Initial import of zappier-edge from zapier monorepo

tests/billing-delta.test.ts

@@ -0,0 +1,19 @@
+import { computeDelta } from '../src/billing/stripe';
+
+describe('computeDelta', () => {
+  it('returns the difference when billable exceeds prior', () => {
+    expect(computeDelta(800, 500)).toBe(300);
+  });
+
+  it('returns 0 when billable equals prior', () => {
+    expect(computeDelta(500, 500)).toBe(0);
+  });
+
+  it('never returns a negative value when billable is below prior', () => {
+    expect(computeDelta(300, 500)).toBe(0);
+  });
+
+  it('returns the full billable amount when nothing was reported before', () => {
+    expect(computeDelta(500, 0)).toBe(500);
+  });
+});