// Copyright IBM Corp. 2014,2016. All Rights Reserved.
// Node module: loopback
// This file is licensed under the MIT License.
// License text available at https://opensource.org/licenses/MIT

/*!
 * Module Dependencies.
 */

var g = require('strong-globalize')();

var isEmail = require('isemail');
var loopback = require('../../lib/loopback');
var utils = require('../../lib/utils');
var path = require('path');
var SALT_WORK_FACTOR = 10;
var crypto = require('crypto');
var MAX_PASSWORD_LENGTH = 72;
var bcrypt;
try {
  // Try the native module first
  bcrypt = require('bcrypt');
  // Browserify returns an empty object
  if (bcrypt && typeof bcrypt.compare !== 'function') {
    bcrypt = require('bcryptjs');
  }
} catch (err) {
  // Fall back to pure JS impl
  bcrypt = require('bcryptjs');
}

var DEFAULT_TTL = 1209600; // 2 weeks in seconds
var DEFAULT_RESET_PW_TTL = 15 * 60; // 15 mins in seconds
var DEFAULT_MAX_TTL = 31556926; // 1 year in seconds
var assert = require('assert');

var debug = require('debug')('loopback:user');

/**
 * Built-in User model.
 * Extends LoopBack [PersistedModel](#persistedmodel-new-persistedmodel).
 *
 * Default `User` ACLs.
 *
 * - DENY EVERYONE `*`
 * - ALLOW EVERYONE `create`
 * - ALLOW OWNER `deleteById`
 * - ALLOW EVERYONE `login`
 * - ALLOW EVERYONE `logout`
 * - ALLOW OWNER `findById`
 * - ALLOW OWNER `updateAttributes`
 *
 * @property {String} username Must be unique.
 * @property {String} password Hidden from remote clients.
 * @property {String} email Must be valid email.
 * @property {Boolean} emailVerified Set when a user's email has been verified via `confirm()`.
 * @property {String} verificationToken Set when `verify()` is called.
 * @property {String} realm The namespace the user belongs to. See [Partitioning users with realms](https://docs.strongloop.com/display/public/LB/Partitioning+users+with+realms) for details.
 * @property {Object} settings Extends the `Model.settings` object.
 * @property {Boolean} settings.emailVerificationRequired Require the email verification
 * process before allowing a login.
 * @property {Number} settings.ttl Default time to live (in seconds) for the `AccessToken` created by `User.login() / user.createAccessToken()`.
 * Default is `1209600` (2 weeks)
 * @property {Number} settings.maxTTL The max value a user can request a token to be alive / valid for.
 * Default is `31556926` (1 year)
 * @property {Boolean} settings.realmRequired Require a realm when logging in a user.
 * @property {String} settings.realmDelimiter When set a realm is required.
 * @property {Number} settings.resetPasswordTokenTTL Time to live for password reset `AccessToken`. Default is `900` (15 minutes).
 * @property {Number} settings.saltWorkFactor The `bcrypt` salt work factor. Default is `10`.
 * @property {Boolean} settings.caseSensitiveEmail Enable case sensitive email.
 *
 * @class User
 * @inherits {PersistedModel}
 */

module.exports = function(User) {
  /**
   * Create access token for the logged in user. This method can be overridden to
   * customize how access tokens are generated
   *
   * @param {Number} ttl The requested ttl
   * @param {Object} [options] The options for access token, such as scope, appId
   * @callback {Function} cb The callback function
   * @param {String|Error} err The error string or object
   * @param {AccessToken} token The generated access token object
   * @promise
   */
  User.prototype.createAccessToken = function(ttl, options, cb) {
    if (cb === undefined && typeof options === 'function') {
      // createAccessToken(ttl, cb)
      cb = options;
      options = undefined;
    }

    cb = cb || utils.createPromiseCallback();

    if (typeof ttl === 'object' && !options) {
      // createAccessToken(options, cb)
      options = ttl;
      ttl = options.ttl;
    }
    options = options || {};
    var userModel = this.constructor;
    ttl = Math.min(ttl || userModel.settings.ttl, userModel.settings.maxTTL);
    this.accessTokens.create({
      ttl: ttl,
    }, cb);
    return cb.promise;
  };

  function splitPrincipal(name, realmDelimiter) {
    var parts = [null, name];
    if (!realmDelimiter) {
      return parts;
    }
    var index = name.indexOf(realmDelimiter);
    if (index !== -1) {
      parts[0] = name.substring(0, index);
      parts[1] = name.substring(index + realmDelimiter.length);
    }
    return parts;
  }

