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package meta

import (
	"crypto/sha1"
	"errors"
	"fmt"
	"os"
	"path"
)

var ErrNotImplemented = errors.New("not implemented")

type Info struct {
	Files       []File `bencode:"files"`
	Length      int64  `bencode:"length"`
	MD5Sum      []byte `bencode:"md5sum,optional"` // never seen in wildlife
	Name        string `bencode:"name"`
	PieceLength int    `bencode:"piece length"`
	Pieces      []byte `bencode:"pieces"`    // compact mode
	Private     bool   `bencode:"private"`   // BEP-0027
	RootHash    []byte `bencode:"root hash"` // BEP-0030
}

// Full returns count of full pieces
func (i Info) Full() int {
	return int(i.TotalLength() / int64(i.PieceLength))
}

// Last returns size of last truncated piece
func (i Info) Last() int {
	return int(i.TotalLength() % int64(i.PieceLength))
}

// TotalLength calculates and retuns the length of all files
func (i Info) TotalLength() int64 {
	if i.Length == 0 {
		for _, f := range i.Files {
			i.Length += f.Length
		}
	}
	return i.Length
}

func (i Info) FileLength(n int) int64 {
	if len(i.Files) == 0 {
		return i.Length
	}
	return i.Files[n].Length
}

func (i Info) GetPieces() []Piece {
	n, last := i.Full(), i.Last()
	if last > 0 {
		n++
	}
	p := make([]Piece, n)
	for k := 0; k < n; k++ {
		p[k].Length = i.PieceLength
		if k+1 == n {
			p[k].Length = last
		}
		off := k * sha1.Size
		copy(p[k].Sum[:], i.Pieces[off:off+sha1.Size])
		p[k].Offset = int64(k) * int64(i.PieceLength)
	}
	return p
}

// Open N-th file and allocate required path on demand
func (i Info) Open(n int) (*os.File, error) {
	p := i.Name
	sz := i.Length
	if len(i.Files) > 0 {
		p = path.Join(p, i.Files[n].Name())
		if err := os.MkdirAll(path.Dir(p), 0755); err != nil {
			return nil, err
		}
		sz = i.Files[n].Length
	}
	fd, err := os.OpenFile(p, os.O_RDWR|os.O_CREATE, 0644)
	if err != nil {
		return nil, err
	}
	return fd, fd.Truncate(sz)
}

// Offset of piece in gross buffer
func (i Info) Offset(piece int) int64 {
	return int64(piece) * int64(i.PieceLength)
}

// findFirstFile looks up first file # corresponding to offset into it
// If piece is bigger then file, next file(s) with offset 0 shall be used
// to fit a piece into
func (i Info) findFirstFile(off int64) (int, int64) {
	for n, l := range i.Files {
		if l.Length > off {
			return n, off
		}
		off -= l.Length
	}
	return 0, off
}

/*
 *  0    1    2    3    4    5    6    7   #piece
 * |----|----|----|----|----|----|----|--| pieces
 * |-|----|--|----|----------|-|----|----| files
 *  0 1    2  3    4          5 6    7     #file
 *
 *  0        2        3        4      #piece
 * |--------|--------|--------|-----| pieces
 * |-|--|--|-|--|--|-------------|--| files
 *  0 1  2  3 4  5  6             7   #file
 */

// FIXME WriteAt ...
func (i Info) WriteAt(b []byte, off int64) (n int, err error) {
	return 0, ErrNotImplemented
}

// FIXME ReadAt ...
func (i Info) ReadAt(b []byte, off int64) (n int, err error) {
	n, foff := i.findFirstFile(off)

	fd, err := i.Open(n)
	if err != nil {
		return 0, err
	}
	defer fd.Close()

	flen := i.FileLength(n)

	// does it fit?
	if int64(len(b))+foff <= flen {
		return fd.ReadAt(b, foff)
	}

	end := flen + foff

	done, err := fd.ReadAt(b[:end], foff)
	if err != nil {
		return done, err
	}
	next, err := i.ReadAt(b[end:], end)

	return done + next, err
}

func (i Info) String() string {
	s := fmt.Sprintf("%s (%d) ", i.Name, i.TotalLength())
	s += fmt.Sprintf("%d × %d + %d\n", i.Full(), i.PieceLength, i.Last())
	for _, f := range i.Files {
		s += fmt.Sprintf("    %v\n", f)
	}
	return s
}