var CHANGES_BATCH_SIZE = 25;
var MAX_SIMULTANEOUS_REVS = 50;
var CHANGES_TIMEOUT_BUFFER = 5000;
var DEFAULT_HEARTBEAT = 10000;

var supportsBulkGetMap = {};

import { generateErrorFromResponse } from 'pouchdb-errors';
import { nextTick } from 'pouchdb-utils';
import { fetch, Headers, AbortController } from 'pouchdb-fetch';

import getArguments from 'argsarray';

import {
  pick,
  filterChange,
  adapterFun as coreAdapterFun,
  explainError,
  clone,
  parseUri,
  bulkGetShim,
  flatten
} from 'pouchdb-utils';

import {
  atob,
  btoa,
  binaryStringToBlobOrBuffer as binStringToBluffer,
  base64StringToBlobOrBuffer as b64StringToBluffer,
  blobOrBufferToBase64 as blufferToBase64
} from 'pouchdb-binary-utils';

import pool from './promise-pool';
import { createError, BAD_ARG } from 'pouchdb-errors';

function readAttachmentsAsBlobOrBuffer(row) {
  var doc = row.doc || row.ok;
  var atts = doc._attachments;
  if (!atts) {
    return;
  }
  Object.keys(atts).forEach(function (filename) {
    var att = atts[filename];
    att.data = b64StringToBluffer(att.data, att.content_type);
  });
}

function encodeDocId(id) {
  if (/^_design/.test(id)) {
    return '_design/' + encodeURIComponent(id.slice(8));
  }
  if (/^_local/.test(id)) {
    return '_local/' + encodeURIComponent(id.slice(7));
  }
  return encodeURIComponent(id);
}

function preprocessAttachments(doc) {
  if (!doc._attachments || !Object.keys(doc._attachments)) {
    return Promise.resolve();
  }

  return Promise.all(Object.keys(doc._attachments).map(function (key) {
    var attachment = doc._attachments[key];
    if (attachment.data && typeof attachment.data !== 'string') {
      return new Promise(function (resolve) {
        blufferToBase64(attachment.data, resolve);
      }).then(function (b64) {
        attachment.data = b64;
      });
    }
  }));
}

function hasUrlPrefix(opts) {
  if (!opts.prefix) {
    return false;
  }
  var protocol = parseUri(opts.prefix).protocol;
  return protocol === 'http' || protocol === 'https';
}

// Get all the information you possibly can about the URI given by name and
// return it as a suitable object.
function getHost(name, opts) {

  // encode db name if opts.prefix is a url (#5574)
  if (hasUrlPrefix(opts)) {
    var dbName = opts.name.substr(opts.prefix.length);
    name = opts.prefix + encodeURIComponent(dbName);
  }

  // Prase the URI into all its little bits
  var uri = parseUri(name);

  // Store the user and password as a separate auth object
  if (uri.user || uri.password) {
    uri.auth = {username: uri.user, password: uri.password};
  }

  // Split the path part of the URI into parts using '/' as the delimiter
  // after removing any leading '/' and any trailing '/'
  var parts = uri.path.replace(/(^\/|\/$)/g, '').split('/');

  // Store the first part as the database name and remove it from the parts
  // array
  uri.db = parts.pop();
  // Prevent double encoding of URI component
  if (uri.db.indexOf('%') === -1) {
    uri.db = encodeURIComponent(uri.db);
  }

  // Restore the path by joining all the remaining parts (all the parts
  // except for the database name) with '/'s
  uri.path = parts.join('/');

  return uri;
}

// Generate a URL with the host data given by opts and the given path
function genDBUrl(opts, path) {
  return genUrl(opts, opts.db + '/' + path);
}

// Generate a URL with the host data given by opts and the given path
function genUrl(opts, path) {
  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  var pathDel = !opts.path ? '' : '/';

  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  return opts.protocol + '://' + opts.host +
         (opts.port ? (':' + opts.port) : '') +
         '/' + opts.path + pathDel + path;
}

function paramsToStr(params) {
  return '?' + Object.keys(params).map(function (k) {
    return k + '=' + encodeURIComponent(params[k]);
  }).join('&');
}