  /**
   * Normalize the credentials
   * @param {Object} credentials The credential object
   * @param {Boolean} realmRequired
   * @param {String} realmDelimiter The realm delimiter, if not set, no realm is needed
   * @returns {Object} The normalized credential object
   */
  User.normalizeCredentials = function(credentials, realmRequired, realmDelimiter) {
    var query = {};
    credentials = credentials || {};
    if (!realmRequired) {
      if (credentials.email) {
        query.email = credentials.email;
      } else if (credentials.username) {
        query.username = credentials.username;
      }
    } else {
      if (credentials.realm) {
        query.realm = credentials.realm;
      }
      var parts;
      if (credentials.email) {
        parts = splitPrincipal(credentials.email, realmDelimiter);
        query.email = parts[1];
        if (parts[0]) {
          query.realm = parts[0];
        }
      } else if (credentials.username) {
        parts = splitPrincipal(credentials.username, realmDelimiter);
        query.username = parts[1];
        if (parts[0]) {
          query.realm = parts[0];
        }
      }
    }
    return query;
  };

  /**
   * Login a user by with the given `credentials`.
   *
   * ```js
   *    User.login({username: 'foo', password: 'bar'}, function (err, token) {
  *      console.log(token.id);
  *    });
   * ```
   *
   * @param {Object} credentials username/password or email/password
   * @param {String[]|String} [include] Optionally set it to "user" to include
   * the user info
   * @callback {Function} callback Callback function
   * @param {Error} err Error object
   * @param {AccessToken} token Access token if login is successful
   * @promise
   */

  User.login = function(credentials, include, fn) {
    var self = this;
    if (typeof include === 'function') {
      fn = include;
      include = undefined;
    }

    fn = fn || utils.createPromiseCallback();

    include = (include || '');
    if (Array.isArray(include)) {
      include = include.map(function(val) {
        return val.toLowerCase();
      });
    } else {
      include = include.toLowerCase();
    }

    var realmDelimiter;
    // Check if realm is required
    var realmRequired = !!(self.settings.realmRequired ||
      self.settings.realmDelimiter);
    if (realmRequired) {
      realmDelimiter = self.settings.realmDelimiter;
    }
    var query = self.normalizeCredentials(credentials, realmRequired,
      realmDelimiter);

    if (realmRequired && !query.realm) {
      var err1 = new Error(g.f('{{realm}} is required'));
      err1.statusCode = 400;
      err1.code = 'REALM_REQUIRED';
      fn(err1);
      return fn.promise;
    }
    if (!query.email && !query.username) {
      var err2 = new Error(g.f('{{username}} or {{email}} is required'));
      err2.statusCode = 400;
      err2.code = 'USERNAME_EMAIL_REQUIRED';
      fn(err2);
      return fn.promise;
    }

    self.findOne({ where: query }, function(err, user) {
      var defaultError = new Error(g.f('login failed'));
      defaultError.statusCode = 401;
      defaultError.code = 'LOGIN_FAILED';

      function tokenHandler(err, token) {
        if (err) return fn(err);
        if (Array.isArray(include) ? include.indexOf('user') !== -1 : include === 'user') {
          // NOTE(bajtos) We can't set token.user here:
          //  1. token.user already exists, it's a function injected by
          //     "AccessToken belongsTo User" relation
          //  2. ModelBaseClass.toJSON() ignores own properties, thus
          //     the value won't be included in the HTTP response
          // See also loopback#161 and loopback#162
          token.__data.user = user;
        }
        fn(err, token);
      }

      if (err) {
        debug('An error is reported from User.findOne: %j', err);
        fn(defaultError);
      } else if (user) {
        user.hasPassword(credentials.password, function(err, isMatch) {
          if (err) {
            debug('An error is reported from User.hasPassword: %j', err);
            fn(defaultError);
          } else if (isMatch) {
            if (self.settings.emailVerificationRequired && !user.emailVerified) {
              // Fail to log in if email verification is not done yet
              debug('User email has not been verified');
              err = new Error(g.f('login failed as the email has not been verified'));
              err.statusCode = 401;
              err.code = 'LOGIN_FAILED_EMAIL_NOT_VERIFIED';
              fn(err);
            } else {
              if (user.createAccessToken.length === 2) {
                user.createAccessToken(credentials.ttl, tokenHandler);
              } else {
                user.createAccessToken(credentials.ttl, credentials, tokenHandler);
              }
            }
          } else {
            debug('The password is invalid for user %s', query.email || query.username);
            fn(defaultError);
          }
        });
      } else {
        debug('No matching record is found for user %s', query.email || query.username);
        fn(defaultError);
      }
    });
    return fn.promise;
  };

