feat(linter/plugins): implement options merging

apps/oxlint/src-js/plugins/options.ts

@@ -2,8 +2,18 @@
  * Options for rules.
  */
 
+import {
+  deepFreezeJsonValue as deepFreezeValue,
+  deepFreezeJsonArray as deepFreezeArray,
+  deepFreezeJsonObject as deepFreezeObject,
+} from "./json.js";
+
 import type { JsonValue } from "./json.ts";
 
+const { freeze } = Object,
+  { isArray } = Array,
+  { min } = Math;
+
 /**
  * Options for a rule on a file.
  */
@@ -17,3 +27,104 @@ export const allOptions: Readonly<Options>[] = [DEFAULT_OPTIONS];
 
 // Index into `allOptions` for default options
 export const DEFAULT_OPTIONS_ID = 0;
+
+/**
+ * Merge user-provided options from config with rule's default options.
+ *
+ * Config options take precedence over default options.
+ *
+ * Returned options are deep frozen.
+ * `ruleOptions` may be frozen in place (or partially frozen) too.
+ * `defaultOptions` must already be deep frozen before calling this function.
+ *
+ * Follows the same merging logic as ESLint's `getRuleOptions`.
+ * https://github.com/eslint/eslint/blob/0f5a94a84beee19f376025c74f703f275d52c94b/lib/linter/linter.js#L443-L454
+ * https://github.com/eslint/eslint/blob/0f5a94a84beee19f376025c74f703f275d52c94b/lib/shared/deep-merge-arrays.js
+ *
+ * Notably, nested arrays are not merged - config options wins. e.g.:
+ * - Config options:  [ [1] ]
+ * - Default options: [ [2, 3], 4 ]
+ * - Merged options:  [ [1], 4 ]
+ *
+ * @param configOptions - Options from config
+ * @param defaultOptions - Default options from `rule.meta.defaultOptions`
+ * @returns Merged options
+ */
+export function mergeOptions(
+  configOptions: Options | null,
+  defaultOptions: Readonly<Options> | null,
+): Readonly<Options> {
+  if (configOptions === null) {
+    return defaultOptions === null ? DEFAULT_OPTIONS : defaultOptions;
+  }
+
+  if (defaultOptions === null) {
+    deepFreezeArray(configOptions);
+    return configOptions;
+  }
+
+  // Both are defined - merge them
+  const merged = [];
+
+  const defaultOptionsLength = defaultOptions.length,
+    ruleOptionsLength = configOptions.length,
+    bothLength = min(defaultOptionsLength, ruleOptionsLength);
+
+  let i = 0;
+  for (; i < bothLength; i++) {
+    merged.push(mergeValues(configOptions[i], defaultOptions[i]));
+  }
+
+  if (defaultOptionsLength > ruleOptionsLength) {
+    for (; i < defaultOptionsLength; i++) {
+      merged.push(defaultOptions[i]);
+    }
+  } else {
+    for (; i < ruleOptionsLength; i++) {
+      const prop = configOptions[i];
+      deepFreezeValue(prop);
+      merged.push(prop);
+    }
+  }
+
+  return freeze(merged);
+}
+
+/**
+ * Merge value from user-provided options with value from default options.
+ *
+ * @param configValue - Value from config
+ * @param defaultValue - Value from default options
+ * @returns Merged value
+ */
+function mergeValues(configValue: JsonValue, defaultValue: JsonValue): JsonValue {
+  // If config value is a primitive, it wins
+  if (configValue === null || typeof configValue !== "object") return configValue;
+
+  // If config value is an array, it wins
+  if (isArray(configValue)) {
+    deepFreezeArray(configValue);
+    return configValue;
+  }
+
+  // If default value is a primitive or an array, config value wins (it's an object)
+  if (defaultValue === null || typeof defaultValue !== "object" || isArray(defaultValue)) {
+    deepFreezeObject(configValue);
+    return configValue;
+  }
+
+  // Both are objects (not arrays)
+  const merged = { ...defaultValue, ...configValue };
+
+  // Symbol properties are not possible in JSON, so no need to handle them here.
+  // All properties are enumerable own properties, so can use simple `for..in` loop.
+  for (const key in configValue) {
+    if (key in defaultValue) {
+      merged[key] = mergeValues(configValue[key], defaultValue[key]);
+    } else {
+      deepFreezeValue(configValue[key]);
+    }
+  }
+
+  return freeze(merged);
+}