test: add unit test for TimeWindow

TimeWindow is used by TrackSpendingService for two purposes:
- tracking cost within a time window
- preventing duplicate alarms within a cooldown period

It is important to test this to ensure alarms related to AI costs are
accurate.

src/backend/src/util/opmath.js

@@ -143,16 +143,17 @@ class MovingMode extends StreamReducer {
 }
 
 class TimeWindow {
-    constructor ({ window_duration, reducer }) {
+    constructor ({ window_duration, reducer, now }) {
         this.window_duration = window_duration;
         this.reducer = reducer;
         this.entries_ = [];
+        this.now = now ?? Date.now;
     }
 
     add (value) {
         this.remove_stale_entries_();
 
-        const timestamp = Date.now();
+        const timestamp = this.now();
         this.entries_.push({
             timestamp,
             value,
@@ -174,7 +175,7 @@ class TimeWindow {
 
     remove_stale_entries_ () {
         let i = 0;
-        const current_ts = Date.now();
+        const current_ts = this.now();
         for ( ; i < this.entries_.length ; i++ ) {
             const entry = this.entries_[i];
             // as soon as an entry is in the window we can break,

src/backend/src/util/opmath.test.js

@@ -0,0 +1,40 @@
+const { expect } = require('chai');
+
+describe('opmath', () => {
+    describe('TimeWindow', () => {
+        it('clears old entries', () => {
+            const { TimeWindow } = require('./opmath');
+            let now_value = 0;
+            const now = () => now_value;
+            const window = new TimeWindow({ window_duration: 1000, now });
+            
+            window.add(1);
+            window.add(2);
+            window.add(3);
+
+            now_value = 900;
+
+            window.add(4);
+            window.add(5);
+            window.add(6);
+
+            expect(window.get()).to.deep.equal([1, 2, 3, 4, 5, 6]);
+
+            now_value = 1100;
+
+            window.add(7);
+            window.add(8);
+            window.add(9);
+
+            expect(window.get()).to.deep.equal([4, 5, 6, 7, 8, 9]);
+
+            now_value = 2000;
+
+            expect(window.get()).to.deep.equal([7, 8, 9]);
+
+            now_value = 2200;
+
+            expect(window.get()).to.deep.equal([]);
+        })
+    })
+});