// Implements the PouchDB API for dealing with CouchDB instances over HTTP
function HttpPouch(opts, callback) {

  // The functions that will be publicly available for HttpPouch
  var api = this;

  var host = getHost(opts.name, opts);
  var dbUrl = genDBUrl(host, '');

  opts = clone(opts);

  var ourFetch = function (url, options) {

    options = options || {};
    options.headers = options.headers || new Headers();

    if (opts.auth || host.auth) {
      var nAuth = opts.auth || host.auth;
      var str = nAuth.username + ':' + nAuth.password;
      var token = btoa(unescape(encodeURIComponent(str)));
      options.headers.set('Authorization', 'Basic ' + token);
    }

    var headers = opts.headers || {};
    Object.keys(headers).forEach(function (key) {
      options.headers.append(key, headers[key]);
    });

    var fetchFun = opts.fetch || fetch;
    return fetchFun(url, options);
  };

  function adapterFun(name, fun) {
    return coreAdapterFun(name, getArguments(function (args) {
      setup().then(function () {
        return fun.apply(this, args);
      }).catch(function (e) {
        var callback = args.pop();
        callback(e);
      });
    }));
  }

  function fetchJSON(url, options, callback) {

    var result = {};

    options = options || {};
    options.headers = options.headers || new Headers();

    if (!options.headers.get('Content-Type')) {
      options.headers.set('Content-Type', 'application/json');
    }
    if (!options.headers.get('Accept')) {
      options.headers.set('Accept', 'application/json');
    }

    return ourFetch(url, options).then(function (response) {
      result.ok = response.ok;
      result.status = response.status;
      return response.json();
    }).then(function (json) {
      result.data = json;
      if (!result.ok) {
        result.data.status = result.status;
        var err = generateErrorFromResponse(result.data);
        if (callback) {
          return callback(err);
        } else {
          throw err;
        }
      }

      if (Array.isArray(result.data)) {
        result.data = result.data.map(function (v) {
          if (v.error || v.missing) {
            return generateErrorFromResponse(v);
          } else {
            return v;
          }
        });
      }

      if (callback) {
        callback(null, result.data);
      } else {
        return result;
      }
    });
  }

  var setupPromise;

  function setup() {
    if (opts.skip_setup) {
      return Promise.resolve();
    }

    // If there is a setup in process or previous successful setup
    // done then we will use that
    // If previous setups have been rejected we will try again
    if (setupPromise) {
      return setupPromise;
    }

    setupPromise = fetchJSON(dbUrl).catch(function (err) {
      if (err && err.status && err.status === 404) {
        // Doesnt exist, create it
        explainError(404, 'PouchDB is just detecting if the remote exists.');
        return fetchJSON(dbUrl, {method: 'PUT'});
      } else {
        return Promise.reject(err);
      }
    }).catch(function (err) {
      // If we try to create a database that already exists, skipped in
      // istanbul since its catching a race condition.
      /* istanbul ignore if */
      if (err && err.status && err.status === 412) {
        return true;
      }
      return Promise.reject(err);
    });

    setupPromise.catch(function () {
      setupPromise = null;
    });

    return setupPromise;
  }

  nextTick(function () {
    callback(null, api);
  });

  api._remote = true;

  /* istanbul ignore next */
  api.type = function () {
    return 'http';
  };

  api.id = adapterFun('id', function (callback) {
    ourFetch(genUrl(host, '')).then(function (response) {
      return response.json();
    }).then(function (result) {
      var uuid = (result && result.uuid) ?
          (result.uuid + host.db) : genDBUrl(host, '');
      callback(null, uuid);
    }).catch(function (err) {
      callback(err);
    });
  });

