The classification filter now offers only the loaded opportunities' classifications

It lists the classifications the opportunities in the days window have,
not every one stored, plus any already chosen, so a selection stays
visible and can be removed when nothing in the window has it. The other
filters don't narrow it, so choosing a status doesn't make
classifications vanish.

web/src/lib/classifications.test.ts

@@ -1,7 +1,8 @@
 import { expect, test } from 'vitest';
 
 import type { ClassificationCode } from '@/types/Opportunities';
-import { applyClassificationChanges } from './classifications';
+import { applyClassificationChanges, classificationsInUse } from './classifications';
+import { testOpportunity } from './testOpportunity';
 
 const code = (id: number, value: string): ClassificationCode => ({
   id,
@@ -27,3 +28,14 @@ test('an added code replaces one with the same id', () => {
   });
   expect(after).toEqual([code(1, '03000001')]);
 });
+
+test('classificationsInUse keeps those the opportunities have, and the selected ones', () => {
+  const all = [code(1, '03000000'), code(2, '45000000'), code(3, '72000000'), code(4, '90000000')];
+  const opportunities = [
+    testOpportunity('a', { classifications: [code(3, '72000000')] }),
+    testOpportunity('b', { classifications: [code(1, '03000000'), code(3, '72000000')] }),
+    testOpportunity('c', { classifications: null }),
+  ];
+  const inUse = classificationsInUse(all, opportunities, new Set([4]));
+  expect(inUse.map((c) => c.id)).toEqual([1, 3, 4]);
+});

web/src/lib/classifications.ts

@@ -1,5 +1,5 @@
 import type { ClassificationsUpdatedPayload } from '@/types/Events';
-import type { ClassificationCode } from '@/types/Opportunities';
+import type { ClassificationCode, Opportunity } from '@/types/Opportunities';
 
 /**
  * The classifications after change: without the removed ids, with the added
@@ -15,3 +15,20 @@ export function applyClassificationChanges(
     (first, second) => first.code.localeCompare(second.code) || first.id - second.id,
   );
 }
+
+/**
+ * The classifications some of the opportunities have, and those whose ids
+ * are selected, so a selection stays visible (and can be removed) when no
+ * opportunity loaded has it.
+ */
+export function classificationsInUse(
+  classifications: readonly ClassificationCode[],
+  opportunities: readonly Opportunity[],
+  selected: ReadonlySet<number>,
+): ClassificationCode[] {
+  const used = new Set(selected);
+  for (const opportunity of opportunities) {
+    for (const code of opportunity.classifications ?? []) used.add(code.id);
+  }
+  return classifications.filter((code) => used.has(code.id));
+}