/**
 * Copyright 2013-2015, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ReactPropTransferer
 */

'use strict';

var assign = require('./Object.assign');
var emptyFunction = require('fbjs/lib/emptyFunction');
var joinClasses = require('fbjs/lib/joinClasses');

/**
 * Creates a transfer strategy that will merge prop values using the supplied
 * `mergeStrategy`. If a prop was previously unset, this just sets it.
 *
 * @param {function} mergeStrategy
 * @return {function}
 */
function createTransferStrategy(mergeStrategy) {
  return function (props, key, value) {
    if (!props.hasOwnProperty(key)) {
      props[key] = value;
    } else {
      props[key] = mergeStrategy(props[key], value);
    }
  };
}

var transferStrategyMerge = createTransferStrategy(function (a, b) {
  // `merge` overrides the first object's (`props[key]` above) keys using the
  // second object's (`value`) keys. An object's style's existing `propA` would
  // get overridden. Flip the order here.
  return assign({}, b, a);
});

/**
 * Transfer strategies dictate how props are transferred by `transferPropsTo`.
 * NOTE: if you add any more exceptions to this list you should be sure to
 * update `cloneWithProps()` accordingly.
 */
var TransferStrategies = {
  /**
   * Never transfer `children`.
   */
  children: emptyFunction,
  /**
   * Transfer the `className` prop by merging them.
   */
  className: createTransferStrategy(joinClasses),
  /**
   * Transfer the `style` prop (which is an object) by merging them.
   */
  style: transferStrategyMerge
};

/**
 * Mutates the first argument by transferring the properties from the second
 * argument.
 *
 * @param {object} props
 * @param {object} newProps
 * @return {object}
 */
function transferInto(props, newProps) {
  for (var thisKey in newProps) {
    if (!newProps.hasOwnProperty(thisKey)) {
      continue;
    }

    var transferStrategy = TransferStrategies[thisKey];

    if (transferStrategy && TransferStrategies.hasOwnProperty(thisKey)) {
      transferStrategy(props, thisKey, newProps[thisKey]);
    } else if (!props.hasOwnProperty(thisKey)) {
      props[thisKey] = newProps[thisKey];
    }
  }
  return props;
}

/**
 * ReactPropTransferer are capable of transferring props to another component
 * using a `transferPropsTo` method.
 *
 * @class ReactPropTransferer
 */
var ReactPropTransferer = {

  /**
   * Merge two props objects using TransferStrategies.
   *
   * @param {object} oldProps original props (they take precedence)
   * @param {object} newProps new props to merge in
   * @return {object} a new object containing both sets of props merged.
   */
  mergeProps: function (oldProps, newProps) {
    return transferInto(assign({}, oldProps), newProps);
  }

};

module.exports = ReactPropTransferer;