  /**
   * Logout a user with the given accessToken id.
   *
   * ```js
   *    User.logout('asd0a9f8dsj9s0s3223mk', function (err) {
  *      console.log(err || 'Logged out');
  *    });
   * ```
   *
   * @param {String} accessTokenID
   * @callback {Function} callback
   * @param {Error} err
   * @promise
   */

  User.logout = function(tokenId, fn) {
    fn = fn || utils.createPromiseCallback();
    this.relations.accessTokens.modelTo.findById(tokenId, function(err, accessToken) {
      if (err) {
        fn(err);
      } else if (accessToken) {
        accessToken.destroy(fn);
      } else {
        fn(new Error(g.f('could not find {{accessToken}}')));
      }
    });
    return fn.promise;
  };

  User.observe('before delete', function(ctx, next) {
    var AccessToken = ctx.Model.relations.accessTokens.modelTo;
    var pkName = ctx.Model.definition.idName() || 'id';
    ctx.Model.find({ where: ctx.where, fields: [pkName] }, function(err, list) {
      if (err) return next(err);

      var ids = list.map(function(u) { return u[pkName]; });
      ctx.where = {};
      ctx.where[pkName] = { inq: ids };

      AccessToken.destroyAll({ userId: { inq: ids }}, next);
    });
  });

  /**
   * Compare the given `password` with the users hashed password.
   *
   * @param {String} password The plain text password
   * @callback {Function} callback Callback function
   * @param {Error} err Error object
   * @param {Boolean} isMatch Returns true if the given `password` matches record
   * @promise
   */

  User.prototype.hasPassword = function(plain, fn) {
    fn = fn || utils.createPromiseCallback();
    if (this.password && plain) {
      bcrypt.compare(plain, this.password, function(err, isMatch) {
        if (err) return fn(err);
        fn(null, isMatch);
      });
    } else {
      fn(null, false);
    }
    return fn.promise;
  };

  /**
   * Verify a user's identity by sending them a confirmation email.
   *
   * ```js
   *    var options = {
   *      type: 'email',
   *      to: user.email,
   *      template: 'verify.ejs',
   *      redirect: '/',
   *      tokenGenerator: function (user, cb) { cb("random-token"); }
   *    };
   *
   *    user.verify(options, next);
   * ```
   *
   * @options {Object} options
   * @property {String} type Must be 'email'.
   * @property {String} to Email address to which verification email is sent.
   * @property {String} from Sender email addresss, for example
   *   `'noreply@myapp.com'`.
   * @property {String} subject Subject line text.
   * @property {String} text Text of email.
   * @property {String} template Name of template that displays verification
   *  page, for example, `'verify.ejs'.
   * @property {String} redirect Page to which user will be redirected after
   *  they verify their email, for example `'/'` for root URI.
   * @property {Function} generateVerificationToken A function to be used to
   *  generate the verification token. It must accept the user object and a
   *  callback function. This function should NOT add the token to the user
   *  object, instead simply execute the callback with the token! User saving
   *  and email sending will be handled in the `verify()` method.
   * @callback {Function} fn Callback function.
   * @param {Error} err Error object.
   * @param {Object} object Contains email, token, uid.
   * @promise
   */

