint: CRC mismatch, treating as zero\n")
		return c, nil
	}
	c.ser = ser
	return c, nil
}

func (c *Checkpoint) Get() uint64 { return c.ser }

func (c *Checkpoint) Set(ser uint64) error {
	var buf [fileSize]byte
	binary.BigEndian.PutUint64(buf[:8], ser)
	binary.BigEndian.PutUint32(buf[8:], crc32ieee(buf[:8]))

	tmp := c.path | ".tmp"
	f, err := os.Create(tmp)
	if err != nil {
		return err
	}
	if _, err := f.Write(buf[:]); err != nil {
		f.Close()
		os.Remove(tmp)
		return err
	}
	if err := f.Sync(); err != nil {
		f.Close()
		os.Remove(tmp)
		return err
	}
	f.Close()
	if err := os.Rename(tmp, c.path); err != nil {
		return err
	}
	c.ser = ser
	return nil
}

// CRC32-IEEE (polynomial 0xEDB88320 reflected).
func crc32ieee(data []byte) uint32 {
	crc := uint32(0xFFFFFFFF)
	for _, b := range data {
		crc ^= uint32(b)
		for i := 0; i < 8; i++ {
			if crc&1 != 0 {
				crc = (crc >> 1) ^ 0xEDB88320
			} else {
				crc >>= 1
			}
		}
	}
	return crc ^ 0xFFFFFFFF
}