feat(pure): computeEstimate with safe division and target-reached handling

tests/pure.test.js

@@ -1,6 +1,6 @@
 import { test } from 'node:test';
 import assert from 'node:assert/strict';
-import { parseTarget } from '../src/pure.js';
+import { parseTarget, computeEstimate } from '../src/pure.js';
 
 test('parseTarget: integer numbers', () => {
   assert.equal(parseTarget(25), 25);
@@ -32,3 +32,38 @@ test('parseTarget: rejects invalid input', () => {
   assert.equal(parseTarget('1.5.5M'), null);
   assert.equal(parseTarget('0'), null);
 });
+
+test('computeEstimate: typical case', () => {
+  const r = computeEstimate(14_328_501, 25_000_000, 247, 4520);
+  assert.equal(r.remaining, 10_671_499);
+  assert.equal(r.trainsToGo, 43_205);
+  assert.equal(r.days, 2_361);
+  assert.ok(r.eta instanceof Date);
+});
+
+test('computeEstimate: target reached', () => {
+  const r = computeEstimate(25_000_000, 25_000_000, 247, 4520);
+  assert.equal(r.remaining, 0);
+  assert.equal(r.trainsToGo, 0);
+  assert.equal(r.days, 0);
+  assert.equal(r.eta, null);
+});
+
+test('computeEstimate: target below current', () => {
+  const r = computeEstimate(30_000_000, 25_000_000, 247, 4520);
+  assert.equal(r.remaining, 0);
+  assert.equal(r.trainsToGo, 0);
+  assert.equal(r.days, 0);
+  assert.equal(r.eta, null);
+});
+
+test('computeEstimate: zero perTrain or perDay does not crash', () => {
+  const a = computeEstimate(100, 200, 0, 50);
+  assert.equal(a.trainsToGo, 0);
+  assert.equal(a.days, 2);
+
+  const b = computeEstimate(100, 200, 50, 0);
+  assert.equal(b.trainsToGo, 2);
+  assert.equal(b.days, 0);
+  assert.equal(b.eta, null);
+});