'use strict';

var utils = require('./utils');
var EE = require('events').EventEmitter;
var Checkpointer = require('./checkpointer');

var MAX_SIMULTANEOUS_REVS = 50;
var RETRY_DEFAULT = false;

function randomNumber(min, max) {
  min = parseInt(min, 10);
  max = parseInt(max, 10);
  if (min !== min) {
    min = 0;
  }
  if (max !== max || max <= min) {
    max = (min || 1) << 1; //doubling
  } else {
    max = max + 1;
  }
  var ratio = Math.random();
  var range = max - min;

  return ~~(range * ratio + min); // ~~ coerces to an int, but fast.
}

function defaultBackOff(min) {
  var max = 0;
  if (!min) {
    max = 2000;
  }
  return randomNumber(min, max);
}

function backOff(repId, src, target, opts, returnValue, result, error) {
  if (opts.retry === false) {
    returnValue.emit('error', error);
    returnValue.removeAllListeners();
    return;
  }
  opts.default_back_off = opts.default_back_off || 0;
  opts.retries = opts.retries || 0;
  if (typeof opts.back_off_function !== 'function') {
    opts.back_off_function = defaultBackOff;
  }
  opts.retries++;
  if (opts.max_retries && opts.retries > opts.max_retries) {
    returnValue.emit('error', new Error('tried ' +
      opts.retries + ' times but replication failed'));
    returnValue.removeAllListeners();
    return;
  }
  returnValue.emit('requestError', error);
  if (returnValue.state === 'active') {
    returnValue.emit('paused', error);
    returnValue.state = 'stopped';
    returnValue.once('active', function () {
      opts.current_back_off = opts.default_back_off;
    });
  }

  opts.current_back_off = opts.current_back_off || opts.default_back_off;
  opts.current_back_off = opts.back_off_function(opts.current_back_off);
  setTimeout(function () {
    replicate(repId, src, target, opts, returnValue);
  }, opts.current_back_off);
}

// We create a basic promise so the caller can cancel the replication possibly
// before we have actually started listening to changes etc
utils.inherits(Replication, EE);
function Replication() {
  EE.call(this);
  this.cancelled = false;
  this.state = 'pending';
  var self = this;
  var promise = new utils.Promise(function (fulfill, reject) {
    self.once('complete', fulfill);
    self.once('error', reject);
  });
  self.then = function (resolve, reject) {
    return promise.then(resolve, reject);
  };
  self.catch = function (reject) {
    return promise.catch(reject);
  };
  // As we allow error handling via "error" event as well,
  // put a stub in here so that rejecting never throws UnhandledError.
  self.catch(function () {});
}

Replication.prototype.cancel = function () {
  this.cancelled = true;
  this.state = 'cancelled';
  this.emit('cancel');
};

Replication.prototype.ready = function (src, target) {
  var self = this;
  this.once('change', function () {
    if (this.state === 'pending' || this.state === 'stopped') {
      self.state = 'active';
      self.emit('active');
    }
  });
  function onDestroy() {
    self.cancel();
  }
  src.once('destroyed', onDestroy);
  target.once('destroyed', onDestroy);
  function cleanup() {
    src.removeListener('destroyed', onDestroy);
    target.removeListener('destroyed', onDestroy);
  }
  this.then(cleanup, cleanup);
};


// TODO: check CouchDB's replication id generation
// Generate a unique id particular to this replication
function genReplicationId(src, target, opts) {
  var filterFun = opts.filter ? opts.filter.toString() : '';
  return src.id().then(function (src_id) {
    return target.id().then(function (target_id) {
      var queryData = src_id + target_id + filterFun +
        JSON.stringify(opts.query_params) + opts.doc_ids;
      return utils.MD5(queryData).then(function (md5) {
        // can't use straight-up md5 alphabet, because
        // the char '/' is interpreted as being for attachments,
        // and + is also not url-safe
        md5 = md5.replace(/\//g, '.').replace(/\+/g, '_');
        return '_local/' + md5;
      });
    });
  });
}

