var bindCallback = require('./bindCallback'),
    isIterateeCall = require('./isIterateeCall'),
    restParam = require('../function/restParam');

/**
 * Creates a function that assigns properties of source object(s) to a given
 * destination object.
 *
 * **Note:** This function is used to create `_.assign`, `_.defaults`, and `_.merge`.
 *
 * @private
 * @param {Function} assigner The function to assign values.
 * @returns {Function} Returns the new assigner function.
 */
function createAssigner(assigner) {
  return restParam(function(object, sources) {
    var index = -1,
        length = object == null ? 0 : sources.length,
        customizer = length > 2 ? sources[length - 2] : undefined,
        guard = length > 2 ? sources[2] : undefined,
        thisArg = length > 1 ? sources[length - 1] : undefined;

    if (typeof customizer == 'function') {
      customizer = bindCallback(customizer, thisArg, 5);
      length -= 2;
    } else {
      customizer = typeof thisArg == 'function' ? thisArg : undefined;
      length -= (customizer ? 1 : 0);
    }
    if (guard && isIterateeCall(sources[0], sources[1], guard)) {
      customizer = length < 3 ? undefined : customizer;
      length = 1;
    }
    while (++index < length) {
      var source = sources[index];
      if (source) {
        assigner(object, source, customizer);
      }
    }
    return object;
  });
}

module.exports = createAssigner;
