var createWrapper = require('./_createWrapper'),
    getPlaceholder = require('./_getPlaceholder'),
    replaceHolders = require('./_replaceHolders'),
    rest = require('./rest');

/** Used to compose bitmasks for wrapper metadata. */
var PARTIAL_FLAG = 32;

/**
 * Creates a function that invokes `func` with `partial` arguments prepended
 * to those provided to the new function. This method is like `_.bind` except
 * it does **not** alter the `this` binding.
 *
 * The `_.partial.placeholder` value, which defaults to `_` in monolithic
 * builds, may be used as a placeholder for partially applied arguments.
 *
 * **Note:** This method doesn't set the "length" property of partially
 * applied functions.
 *
 * @static
 * @memberOf _
 * @category Function
 * @param {Function} func The function to partially apply arguments to.
 * @param {...*} [partials] The arguments to be partially applied.
 * @returns {Function} Returns the new partially applied function.
 * @example
 *
 * var greet = function(greeting, name) {
 *   return greeting + ' ' + name;
 * };
 *
 * var sayHelloTo = _.partial(greet, 'hello');
 * sayHelloTo('fred');
 * // => 'hello fred'
 *
 * // Partially applied with placeholders.
 * var greetFred = _.partial(greet, _, 'fred');
 * greetFred('hi');
 * // => 'hi fred'
 */
var partial = rest(function(func, partials) {
  var holders = replaceHolders(partials, getPlaceholder(partial));
  return createWrapper(func, PARTIAL_FLAG, undefined, partials, holders);
});

// Assign default placeholders.
partial.placeholder = {};

module.exports = partial;