'use strict';

var utils = require('./utils');
var Pouch = require('./index');
var EE = require('events').EventEmitter;

// 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(opts) {
  EE.call(this);
  this.cancelled = false;
  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 (err) {});
}

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

Replication.prototype.ready = function (src, target) {
  var self = this;
  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) {
        return '_local/' + md5;
      });
    });
  });
}


function updateCheckpoint(db, id, checkpoint) {
    return db.get(id).catch(function (err) {
      if (err.status === 404) {
        return {_id: id};
      }
      throw err;
    }).then(function (doc) {
      doc.last_seq = checkpoint;
      return db.put(doc);
    });
  }

function Checkpointer(src, target, id) {
  this.src = src;
  this.target = target;
  this.id = id;
}

Checkpointer.prototype.writeCheckpoint = function (checkpoint) {
  var self = this;
  return this.updateTarget(checkpoint).then(function () {
    return self.updateSource(checkpoint);
  });
};
Checkpointer.prototype.updateTarget = function (checkpoint) {
  return updateCheckpoint(this.target, this.id, checkpoint);
};
Checkpointer.prototype.updateSource = function (checkpoint) {
  var self = this;
  if (this.readOnlySource) {
    return utils.Promise.resolve(true);
  }
  return updateCheckpoint(this.src, this.id, checkpoint).catch(function (err) {
    var isForbidden = typeof err.status === 'number' &&
      Math.floor(err.status / 100) === 4;
    if (isForbidden) {
      self.readOnlySource = true;
      return true;
    }
    throw err;
  });
};
Checkpointer.prototype.getCheckpoint = function () {
  var self = this;
  return self.target.get(self.id).then(function (targetDoc) {
    return self.src.get(self.id).then(function (sourceDoc) {
      if (targetDoc.last_seq === sourceDoc.last_seq) {
        return sourceDoc.last_seq;
      }
      return 0;
    }, function (err) {
      if (err.status === 404 && targetDoc.last_seq) {
        return self.src.put({
          _id: self.id,
          last_seq: 0
        }).then(function () {
          return 0;
        }, function (err) {
          if (err.status === 401) {
            self.readOnlySource = true;
            return targetDoc.last_seq;
          }
          return 0;
        });
      }
      throw err;
    });
  }).catch(function (err) {
    if (err.status !== 404) {
      throw err;
    }
    return 0;
  });
};
function replicate(repId, src, target, opts, returnValue) {
  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 changesCount = 0; // number of changes received since calling src.changes
  var doc_ids = opts.doc_ids;
  var checkpointer = new Checkpointer(src, target, repId);
  var 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 (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      var errors = [];
      res.forEach(function (res) {
        if (!res.ok) {
          result.doc_write_failures++;
          errors.push(new Error(res.reason || res.message || 'Unknown reason'));
        }
      });
      if (errors.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 getDocs() {
    var docIds = Object.keys(currentBatch.diffs);
    if (!docIds.length) {
      return utils.Promise.resolve();
    }
    var idsToDocs = new utils.Map();
    currentBatch.changes.forEach(function (change) {
      idsToDocs.set(change.id, change.doc);
    });
    return utils.Promise.all(docIds.map(function (docId) {
      return mapToDocWithOpenRevs(docId, idsToDocs.get(docId));
    })).then(function (docLists) {
      docLists.forEach(function (docs) {
        docs.forEach(processSourceDoc);
      });
    });
  }

  function mapToDocWithOpenRevs(docId, doc) {
    // as an optimization, we optimistically try to fetch all the open
    // revs using just the docs returned from changes()
    // if that's not possible, we do separate GETs for the open_revs
    if (doc._deleted) {
      return [{ok: doc}];
    }
    var diffs = currentBatch.diffs;
    var missing = diffs[docId].missing;
    var needAttachments = doc._attachments;
    var needOtherRevs = missing.length > 1 || missing[0] !== doc._rev;
    if (needAttachments || needOtherRevs) {
      // fetch individually (slower)
      return src.get(docId, {
        revs: true,
        open_revs: "all", // avoid url too long error, don't send revs array
        attachments: true
      }).then(function (results) {
        var missingMap = {};
        missing.forEach(function (missingRev) {
          missingMap[missingRev] = true;
        });
        return results.filter(function (res) {
          var rev = res.ok ? res.ok._rev : res.missing;
          return missingMap[rev];
        });
      });
    }
    return [{ok: doc}];
  }

  function processSourceDoc(doc) {
    var diffs = currentBatch.diffs;
    if (returnValue.cancelled) {
      return completeReplication();
    }
    if (doc.ok) {
      result.docs_read++;
      currentBatch.pendingRevs++;
      currentBatch.docs.push(doc.ok);
      delete diffs[doc.ok._id];
    }
  }


  function finishBatch() {
    writingCheckpoint = true;
    return checkpointer.writeCheckpoint(
      currentBatch.seq
    ).then(function (res) {
      writingCheckpoint = false;
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      result.last_seq = last_seq = currentBatch.seq;
      result.docs_written += currentBatch.docs.length;
      returnValue.emit('change', utils.clone(result));
      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 (returnValue.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 (returnValue.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('uptodate', utils.clone(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;
    }
    result.ok = false;
    result.status = 'aborted';
    result.errors.push(err);
    batches = [];
    pendingBatch = {
      seq: 0,
      changes: [],
      docs: []
    };
    completeReplication();
  }


  function completeReplication() {
    if (replicationCompleted) {
      return;
    }
    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 = returnValue.cancelled = true;
    if (result.errors.length > 0) {
      var error = result.errors.pop();
      if (result.errors.length > 0) {
        error.other_errors = result.errors;
      }
      error.result = result;
      returnValue.emit('error', error);
    } else {
      returnValue.emit('complete', result);
    }
    returnValue.removeAllListeners();
  }


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


  function onChangesComplete(changes) {
    changesPending = false;
    if (returnValue.cancelled) {
      return completeReplication();
    }
    if (changesCount > 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 (returnValue.cancelled) {
      return completeReplication();
    }
    abortReplication('changes rejected', err);
  }


  function getChanges() {
    if (
      !changesPending &&
      !changesCompleted &&
      batches.length < batches_limit
    ) {
      changesPending = true;
      changesCount = 0;
      src.changes(changesOpts)
      .on('change', onChange)
      .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: false,
        include_docs: true
      };
      if (opts.filter) {
        changesOpts.filter = opts.filter;
      }
      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 (res) {
      writingCheckpoint = false;
      if (returnValue.cancelled) {
        completeReplication();
        return;
      }
      last_seq = opts.since;
      startChanges();
    }).catch(function (err) {
      writingCheckpoint = false;
      abortReplication('writeCheckpoint completed with error', err);
      throw err;
    });
  }
}


function toPouch(db) {
  if (typeof db === 'string') {
    return new Pouch(db);
  } else if (db.then) {
    return db;
  } else {
    return utils.Promise.resolve(db);
  }
}


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;
  var replicateRet = new Replication(opts);
  toPouch(src).then(function (src) {
    return toPouch(target).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;
}

exports.replicate = replicateWrapper;