feat: add configurable credit calculation with validation

test/credits.test.js

@@ -0,0 +1,35 @@
+"use strict";
+
+const test = require("node:test");
+const assert = require("node:assert/strict");
+const { calculateCredits } = require("../src/lib/credits");
+
+const config = {
+  baseCredits: 1,
+  includedChars: 25000,
+  stepChars: 10000,
+  stepCredits: 1,
+  maxCharsPerArticle: 120000,
+};
+
+test("charges base credit up to included chars", () => {
+  assert.equal(calculateCredits(1, config), 1);
+  assert.equal(calculateCredits(25000, config), 1);
+});
+
+test("charges one extra credit after crossing step boundary", () => {
+  assert.equal(calculateCredits(25001, config), 2);
+  assert.equal(calculateCredits(35000, config), 2);
+});
+
+test("charges multiple steps correctly", () => {
+  assert.equal(calculateCredits(45001, config), 4);
+});
+
+test("rejects oversized articles", () => {
+  assert.throws(() => calculateCredits(120001, config), /article_too_long/);
+});
+
+test("rejects non-positive charCount", () => {
+  assert.throws(() => calculateCredits(0, config), /charCount must be a positive integer/);
+});