perf(agent): unranked BM25 scores and a top-k merge — same rankings, 4-5x faster
Fusion min-max normalises over every keyword match, so hybrid_search asked BM25 for a ranked list of the whole corpus: a hash insert per posting, then a sort of every match, then the merge sorted every candidate again to keep k. - BM25Index::scores returns every match unsorted, accumulated in a dense array (contributions are strictly positive, so zero means untouched). search() is built on it with the bounded heap. - merge_vector_keyword partitions out its top k (select_nth) and orders only those, with the same score-then-id order. - Both hybrid paths use scores(). Rankings are identical (equivalence tests for both changes). p50 0.24 -> 0.07 ms (1K), 2.1 -> 0.49 ms (10K), 23 -> 4.65 ms (100K). The harness gains --fusion-study, which measured the alternative — capping the keyword pool — and found it changes the top-10 for most queries (overlap 0.83-0.92, different #1 for 10-35%) for only a 2x saving. Not adopted. Co-Authored-By: Claude Fable 5.1 <[email protected]>
This commit is contained in:
osobh
co-authored by
Claude Fable 5.1
Claude Fable 5.1
parent
f15bf2eb22
commit
390a2e3836
@@ -83,35 +83,15 @@ impl BM25Index {
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/// Uses Block-Max WAND for early termination when remaining documents
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/// cannot beat the current top-k threshold.
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pub fn search(&self, query: &str, k: usize) -> Vec<(usize, f32)> {
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if self.num_docs == 0 || k == 0 {
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if k == 0 {
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return Vec::new();
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}
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// Term-at-a-time accumulation. IDF is computed here rather than cached
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// at build time: it depends on the live document count, which changes
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// with every incremental add/remove, and costs one `ln` per query term.
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let mut scores: HashMap<usize, f32> = HashMap::new();
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for token in tokenize(query) {
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let Some(postings) = self.inverted.get(token.as_str()) else {
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continue;
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};
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let df = postings.len() as f32;
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let idf = ((self.num_docs as f32 - df + 0.5) / (df + 0.5) + 1.0).ln();
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for &(doc_id, freq) in postings {
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let dl = self.doc_lengths[doc_id] as f32;
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let freq_f = freq as f32;
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let tf = (freq_f * (self.k1 + 1.0))
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/ (freq_f + self.k1 * (1.0 - self.b + self.b * dl / self.avg_dl));
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*scores.entry(doc_id).or_insert(0.0) += idf * tf;
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}
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}
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// Top-k with a bounded min-heap: O(matches * log k) instead of sorting
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// every match. Ties break towards the lower doc id so results are
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// deterministic (the accumulator is a HashMap).
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// deterministic.
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let mut heap: BinaryHeap<Reverse<(HeapScore, Reverse<usize>)>> =
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BinaryHeap::with_capacity(k + 1);
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for (doc_id, score) in scores {
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BinaryHeap::with_capacity(k.min(1024) + 1);
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for (doc_id, score) in self.scores(query) {
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heap.push(Reverse((HeapScore(score), Reverse(doc_id))));
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if heap.len() > k {
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heap.pop();
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@@ -125,6 +105,47 @@ impl BM25Index {
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results
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}
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/// The BM25 score of **every** matching document, in doc-id order, unsorted
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/// by score. Score fusion normalises over the whole matching set, so it
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/// needs all of these but not their ranking; producing a ranked list of
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/// every match (`search(query, corpus_len)`) spent most of its time sorting.
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pub fn scores(&self, query: &str) -> Vec<(usize, f32)> {
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if self.num_docs == 0 {
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return Vec::new();
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}
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// Term-at-a-time accumulation into a dense array: a common term has a
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// posting per document, and hashing each one dominated query time.
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// IDF is computed here rather than cached at build time: it depends on
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// the live document count, which changes with every incremental
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// add/remove, and costs one `ln` per query term.
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let mut acc = vec![0.0f32; self.doc_lengths.len()];
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let mut matched = false;
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for token in tokenize(query) {
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let Some(postings) = self.inverted.get(token.as_str()) else {
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continue;
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};
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matched = true;
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let df = postings.len() as f32;
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let idf = ((self.num_docs as f32 - df + 0.5) / (df + 0.5) + 1.0).ln();
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for &(doc_id, freq) in postings {
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let dl = self.doc_lengths[doc_id] as f32;
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let freq_f = freq as f32;
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let tf = (freq_f * (self.k1 + 1.0))
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/ (freq_f + self.k1 * (1.0 - self.b + self.b * dl / self.avg_dl));
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acc[doc_id] += idf * tf;
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}
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}
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if !matched {
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return Vec::new();
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}
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// Every contribution is strictly positive (idf = ln(1 + x), x > 0), so
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// a zero entry is a document no query term touched.