  // Sends a POST request to the host calling the couchdb _compact function
  //    version: The version of CouchDB it is running
  api.compact = adapterFun('compact', function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    fetchJSON(genDBUrl(host, '_compact'), {method: 'POST'}).then(function () {
      function ping() {
        api.info(function (err, res) {
          // CouchDB may send a "compact_running:true" if it's
          // already compacting. PouchDB Server doesn't.
          /* istanbul ignore else */
          if (res && !res.compact_running) {
            callback(null, {ok: true});
          } else {
            setTimeout(ping, opts.interval || 200);
          }
        });
      }
      // Ping the http if it's finished compaction
      ping();
    });
  });

  api.bulkGet = coreAdapterFun('bulkGet', function (opts, callback) {
    var self = this;

    function doBulkGet(cb) {
      var params = {};
      if (opts.revs) {
        params.revs = true;
      }
      if (opts.attachments) {
        /* istanbul ignore next */
        params.attachments = true;
      }
      if (opts.latest) {
        params.latest = true;
      }
      fetchJSON(genDBUrl(host, '_bulk_get' + paramsToStr(params)), {
        method: 'POST',
        body: JSON.stringify({ docs: opts.docs})
      }).then(function (result) {
        if (opts.attachments && opts.binary) {
          result.data.results.forEach(function (res) {
            res.docs.forEach(readAttachmentsAsBlobOrBuffer);
          });
        }
        cb(null, result.data);
      }).catch(cb);
    }

    /* istanbul ignore next */
    function doBulkGetShim() {
      // avoid "url too long error" by splitting up into multiple requests
      var batchSize = MAX_SIMULTANEOUS_REVS;
      var numBatches = Math.ceil(opts.docs.length / batchSize);
      var numDone = 0;
      var results = new Array(numBatches);

      function onResult(batchNum) {
        return function (err, res) {
          // err is impossible because shim returns a list of errs in that case
          results[batchNum] = res.results;
          if (++numDone === numBatches) {
            callback(null, {results: flatten(results)});
          }
        };
      }

      for (var i = 0; i < numBatches; i++) {
        var subOpts = pick(opts, ['revs', 'attachments', 'binary', 'latest']);
        subOpts.docs = opts.docs.slice(i * batchSize,
          Math.min(opts.docs.length, (i + 1) * batchSize));
        bulkGetShim(self, subOpts, onResult(i));
      }
    }

    // mark the whole database as either supporting or not supporting _bulk_get
    var dbUrl = genUrl(host, '');
    var supportsBulkGet = supportsBulkGetMap[dbUrl];

    /* istanbul ignore next */
    if (typeof supportsBulkGet !== 'boolean') {
      // check if this database supports _bulk_get
      doBulkGet(function (err, res) {
        if (err) {
          supportsBulkGetMap[dbUrl] = false;
          explainError(
            err.status,
            'PouchDB is just detecting if the remote ' +
            'supports the _bulk_get API.'
          );
          doBulkGetShim();
        } else {
          supportsBulkGetMap[dbUrl] = true;
          callback(null, res);
        }
      });
    } else if (supportsBulkGet) {
      doBulkGet(callback);
    } else {
      doBulkGetShim();
    }
  });

  // Calls GET on the host, which gets back a JSON string containing
  //    couchdb: A welcome string
  //    version: The version of CouchDB it is running
  api._info = function (callback) {
    setup().then(function () {
      return ourFetch(genDBUrl(host, ''));
    }).then(function (response) {
      return response.json();
    }).then(function (info) {
      info.host = genDBUrl(host, '');
      callback(null, info);
    }).catch(callback);
  };

  api.fetch = function (path, options) {
    return setup().then(function () {
      return ourFetch(genDBUrl(host, path), options);
    });
  };

  // Get the document with the given id from the database given by host.
  // The id could be solely the _id in the database, or it may be a
  // _design/ID or _local/ID path
  api.get = adapterFun('get', function (id, opts, callback) {
    // If no options were given, set the callback to the second parameter
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    // List of parameters to add to the GET request
    var params = {};

    if (opts.revs) {
      params.revs = true;
    }

    if (opts.revs_info) {
      params.revs_info = true;
    }

    if (opts.latest) {
      params.latest = true;
    }

    if (opts.open_revs) {
      if (opts.open_revs !== "all") {
        opts.open_revs = JSON.stringify(opts.open_revs);
      }
      params.open_revs = opts.open_revs;
    }

    if (opts.rev) {
      params.rev = opts.rev;
    }

    if (opts.conflicts) {
      params.conflicts = opts.conflicts;
    }

    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = opts.update_seq;
    }

    id = encodeDocId(id);

