'use strict';

var as = require('../../lib/as.js');
var debug = require('debug')('express-http-proxy');
var zlib = require('zlib');

function isResGzipped(res) {
  return res.headers['content-encoding'] === 'gzip';
}

function zipOrUnzip(method) {
  return function (rspData, res) {
    return new Promise(function (resolve, reject) {
      if (isResGzipped(res) && rspData.length) {
        zlib[method](rspData, function (err, buffer) {
          if (err) {
            reject(err);
          } else {
            resolve(buffer);
          }
        });
      } else {
        resolve(rspData);
      }
    });
  };
}

var maybeUnzipPromise = zipOrUnzip('gunzip');
var maybeZipPromise = zipOrUnzip('gzip');

function verifyBuffer(rspd, reject) {
  if (!Buffer.isBuffer(rspd)) {
    return reject(new Error('userResDecorator should return string or buffer as data'));
  }
}

function updateHeaders(res, rspdBefore, rspdAfter, reject) {
  if (!res.headersSent) {
    res.set('content-length', rspdAfter.length);
  } else if (rspdAfter.length !== rspdBefore.length) {
    var error = '"Content-Length" is already sent,' +
          'the length of response data can not be changed';
    return reject(new Error(error));
  }
}

function decorateProxyResBody(container) {
  var resolverFn = container.options.userResDecorator;

  if (!resolverFn) {
    return Promise.resolve(container);
  }

  var proxyResDataPromise = maybeUnzipPromise(container.proxy.resData, container.proxy.res);
  var proxyRes = container.proxy.res;
  var req = container.user.req;
  var res = container.user.res;
  var originalResData;

  if (res.statusCode === 304) {
    debug('Skipping userResDecorator on response 304');
    return Promise.resolve(container);
  }

  return proxyResDataPromise
    .then(function (proxyResData) {
      originalResData = proxyResData;
      return resolverFn(proxyRes, proxyResData, req, res);
    })
    .then(function (modifiedResData) {
      return new Promise(function (resolve, reject) {
        var rspd = as.buffer(modifiedResData, container.options);
        verifyBuffer(rspd, reject);
        updateHeaders(res, originalResData, rspd, reject);
        maybeZipPromise(rspd, container.proxy.res).then(function (buffer) {
          container.proxy.resData = buffer;
          resolve(container);
        }).catch(function (error) {
          reject(error);
        });
      });
    });
}

module.exports = decorateProxyResBody;