function replicate(repId, src, target, opts, returnValue, result) {
  var batches = [];               // list of batches to be processed
  var currentBatch;               // the batch currently being processed
  var pendingBatch = {
    seq: 0,
    changes: [],
    docs: []
  }; // next batch, not yet ready to be processed
  var writingCheckpoint = false;  // true while checkpoint is being written
  var changesCompleted = false;   // true when all changes received
  var replicationCompleted = false; // true when replication has completed
  var last_seq = 0;
  var continuous = opts.continuous || opts.live || false;
  var batch_size = opts.batch_size || 100;
  var batches_limit = opts.batches_limit || 10;
  var changesPending = false;     // true while src.changes is running
  var doc_ids = opts.doc_ids;
  var state = {
    cancelled: false
  };
  var checkpointer = new Checkpointer(src, target, repId, state);
  var allErrors = [];
  var changedDocs = [];

  result = result || {
    ok: true,
    start_time: new Date(),
    docs_read: 0,
    docs_written: 0,
    doc_write_failures: 0,
    errors: []
  };

  var changesOpts = {};
  returnValue.ready(src, target);

  function writeDocs() {
    if (currentBatch.docs.length === 0) {
      return;
    }
    var docs = currentBatch.docs;
    return target.bulkDocs({docs: docs, new_edits: false}).then(function (res) {
      if (state.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      var errors = [];
      var errorsById = {};
      res.forEach(function (res) {
        if (res.error) {
          result.doc_write_failures++;
          errors.push(res);
          errorsById[res.id] = res;
        }
      });
      result.errors = errors;
      allErrors = allErrors.concat(errors);
      result.docs_written += currentBatch.docs.length - errors.length;
      var non403s = errors.filter(function (error) {
        return error.name !== 'unauthorized' && error.name !== 'forbidden';
      });

      changedDocs = [];
      docs.forEach(function(doc) {
        var error = errorsById[doc._id];
        if (error) {
          returnValue.emit('denied', utils.clone(error));
        } else {
          changedDocs.push(doc);
        }
      });

      if (non403s.length > 0) {
        var error = new Error('bulkDocs error');
        error.other_errors = errors;
        abortReplication('target.bulkDocs failed to write docs', error);
        throw new Error('bulkWrite partial failure');
      }
    }, function (err) {
      result.doc_write_failures += docs.length;
      throw err;
    });
  }

  function processDiffDoc(id) {
    var diffs = currentBatch.diffs;
    var allMissing = diffs[id].missing;
    // avoid url too long error by batching
    var missingBatches = [];
    for (var i = 0; i < allMissing.length; i += MAX_SIMULTANEOUS_REVS) {
      missingBatches.push(allMissing.slice(i, Math.min(allMissing.length,
        i + MAX_SIMULTANEOUS_REVS)));
    }

    return utils.Promise.all(missingBatches.map(function (missing) {
      var opts = {
        revs: true,
        open_revs: missing,
        attachments: true
      };
      return src.get(id, opts).then(function (docs) {
        docs.forEach(function (doc) {
          if (state.cancelled) {
            return completeReplication();
          }
          if (doc.ok) {
            result.docs_read++;
            currentBatch.pendingRevs++;
            currentBatch.docs.push(doc.ok);
          }
        });
        delete diffs[id];
      });
    }));
  }

  function getAllDocs() {
    var diffKeys = Object.keys(currentBatch.diffs);
    return utils.Promise.all(diffKeys.map(processDiffDoc));
  }


  function getRevisionOneDocs() {
    // filter out the generation 1 docs and get them
    // leaving the non-generation one docs to be got otherwise
    var ids = Object.keys(currentBatch.diffs).filter(function (id) {
      var missing = currentBatch.diffs[id].missing;
      return missing.length === 1 && missing[0].slice(0, 2) === '1-';
    });
    if (!ids.length) { // nothing to fetch
      return utils.Promise.resolve();
    }
    return src.allDocs({
      keys: ids,
      include_docs: true
    }).then(function (res) {
      if (state.cancelled) {
        completeReplication();
        throw (new Error('cancelled'));
      }
      res.rows.forEach(function (row) {
        if (row.doc && !row.deleted &&
          row.value.rev.slice(0, 2) === '1-' && (
            !row.doc._attachments ||
            Object.keys(row.doc._attachments).length === 0
          )
        ) {
          result.docs_read++;
          currentBatch.pendingRevs++;
          currentBatch.docs.push(row.doc);
          delete currentBatch.diffs[row.id];
        }
      });
    });
  }