    function fetchAttachments(doc) {
      var atts = doc._attachments;
      var filenames = atts && Object.keys(atts);
      if (!atts || !filenames.length) {
        return;
      }
      // we fetch these manually in separate XHRs, because
      // Sync Gateway would normally send it back as multipart/mixed,
      // which we cannot parse. Also, this is more efficient than
      // receiving attachments as base64-encoded strings.
      function fetchData(filename) {
        var att = atts[filename];
        var path = encodeDocId(doc._id) + '/' + encodeAttachmentId(filename) +
            '?rev=' + doc._rev;
        return ourFetch(genDBUrl(host, path)).then(function (response) {
          if (typeof process !== 'undefined' && !process.browser) {
            return response.buffer();
          } else {
            /* istanbul ignore next */
            return response.blob();
          }
        }).then(function (blob) {
          if (opts.binary) {
            // TODO: Can we remove this?
            if (typeof process !== 'undefined' && !process.browser) {
              blob.type = att.content_type;
            }
            return blob;
          }
          return new Promise(function (resolve) {
            blufferToBase64(blob, resolve);
          });
        }).then(function (data) {
          delete att.stub;
          delete att.length;
          att.data = data;
        });
      }

      var promiseFactories = filenames.map(function (filename) {
        return function () {
          return fetchData(filename);
        };
      });

      // This limits the number of parallel xhr requests to 5 any time
      // to avoid issues with maximum browser request limits
      return pool(promiseFactories, 5);
    }

    function fetchAllAttachments(docOrDocs) {
      if (Array.isArray(docOrDocs)) {
        return Promise.all(docOrDocs.map(function (doc) {
          if (doc.ok) {
            return fetchAttachments(doc.ok);
          }
        }));
      }
      return fetchAttachments(docOrDocs);
    }

    var url = genDBUrl(host, id + paramsToStr(params));
    fetchJSON(url).then(function (res) {
      return Promise.resolve().then(function () {
        if (opts.attachments) {
          return fetchAllAttachments(res.data);
        }
      }).then(function () {
        callback(null, res.data);
      });
    }).catch(function (e) {
      e.docId = id;
      callback(e);
    });
  });


  // Delete the document given by doc from the database given by host.
  api.remove = adapterFun('remove', function (docOrId, optsOrRev, opts, cb) {
    var doc;
    if (typeof optsOrRev === 'string') {
      // id, rev, opts, callback style
      doc = {
        _id: docOrId,
        _rev: optsOrRev
      };
      if (typeof opts === 'function') {
        cb = opts;
        opts = {};
      }
    } else {
      // doc, opts, callback style
      doc = docOrId;
      if (typeof optsOrRev === 'function') {
        cb = optsOrRev;
        opts = {};
      } else {
        cb = opts;
        opts = optsOrRev;
      }
    }

    var rev = (doc._rev || opts.rev);
    var url = genDBUrl(host, encodeDocId(doc._id)) + '?rev=' + rev;

    fetchJSON(url, {method: 'DELETE'}, cb);
  });

  function encodeAttachmentId(attachmentId) {
    return attachmentId.split("/").map(encodeURIComponent).join("/");
  }

  // Get the attachment
  api.getAttachment = adapterFun('getAttachment', function (docId, attachmentId,
                                                            opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    var params = opts.rev ? ('?rev=' + opts.rev) : '';
    var url = genDBUrl(host, encodeDocId(docId)) + '/' +
        encodeAttachmentId(attachmentId) + params;
    var contentType;
    ourFetch(url, {method: 'GET'}).then(function (response) {
      contentType = response.headers.get('content-type');
      if (!response.ok) {
        throw response;
      } else {
        if (typeof process !== 'undefined' && !process.browser) {
          return response.buffer();
        } else {
          /* istanbul ignore next */
          return response.blob();
        }
      }
    }).then(function (blob) {
      // TODO: also remove
      if (typeof process !== 'undefined' && !process.browser) {
        blob.type = contentType;
      }
      callback(null, blob);
    }).catch(function (err) {
      callback(err);
    });
  });

