[Monthly credit accml] fix crash caused by date change mid-computation

uses months to compute keys rather than dates

services/backend/src/services/studyProgramme/studyProgrammeHelpers.ts

@@ -139,17 +139,19 @@ export const getYearlyMonthlyCreditsObj = () => {
   return yearlyMonthlyCredits
 }
 
-const getMonthlyCreditsObj = (academicStartYear?: number) => {
+const getMonthlyCreditsObj = (academicStartYear = 2017) => {
   const today = now()
-  const time = getStartDate(true, academicStartYear)
+
+  /* Compare whole months instead of dates, so the result does not depend on the time of day.
+    Academic year starts in August (month index 7). */
+  const firstMonth = academicStartYear * 12 + 7
+  const lastMonth = today.getFullYear() * 12 + today.getMonth()
 
   const monthlyCredits: Record<string, number[]> = {}
-  today.setDate(1)
 
   /* NB: JS months start at 0. We need them to start at 1. */
-  while (time < today) {
-    monthlyCredits[`${time.getFullYear()}-${time.getMonth() + 1}`] = []
-    time.setMonth(time.getMonth() + 1)
+  for (let month = firstMonth; month <= lastMonth; month++) {
+    monthlyCredits[`${Math.floor(month / 12)}-${(month % 12) + 1}`] = []
   }
 
   return monthlyCredits

services/backend/tests/unit/services/studyProgramme/studyProgrammeHelpers.test.ts

@@ -1,5 +1,12 @@
 import { describe, it, assert, vi, beforeAll } from 'vitest'
-import { getPercentage, getYearsArray, getYearsObject } from '@/services/studyProgramme/studyProgrammeHelpers'
+import {
+  getMonthlyCredits,
+  getPercentage,
+  getYearlyMonthlyCreditsObj,
+  getYearsArray,
+  getYearsObject,
+} from '@/services/studyProgramme/studyProgrammeHelpers'
+import { clockStorage } from '@/util/clock'
 
 void describe('Get years object', () => {
   it('should return nothing with empty years', () => {
@@ -95,3 +102,36 @@ void describe('Get percentage', () => {
     assert.strictEqual(getPercentage(-5, 10), '-50.0 %')
   })
 })
+
+void describe('Get monthly credits', () => {
+  const atTime = <T>(date: string, fn: () => T) => clockStorage.run({ now: new Date(date) }, fn)
+
+  it('should include the current month regardless of the time of day', () => {
+    const lastKey = (date: string) => Object.keys(atTime(date, () => getYearlyMonthlyCreditsObj())[2024]).at(-1)
+
+    assert.strictEqual(lastKey('2026-10-07T00:05:00'), '2026-10')
+    assert.strictEqual(lastKey('2026-10-07T23:40:00'), '2026-10')
+  })
+
+  it('should not throw when computation continues past midnight', () => {
+    const monthlyCreditsByStartingYear = atTime('2026-10-06T23:40:00', () => getYearlyMonthlyCreditsObj())
+
+    assert.doesNotThrow(() =>
+      atTime('2026-10-07T00:05:00', () =>
+        getMonthlyCredits([], new Date('2024-08-01'), 2024, monthlyCreditsByStartingYear[2024])
+      )
+    )
+  })
+
+  it('should accumulate credits cumulatively by month', () => {
+    const monthlyCredits = atTime('2024-10-15T12:00:00', () => getYearlyMonthlyCreditsObj())[2024]
+    const credits = [
+      { attainment_date: new Date('2024-08-20'), credits: 5 },
+      { attainment_date: new Date('2024-10-01'), credits: 3 },
+    ] as Parameters<typeof getMonthlyCredits>[0]
+
+    atTime('2024-10-15T12:00:00', () => getMonthlyCredits(credits, new Date('2024-08-01'), 2024, monthlyCredits))
+
+    assert.deepStrictEqual(monthlyCredits, { '2024-8': [5], '2024-9': [5], '2024-10': [8] })
+  })
+})