/**
 * hybrid.ts — Lexical scoring and score-fusion for logosdb hybrid search (#85).
 *
 * Two fusion strategies:
 *   "rrf"      — Reciprocal Rank Fusion  (rank-based, distribution-free)
 *   "weighted" — Linear interpolation    combined = (1-w)*ann + w*lexical
 *
 * Lexical scoring uses a BM25-lite implementation (BM25 without IDF, i.e. purely
 * based on term frequency within each document relative to average document length).
 * This runs entirely in the MCP process over the ANN result set — no core changes.
 */
import type { SearchHit } from 'logosdb';
export type FusionStrategy = 'rrf' | 'weighted';
export interface HybridOptions {
    /** Fusion strategy (default: "rrf") */
    fusion?: FusionStrategy;
    /**
     * Weight of the lexical score in "weighted" mode (0 = pure ANN, 1 = pure lexical).
     * Default: 0.5
     */
    lexical_weight?: number;
    /**
     * RRF rank constant k (default: 60).
     * Score = 1/(k + rank). Higher k dampens rank differences.
     */
    rrf_k?: number;
}
export interface HybridHit extends SearchHit {
    ann_score: number;
    lexical_score: number;
    hybrid_score: number;
}
/**
 * Re-rank `hits` (already ordered by ANN score desc) using hybrid scoring.
 * Returns a new array sorted by hybrid_score desc, trimmed to `topK`.
 */
export declare function hybridRerank(hits: SearchHit[], query: string, topK: number, opts?: HybridOptions): HybridHit[];
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