package metainfo

import (
	"os"
	"path/filepath"
)

func (info *Info) Write(dir string, i int, data []byte) error {
	off := int64(i) * info.PieceSize
	if info.Size > 0 {
		return writeAt(filepath.Join(dir, info.Name), off, data)
	}
	for _, seg := range info.Segments(off, int64(len(data))) {
		f := info.Files[seg.File]
		path := filepath.Join(dir, info.Name, filepath.Join(f.Path...))
		if err := writeAt(path, seg.Offset, data[:seg.Size]); err != nil {
			return err
		}
		data = data[seg.Size:]
	}
	return nil
}

func (info *Info) Read(dir string, i int, size int64) []byte {
	off := int64(i) * info.PieceSize
	if info.Size > 0 {
		return readAt(filepath.Join(dir, info.Name), off, size)
	}
	data := make([]byte, size)
	pos := int64(0)
	for _, seg := range info.Segments(off, size) {
		f := info.Files[seg.File]
		path := filepath.Join(dir, info.Name, filepath.Join(f.Path...))
		chunk := readAt(path, seg.Offset, seg.Size)
		if chunk == nil {
			return nil
		}
		copy(data[pos:], chunk)
		pos += seg.Size
	}
	return data
}

func writeAt(path string, off int64, data []byte) error {
	if err := os.MkdirAll(filepath.Dir(path), 0755); err != nil {
		return err
	}
	f, err := os.OpenFile(path, os.O_CREATE|os.O_WRONLY, 0644)
	if err != nil {
		return err
	}
	defer f.Close()
	_, err = f.WriteAt(data, off)
	return err
}

func readAt(path string, off, size int64) []byte {
	f, err := os.Open(path)
	if err != nil {
		return nil
	}
	defer f.Close()
	data := make([]byte, size)
	if _, err := f.ReadAt(data, off); err != nil {
		return nil
	}
	return data
}