velesdb-memory 0.9.2

//! Priority packing under a token budget.
//!
//! Invariant: the sum of per-piece token estimates of everything packed,
//! plus one joiner token per packed fragment, never exceeds the usable
//! budget. For a superadditive estimator (the default char-ratio one rounds
//! every piece up) this bounds the estimate of the assembled text, so the
//! output provably fits.
//!
//! Selection is a deterministic total order — critical first, then caller
//! priority, then relevance, then input order — with `seq` as the final
//! tie-break so equal fragments can never swap places between runs.

use super::estimator::TokenEstimator;

/// The separator emitted between packed fragments — the single source both
/// the packing accountant and the assembly joins use, so the accounted cost
/// and the emitted bytes can never drift apart.
pub(crate) const JOINER: &str = "\n\n";

/// One emission piece: its text, and — for a piece whose token cost is
/// precomputed outside the injected estimator (a media fragment's single
/// atomic piece, US-009 PR1) — that fixed cost. `None` (every non-media
/// piece) means "measure `text` with the injected estimator", the unchanged
/// pre-media behavior; `Some` must never be re-derived from `text` (for
/// media, `text` is only the caption, which on its own would grossly
/// under-price the piece).
#[derive(Debug, Clone)]
pub(crate) struct Piece {
    pub text: String,
    pub cost: Option<u64>,
}

/// One packable fragment: its emission pieces plus its selection keys.
#[derive(Debug)]
pub(crate) struct PackItem {
    /// Input position — the last tie-break, and the emission order.
    pub seq: usize,
    /// Critical fragments pack before everything else.
    pub critical: bool,
    /// Caller priority (higher first).
    pub priority: u8,
    /// Lexical relevance to the query (higher first).
    pub relevance: f32,
    /// The fragment's text, pre-cut into orderly pieces (chunks); packing
    /// takes a prefix of them.
    pub pieces: Vec<Piece>,
}

/// How many leading pieces of each item fit under `usable` tokens. The
/// result is aligned with `items` (input order), not with selection order.
pub(crate) fn pack(items: &[PackItem], usable: u64, estimator: &dyn TokenEstimator) -> Vec<usize> {
    let mut taken = vec![0_usize; items.len()];
    let mut remaining = usable;
    // Priced by the *injected* estimator, not a constant: a tokenizer that
    // prices "\n\n" higher than the default would otherwise overflow the
    // budget by one under-counted joiner per fragment.
    let joiner_tokens = estimator.estimate(JOINER);
    for &index in &selection_order(items) {
        let item = &items[index];
        taken[index] = take_pieces(&item.pieces, &mut remaining, joiner_tokens, estimator);
    }
    taken
}

/// Item indices in packing order: critical desc, priority desc, relevance
/// desc, seq asc.
fn selection_order(items: &[PackItem]) -> Vec<usize> {
    let mut order: Vec<usize> = (0..items.len()).collect();
    order.sort_by(|&a, &b| {
        let (left, right) = (&items[a], &items[b]);
        right
            .critical
            .cmp(&left.critical)
            .then_with(|| right.priority.cmp(&left.priority))
            .then_with(|| right.relevance.total_cmp(&left.relevance))
            .then_with(|| left.seq.cmp(&right.seq))
    });
    order
}

/// Greedily take leading pieces while they fit; the first piece also pays
/// the fragment's joiner cost. A piece's own cost is its precomputed
/// [`Piece::cost`] when set, otherwise the injected estimator over its text.
fn take_pieces(
    pieces: &[Piece],
    remaining: &mut u64,
    joiner_tokens: u64,
    estimator: &dyn TokenEstimator,
) -> usize {
    let mut count = 0_usize;
    for piece in pieces {
        let joiner = if count == 0 { joiner_tokens } else { 0 };
        let base = piece
            .cost
            .unwrap_or_else(|| estimator.estimate(&piece.text));
        let cost = base.saturating_add(joiner);
        if cost > *remaining {
            break;
        }
        *remaining -= cost;
        count += 1;
    }
    count
}

#[cfg(test)]
#[path = "budget_tests.rs"]
mod tests;