import { isUnsafeProperty } from '../_internal/isUnsafeProperty.ts';
import { isPlainObject } from '../predicate/isPlainObject.ts';

/**
 * Merges the properties of the source object into the target object.
 *
 * You can provide a custom `merge` function to control how properties are merged. It should return the value to be set in the target object.
 *
 * If it returns `undefined`, a default deep merge will be applied for arrays and objects:
 *
 * - If a property in the source object is an array or an object and the corresponding property in the target object is also an array or object, they will be merged.
 * - If a property in the source object is undefined, it will not overwrite a defined property in the target object.
 *
 * Note that this function mutates the target object.
 *
 * @param target - The target object into which the source object properties will be merged. This object is modified in place.
 * @param source - The source object whose properties will be merged into the target object.
 * @param merge - A custom merge function that defines how properties should be combined. It receives the following arguments:
 *   - `targetValue`: The current value of the property in the target object.
 *   - `sourceValue`: The value of the property in the source object.
 *   - `key`: The key of the property being merged.
 *   - `target`: The target object.
 *   - `source`: The source object.
 *
 * @returns The updated target object with properties from the source object merged in.
 *
 * @template T - Type of the target object.
 * @template S - Type of the source object.
 *
 * @example
 * const target = { a: 1, b: 2 };
 * const source = { b: 3, c: 4 };
 *
 * mergeWith(target, source, (targetValue, sourceValue) => {
 *   if (typeof targetValue === 'number' && typeof sourceValue === 'number') {
 *     return targetValue + sourceValue;
 *   }
 * });
 * // Returns { a: 1, b: 5, c: 4 }
 * @example
 * const target = { a: [1], b: [2] };
 * const source = { a: [3], b: [4] };
 *
 * const result = mergeWith(target, source, (objValue, srcValue) => {
 *   if (Array.isArray(objValue)) {
 *     return objValue.concat(srcValue);
 *   }
 * });
 *
 * expect(result).toEqual({ a: [1, 3], b: [2, 4] });
 */
export function mergeWith<T extends Record<PropertyKey, any>, S extends Record<PropertyKey, any>>(
  target: T,
  source: S,
  merge: (targetValue: any, sourceValue: any, key: string, target: T, source: S) => any
): T & S {
  const sourceKeys = Object.keys(source) as Array<keyof T>;

  for (let i = 0; i < sourceKeys.length; i++) {
    const key = sourceKeys[i];

    if (isUnsafeProperty(key)) {
      continue;
    }

    const sourceValue = source[key];
    const targetValue = target[key];

    const merged = merge(targetValue, sourceValue, key as string, target, source);

    if (merged !== undefined) {
      target[key] = merged;
    } else if (Array.isArray(sourceValue)) {
      if (Array.isArray(targetValue)) {
        target[key] = mergeWith<any, S[keyof T]>(targetValue, sourceValue, merge);
      } else {
        target[key] = mergeWith<any, S[keyof T]>([], sourceValue, merge);
      }
    } else if (isPlainObject(sourceValue)) {
      if (isPlainObject(targetValue)) {
        target[key] = mergeWith<any, S[keyof T]>(targetValue, sourceValue, merge);
      } else {
        target[key] = mergeWith<any, S[keyof T]>({}, sourceValue, merge);
      }
    } else if (targetValue === undefined || sourceValue !== undefined) {
      target[key] = sourceValue;
    }
  }

  return target;
}
