Function kendallTau

packages/plugin/scripts/embedding-baseline-diff.ts:62–83

Kendall tau-b rank correlation over the intersection of two top-K lists. * Returns null when the intersection is < 2 items (undefined correlation).

(oldIds: number[], newIds: number[])

/** Kendall tau-b rank correlation over the intersection of two top-K lists.
 *  Returns null when the intersection is < 2 items (undefined correlation). */
function kendallTau(oldIds: number[], newIds: number[]): number | null {
    const common = oldIds.filter((id) => newIds.includes(id));
    if (common.length < 2) return null;

    const oldRank = new Map(oldIds.map((id, idx) => [id, idx]));
    const newRank = new Map(newIds.map((id, idx) => [id, idx]));

    let concordant = 0;
    let discordant = 0;
    for (let i = 0; i < common.length; i++) {
        for (let j = i + 1; j < common.length; j++) {
            const a = common[i];
            const b = common[j];
            const oldOrder = (oldRank.get(a) ?? 0) - (oldRank.get(b) ?? 0);
            const newOrder = (newRank.get(a) ?? 0) - (newRank.get(b) ?? 0);
            if (oldOrder * newOrder > 0) concordant++;
            else if (oldOrder * newOrder < 0) discordant++;
        }
    }
    const total = concordant + discordant;
    return total === 0 ? null : (concordant - discordant) / total;
}

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