storrent/metainfo/metainfo.go

package metainfo

import (
	"crypto/sha1"
	"fmt"
	"os"

	"storrent/bencode"
)

type File struct {
	Info     Info
	InfoHash [20]byte
	Size     int64
}

type Info struct {
	Name      string
	PieceSize int64
	Pieces    [][20]byte
	Size      int64
	Files     []Entry
}

type Entry struct {
	Size   int64
	Offset int64
	Path   []string
}

type Segment struct {
	File   int
	Offset int64
	Size   int64
}

func (i *Info) Segments(off, size int64) []Segment {
	var segs []Segment
	for idx, f := range i.Files {
		end := f.Offset + f.Size
		if off >= end {
			continue
		}
		n := min(size, end-off)
		segs = append(segs, Segment{File: idx, Offset: off - f.Offset, Size: n})
		off += n
		size -= n
		if size == 0 {
			break
		}
	}
	return segs
}

func Parse(path string) (*File, error) {
	data, err := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	return ParseBytes(data)
}

func ParseBytes(data []byte) (*File, error) {
	v, _, err := bencode.Decode(data)
	if err != nil {
		return nil, err
	}
	d, ok := v.(map[string]any)
	if !ok {
		return nil, fmt.Errorf("invalid torrent dict")
	}

	f := &File{}
	raw, err := findInfoBytes(data)
	if err != nil {
		return nil, err
	}
	f.InfoHash = sha1.Sum(raw)
	dict, ok := d["info"].(map[string]any)
	if !ok {
		return nil, fmt.Errorf("invalid info dict")
	}
	f.Info, err = parseInfo(dict)
	if err != nil {
		return nil, err
	}
	if f.Info.Size > 0 {
		f.Size = f.Info.Size
	} else {
		for _, e := range f.Info.Files {
			f.Size += e.Size
		}
	}
	return f, nil
}

func findInfoBytes(data []byte) ([]byte, error) {
	if len(data) == 0 || data[0] != 'd' {
		return nil, fmt.Errorf("not a dict")
	}

	i := 1
	for i < len(data) && data[i] != 'e' {
		k, n, err := bencode.DecodeString(data[i:])
		if err != nil {
			return nil, err
		}
		i += n
		if k == "info" {
			_, n, err := bencode.Decode(data[i:])
			if err != nil {
				return nil, err
			}
			return data[i : i+n], nil
		}
		_, n, err = bencode.Decode(data[i:])
		if err != nil {
			return nil, err
		}
		i += n
	}
	return nil, fmt.Errorf("info not found")
}

func parseInfo(d map[string]any) (Info, error) {
	var info Info
	name, ok := d["name"].(string)
	if !ok {
		return info, fmt.Errorf("invalid name")
	}
	info.Name = name
	pl, ok := d["piece length"].(int64)
	if !ok {
		return info, fmt.Errorf("invalid piece length")
	}
	info.PieceSize = pl
	ps, ok := d["pieces"].(string)
	if !ok || len(ps)%20 != 0 {
		return info, fmt.Errorf("invalid pieces")
	}
	npieces := len(ps) / 20
	info.Pieces = make([][20]byte, npieces)
	for i := range npieces {
		copy(info.Pieces[i][:], ps[i*20:(i+1)*20])
	}
	if n, ok := d["length"].(int64); ok {
		info.Size = n
		return info, nil
	}

	fs, ok := d["files"].([]any)
	if !ok {
		return info, fmt.Errorf("invalid files")
	}

	off := int64(0)
	for _, f := range fs {
		fd, ok := f.(map[string]any)
		if !ok {
			return info, fmt.Errorf("invalid file entry")
		}
		n, ok := fd["length"].(int64)
		if !ok {
			return info, fmt.Errorf("invalid file length")
		}
		list, ok := fd["path"].([]any)
		if !ok {
			return info, fmt.Errorf("invalid file path")
		}
		var path []string
		for _, p := range list {
			s, ok := p.(string)
			if !ok {
				return info, fmt.Errorf("invalid path element")
			}
			path = append(path, s)
		}
		info.Files = append(info.Files, Entry{Size: n, Offset: off, Path: path})
		off += n
	}
	return info, nil
}