import { describe, expect, it } from "vitest";

import { RRF_K, fuseHybrid, reciprocalRankFusion } from "./rrf.js";

describe("reciprocalRankFusion", () => {
  it("uses the standard constant k=60", () => {
    expect(RRF_K).toBe(60);
  });

  it("scores a doc in both lists higher than a doc in one", () => {
    const fused = reciprocalRankFusion([
      { name: "bm25", docIds: ["a", "b"] },
      { name: "vec", docIds: ["a", "c"] },
    ]);
    // a: 1/61 + 1/61 ; b: bm25 rank 2 → 1/62 ; c: vec rank 2 → 1/62 (tie → docId)
    expect(fused[0]!.docId).toBe("a");
    expect(fused[0]!.score).toBeCloseTo(2 / 61, 12);
    expect(fused[0]!.sources).toEqual(["bm25", "vec"]);
    expect(fused[1]!.score).toBeCloseTo(1 / 62, 12);
    expect(fused[2]!.score).toBeCloseTo(1 / 62, 12);
  });

  it("rank matters: 2nd place contributes 1/(k+2)", () => {
    const fused = reciprocalRankFusion([{ name: "x", docIds: ["a", "b", "c"] }]);
    expect(fused[0]!.docId).toBe("a");
    expect(fused[0]!.score).toBeCloseTo(1 / 61, 12);
    expect(fused[1]!.score).toBeCloseTo(1 / 62, 12);
    expect(fused[2]!.score).toBeCloseTo(1 / 63, 12);
  });

  it("a doc ranked 1 in one list beats a doc ranked 1+1 in both only when the sum is higher", () => {
    // x: 1/61 + 1/62 ≈ 0.03252 ; y: 1/61
    const fused = reciprocalRankFusion([
      { name: "l1", docIds: ["x", "y"] },
      { name: "l2", docIds: ["x"] },
    ]);
    expect(fused[0]!.docId).toBe("x");
    expect(fused[1]!.docId).toBe("y");
  });

  it("breaks score ties deterministically by docId", () => {
    const fused = reciprocalRankFusion([
      { name: "a", docIds: ["z"] },
      { name: "b", docIds: ["m"] },
    ]);
    expect(fused.map((f) => f.docId)).toEqual(["m", "z"]);
  });

  it("respects limit", () => {
    const fused = reciprocalRankFusion(
      [{ name: "x", docIds: ["a", "b", "c", "d"] }],
      60,
      2,
    );
    expect(fused).toHaveLength(2);
  });

  it("handles empty lists", () => {
    expect(reciprocalRankFusion([{ name: "x", docIds: [] }])).toEqual([]);
  });

  it("throws on non-positive k", () => {
    expect(() => reciprocalRankFusion([], 0)).toThrow();
  });

  it("fuseHybrid names the sources bm25/vec", () => {
    const fused = fuseHybrid(["a", "b"], ["b", "c"]);
    const b = fused.find((f) => f.docId === "b");
    expect(b?.sources).toEqual(["bm25", "vec"]);
    expect(fused[0]!.docId).toBe("b");
  });
});