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acc.into_iter()
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.enumerate()
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.filter(|&(_, score)| score > 0.0)
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.collect()
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}
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/// Number of document slots (live or not) the index covers. Ids are
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/// positions in the document list it mirrors.
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pub fn len(&self) -> usize {
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@@ -564,6 +585,20 @@ mod tests {
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}
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}
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#[test]
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fn scores_is_the_unranked_form_of_a_full_search() {
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let mut state = 99u64;
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let docs: Vec<String> = (0..200).map(|_| random_doc(&mut state)).collect();
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let tombstones: Vec<u8> = (0..200).map(|i| u8::from(i % 7 == 0)).collect();
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let index = BM25Index::build(&docs, &tombstones);
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for query in ["alpha", "beta gamma x1", "missing", ""] {
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let mut all = index.scores(query);
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all.sort_by(|a, b| b.1.total_cmp(&a.1).then(a.0.cmp(&b.0)));
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assert_eq!(all, index.search(query, docs.len()), "{query:?}");
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assert!(all.iter().all(|(id, _)| tombstones[*id] == 0));
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}
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}
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#[test]
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fn ties_break_towards_the_lower_doc_id() {
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let docs: Vec<String> = (0..6).map(|_| "same text".to_string()).collect();
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@@ -58,7 +58,7 @@ pub fn hybrid_search(
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vector_search::cosine_similarity_batch(query_embedding, vectors, tombstones)
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}
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};
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let kw_scores = bm25_index.search(query_text, vectors.len());
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let kw_scores = bm25_index.scores(query_text);
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merge_vector_keyword(vec_scores, kw_scores, vector_weight, keyword_weight, k)
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}
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@@ -92,13 +92,22 @@ pub fn merge_vector_keyword(
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let mut results: Vec<(usize, f32)> = merged.into_iter().collect();
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// Index tie-break: `merged` is a HashMap, so without it the ties that
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// survive `truncate` differ from run to run.
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results.sort_by(|a, b| {
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// survive differ from run to run.
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let by_score_then_id = |a: &(usize, f32), b: &(usize, f32)| {
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b.1.partial_cmp(&a.1)
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.unwrap_or(std::cmp::Ordering::Equal)
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.then(a.0.cmp(&b.0))
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});
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results.truncate(k);
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};
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// Only the top k are wanted: partition them out, then order just those,
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// instead of sorting every candidate (the keyword side can be the corpus).
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if k == 0 {
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return Vec::new();
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}
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if results.len() > k {
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results.select_nth_unstable_by(k - 1, by_score_then_id);
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results.truncate(k);
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}
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results.sort_by(by_score_then_id);
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results
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}
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@@ -344,6 +353,25 @@ mod tests {
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assert_eq!(result[0].1, 1.0);
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}
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#[test]
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fn merge_top_k_matches_a_full_sort() {
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// Many ties (scores repeat) so the index tie-break is exercised.
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let vec_scores: Vec<(usize, f32)> = (0..300).map(|i| (i, ((i * 7) % 13) as f32)).collect();
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let kw_scores: Vec<(usize, f32)> = (100..500).map(|i| (i, ((i * 5) % 11) as f32)).collect();
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let everything =
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merge_vector_keyword(vec_scores.clone(), kw_scores.clone(), 0.7, 0.3, 10_000);
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assert_eq!(everything.len(), 500);
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assert!(
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everything
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.windows(2)
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.all(|w| { w[0].1 > w[1].1 || (w[0].1 == w[1].1 && w[0].0 < w[1].0) })
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);
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for k in [0, 1, 7, 50, 499, 500, 501] {
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let top = merge_vector_keyword(vec_scores.clone(), kw_scores.clone(), 0.7, 0.3, k);
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assert_eq!(top, everything[..k.min(500)], "k = {k}");
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}
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}
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#[test]
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fn normalize_scores_all_equal() {
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let matched = normalize_scores(&[(0, 0.4), (1, 0.4)]);
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@@ -35,7 +35,9 @@ impl HDF5Memory {
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.into_iter()
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.map(|(id, dist)| (id, 1.0 - dist))
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.collect();
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let kw_scores = bm25.search(query_text, self.cache.len());
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// Fusion normalises over every keyword match, so it needs all
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// the scores — but not ranked.