  User.prototype.verify = function(options, fn) {
    fn = fn || utils.createPromiseCallback();

    var user = this;
    var userModel = this.constructor;
    var registry = userModel.registry;
    assert(typeof options === 'object', 'options required when calling user.verify()');
    assert(options.type, 'You must supply a verification type (options.type)');
    assert(options.type === 'email', 'Unsupported verification type');
    assert(options.to || this.email,
      'Must include options.to when calling user.verify() ' +
      'or the user must have an email property');
    assert(options.from, 'Must include options.from when calling user.verify()');

    options.redirect = options.redirect || '/';
    var defaultTemplate = path.join(__dirname, '..', '..', 'templates', 'verify.ejs');
    options.template = path.resolve(options.template || defaultTemplate);
    options.user = this;
    options.protocol = options.protocol || 'http';

    var app = userModel.app;
    options.host = options.host || (app && app.get('host')) || 'localhost';
    options.port = options.port || (app && app.get('port')) || 3000;
    options.restApiRoot = options.restApiRoot || (app && app.get('restApiRoot')) || '/api';

    var displayPort = (
      (options.protocol === 'http' && options.port == '80') ||
      (options.protocol === 'https' && options.port == '443')
    ) ? '' : ':' + options.port;

    var urlPath = joinUrlPath(
      options.restApiRoot,
      userModel.http.path,
      userModel.sharedClass.findMethodByName('confirm').http.path
    );

    options.verifyHref = options.verifyHref ||
      options.protocol +
      '://' +
      options.host +
      displayPort +
      urlPath +
      '?uid=' +
      options.user.id +
      '&redirect=' +
      options.redirect;

    // Email model
    var Email = options.mailer || this.constructor.email || registry.getModelByType(loopback.Email);

    // Set a default token generation function if one is not provided
    var tokenGenerator = options.generateVerificationToken || User.generateVerificationToken;

    tokenGenerator(user, function(err, token) {
      if (err) { return fn(err); }

      user.verificationToken = token;
      user.save(function(err) {
        if (err) {
          fn(err);
        } else {
          sendEmail(user);
        }
      });
    });

    // TODO - support more verification types
    function sendEmail(user) {
      options.verifyHref += '&token=' + user.verificationToken;

      options.text = options.text || g.f('Please verify your email by opening ' +
        'this link in a web browser:\n\t%s', options.verifyHref);

      options.text = options.text.replace(/\{href\}/g, options.verifyHref);

      options.to = options.to || user.email;

      options.subject = options.subject || g.f('Thanks for Registering');

      options.headers = options.headers || {};

      var template = loopback.template(options.template);
      options.html = template(options);

      Email.send(options, function(err, email) {
        if (err) {
          fn(err);
        } else {
          fn(null, { email: email, token: user.verificationToken, uid: user.id });
        }
      });
    }
    return fn.promise;
  };

  /**
   * A default verification token generator which accepts the user the token is
   * being generated for and a callback function to indicate completion.
   * This one uses the crypto library and 64 random bytes (converted to hex)
   * for the token. When used in combination with the user.verify() method this
   * function will be called with the `user` object as it's context (`this`).
   *
   * @param {object} user The User this token is being generated for.
   * @param {Function} cb The generator must pass back the new token with this function call
   */
  User.generateVerificationToken = function(user, cb) {
    crypto.randomBytes(64, function(err, buf) {
      cb(err, buf && buf.toString('hex'));
    });
  };

  /**
   * Confirm the user's identity.
   *
   * @param {Any} userId
   * @param {String} token The validation token
   * @param {String} redirect URL to redirect the user to once confirmed
   * @callback {Function} callback
   * @param {Error} err
   * @promise
   */
  User.confirm = function(uid, token, redirect, fn) {
    fn = fn || utils.createPromiseCallback();
    this.findById(uid, function(err, user) {
      if (err) {
        fn(err);
      } else {
        if (user && user.verificationToken === token) {
          user.verificationToken = null;
          user.emailVerified = true;
          user.save(function(err) {
            if (err) {
              fn(err);
            } else {
              fn();
            }
          });
        } else {
          if (user) {
            err = new Error(g.f('Invalid token: %s', token));
            err.statusCode = 400;
            err.code = 'INVALID_TOKEN';
          } else {
            err = new Error(g.f('User not found: %s', uid));
            err.statusCode = 404;
            err.code = 'USER_NOT_FOUND';
          }
          fn(err);
        }
      }
    });
    return fn.promise;
  };

  /**
   * Create a short lived acess token for temporary login. Allows users
   * to change passwords if forgotten.
   *
   * @options {Object} options
   * @prop {String} email The user's email address
   * @callback {Function} callback
   * @param {Error} err
   * @promise
   */

