/**
* @license Apache-2.0
*
* Copyright (c) 2018 The Stdlib Authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*    http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

'use strict';

// MODULES //

var objectKeys = require( './../../keys' );
var isObject = require( '@stdlib/assert/is-object' );
var hasOwnProp = require( '@stdlib/assert/has-own-property' );
var isBuffer = require( '@stdlib/assert/is-buffer' );
var isFunction = require( '@stdlib/assert/is-function' );
var typeOf = require( './../../type-of' );
var deepCopy = require( './../../copy' );


// MAIN //

/**
* Merges a source object into a target object.
*
* @private
* @param {Object} target - target object
* @param {Object} source - source object
* @param {number} level - merge level
* @param {boolean} copy - indicates whether to perform a deep copy of merged values
* @param {(boolean|Function)} override - defines the merge strategy
* @param {boolean} extend - indicates whether new properties can be added to the target object
*/
function deepMerge( target, source, level, copy, override, extend ) {
	var hasProp;
	var isFunc;
	var name;
	var keys;
	var curr;
	var key;
	var val;
	var tmp;
	var i;

	// Determine if we were provided a custom override strategy:
	isFunc = isFunction( override );

	// Decrement the level:
	level -= 1;

	// Loop through the source keys and implement the merge strategy...
	keys = objectKeys( source );
	for ( i = 0; i < keys.length; i++ ) {
		key = keys[ i ];
		hasProp = hasOwnProp( target, key );

		// Can we add new properties to the target?
		if ( !hasProp && !extend ) {
			continue;
		}
		val = source[ key ];

		if ( hasProp ) {
			curr = target[ key ];
			name = typeOf( curr );

			// Should we recurse to perform a deep(er) merge? (only if both the current value and the proposed value are objects and the level is > 0)
			if (
				!isBuffer( curr ) &&
				name === 'object' &&
				isObject( val ) &&
				level
			) {
				deepMerge( curr, val, level, copy, override, extend );
				continue;
			}
			// Should we apply a custom merge (override) strategy?
			if ( isFunc ) {
				tmp = override( curr, val, key );

				// WARNING: the following check does NOT prevent shared (leaky) nested references. We only check for top-level reference equality. We will assume that the user knows best, given their having provided a custom override strategy.
				if ( copy && tmp !== curr && tmp === val ) {
					tmp = deepCopy( tmp );
				}
				target[ key ] = tmp;
			}
			// Are we allowed to override an existing target value?
			else if ( override ) {
				if ( copy ) {
					target[ key ] = deepCopy( val );
				} else {
					target[ key ] = val;
				}
			}
		}
		// New property to be added to target object. Should we deep copy the source value?
		else if ( copy ) {
			target[ key ] = deepCopy( val );
		}
		// Perform a simple assignment...
		else {
			target[ key ] = val;
		}
	}
}


// EXPORTS //

module.exports = deepMerge;