  // Remove the attachment given by the id and rev
  api.removeAttachment =  adapterFun('removeAttachment', function (docId,
                                                                   attachmentId,
                                                                   rev,
                                                                   callback) {
    var url = genDBUrl(host, encodeDocId(docId) + '/' +
                       encodeAttachmentId(attachmentId)) + '?rev=' + rev;
    fetchJSON(url, {method: 'DELETE'}, callback);
  });

  // Add the attachment given by blob and its contentType property
  // to the document with the given id, the revision given by rev, and
  // add it to the database given by host.
  api.putAttachment = adapterFun('putAttachment', function (docId, attachmentId,
                                                            rev, blob,
                                                            type, callback) {
    if (typeof type === 'function') {
      callback = type;
      type = blob;
      blob = rev;
      rev = null;
    }
    var id = encodeDocId(docId) + '/' + encodeAttachmentId(attachmentId);
    var url = genDBUrl(host, id);
    if (rev) {
      url += '?rev=' + rev;
    }

    if (typeof blob === 'string') {
      // input is assumed to be a base64 string
      var binary;
      try {
        binary = atob(blob);
      } catch (err) {
        return callback(createError(BAD_ARG,
                        'Attachment is not a valid base64 string'));
      }
      blob = binary ? binStringToBluffer(binary, type) : '';
    }

    // Add the attachment
    fetchJSON(url, {
      headers: new Headers({'Content-Type': type}),
      method: 'PUT',
      body: blob
    }, callback);
  });

  // Update/create multiple documents given by req in the database
  // given by host.
  api._bulkDocs = function (req, opts, callback) {
    // If new_edits=false then it prevents the database from creating
    // new revision numbers for the documents. Instead it just uses
    // the old ones. This is used in database replication.
    req.new_edits = opts.new_edits;

    setup().then(function () {
      return Promise.all(req.docs.map(preprocessAttachments));
    }).then(function () {
      // Update/create the documents
      return fetchJSON(genDBUrl(host, '_bulk_docs'), {
        method: 'POST',
        body: JSON.stringify(req)
      }, callback);
    }).catch(callback);
  };


  // Update/create document
  api._put = function (doc, opts, callback) {
    setup().then(function () {
      return preprocessAttachments(doc);
    }).then(function () {
      return fetchJSON(genDBUrl(host, encodeDocId(doc._id)), {
        method: 'PUT',
        body: JSON.stringify(doc)
      });
    }).then(function (result) {
      callback(null, result.data);
    }).catch(function (err) {
      err.docId = doc && doc._id;
      callback(err);
    });
  };


  // Get a listing of the documents in the database given
  // by host and ordered by increasing id.
  api.allDocs = adapterFun('allDocs', function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    // List of parameters to add to the GET request
    var params = {};
    var body;
    var method = 'GET';

    if (opts.conflicts) {
      params.conflicts = true;
    }

    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = true;
    }

    if (opts.descending) {
      params.descending = true;
    }

    if (opts.include_docs) {
      params.include_docs = true;
    }

    // added in CouchDB 1.6.0
    if (opts.attachments) {
      params.attachments = true;
    }

    if (opts.key) {
      params.key = JSON.stringify(opts.key);
    }

    if (opts.start_key) {
      opts.startkey = opts.start_key;
    }

    if (opts.startkey) {
      params.startkey = JSON.stringify(opts.startkey);
    }

    if (opts.end_key) {
      opts.endkey = opts.end_key;
    }

    if (opts.endkey) {
      params.endkey = JSON.stringify(opts.endkey);
    }

    if (typeof opts.inclusive_end !== 'undefined') {
      params.inclusive_end = !!opts.inclusive_end;
    }

    if (typeof opts.limit !== 'undefined') {
      params.limit = opts.limit;
    }

    if (typeof opts.skip !== 'undefined') {
      params.skip = opts.skip;
    }

    var paramStr = paramsToStr(params);

    if (typeof opts.keys !== 'undefined') {
      method = 'POST';
      body = {keys: opts.keys};
    }

    fetchJSON(genDBUrl(host, '_all_docs' + paramStr), {
       method: method,
      body: JSON.stringify(body)
    }).then(function (result) {
      if (opts.include_docs && opts.attachments && opts.binary) {
        result.data.rows.forEach(readAttachmentsAsBlobOrBuffer);
      }
      callback(null, result.data);
    }).catch(callback);
  });