  User.resetPassword = function(options, cb) {
    cb = cb || utils.createPromiseCallback();
    var UserModel = this;
    var ttl = UserModel.settings.resetPasswordTokenTTL || DEFAULT_RESET_PW_TTL;
    options = options || {};
    if (typeof options.email !== 'string') {
      var err = new Error(g.f('Email is required'));
      err.statusCode = 400;
      err.code = 'EMAIL_REQUIRED';
      cb(err);
      return cb.promise;
    }

    try {
      if (options.password) {
        UserModel.validatePassword(options.password);
      }
    } catch (err) {
      return cb(err);
    }
    UserModel.findOne({ where: { email: options.email }}, function(err, user) {
      if (err) {
        return cb(err);
      }
      if (!user) {
        err = new Error(g.f('Email not found'));
        err.statusCode = 404;
        err.code = 'EMAIL_NOT_FOUND';
        return cb(err);
      }
      // create a short lived access token for temp login to change password
      // TODO(ritch) - eventually this should only allow password change
      if (UserModel.settings.emailVerificationRequired && !user.emailVerified) {
        err = new Error(g.f('Email has not been verified'));
        err.statusCode = 401;
        err.code = 'RESET_FAILED_EMAIL_NOT_VERIFIED';
        return cb(err);
      }

      user.accessTokens.create({ ttl: ttl }, function(err, accessToken) {
        if (err) {
          return cb(err);
        }
        cb();
        UserModel.emit('resetPasswordRequest', {
          email: options.email,
          accessToken: accessToken,
          user: user,
        });
      });
    });

    return cb.promise;
  };

  /*!
   * Hash the plain password
   */
  User.hashPassword = function(plain) {
    this.validatePassword(plain);
    var salt = bcrypt.genSaltSync(this.settings.saltWorkFactor || SALT_WORK_FACTOR);
    return bcrypt.hashSync(plain, salt);
  };

  User.validatePassword = function(plain) {
    var err;
    if (plain && typeof plain === 'string' && plain.length <= MAX_PASSWORD_LENGTH) {
      return true;
    }
    if (plain.length > MAX_PASSWORD_LENGTH) {
      err = new Error (g.f('Password too long: %s', plain));
      err.code = 'PASSWORD_TOO_LONG';
    } else {
      err =  new Error(g.f('Invalid password: %s', plain));
      err.code = 'INVALID_PASSWORD';
    }
    err.statusCode = 422;
    throw err;
  };
  /*!
   * Setup an extended user model.
   */

  User.setup = function() {
    // We need to call the base class's setup method
    User.base.setup.call(this);
    var UserModel = this;

    // max ttl
    this.settings.maxTTL = this.settings.maxTTL || DEFAULT_MAX_TTL;
    this.settings.ttl = this.settings.ttl || DEFAULT_TTL;

    UserModel.setter.email = function(value) {
      if (!UserModel.settings.caseSensitiveEmail) {
        this.$email = value.toLowerCase();
      } else {
        this.$email = value;
      }
    };

    UserModel.setter.password = function(plain) {
      if (typeof plain !== 'string') {
        return;
      }
      if (plain.indexOf('$2a$') === 0 && plain.length === 60) {
        // The password is already hashed. It can be the case
        // when the instance is loaded from DB
        this.$password = plain;
      } else {
        this.$password = this.constructor.hashPassword(plain);
      }
    };

    // Access token to normalize email credentials
    UserModel.observe('access', function normalizeEmailCase(ctx, next) {
      if (!ctx.Model.settings.caseSensitiveEmail && ctx.query.where && ctx.query.where.email) {
        ctx.query.where.email = ctx.query.where.email.toLowerCase();
      }
      next();
    });

    // Make sure emailVerified is not set by creation
    UserModel.beforeRemote('create', function(ctx, user, next) {
      var body = ctx.req.body;
      if (body && body.emailVerified) {
        body.emailVerified = false;
      }
      next();
    });

    // Delete old sessions once email is updated
    UserModel.observe('before save', function beforeEmailUpdate(ctx, next) {
      if (ctx.isNewInstance) return next();
      if (!ctx.where && !ctx.instance) return next();
      var where = ctx.where || { id: ctx.instance.id };
      ctx.Model.find({ where: where }, function(err, userInstances) {
        if (err) return next(err);
        ctx.hookState.originalUserData = userInstances.map(function(u) {
          return { id: u.id, email: u.email };
        });
        next();
      });
    });