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let kw_scores = bm25.scores(query_text);
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hybrid::merge_vector_keyword(
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vec_scores,
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kw_scores,
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@@ -369,10 +369,101 @@ fn bench_end_to_end(n: usize, json: &mut Vec<serde_json::Value>) {
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}));
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}
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// ---------------------------------------------------------------------------
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// Fusion study: does capping the keyword candidate pool change the ranking?
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// ---------------------------------------------------------------------------
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/// `hybrid_search` min-max normalises each signal over the candidates it is
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/// given. The vector stage supplies a pool of `max(8k, 64)`; the keyword stage
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/// supplies *every* matching record, which is what now dominates query time.
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/// This compares the current fusion with one whose keyword stage is capped to
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/// a pool, reporting how often the final top-k agree and what each costs.
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fn fusion_study(n: usize) {
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use clawhdf5_agent::bm25::BM25Index;
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use clawhdf5_agent::hybrid::merge_vector_keyword;
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let data = make_dataset(n, 0xE2E ^ n as u64);
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let mut rng = Rng(7);
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let texts: Vec<String> = (0..n)
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.map(|i| text_for(data.cluster_of[i], i, &mut rng))
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.collect();
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let query_texts: Vec<String> = data
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.query_cluster
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.iter()
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.enumerate()
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.map(|(i, c)| text_for(*c, i, &mut rng))
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.collect();
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let bm25 = BM25Index::build(&texts, &vec![0u8; n]);
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let index = HnswIndex::build_with_metric(
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&data.vectors,
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HNSW_M,
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HNSW_EF_CONSTRUCTION,
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DistanceMetric::Cosine,
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);
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let vec_pool = (K * 8).max(64);
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println!("\n### Fusion study, N = {n} (k = {K}, weights 0.7 / 0.3, vector pool {vec_pool})\n");
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println!(
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"| keyword pool | top-{K} overlap vs full | identical top-{K} | same #1 | keyword+merge µs |"
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);
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println!("|---:|---:|---:|---:|---:|");
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let fuse = |q: usize, kw_pool: usize| -> (Vec<usize>, Duration) {
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let vec_scores: Vec<(usize, f32)> = index
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.search(&data.queries[q], vec_pool, vec_pool)
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.into_iter()
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.map(|(id, d)| (id, 1.0 - d))
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.collect();
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let t = Instant::now();
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let kw = bm25.search(&query_texts[q], kw_pool);
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let merged = merge_vector_keyword(vec_scores, kw, 0.7, 0.3, K);
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let took = t.elapsed();
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(merged.into_iter().map(|(id, _)| id).collect(), took)
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};
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let full: Vec<(Vec<usize>, Duration)> = (0..N_QUERIES).map(|q| fuse(q, n)).collect();
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let full_time: Duration = full.iter().map(|f| f.1).sum();
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println!(
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"| all ({n}) | 1.0000 | 100.0% | 100.0% | {:.0} |",
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micros(full_time) / N_QUERIES as f64
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);
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for pool in [vec_pool, vec_pool * 4, 1000] {
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if pool >= n {
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continue;
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}
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let (mut overlap, mut identical, mut same_first) = (0usize, 0usize, 0usize);
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let mut time = Duration::ZERO;
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for (q, (want, _)) in full.iter().enumerate() {
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let (got, took) = fuse(q, pool);
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time += took;
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overlap += got.iter().filter(|id| want.contains(id)).count();
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identical += usize::from(&got == want);
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same_first += usize::from(got.first() == want.first());
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}
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println!(
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"| {pool} | {:.4} | {:.1}% | {:.1}% | {:.0} |",
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overlap as f64 / (K * N_QUERIES) as f64,
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100.0 * identical as f64 / N_QUERIES as f64,
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100.0 * same_first as f64 / N_QUERIES as f64,
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micros(time) / N_QUERIES as f64
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);
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}
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}
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fn main() {
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let args: Vec<String> = std::env::args().skip(1).collect();
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let full = args.iter().any(|a| a == "--full");
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let ann_only = args.iter().any(|a| a == "--ann-only");
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if args.iter().any(|a| a == "--fusion-study") {
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for &n in if full {
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&[10_000, 100_000][..]
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} else {
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&[10_000][..]
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} {
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fusion_study(n);
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}
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return;
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}
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if args.iter().any(|a| a == "--uniform") {
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UNIFORM.store(true, std::sync::atomic::Ordering::Relaxed);
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println!("(uniform random data)");
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