feat: initial open marketplace with llm-security, config-audit, ultraplan-local

plugins/llm-security/scanners/lib/distribution-stats.mjs

@@ -0,0 +1,58 @@
+// distribution-stats.mjs — Statistical divergence utilities for behavioral drift detection.
+// Zero external dependencies. <50 lines.
+//
+// Jensen-Shannon divergence measures how different two probability distributions are.
+// Used by post-session-guard.mjs to detect tool distribution shifts within a session.
+//
+// OWASP: ASI01 (Excessive Agency — behavioral pattern changes may indicate hijacking)
+
+/**
+ * Kullback-Leibler divergence KL(P || Q).
+ * @param {Map<string, number>} P
+ * @param {Map<string, number>} Q
+ * @returns {number}
+ */
+function klDivergence(P, Q) {
+  let kl = 0;
+  for (const [key, p] of P) {
+    if (p === 0) continue;
+    const q = Q.get(key) || 0;
+    if (q === 0) return Infinity;
+    kl += p * Math.log2(p / q);
+  }
+  return kl;
+}
+
+/**
+ * Jensen-Shannon divergence. 0 = identical, 1 = fully disjoint (log2 basis).
+ * Always finite, symmetric: JSD(P,Q) = JSD(Q,P).
+ * @param {Map<string, number>} P - Normalized probability distribution
+ * @param {Map<string, number>} Q - Normalized probability distribution
+ * @returns {number}
+ */
+export function jensenShannonDivergence(P, Q) {
+  const allKeys = new Set([...P.keys(), ...Q.keys()]);
+  const M = new Map();
+  for (const key of allKeys) {
+    M.set(key, 0.5 * (P.get(key) || 0) + 0.5 * (Q.get(key) || 0));
+  }
+  return 0.5 * klDivergence(P, M) + 0.5 * klDivergence(Q, M);
+}
+
+/**
+ * Build normalized probability distribution from category labels.
+ * @param {string[]} labels
+ * @returns {Map<string, number>} Values sum to 1.0 (empty input → empty map)
+ */
+export function buildDistribution(labels) {
+  if (labels.length === 0) return new Map();
+  const counts = new Map();
+  for (const label of labels) {
+    counts.set(label, (counts.get(label) || 0) + 1);
+  }
+  const dist = new Map();
+  for (const [key, count] of counts) {
+    dist.set(key, count / labels.length);
+  }
+  return dist;
+}