feat(ultraplan-local): add lib/parsers/jaccard.mjs

plugins/ultraplan-local/lib/parsers/jaccard.mjs

@@ -0,0 +1,41 @@
+// lib/parsers/jaccard.mjs
+// Jaccard similarity for SC4 determinism floor.
+//
+// jaccard(A, B) = |A ∩ B| / |A ∪ B|
+// Inputs are arrays of strings; deduplicated internally.
+// Both empty → 1.0 (vacuously identical). One empty → 0.0.
+
+/**
+ * Compute Jaccard similarity between two string sets.
+ * @param {string[]} setA
+ * @param {string[]} setB
+ * @returns {number} similarity in [0, 1]
+ */
+export function jaccardSimilarity(setA, setB) {
+  if (!Array.isArray(setA) || !Array.isArray(setB)) {
+    throw new TypeError('jaccardSimilarity: both inputs must be arrays');
+  }
+  const a = new Set(setA);
+  const b = new Set(setB);
+  if (a.size === 0 && b.size === 0) return 1.0;
+  if (a.size === 0 || b.size === 0) return 0.0;
+
+  let intersection = 0;
+  for (const x of a) {
+    if (b.has(x)) intersection += 1;
+  }
+  const union = a.size + b.size - intersection;
+  return intersection / union;
+}
+
+/**
+ * Check whether a similarity meets a threshold.
+ * @param {number} similarity
+ * @param {number} threshold
+ * @returns {boolean}
+ */
+export function meetsThreshold(similarity, threshold) {
+  if (typeof similarity !== 'number' || typeof threshold !== 'number') return false;
+  if (!Number.isFinite(similarity) || !Number.isFinite(threshold)) return false;
+  return similarity >= threshold;
+}