/**
 * Trigram + Jaccard similarity utilities for MI knowledge ranking.
 * Pure functions, zero dependencies.
 */

/** Extract character trigrams from text (lowercased). */
export function trigrams(text: string): Set<string> {
	const s = text.toLowerCase().replace(/[^\w\s]/g, "");
	const set = new Set<string>();
	for (let i = 0; i <= s.length - 3; i++) {
		set.add(s.slice(i, i + 3));
	}
	return set;
}

/** Jaccard similarity between two trigram sets (0..1). */
export function jaccardSimilarity(a: Set<string>, b: Set<string>): number {
	if (a.size === 0 && b.size === 0) return 0;
	let intersection = 0;
	for (const t of a) {
		if (b.has(t)) intersection++;
	}
	const union = a.size + b.size - intersection;
	return union === 0 ? 0 : intersection / union;
}