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

src/pure.js

@@ -23,3 +23,16 @@ export function parseTarget(input) {
   const multiplier = suffix ? SUFFIXES[suffix] : 1;
   return Math.floor(num * multiplier);
 }
+
+export function computeEstimate(current, target, perTrain, perDay) {
+  const remaining = Math.max(0, target - current);
+  if (remaining === 0) {
+    return { remaining: 0, trainsToGo: 0, days: 0, eta: null };
+  }
+
+  const trainsToGo = perTrain > 0 ? Math.ceil(remaining / perTrain) : 0;
+  const days = perDay > 0 ? Math.ceil(remaining / perDay) : 0;
+  const eta = days > 0 ? new Date(Date.now() + days * 86_400_000) : null;
+
+  return { remaining, trainsToGo, days, eta };
+}

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);
+});