FedP2P/metainfo/piece-length.go

// From https://github.com/jackpal/Taipei-Torrent

// Copyright (c) 2010 Jack Palevich. All rights reserved.
//
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// modification, are permitted provided that the following conditions are
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package metainfo

import (
	"math"
)

// For more context on why these numbers, see http://wiki.vuze.com/w/Torrent_Piece_Size
const (
	minimumPieceLength   = 64 * 1024
	maximumPieceLength   = 2 * 1024 * 1024
	targetPieceCountLog2 = 10
	targetPieceCountMin  = 1 << targetPieceCountLog2
)

// Target piece count should be < targetPieceCountMax
const targetPieceCountMax = targetPieceCountMin << 1

// Choose a good piecelength.
// piecelength >= 64KB and piecelength <= 2MB
// piecelength = 64KB*X
// totalLength / piecelength = piecenumber <= 2048
func ChoosePieceLength(totalLength int64) (pieceLength int64) {
	// Must be a power of 2.
	// Must be a multiple of 64KB
	// Prefer to provide around 1024..2048 pieces.
	pieceLength = minimumPieceLength
	pieces := int64(math.Ceil(float64(totalLength) / float64(pieceLength)))
	for pieces > targetPieceCountMax {
		if (pieceLength << 1) > maximumPieceLength {
			break
		}
		pieceLength <<= 1
		pieces >>= 1
	}
	return
}