/**
 * ADR-121 Phase 17 — Hybrid sparse+dense retrieval.
 *
 * Composes BM25 (sparse lexical) + dense vector search via RRF
 * (Phase 11) into a single fused ranking. The standard production
 * pattern for catching documents with rare/technical terms that
 * dense embeddings underweight while preserving the semantic
 * matching that BM25 misses.
 *
 * The two signals are not score-comparable directly (BM25 is
 * unbounded positive, cosine ∈ [-1, 1]) — RRF operates on ranks,
 * which makes it the right fusion primitive here.
 *
 * Pipeline:
 *   1. denseHits = denseTopK(corpus, queryVec, fetchK)
 *   2. sparseHits = bm25TopK(queryText, bm25Index, fetchK)
 *   3. fused = RRF([denseHits, sparseHits], { k, kRrf, listWeights })
 *
 * Caller provides:
 *   - the dense search function (any backing: AnnRouter, in-memory,
 *     DiskANN snapshot)
 *   - the BM25 index (built once per corpus via buildBm25Index)
 *
 * Composable with all earlier primitives: HyDE produces the query
 * vector, MMR-rerank the fused output, etc.
 */
import { type RrfFusedHit } from './rrf.js';
import { type Bm25Index } from './bm25.js';
export interface HybridRetrievalOptions {
    /** Final number of fused results to return. */
    readonly k: number;
    /**
     * Top-N to pull from each sub-retriever (BM25 + dense) before RRF.
     * Default 3*k. Larger = wider input pool, more compute.
     */
    readonly fetchK?: number;
    /** RRF smoothing constant (Cormack-Clarke-Büttcher default 60). */
    readonly kRrf?: number;
    /**
     * Per-list weights for RRF, in [dense_weight, sparse_weight] order.
     * Default [1, 1]. Use to bias toward one signal — e.g. [1.5, 1.0]
     * to prefer dense semantic matches in general while letting BM25
     * still contribute on rare-term docs.
     */
    readonly listWeights?: readonly [number, number];
}
export interface HybridRetrievalDiagnostics {
    readonly denseHitCount: number;
    readonly sparseHitCount: number;
    /** IDs that appeared in BOTH lists (overlap signal). */
    readonly overlapIds: ReadonlyArray<string>;
    /** Mean BM25 score across the sparse hits, for cost attribution. */
    readonly meanSparseScore: number;
}
export interface HybridRetrievalResult {
    readonly hits: ReadonlyArray<RrfFusedHit>;
    readonly diagnostics: HybridRetrievalDiagnostics;
}
/**
 * Run hybrid sparse+dense retrieval. Returns RRF-fused hits plus
 * diagnostics for observability.
 *
 * The dense search is async (matches real backings); the BM25 search
 * is sync (pure in-memory). Caller passes the dense search function
 * so this module stays decoupled from any specific ANN backing.
 */
export declare function hybridRetrieval(queryText: string, queryVector: Float32Array | number[], bm25Index: Bm25Index, denseSearch: (queryVec: Float32Array | number[], k: number) => Promise<ReadonlyArray<{
    id: string;
    score?: number;
}>> | ReadonlyArray<{
    id: string;
    score?: number;
}>, options: HybridRetrievalOptions): Promise<HybridRetrievalResult>;
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