Add Rate.msSinceLastEvent

src/Rate.spec.ts

@@ -36,20 +36,24 @@ describe("Rate", () => {
     expectRate(r, 0)
     tk.freeze(now + minuteMs)
     expectRate(r, 0)
+    expect(r.msSinceLastEvent).to.eql(minuteMs)
   })
 
   it("decays the rate as time elapses", () => {
     expectRate(r, 0)
     r.onEvent()
     expectRate(r, 0)
     tk.freeze(now + 10)
+    expect(r.msSinceLastEvent).to.eql(10)
     r.onEvent()
     expectRate(r, 1 / 10)
     tk.freeze(now + secondMs)
     expectRate(r, 2 / secondMs)
-    tk.freeze(now + 2 * secondMs)
+    tk.freeze(now + 2 * secondMs + 10)
+    expect(r.msSinceLastEvent).to.eql(2 * secondMs)
     expectRate(r, 1 / secondMs)
-    tk.freeze(now + minuteMs)
+    tk.freeze(now + minuteMs + 10)
+    expect(r.msSinceLastEvent).to.eql(minuteMs)
     expectRate(r, 2 / minuteMs)
   })
 

src/Rate.ts

@@ -24,6 +24,10 @@ export class Rate {
     return this.#eventCount
   }
 
+  get msSinceLastEvent(): number | null {
+    return this.#lastEventTs == null ? null : Date.now() - this.#lastEventTs
+  }
+
   get msPerEvent(): number | null {
     if (this.#weightedTotalAvg == null || this.#lastEventTs == null) return null
     // If we haven't seen an event for a while, include that in the estimate: