Initial import of verae-fleet from zapier monorepo

src/rtt.js

@@ -0,0 +1,57 @@
+/**
+ * Ring buffer of message-processing RTTs (ms) for planning percentiles.
+ * @module rtt
+ */
+
+export class RttWindow {
+  /**
+   * @param {number} [cap=400]
+   */
+  constructor(cap = 400) {
+    this.cap = cap;
+    /** @type {number[]} */
+    this.samples = [];
+  }
+
+  add(ms) {
+    const n = Number(ms);
+    if (!Number.isFinite(n) || n < 0) return;
+    this.samples.push(n);
+    if (this.samples.length > this.cap) this.samples.splice(0, this.samples.length - this.cap);
+  }
+
+  stats() {
+    return summarizeRtt(this.samples);
+  }
+}
+
+/**
+ * @param {number[]} samples
+ */
+export function summarizeRtt(samples) {
+  const xs = samples.filter((n) => Number.isFinite(n) && n >= 0).slice().sort((a, b) => a - b);
+  if (!xs.length) {
+    return { count: 0, minMs: null, avgMs: null, p50Ms: null, p90Ms: null };
+  }
+  const sum = xs.reduce((a, b) => a + b, 0);
+  return {
+    count: xs.length,
+    minMs: round1(xs[0]),
+    avgMs: round1(sum / xs.length),
+    p50Ms: round1(percentile(xs, 0.5)),
+    p90Ms: round1(percentile(xs, 0.9)),
+  };
+}
+
+function percentile(sorted, p) {
+  if (sorted.length === 1) return sorted[0];
+  const idx = (sorted.length - 1) * p;
+  const lo = Math.floor(idx);
+  const hi = Math.ceil(idx);
+  if (lo === hi) return sorted[lo];
+  return sorted[lo] + (sorted[hi] - sorted[lo]) * (idx - lo);
+}
+
+function round1(n) {
+  return Math.round(n * 10) / 10;
+}