1 // From https://github.com/jackpal/Taipei-Torrent
3 // Copyright (c) 2010 Jack Palevich. All rights reserved.
5 // Redistribution and use in source and binary forms, with or without
6 // modification, are permitted provided that the following conditions are
9 // * Redistributions of source code must retain the above copyright
10 // notice, this list of conditions and the following disclaimer.
11 // * Redistributions in binary form must reproduce the above
12 // copyright notice, this list of conditions and the following disclaimer
13 // in the documentation and/or other materials provided with the
15 // * Neither the name of Google Inc. nor the names of its
16 // contributors may be used to endorse or promote products derived from
17 // this software without specific prior written permission.
19 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
21 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
22 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
23 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
24 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
25 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
26 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
27 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
28 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
29 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
33 // For more context on why these numbers, see http://wiki.vuze.com/w/Torrent_Piece_Size
34 const minimumPieceLength = 16 * 1024
35 const targetPieceCountLog2 = 10
36 const targetPieceCountMin = 1 << targetPieceCountLog2
38 // Target piece count should be < targetPieceCountMax
39 const targetPieceCountMax = targetPieceCountMin << 1
41 // Choose a good piecelength.
42 func ChoosePieceLength(totalLength int64) (pieceLength int64) {
43 // Must be a power of 2.
44 // Must be a multiple of 16KB
45 // Prefer to provide around 1024..2048 pieces.
46 pieceLength = minimumPieceLength
47 pieces := totalLength / pieceLength
48 for pieces >= targetPieceCountMax {