"use strict";

module.exports = function immutableDeepMerge(object1, object2) {
	let hasChanges = false;
	let changedProperties = { };
	let deletedProperties = [];

	if (object2 != null) {
		for (let key of Object.keys(object2)) {
			let value = object2[key];
			let originalValue = object1[key];

			if (value === Delete) {
				deletedProperties.push(key);
				hasChanges = true;
			} else {
				let transformedValue;

				let normalizedValue = (typeof value === "function")
					? value(originalValue)
					: value;

				if (typeof normalizedValue === "object" && normalizedValue !== null) {
					// NOTE: We default to an empty object for the original value because from the perspective of a deep-merge, any nested paths required by the new input that don't exist in the original input should be imagined into existence.
					transformedValue = immutableDeepMerge(originalValue ?? {}, normalizedValue);
				} else {
					transformedValue = normalizedValue;
				}

				changedProperties[key] = transformedValue;

				if (transformedValue !== originalValue) {
					hasChanges = true;
				}
			}
		}
	}

	if (hasChanges) {
		let merged = { ... object1, ... changedProperties };

		for (let property of deletedProperties) {
			delete merged[property];
		}

		return merged;
	} else {
		return object1;
	}
};