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package main
import (
"bytes"
"crypto/sha1"
"fmt"
"math"
bencode "github.com/jackpal/bencode-go"
)
const BlockSize = 16 * 1024
type Meta struct {
Announce string `bencode:"announce"`
Info FileInfo `bencode:"info"`
}
type FileInfo struct {
Length int `bencode:"length"`
Name string `bencode:"name"`
PieceLength int `bencode:"piece length"`
Pieces string `bencode:"pieces"`
}
type Piece struct {
Index int
Len int
Hash string
Blocks []uint32
}
func (m Meta) InfoHash() ([]byte, error) {
sha := sha1.New()
if err := bencode.Marshal(sha, m.Info); err != nil {
return nil, err
}
return sha.Sum(nil), nil
}
func (m Meta) PieceHashes() []string {
hashes := make([]string, 0, len(m.Info.Pieces)/20)
for i := 0; i < len(m.Info.Pieces); i += 20 {
hashes = append(hashes, m.Info.Pieces[i:i+20])
}
return hashes
}
func (m Meta) PieceCount() int {
return len(m.Info.Pieces) / 20
}
func (m Meta) PieceLens() []int {
pieceCnt := m.PieceCount()
pieces := make([]int, pieceCnt)
for i := 0; i < pieceCnt; i++ {
if i < pieceCnt-1 {
pieces[i] = m.Info.PieceLength
} else {
pieces[i] = m.Info.Length - i*m.Info.PieceLength
}
}
return pieces
}
func (m Meta) BlockLens(pieceIdx int) []uint32 {
pieceLen := m.PieceLens()[pieceIdx]
blockCnt := int(math.Ceil(float64(pieceLen) / float64(BlockSize)))
blocks := make([]uint32, blockCnt)
for i := 0; i < blockCnt; i++ {
if i < blockCnt-1 {
blocks[i] = uint32(BlockSize)
} else {
blocks[i] = uint32(pieceLen - i*BlockSize)
}
}
return blocks
}
func (p Piece) CheckHash(data []byte) error {
sha := sha1.New()
if _, err := bytes.NewBuffer(data).WriteTo(sha); err != nil {
return err
}
exp, act := []byte(p.Hash), sha.Sum(nil)
if !bytes.Equal(exp, act) {
return fmt.Errorf("expected hash: %x, actual: %x", exp, act)
}
return nil
}
func (m Meta) Pieces() []Piece {
cnt := m.PieceCount()
pieces := make([]Piece, cnt)
for i := 0; i < cnt; i++ {
p := Piece{
Index: i,
Hash: m.Info.Pieces[i*20 : i*20+20],
Blocks: m.BlockLens(i),
}
if i < cnt-1 {
p.Len = m.Info.PieceLength
} else {
p.Len = m.Info.Length - i*m.Info.PieceLength
}
pieces[i] = p
}
return pieces
}
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