var forOwn = require('./forOwn');
var isPlainObject = require('../lang/isPlainObject');

    /*
     * Helper function to flatten to a destination object.
     * Used to remove the need to create intermediate objects while flattening.
     */
    function flattenTo(obj, result, prefix, level) {
        forOwn(obj, function (value, key) {
            var nestedPrefix = prefix ? prefix + '.' + key : key;

            if (level !== 0 && isPlainObject(value)) {
                flattenTo(value, result, nestedPrefix, level - 1);
            } else {
                result[nestedPrefix] = value;
            }
        });

        return result;
    }

    /**
     * Recursively flattens an object.
     * A new object containing all the elements is returned.
     * If level is specified, it will only flatten up to that level.
     */
    function flatten(obj, level) {
        if (obj == null) {
            return {};
        }

        level = level == null ? -1 : level;
        return flattenTo(obj, {}, '', level);
    }

    module.exports = flatten;