feat: 🎸 add first implementation

src/__tests__/index.spec.ts

@@ -1 +1,85 @@
-xit('Jest working', () => {});
+import {hotReddit, bestReddit, hotYCombinator} from '..';
+
+describe('hotReddit', () => {
+  it('gives higher rating to never content', async () => {
+    const ts = Date.now();
+    const score1 = hotReddit(10, 2, ts - 10000);
+    const score2 = hotReddit(10, 2, ts);
+
+    expect(score2).toBeGreaterThan(score1);
+  });
+
+  it('gives higher rating to content with more up votes', async () => {
+    const ts = Date.now();
+    const score1 = hotReddit(12, 2, ts);
+    const score2 = hotReddit(10, 2, ts);
+
+    expect(score1).toBeGreaterThan(score2);
+  });
+
+  it('gives higher rating to content with less down votes', async () => {
+    const ts = Date.now();
+    const score1 = hotReddit(12, 2, ts);
+    const score2 = hotReddit(12, 1, ts);
+
+    expect(score2).toBeGreaterThan(score1);
+  });
+
+  it('gives higher rating to content with more up votes - 2', async () => {
+    const ts = Date.now();
+    const score1 = hotReddit(1, 0, ts);
+    const score2 = hotReddit(2, 0, ts);
+
+    expect(score2).toBeGreaterThan(score1);
+  });
+});
+
+describe('bestReddit', () => {
+  it('gives higher score to content with more up votes', async () => {
+    const score1 = bestReddit(11, 2);
+    const score2 = bestReddit(10, 2);
+
+    expect(score1).toBeGreaterThan(score2);
+  });
+
+  it('gives higher score to content with less down votes', async () => {
+    const score1 = bestReddit(1000, 10);
+    const score2 = bestReddit(1000, 11);
+
+    expect(score1).toBeGreaterThan(score2);
+  });
+});
+
+describe('hotYCombinator', () => {
+  it('gives higher score to more recent content', async () => {
+    const ts = Date.now();
+    const score1 = hotYCombinator(10, ts);
+    const score2 = hotYCombinator(10, ts - 10000);
+
+    expect(score1).toBeGreaterThan(score2);
+  });
+
+  it('gives higher score to content with more up votes', async () => {
+    const ts = Date.now();
+    const score1 = hotYCombinator(110, ts);
+    const score2 = hotYCombinator(111, ts);
+
+    expect(score2).toBeGreaterThan(score1);
+  });
+
+  it('gives higher score to content with more up votes - 2', async () => {
+    const ts = Date.now();
+    const score1 = hotYCombinator(0, ts);
+    const score2 = hotYCombinator(111, ts);
+
+    expect(score2).toBeGreaterThan(score1);
+  });
+
+  it('gives higher score to content with more up votes - 3', async () => {
+    const ts = Date.now();
+    const score1 = hotYCombinator(1, ts);
+    const score2 = hotYCombinator(111, ts);
+
+    expect(score2).toBeGreaterThan(score1);
+  });
+});

src/index.ts

@@ -1 +1,50 @@
-console.log('Hello world!'); // tslint:disable-line no-console
+/**
+ * Reddit's hot content scoring function. Used for raking higher most recent and
+ * highest voted content. More recent content items get higher score automatically.
+ *
+ * - See: https://medium.com/hacking-and-gonzo/how-reddit-ranking-algorithms-work-ef111e33d0d9
+ *
+ * @param ups Number of up votes.
+ * @param downs Number of down votes.
+ * @param ts Timestamp in milliseconds when content was created.
+ */
+export function hotReddit (ups: number, downs: number, ts = Date.now()) {
+  const s = ups - downs;
+  const order = Math.log(Math.max(Math.abs(s), 1)) / Math.LN10;
+  const sign = s > 0 ? 1 : (s < 0 ? -1 : 0);
+  const seconds = (ts / 1e3) - 1134028003;
+  return sign * order + seconds / 45000;
+}
+
+/**
+ * YCombinator's hot content scoring formula. Does not require down votes.
+ *
+ * - See: https://moz.com/blog/reddit-stumbleupon-delicious-and-hacker-news-algorithms-exposed
+ *
+ * @param ups Number of up votes.
+ * @param ts Timestamp when content was created in milliseconds.
+ */
+export function hotYCombinator (ups: number, ts: number = Date.now()) {
+  const hoursSinceCreated = (Date.now() - ts) / (1000 * 60 * 60);
+  return ups / Math.pow(hoursSinceCreated + 2, 1.5);
+}
+
+/**
+ * Reddit's content scoring function for finding best replies regardless of time
+ * when content was created. This function does not penalize old content.
+ *
+ * - See: https://medium.com/hacking-and-gonzo/how-reddit-ranking-algorithms-work-ef111e33d0d9
+ *
+ * @param ups Number of up votes.
+ * @param downs Number of down votes.
+ */
+export function bestReddit (ups: number, downs: number) {
+  const n = ups + downs;
+  if (!n) return 0;
+  const z = 1.281551565545;
+  const p = ups / n;
+  const left = p + 1 / (2 * n) * z * z;
+  const right = z * Math.sqrt(p * (1 - p) / n + z * z / (4 * n * n));
+  const under = 1 + 1 / n * z * z;
+  return (left - right) / under;
+}