76 lines
2.3 KiB
Rust
76 lines
2.3 KiB
Rust
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// The piece length picker attempts to pick a reasonable piece length
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// for a torrent given the size of the torrent's contents.
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//
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// Constraints:
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// - Decreasing piece length increases protocol overhead.
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// - Decreasing piece length increases torrent metainfo size.
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// - Increasing piece length increases the amount of data that must be
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// thrown away in case of corruption.
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// - Increasing piece length increases the amount of data that must be
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// downloaded before it can be verified and uploaded to other peers.
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// - Decreasing piece length increases the proportion of disk seeks to
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// disk reads. This can be an issue for spinning disks.
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// - The BitTorrent v2 specification requires that piece sizes be
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// larger than 16 KiB.
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//
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// These constraints could probably be exactly defined and optimized
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// using an integer programming solver, but instead we just copy what
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// libtorrent does.
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use crate::common::*;
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pub(crate) struct PieceLengthPicker;
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impl PieceLengthPicker {
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pub(crate) fn from_content_size(content_size: Bytes) -> Bytes {
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#![allow(
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clippy::as_conversions,
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clippy::cast_sign_loss,
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clippy::cast_precision_loss,
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clippy::cast_possible_truncation
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)]
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let exponent = (content_size.count() as f64).log2().ceil() as u128;
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Bytes::from(1u128 << (exponent / 2 + 4))
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.max(Bytes::kib() * 16)
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.min(Bytes::mib() * 16)
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}
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pub(crate) fn piece_count(content_size: Bytes, piece_length: Bytes) -> u128 {
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if content_size == Bytes::from(0u128) {
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0
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} else {
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(content_size / piece_length).max(1)
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}
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}
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pub(crate) fn metainfo_size(content_size: Bytes, piece_length: Bytes) -> Bytes {
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let digest_length: u128 = sha1::DIGEST_LENGTH.into_u64().into();
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Bytes::from(Self::piece_count(content_size, piece_length) * digest_length)
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}
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn limits() {
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assert_eq!(
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PieceLengthPicker::from_content_size(Bytes::mib() * 2),
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Bytes::kib() * 16
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);
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assert_eq!(
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PieceLengthPicker::from_content_size(Bytes::mib() * 4),
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Bytes::kib() * 32
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);
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assert_eq!(
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PieceLengthPicker::from_content_size(Bytes::mib() * 8),
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Bytes::kib() * 32
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);
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assert_eq!(
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PieceLengthPicker::from_content_size(Bytes::mib() * 16),
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Bytes::kib() * 64
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);
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}
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}
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