  function getDocs() {
    return getRevisionOneDocs().then(getAllDocs);
  }

  function finishBatch() {
    writingCheckpoint = true;
    return checkpointer.writeCheckpoint(currentBatch.seq).then(function () {
      writingCheckpoint = false;
      if (state.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      result.last_seq = last_seq = currentBatch.seq;
      var outResult = utils.clone(result);
      outResult.docs = changedDocs;
      returnValue.emit('change', outResult);
      currentBatch = undefined;
      getChanges();
    }).catch(function (err) {
      writingCheckpoint = false;
      abortReplication('writeCheckpoint completed with error', err);
      throw err;
    });
  }

  function getDiffs() {
    var diff = {};
    currentBatch.changes.forEach(function (change) {
      diff[change.id] = change.changes.map(function (x) {
        return x.rev;
      });
    });
    return target.revsDiff(diff).then(function (diffs) {
      if (state.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      // currentBatch.diffs elements are deleted as the documents are written
      currentBatch.diffs = diffs;
      currentBatch.pendingRevs = 0;
    });
  }

  function startNextBatch() {
    if (state.cancelled || currentBatch) {
      return;
    }
    if (batches.length === 0) {
      processPendingBatch(true);
      return;
    }
    currentBatch = batches.shift();
    getDiffs()
      .then(getDocs)
      .then(writeDocs)
      .then(finishBatch)
      .then(startNextBatch)
      .catch(function (err) {
        abortReplication('batch processing terminated with error', err);
      });
  }


  function processPendingBatch(immediate) {
    if (pendingBatch.changes.length === 0) {
      if (batches.length === 0 && !currentBatch) {
        if ((continuous && changesOpts.live) || changesCompleted) {
          returnValue.emit('paused');
          returnValue.emit('uptodate', result);
        }
        if (changesCompleted) {
          completeReplication();
        }
      }
      return;
    }
    if (
      immediate ||
      changesCompleted ||
      pendingBatch.changes.length >= batch_size
    ) {
      batches.push(pendingBatch);
      pendingBatch = {
        seq: 0,
        changes: [],
        docs: []
      };
      startNextBatch();
    }
  }


  function abortReplication(reason, err) {
    if (replicationCompleted) {
      return;
    }
    if (!err.message) {
      err.message = reason;
    }
    result.ok = false;
    result.status = 'aborting';
    result.errors.push(err);
    allErrors = allErrors.concat(err);
    batches = [];
    pendingBatch = {
      seq: 0,
      changes: [],
      docs: []
    };
    completeReplication();
  }


  function completeReplication() {
    if (replicationCompleted) {
      return;
    }
    if (state.cancelled) {
      result.status = 'cancelled';
      if (writingCheckpoint) {
        return;
      }
    }
    result.status = result.status || 'complete';
    result.end_time = new Date();
    result.last_seq = last_seq;
    replicationCompleted = state.cancelled = true;
    var non403s = allErrors.filter(function (error) {
      return error.name !== 'unauthorized' && error.name !== 'forbidden';
    });
    if (non403s.length > 0) {
      var error = allErrors.pop();
      if (allErrors.length > 0) {
        error.other_errors = allErrors;
      }
      error.result = result;
      backOff(repId, src, target, opts, returnValue, result, error);
    } else {
      result.errors = allErrors;
      returnValue.emit('complete', result);
      returnValue.removeAllListeners();
    }
  }


  function onChange(change) {
    if (state.cancelled) {
      return completeReplication();
    }
    var filter = utils.filterChange(opts)(change);
    if (!filter) {
      return;
    }
    if (
      pendingBatch.changes.length === 0 &&
      batches.length === 0 &&
      !currentBatch
    ) {
      returnValue.emit('outofdate', result);
    }
    pendingBatch.seq = change.seq;
    pendingBatch.changes.push(change);
    processPendingBatch(batches.length === 0);
  }


  function onChangesComplete(changes) {
    changesPending = false;
    if (state.cancelled) {
      return completeReplication();
    }