  // Get a list of changes made to documents in the database given by host.
  // TODO According to the README, there should be two other methods here,
  // api.changes.addListener and api.changes.removeListener.
  api._changes = function (opts) {

    // We internally page the results of a changes request, this means
    // if there is a large set of changes to be returned we can start
    // processing them quicker instead of waiting on the entire
    // set of changes to return and attempting to process them at once
    var batchSize = 'batch_size' in opts ? opts.batch_size : CHANGES_BATCH_SIZE;

    opts = clone(opts);

    if (opts.continuous && !('heartbeat' in opts)) {
      opts.heartbeat = DEFAULT_HEARTBEAT;
    }

    var requestTimeout = ('timeout' in opts) ? opts.timeout : 30 * 1000;

    // ensure CHANGES_TIMEOUT_BUFFER applies
    if ('timeout' in opts && opts.timeout &&
      (requestTimeout - opts.timeout) < CHANGES_TIMEOUT_BUFFER) {
        requestTimeout = opts.timeout + CHANGES_TIMEOUT_BUFFER;
    }

    /* istanbul ignore if */
    if ('heartbeat' in opts && opts.heartbeat &&
       (requestTimeout - opts.heartbeat) < CHANGES_TIMEOUT_BUFFER) {
        requestTimeout = opts.heartbeat + CHANGES_TIMEOUT_BUFFER;
    }

    var params = {};
    if ('timeout' in opts && opts.timeout) {
      params.timeout = opts.timeout;
    }

    var limit = (typeof opts.limit !== 'undefined') ? opts.limit : false;
    var returnDocs;
    if ('return_docs' in opts) {
      returnDocs = opts.return_docs;
    } else if ('returnDocs' in opts) {
      // TODO: Remove 'returnDocs' in favor of 'return_docs' in a future release
      returnDocs = opts.returnDocs;
    } else {
      returnDocs = true;
    }
    //
    var leftToFetch = limit;

    if (opts.style) {
      params.style = opts.style;
    }

    if (opts.include_docs || opts.filter && typeof opts.filter === 'function') {
      params.include_docs = true;
    }

    if (opts.attachments) {
      params.attachments = true;
    }

    if (opts.continuous) {
      params.feed = 'longpoll';
    }

    if (opts.seq_interval) {
      params.seq_interval = opts.seq_interval;
    }

    if (opts.conflicts) {
      params.conflicts = true;
    }

    if (opts.descending) {
      params.descending = true;
    }
    
    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = true;
    }

    if ('heartbeat' in opts) {
      // If the heartbeat value is false, it disables the default heartbeat
      if (opts.heartbeat) {
        params.heartbeat = opts.heartbeat;
      }
    }

    if (opts.filter && typeof opts.filter === 'string') {
      params.filter = opts.filter;
    }

    if (opts.view && typeof opts.view === 'string') {
      params.filter = '_view';
      params.view = opts.view;
    }

    // If opts.query_params exists, pass it through to the changes request.
    // These parameters may be used by the filter on the source database.
    if (opts.query_params && typeof opts.query_params === 'object') {
      for (var param_name in opts.query_params) {
        /* istanbul ignore else */
        if (opts.query_params.hasOwnProperty(param_name)) {
          params[param_name] = opts.query_params[param_name];
        }
      }
    }

    var method = 'GET';
    var body;

    if (opts.doc_ids) {
      // set this automagically for the user; it's annoying that couchdb
      // requires both a "filter" and a "doc_ids" param.
      params.filter = '_doc_ids';
      method = 'POST';
      body = {doc_ids: opts.doc_ids };
    }
    /* istanbul ignore next */
    else if (opts.selector) {
      // set this automagically for the user, similar to above
      params.filter = '_selector';
      method = 'POST';
      body = {selector: opts.selector };
    }

    var controller = new AbortController();
    var lastFetchedSeq;

