feat: add TTR simulator and calculation engine, enhance member model with TTR adjustment constant

frontend/tests/ttrCalculator.test.mjs

@@ -0,0 +1,44 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  calculateExpectedWinProbability,
+  calculateTtrChange,
+  calculateTtrEvent,
+  roundTtr,
+} from '../src/services/ttrCalculator.mjs';
+
+test('equal ratings have a 50 percent expected win probability', () => {
+  assert.equal(calculateExpectedWinProbability(1500, 1500), 0.5);
+  assert.equal(calculateTtrChange(1500, 1500, 'win', 16).displayChange, 8);
+  assert.equal(calculateTtrChange(1500, 1500, 'loss', 16).displayChange, -8);
+});
+
+test('a win against a stronger player gains more than a win against a weaker player', () => {
+  const strong = calculateTtrChange(1318, 1429, 'win', 16);
+  const weak = calculateTtrChange(1318, 1180, 'win', 16);
+  assert.ok(strong.rawChange > weak.rawChange);
+  assert.ok(calculateTtrChange(1318, 1429, 'loss', 16).rawChange > calculateTtrChange(1318, 1180, 'loss', 16).rawChange);
+});
+
+test('an event uses one immutable snapshot and rounds only the total', () => {
+  const event = calculateTtrEvent({
+    ownTtr: 1318,
+    adjustmentConstant: 16,
+    matches: [
+      { opponentTtr: 1382, result: 'win' },
+      { opponentTtr: 1429, result: 'win' },
+    ],
+  });
+  assert.equal(event.calculations[0].ownTtrSnapshot, 1318);
+  assert.equal(event.ownTtrSnapshot, 1318);
+  assert.equal(event.displayChange, 25);
+  assert.equal(event.projectedTtr, 1343);
+  assert.equal(event.displayChange, roundTtr(event.calculations[0].rawChange + event.calculations[1].rawChange));
+  assert.equal(event.projectedTtr, 1318 + event.displayChange);
+});
+
+test('the adjustment constant is player-specific input', () => {
+  const with16 = calculateTtrChange(1500, 1500, 'win', 16).rawChange;
+  const with8 = calculateTtrChange(1500, 1500, 'win', 8).rawChange;
+  assert.equal(with16, with8 * 2);
+});