// A router that is based on promises rather than req/res/next.
// This is intended to replace the use of express.Router to handle
// subsections of the API surface.
// This will make it easier to have methods like 'batch' that
// themselves use our routing information, without disturbing express
// components that external developers may be modifying.

import express from 'express';

export default class PromiseRouter {
  // Each entry should be an object with:
  // path: the path to route, in express format
  // method: the HTTP method that this route handles.
  //   Must be one of: POST, GET, PUT, DELETE
  // handler: a function that takes request, and returns a promise.
  //   Successful handlers should resolve to an object with fields:
  //     status: optional. the http status code. defaults to 200
  //     response: a json object with the content of the response
  //     location: optional. a location header
  constructor(routes = []) {
    this.routes = routes;
    this.mountRoutes();
  }

  // Leave the opportunity to
  // subclasses to mount their routes by overriding
  mountRoutes() {}

  // Merge the routes into this one
  merge(router) {
    for (var route of router.routes) {
      this.routes.push(route);
    }
  };

  route(method, path, ...handlers) {
    switch(method) {
    case 'POST':
    case 'GET':
    case 'PUT':
    case 'DELETE':
      break;
    default:
      throw 'cannot route method: ' + method;
    }

    let handler = handlers[0];

    if (handlers.length > 1) {
      const length = handlers.length;
      handler = function(req) {
        return handlers.reduce((promise, handler) => {
          return promise.then((result) => {
            return handler(req);
          });
        }, Promise.resolve());
      }
    }

    this.routes.push({
      path: path,
      method: method,
      handler: handler
    });
  };

  // Returns an object with:
  //   handler: the handler that should deal with this request
  //   params: any :-params that got parsed from the path
  // Returns undefined if there is no match.
  match(method, path) {
    for (var route of this.routes) {
      if (route.method != method) {
        continue;
      }
      // NOTE: we can only route the specific wildcards :className and
      // :objectId, and in that order.
      // This is pretty hacky but I don't want to rebuild the entire
      // express route matcher. Maybe there's a way to reuse its logic.
      var pattern = '^' + route.path + '$';

      pattern = pattern.replace(':className',
                                '(_?[A-Za-z][A-Za-z_0-9]*)');
      pattern = pattern.replace(':objectId',
                                '([A-Za-z0-9]+)');
      var re = new RegExp(pattern);
      var m = path.match(re);
      if (!m) {
        continue;
      }
      var params = {};
      if (m[1]) {
        params.className = m[1];
      }
      if (m[2]) {
        params.objectId = m[2];
      }

      return {params: params, handler: route.handler};
    }
  };

  // Mount the routes on this router onto an express app (or express router)
  mountOnto(expressApp) {
    for (var route of this.routes) {
      switch(route.method) {
      case 'POST':
        expressApp.post(route.path, makeExpressHandler(route.handler));
        break;
      case 'GET':
        expressApp.get(route.path, makeExpressHandler(route.handler));
        break;
      case 'PUT':
        expressApp.put(route.path, makeExpressHandler(route.handler));
        break;
      case 'DELETE':
        expressApp.delete(route.path, makeExpressHandler(route.handler));
        break;
      default:
        throw 'unexpected code branch';
      }
    }
  };

  expressApp() {
    var expressApp = express();
    for (var route of this.routes) {
      switch(route.method) {
      case 'POST':
        expressApp.post(route.path, makeExpressHandler(route.handler));
        break;
      case 'GET':
        expressApp.get(route.path, makeExpressHandler(route.handler));
        break;
      case 'PUT':
        expressApp.put(route.path, makeExpressHandler(route.handler));
        break;
      case 'DELETE':
        expressApp.delete(route.path, makeExpressHandler(route.handler));
        break;
      default:
        throw 'unexpected code branch';
      }
    }
    return expressApp;
  }
}

// Global flag. Set this to true to log every request and response.
PromiseRouter.verbose = process.env.VERBOSE || false;

// A helper function to make an express handler out of a a promise
// handler.
// Express handlers should never throw; if a promise handler throws we
// just treat it like it resolved to an error.
function makeExpressHandler(promiseHandler) {
  return function(req, res, next) {
    try {
      if (PromiseRouter.verbose) {
        console.log(req.method, req.originalUrl, req.headers,
                    JSON.stringify(req.body, null, 2));
      }
      promiseHandler(req).then((result) => {
        if (!result.response && !result.location && !result.text) {
          console.log('BUG: the handler did not include a "response" or a "location" field');
          throw 'control should not get here';
        }
        if (PromiseRouter.verbose) {
          console.log('response:', JSON.stringify(result, null, 2));
        }

        var status = result.status || 200;
        res.status(status);

        if (result.text) {
          return res.send(result.text);
        }

        if (result.location) {
          res.set('Location', result.location);
          // Override the default expressjs response
          // as it double encodes %encoded chars in URL
          if (!result.response) {
            return res.send('Found. Redirecting to '+result.location);
          }
        }
        res.json(result.response);
      }, (e) => {
        if (PromiseRouter.verbose) {
          console.log('error:', e);
        }
        next(e);
      });
    } catch (e) {
      if (PromiseRouter.verbose) {
        console.log('error:', e);
      }
      next(e);
    }
  }
}