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//! Piece/block picker.
//!
//! Consumes the scheduler's per-piece utility vector plus per-piece rarity,
//! and selects (a) which piece to request from a peer and (b) which block
//! of that piece to request next. Supports the **endgame** mode that
//! prevents the classic 99% stall: once few pieces remain, duplicate
//! requests are allowed and cancelled as blocks arrive.
use crate::bitfield::Bitfield;
use crate::piece::PieceTracker;
/// Options for piece picking.
#[derive(Debug, Clone, Copy, Default)]
pub struct PickOptions {
/// Enable endgame (allow re-requesting in-flight pieces).
pub endgame: bool,
}
/// The picker is stateless: everything it needs is passed in.
#[derive(Debug, Clone, Copy)]
pub struct Picker;
impl Picker {
/// Pick the best piece to request from a peer.
///
/// `utilities` comes from the scheduler; `availability` is the per-piece
/// peer count (used for rare-first tie-breaking); `peer_have` is the
/// peer's piece bitfield and `peer_has_all` whether it declared
/// `have_all` (fast extension, BEP-6) — a `have_all` seed owns every
/// piece while its bitfield stays empty, so the picker MUST NOT consult
/// `peer_have` for it or it would refuse to pick anything and stall the
/// download at 0% against seeds. `priorities` carries per-piece
/// multipliers from the file priorities (0 = skipped file — such pieces
/// are never picked). Pieces we already have or (unless endgame) already
/// have in flight are skipped.
pub fn pick_piece(
tracker: &PieceTracker,
utilities: &[i64],
availability: &[u32],
peer_have: &Bitfield,
peer_has_all: bool,
priorities: &[i64],
opts: PickOptions,
) -> Option<u32> {
let n = tracker.piece_count();
let max_avail = availability.iter().copied().max().unwrap_or(0);
let mut best: Option<(i64, i64, u32)> = None; // (score, rarity, piece)
for p in 0..n {
if tracker.is_have(p) {
continue;
}
if !opts.endgame && tracker.is_in_flight(p) {
continue;
}
if !peer_has_all && !peer_have.get(p) {
continue;
}
let prio = priorities.get(p as usize).copied().unwrap_or(1);
if prio <= 0 {
continue; // piece belongs only to skipped files
}
let util = utilities.get(p as usize).copied().unwrap_or(0);
let rarity =
(max_avail as i64) - (availability.get(p as usize).copied().unwrap_or(0) as i64);
let score = util
.saturating_mul(prio)
.saturating_mul(1024)
.saturating_add(rarity);
if let Some((bs, br, _)) = best {
if score > bs || (score == bs && rarity > br) {
best = Some((score, rarity, p));
}
} else {
best = Some((score, rarity, p));
}
}
best.map(|(_, _, p)| p)
}
/// Pick the next block of `piece` that is neither received nor already
/// requested. Returns the block index.
pub fn pick_block(
tracker: &PieceTracker,
piece: u32,
total_blocks: u16,
prefer_contiguous: bool,
) -> Option<u16> {
if tracker.is_have(piece) {
return None;
}
// contiguous: start scanning from the first missing position for
// locality; otherwise from a pseudo-random-ish offset derived from
// the piece index to spread load.
let start = if prefer_contiguous {
0u16
} else {
(piece % 97) as u16 % total_blocks.max(1)
};
for i in 0..total_blocks {
let b = (start + i) % total_blocks;
if !tracker.block_received(piece, b) && !tracker.block_requested(piece, b) {
return Some(b);
}
}
None
}
/// Whether endgame should activate: the number of not-yet-have pieces
/// is below the threshold.
pub fn should_endgame(tracker: &PieceTracker, threshold: u32) -> bool {
tracker.outstanding_pieces() <= threshold
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::piece::PieceTracker;
#[test]
fn picks_utility_winner() {
let t = PieceTracker::new(8, 16 * 1024);
let mut peer = Bitfield::new(8);
peer.set(3);
peer.set(5);
let util = [0i64, 0, 0, 100, 0, 90, 0, 0];
let avail = [0u32; 8];
let prio = [1i64; 8];
let p = Picker::pick_piece(
&t,
&util,
&avail,
&peer,
false,
&prio,
PickOptions::default(),
)
.unwrap();
assert_eq!(p, 3);
}
#[test]
fn respects_have_and_peer() {
let mut t = PieceTracker::new(4, 16 * 1024);
t.mark_piece_have(0);
let mut peer = Bitfield::new(4);
peer.set(0);
peer.set(1);
let util = [0i64; 4];
let avail = [0u32; 4];
let prio = [1i64; 4];
let p = Picker::pick_piece(
&t,
&util,
&avail,
&peer,
false,
&prio,
PickOptions::default(),
)
.unwrap();
assert_eq!(p, 1);
}
#[test]
fn skips_disabled_pieces() {
let t = PieceTracker::new(4, 16 * 1024);
let mut peer = Bitfield::new(4);
peer.set_all();
let util = [0i64; 4];
let avail = [0u32; 4];
// piece 2 belongs to a skipped file
let prio = [1i64, 1, 0, 1];
let p = Picker::pick_piece(
&t,
&util,
&avail,
&peer,
false,
&prio,
PickOptions::default(),
)
.unwrap();
assert_ne!(p, 2);
// all skipped → nothing to pick (even for a have_all seed)
let prio = [0i64; 4];
assert!(Picker::pick_piece(
&t,
&util,
&avail,
&peer,
true,
&prio,
PickOptions::default()
)
.is_none());
}
#[test]
fn high_priority_wins_tie() {
let t = PieceTracker::new(4, 16 * 1024);
let mut peer = Bitfield::new(4);
peer.set_all();
let util = [10i64, 10, 10, 10];
let avail = [0u32; 4];
// high-priority piece 1 beats equal-utility normal pieces
let prio = [1i64, 4, 1, 1];
let p = Picker::pick_piece(
&t,
&util,
&avail,
&peer,
false,
&prio,
PickOptions::default(),
)
.unwrap();
assert_eq!(p, 1);
}
#[test]
fn have_all_seed_is_pickable_with_empty_bitfield() {
// A fast-extension seed declares have_all but keeps its bitfield
// empty. The picker must treat it as owning every piece — otherwise
// no request is ever issued and the download stalls at 0%.
let t = PieceTracker::new(8, 16 * 1024);
let empty = Bitfield::new(8); // have_all: bitfield stays empty
let util = [0i64; 8];
let avail = [1u32; 8];
let prio = [1i64; 8];
let p = Picker::pick_piece(
&t,
&util,
&avail,
&empty,
true,
&prio,
PickOptions::default(),
)
.expect("have_all seed must be pickable");
assert!(p < 8);
// without the have_all flag the same empty bitfield is unpickable
assert!(Picker::pick_piece(
&t,
&util,
&avail,
&empty,
false,
&prio,
PickOptions::default()
)
.is_none());
}
#[test]
fn block_selection_skips_requested() {
let mut t = PieceTracker::new(4, 32 * 1024);
t.mark_block_received(0, 0, 2);
t.mark_block_requested(0, 1, 2);
assert_eq!(Picker::pick_block(&t, 0, 2, true), None);
// free it up
t.clear_block_requested(0, 1, 2);
assert_eq!(Picker::pick_block(&t, 0, 2, true), Some(1));
}
}