    // Get all the changes starting wtih the one immediately after the
    // sequence number given by since.
    var fetchData = function (since, callback) {
      if (opts.aborted) {
        return;
      }
      params.since = since;
      // "since" can be any kind of json object in Coudant/CouchDB 2.x
      /* istanbul ignore next */
      if (typeof params.since === "object") {
        params.since = JSON.stringify(params.since);
      }

      if (opts.descending) {
        if (limit) {
          params.limit = leftToFetch;
        }
      } else {
        params.limit = (!limit || leftToFetch > batchSize) ?
          batchSize : leftToFetch;
      }

      // Set the options for the ajax call
      var url = genDBUrl(host, '_changes' + paramsToStr(params));
      var fetchOpts = {
        signal: controller.signal,
        method: method,
        body: JSON.stringify(body)
      };
      lastFetchedSeq = since;

      /* istanbul ignore if */
      if (opts.aborted) {
        return;
      }

      // Get the changes
      setup().then(function () {
        return fetchJSON(url, fetchOpts, callback);
      }).catch(callback);
    };

    // If opts.since exists, get all the changes from the sequence
    // number given by opts.since. Otherwise, get all the changes
    // from the sequence number 0.
    var results = {results: []};

    var fetched = function (err, res) {
      if (opts.aborted) {
        return;
      }
      var raw_results_length = 0;
      // If the result of the ajax call (res) contains changes (res.results)
      if (res && res.results) {
        raw_results_length = res.results.length;
        results.last_seq = res.last_seq;
        var pending = null;
        var lastSeq = null;
        // Attach 'pending' property if server supports it (CouchDB 2.0+)
        /* istanbul ignore if */
        if (typeof res.pending === 'number') {
          pending = res.pending;
        }
        if (typeof results.last_seq === 'string' || typeof results.last_seq === 'number') {
          lastSeq = results.last_seq;
        }
        // For each change
        var req = {};
        req.query = opts.query_params;
        res.results = res.results.filter(function (c) {
          leftToFetch--;
          var ret = filterChange(opts)(c);
          if (ret) {
            if (opts.include_docs && opts.attachments && opts.binary) {
              readAttachmentsAsBlobOrBuffer(c);
            }
            if (returnDocs) {
              results.results.push(c);
            }
            opts.onChange(c, pending, lastSeq);
          }
          return ret;
        });
      } else if (err) {
        // In case of an error, stop listening for changes and call
        // opts.complete
        opts.aborted = true;
        opts.complete(err);
        return;
      }

      // The changes feed may have timed out with no results
      // if so reuse last update sequence
      if (res && res.last_seq) {
        lastFetchedSeq = res.last_seq;
      }

      var finished = (limit && leftToFetch <= 0) ||
        (res && raw_results_length < batchSize) ||
        (opts.descending);

      if ((opts.continuous && !(limit && leftToFetch <= 0)) || !finished) {
        // Queue a call to fetch again with the newest sequence number
        nextTick(function () { fetchData(lastFetchedSeq, fetched); });
      } else {
        // We're done, call the callback
        opts.complete(null, results);
      }
    };

    fetchData(opts.since || 0, fetched);

    // Return a method to cancel this method from processing any more
    return {
      cancel: function () {
        opts.aborted = true;
        controller.abort();
      }
    };
  };

  // Given a set of document/revision IDs (given by req), tets the subset of
  // those that do NOT correspond to revisions stored in the database.
  // See http://wiki.apache.org/couchdb/HttpPostRevsDiff
  api.revsDiff = adapterFun('revsDiff', function (req, opts, callback) {
    // If no options were given, set the callback to be the second parameter
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }

    // Get the missing document/revision IDs
    fetchJSON(genDBUrl(host, '_revs_diff'), {
      method: 'POST',
      body: JSON.stringify(req)
    }, callback);
  });

  api._close = function (callback) {
    callback();
  };

  api._destroy = function (options, callback) {
    fetchJSON(genDBUrl(host, ''), {method: 'DELETE'}).then(function (json) {
      callback(null, json);
    }).catch(function (err) {
      /* istanbul ignore if */
      if (err.status === 404) {
        callback(null, {ok: true});
      } else {
        callback(err);
      }
    });
  };
}

// HttpPouch is a valid adapter.
HttpPouch.valid = function () {
  return true;
};

export default function (PouchDB) {
  PouchDB.adapter('http', HttpPouch, false);
  PouchDB.adapter('https', HttpPouch, false);
}