feat(api): add concept temperature bands and expose in stats

apps/api/src/srs.test.ts

@@ -19,6 +19,8 @@ import {
   isoWeek,
   rng,
   sample,
+  TEMPERATURES,
+  temperatureOf,
   weekdayOf,
   workingDay,
 } from "./srs.ts";
@@ -132,6 +134,31 @@ describe("the ladder", () => {
   });
 });
 
+describe("concept temperature", () => {
+  test("the bands are the ladder's own windows", () => {
+    // Boundaries are inclusive, and the bands tile the whole age axis with no
+    // gap: whatever the windows become, every age still lands in exactly one.
+    expect(TEMPERATURES).toEqual(["hot", "fresh", "fading", "cold"]);
+    expect(temperatureOf(0)).toBe("hot");
+    expect(temperatureOf(WINDOWS[0])).toBe("hot");
+    expect(temperatureOf(WINDOWS[0] + 1)).toBe("fresh");
+    expect(temperatureOf(WINDOWS[1])).toBe("fresh");
+    expect(temperatureOf(WINDOWS[1] + 1)).toBe("fading");
+    expect(temperatureOf(WINDOWS[1] * 2)).toBe("fading");
+    expect(temperatureOf(WINDOWS[1] * 2 + 1)).toBe("cold");
+  });
+
+  test("never attempted is cold, not unknown", () => {
+    expect(temperatureOf(null)).toBe("cold");
+  });
+
+  test("every age of the campaign has exactly one band", () => {
+    for (let age = 0; age <= CALENDAR.length; age++) {
+      expect(TEMPERATURES).toContain(temperatureOf(age));
+    }
+  });
+});
+
 describe("deterministic sampling", () => {
   test("same seed, same picks", () => {
     const pool = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];