    // if no results were returned then we're done,
    // else fetch more
    if (changes.results.length > 0) {
      changesOpts.since = changes.last_seq;
      getChanges();
    } else {
      if (continuous) {
        changesOpts.live = true;
        getChanges();
      } else {
        changesCompleted = true;
      }
    }
    processPendingBatch(true);
  }


  function onChangesError(err) {
    changesPending = false;
    if (state.cancelled) {
      return completeReplication();
    }
    abortReplication('changes rejected', err);
  }


  function getChanges() {
    if (!(
      !changesPending &&
      !changesCompleted &&
      batches.length < batches_limit
    )) {
      return;
    }
    changesPending = true;
    function abortChanges() {
      changes.cancel();
    }
    function removeListener() {
      returnValue.removeListener('cancel', abortChanges);
    }
    returnValue.once('cancel', abortChanges);
    var changes = src.changes(changesOpts)
    .on('change', onChange);
    changes.then(removeListener, removeListener);
    changes.then(onChangesComplete)
    .catch(onChangesError);
  }


  function startChanges() {
    checkpointer.getCheckpoint().then(function (checkpoint) {
      last_seq = checkpoint;
      changesOpts = {
        since: last_seq,
        limit: batch_size,
        batch_size: batch_size,
        style: 'all_docs',
        doc_ids: doc_ids,
        returnDocs: true // required so we know when we're done
      };
      if (opts.filter) {
        // required for the client-side filter in onChange
        changesOpts.include_docs = true;
      }
      if (opts.query_params) {
        changesOpts.query_params = opts.query_params;
      }
      getChanges();
    }).catch(function (err) {
      abortReplication('getCheckpoint rejected with ', err);
    });
  }


  returnValue.once('cancel', completeReplication);

  if (typeof opts.onChange === 'function') {
    returnValue.on('change', opts.onChange);
  }

  if (typeof opts.complete === 'function') {
    returnValue.once('error', opts.complete);
    returnValue.once('complete', function (result) {
      opts.complete(null, result);
    });
  }

  if (typeof opts.since === 'undefined') {
    startChanges();
  } else {
    writingCheckpoint = true;
    checkpointer.writeCheckpoint(opts.since).then(function () {
      writingCheckpoint = false;
      if (state.cancelled) {
        completeReplication();
        return;
      }
      last_seq = opts.since;
      startChanges();
    }).catch(function (err) {
      writingCheckpoint = false;
      abortReplication('writeCheckpoint completed with error', err);
      throw err;
    });
  }
}

exports.toPouch = toPouch;
function toPouch(db, opts) {
  var PouchConstructor = opts.PouchConstructor;
  if (typeof db === 'string') {
    return new PouchConstructor(db);
  } else if (db.then) {
    return db;
  } else {
    return utils.Promise.resolve(db);
  }
}


exports.replicate = replicateWrapper;
function replicateWrapper(src, target, opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  if (typeof opts === 'undefined') {
    opts = {};
  }
  if (!opts.complete) {
    opts.complete = callback || function () {};
  }
  opts = utils.clone(opts);
  opts.continuous = opts.continuous || opts.live;
  opts.retry = ('retry' in opts) ? opts.retry : RETRY_DEFAULT;
  /*jshint validthis:true */
  opts.PouchConstructor = opts.PouchConstructor || this;
  var replicateRet = new Replication(opts);
  toPouch(src, opts).then(function (src) {
    return toPouch(target, opts).then(function (target) {
      return genReplicationId(src, target, opts).then(function (repId) {
        replicate(repId, src, target, opts, replicateRet);
      });
    });
  }).catch(function (err) {
    replicateRet.emit('error', err);
    opts.complete(err);
  });
  return replicateRet;
}