feat: add configurable credit calculation with validation

src/lib/credits.js

@@ -0,0 +1,36 @@
+"use strict";
+
+function assertPositiveInt(name, value) {
+  if (!Number.isInteger(value) || value <= 0) {
+    throw new Error(`${name} must be a positive integer`);
+  }
+}
+
+function validateCreditConfig(config) {
+  assertPositiveInt("baseCredits", config.baseCredits);
+  assertPositiveInt("includedChars", config.includedChars);
+  assertPositiveInt("stepChars", config.stepChars);
+  assertPositiveInt("stepCredits", config.stepCredits);
+  assertPositiveInt("maxCharsPerArticle", config.maxCharsPerArticle);
+}
+
+function calculateCredits(charCount, config) {
+  validateCreditConfig(config);
+
+  if (!Number.isInteger(charCount) || charCount <= 0) {
+    throw new Error("charCount must be a positive integer");
+  }
+
+  if (charCount > config.maxCharsPerArticle) {
+    throw new Error("article_too_long");
+  }
+
+  const overage = Math.max(0, charCount - config.includedChars);
+  const extraSteps = overage === 0 ? 0 : Math.ceil(overage / config.stepChars);
+  return config.baseCredits + (extraSteps * config.stepCredits);
+}
+
+module.exports = {
+  calculateCredits,
+  validateCreditConfig,
+};