package wal

import (
	"encoding/binary"
	"errors"
	"hash/crc32"
	"io"
	"git.crumpington.com/public/jldb/lib/errs"
)

func ioErrOrEOF(err error) error {
	if err == nil {
		return nil
	}
	if errors.Is(err, io.EOF) {
		return err
	}
	return errs.IO.WithErr(err)

}

// ----------------------------------------------------------------------------

type readAtReader struct {
	f      io.ReaderAt
	offset int64
}

func readerAtToReader(f io.ReaderAt, offset int64) io.Reader {
	return &readAtReader{f: f, offset: offset}
}

func (r *readAtReader) Read(b []byte) (int, error) {
	n, err := r.f.ReadAt(b, r.offset)
	r.offset += int64(n)
	return n, ioErrOrEOF(err)
}

// ----------------------------------------------------------------------------

type writeAtWriter struct {
	w      io.WriterAt
	offset int64
}

func writerAtToWriter(w io.WriterAt, offset int64) io.Writer {
	return &writeAtWriter{w: w, offset: offset}
}

func (w *writeAtWriter) Write(b []byte) (int, error) {
	n, err := w.w.WriteAt(b, w.offset)
	w.offset += int64(n)
	return n, ioErrOrEOF(err)
}

// ----------------------------------------------------------------------------

type crcWriter struct {
	w   io.Writer
	crc uint32
}

func newCRCWriter(w io.Writer) *crcWriter {
	return &crcWriter{w: w}
}

func (w *crcWriter) Write(b []byte) (int, error) {
	n, err := w.w.Write(b)
	w.crc = crc32.Update(w.crc, crc32.IEEETable, b[:n])
	return n, ioErrOrEOF(err)
}

func (w *crcWriter) CRC() uint32 {
	return w.crc
}

// ----------------------------------------------------------------------------

type dataReader struct {
	r         io.Reader
	remaining int64
	crc       uint32
}

func newDataReader(r io.Reader, dataSize int64) *dataReader {
	return &dataReader{r: r, remaining: dataSize}
}

func (r *dataReader) Read(b []byte) (int, error) {
	if r.remaining == 0 {
		return 0, io.EOF
	}

	if int64(len(b)) > r.remaining {
		b = b[:r.remaining]
	}

	n, err := r.r.Read(b)
	r.crc = crc32.Update(r.crc, crc32.IEEETable, b[:n])
	r.remaining -= int64(n)

	if r.remaining == 0 {
		if err := r.checkCRC(); err != nil {
			return n, err
		}
	}

	if err != nil && !errors.Is(err, io.EOF) {
		return n, errs.IO.WithErr(err)
	}

	return n, nil
}

func (r *dataReader) checkCRC() error {
	buf := make([]byte, 4)
	if _, err := r.r.Read(buf); err != nil {
		return errs.Corrupt.WithErr(err)
	}
	crc := binary.LittleEndian.Uint32(buf)
	if crc != r.crc {
		return errs.Corrupt.WithMsg("crc mismatch")
	}
	return nil
}