import { clone, filterChange, uuid } from 'pouchdb-utils';
import getDocs from './getDocs';
import Promise from 'pouchdb-promise';
import Checkpointer from 'pouchdb-checkpointer';
import backOff from './backoff';
import generateReplicationId from 'pouchdb-generate-replication-id';

function replicate(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 repId;
  var checkpointer;
  var changedDocs = [];
  // Like couchdb, every replication gets a unique session id
  var session = uuid();

  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 initCheckpointer() {
    if (checkpointer) {
      return Promise.resolve();
    }
    return generateReplicationId(src, target, opts).then(function (res) {
      repId = res;
      checkpointer = new Checkpointer(src, target, repId, returnValue);
    });
  }

  function writeDocs() {
    changedDocs = [];

    if (currentBatch.docs.length === 0) {
      return;
    }
    var docs = currentBatch.docs;
    var bulkOpts = {timeout: opts.timeout};
    return target.bulkDocs({docs: docs, new_edits: false}, bulkOpts).then(function (res) {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }

      // `res` doesn't include full documents (which live in `docs`), so we create a map of 
      // (id -> error), and check for errors while iterating over `docs`
      var errorsById = Object.create(null);
      res.forEach(function (res) {
        if (res.error) {
          errorsById[res.id] = res;
        }
      });

      var errorsNo = Object.keys(errorsById).length;
      result.doc_write_failures += errorsNo;
      result.docs_written += docs.length - errorsNo;

      docs.forEach(function (doc) {
        var error = errorsById[doc._id];
        if (error) {
          result.errors.push(error);
          if (error.name === 'unauthorized' || error.name === 'forbidden') {
            returnValue.emit('denied', clone(error));
          } else {
            throw error;
          }
        } else {
          changedDocs.push(doc);
        }
      });

    }, function (err) {
      result.doc_write_failures += docs.length;
      throw err;
    });
  }

  function finishBatch() {
    if (currentBatch.error) {
      throw new Error('There was a problem getting docs.');
    }
    result.last_seq = last_seq = currentBatch.seq;
    var outResult = clone(result);
    if (changedDocs.length) {
      outResult.docs = changedDocs;
      returnValue.emit('change', outResult);
    }
    writingCheckpoint = true;
    return checkpointer.writeCheckpoint(currentBatch.seq,
        session).then(function () {
      writingCheckpoint = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      currentBatch = undefined;
      getChanges();
    }).catch(function (err) {
      onCheckpointError(err);
      throw err;
    });
  }

  function getDiffs() {
    var diff = {};
    currentBatch.changes.forEach(function (change) {
      // Couchbase Sync Gateway emits these, but we can ignore them
      /* istanbul ignore if */
      if (change.id === "_user/") {
        return;
      }
      diff[change.id] = change.changes.map(function (x) {
        return x.rev;
      });
    });
    return target.revsDiff(diff).then(function (diffs) {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      // currentBatch.diffs elements are deleted as the documents are written
      currentBatch.diffs = diffs;
    });
  }

  function getBatchDocs() {
    return getDocs(src, target, currentBatch.diffs, returnValue).then(function (got) {
      currentBatch.error = !got.ok;
      got.docs.forEach(function (doc) {
        delete currentBatch.diffs[doc._id];
        result.docs_read++;
        currentBatch.docs.push(doc);
      });
    });
  }

  function startNextBatch() {
    if (returnValue.cancelled || currentBatch) {
      return;
    }
    if (batches.length === 0) {
      processPendingBatch(true);
      return;
    }
    currentBatch = batches.shift();
    getDiffs()
      .then(getBatchDocs)
      .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.state = 'pending';
          returnValue.emit('paused');
        }
        if (changesCompleted) {
          completeReplication();
        }
      }
      return;
    }
    if (
      immediate ||
      changesCompleted ||
      pendingBatch.changes.length >= batch_size
    ) {
      batches.push(pendingBatch);
      pendingBatch = {
        seq: 0,
        changes: [],
        docs: []
      };
      if (returnValue.state === 'pending' || returnValue.state === 'stopped') {
        returnValue.state = 'active';
        returnValue.emit('active');
      }
      startNextBatch();
    }
  }