    UserModel.observe('after save', function afterEmailUpdate(ctx, next) {
      if (!ctx.Model.relations.accessTokens) return next();
      var AccessToken = ctx.Model.relations.accessTokens.modelTo;
      var newEmail = (ctx.instance || ctx.data).email;
      if (!ctx.hookState.originalUserData) return next();
      var idsToExpire = ctx.hookState.originalUserData.filter(function(u) {
        return u.email !== newEmail;
      }).map(function(u) {
        return u.id;
      });
      if (!idsToExpire.length) return next();
      AccessToken.deleteAll({ userId: { inq: idsToExpire }}, next);
    });

    UserModel.remoteMethod(
      'login',
      {
        description: 'Login a user with username/email and password.',
        accepts: [
          { arg: 'credentials', type: 'object', required: true, http: { source: 'body' }},
          { arg: 'include', type: ['string'], http: { source: 'query' },
            description: 'Related objects to include in the response. ' +
            'See the description of return value for more details.' },
        ],
        returns: {
          arg: 'accessToken', type: 'object', root: true,
          description:
            g.f('The response body contains properties of the {{AccessToken}} created on login.\n' +
            'Depending on the value of `include` parameter, the body may contain ' +
            'additional properties:\n\n' +
            '  - `user` - `U+007BUserU+007D` - Data of the currently logged in user. ' +
            '{{(`include=user`)}}\n\n'),
        },
        http: { verb: 'post' },
      }
    );

    UserModel.remoteMethod(
      'logout',
      {
        description: 'Logout a user with access token.',
        accepts: [
          { arg: 'access_token', type: 'string', required: true, http: function(ctx) {
            var req = ctx && ctx.req;
            var accessToken = req && req.accessToken;
            var tokenID = accessToken && accessToken.id;

            return tokenID;
          }, description: 'Do not supply this argument, it is automatically extracted ' +
            'from request headers.',
          },
        ],
        http: { verb: 'all' },
      }
    );

    UserModel.remoteMethod(
      'confirm',
      {
        description: 'Confirm a user registration with email verification token.',
        accepts: [
          { arg: 'uid', type: 'string', required: true },
          { arg: 'token', type: 'string', required: true },
          { arg: 'redirect', type: 'string' },
        ],
        http: { verb: 'get', path: '/confirm' },
      }
    );

    UserModel.remoteMethod(
      'resetPassword',
      {
        description: 'Reset password for a user with email.',
        accepts: [
          { arg: 'options', type: 'object', required: true, http: { source: 'body' }},
        ],
        http: { verb: 'post', path: '/reset' },
      }
    );

    UserModel.afterRemote('confirm', function(ctx, inst, next) {
      if (ctx.args.redirect !== undefined) {
        if (!ctx.res) {
          return next(new Error(g.f('The transport does not support HTTP redirects.')));
        }
        ctx.res.location(ctx.args.redirect);
        ctx.res.status(302);
      }
      next();
    });

    // default models
    assert(loopback.Email, 'Email model must be defined before User model');
    UserModel.email = loopback.Email;

    assert(loopback.AccessToken, 'AccessToken model must be defined before User model');
    UserModel.accessToken = loopback.AccessToken;

    UserModel.validate('email', emailValidator, {
      message: g.f('Must provide a valid email'),
    });

    // Realm users validation
    if (UserModel.settings.realmRequired && UserModel.settings.realmDelimiter) {
      UserModel.validatesUniquenessOf('email', {
        message: 'Email already exists',
        scopedTo: ['realm'],
      });
      UserModel.validatesUniquenessOf('username', {
        message: 'User already exists',
        scopedTo: ['realm'],
      });
    } else {
      // Regular(Non-realm) users validation
      UserModel.validatesUniquenessOf('email', { message: 'Email already exists' });
      UserModel.validatesUniquenessOf('username', { message: 'User already exists' });
    }

    return UserModel;
  };

  /*!
   * Setup the base user.
   */

  User.setup();
};

function emailValidator(err, done) {
  var value = this.email;
  if (value == null)
    return;
  if (typeof value !== 'string')
    return err('string');
  if (value === '') return;
  if (!isEmail(value))
    return err('email');
}

function joinUrlPath(args) {
  var result = arguments[0];
  for (var ix = 1; ix < arguments.length; ix++) {
    var next = arguments[ix];
    result += result[result.length - 1] === '/' && next[0] === '/' ?
      next.slice(1) : next;
  }
  return result;
}