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


  function completeReplication(fatalError) {
    if (replicationCompleted) {
      return;
    }
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      result.status = 'cancelled';
      if (writingCheckpoint) {
        return;
      }
    }
    result.status = result.status || 'complete';
    result.end_time = new Date();
    result.last_seq = last_seq;
    replicationCompleted = true;

    if (fatalError) {
      fatalError.result = result;

      if (fatalError.name === 'unauthorized' || fatalError.name === 'forbidden') {
        returnValue.emit('error', fatalError);
        returnValue.removeAllListeners();
      } else {
        backOff(opts, returnValue, fatalError, function () {
          replicate(src, target, opts, returnValue);
        });
      }
    } else {
      returnValue.emit('complete', result);
      returnValue.removeAllListeners();
    }
  }


  function onChange(change) {
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      return completeReplication();
    }
    var filter = filterChange(opts)(change);
    if (!filter) {
      return;
    }
    pendingBatch.seq = change.seq;
    pendingBatch.changes.push(change);
    processPendingBatch(batches.length === 0 && changesOpts.live);
  }


  function onChangesComplete(changes) {
    changesPending = false;
    /* istanbul ignore if */
    if (returnValue.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();
      processPendingBatch(true);
    } else {

      var complete = function () {
        if (continuous) {
          changesOpts.live = true;
          getChanges();
        } else {
          changesCompleted = true;
        }
        processPendingBatch(true);
      };

      // update the checkpoint so we start from the right seq next time
      if (!currentBatch && changes.results.length === 0) {
        writingCheckpoint = true;
        checkpointer.writeCheckpoint(changes.last_seq,
            session).then(function () {
          writingCheckpoint = false;
          result.last_seq = last_seq = changes.last_seq;
          complete();
        })
        .catch(onCheckpointError);
      } else {
        complete();
      }
    }
  }


  function onChangesError(err) {
    changesPending = false;
    /* istanbul ignore if */
    if (returnValue.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);
    }

    if (returnValue._changes) { // remove old changes() and listeners
      returnValue.removeListener('cancel', returnValue._abortChanges);
      returnValue._changes.cancel();
    }
    returnValue.once('cancel', abortChanges);

    var changes = src.changes(changesOpts)
      .on('change', onChange);
    changes.then(removeListener, removeListener);
    changes.then(onChangesComplete)
      .catch(onChangesError);

    if (opts.retry) {
      // save for later so we can cancel if necessary
      returnValue._changes = changes;
      returnValue._abortChanges = abortChanges;
    }
  }


  function startChanges() {
    initCheckpointer().then(function () {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        return;
      }
      return 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,
          return_docs: true // required so we know when we're done
        };
        if (opts.filter) {
          if (typeof opts.filter !== 'string') {
            // required for the client-side filter in onChange
            changesOpts.include_docs = true;
          } else { // ddoc filter
            changesOpts.filter = opts.filter;
          }
        }
        if ('heartbeat' in opts) {
          changesOpts.heartbeat = opts.heartbeat;
        }
        if ('timeout' in opts) {
          changesOpts.timeout = opts.timeout;
        }
        if (opts.query_params) {
          changesOpts.query_params = opts.query_params;
        }
        if (opts.view) {
          changesOpts.view = opts.view;
        }
        getChanges();
      });
    }).catch(function (err) {
      abortReplication('getCheckpoint rejected with ', err);
    });
  }

  /* istanbul ignore next */
  function onCheckpointError(err) {
    writingCheckpoint = false;
    abortReplication('writeCheckpoint completed with error', err);
  }

  /* istanbul ignore if */
  if (returnValue.cancelled) { // cancelled immediately
    completeReplication();
    return;
  }

  if (!returnValue._addedListeners) {
    returnValue.once('cancel', completeReplication);

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

  if (typeof opts.since === 'undefined') {
    startChanges();
  } else {
    initCheckpointer().then(function () {
      writingCheckpoint = true;
      return checkpointer.writeCheckpoint(opts.since, session);
    }).then(function () {
      writingCheckpoint = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        return;
      }
      last_seq = opts.since;
      startChanges();
    }).catch(onCheckpointError);
  }
}

export default replicate;