// Copyright IBM Corp. 2014,2016. All Rights Reserved.
  // Node module: loopback
  // This file is licensed under the MIT License.
  // License text available at https://opensource.org/licenses/MIT
  
  /*!
   * Module Dependencies.
   */
  
  var g = require('strong-globalize')();
  
  var PersistedModel = require('../../lib/loopback').PersistedModel;
  var loopback = require('../../lib/loopback');
  var utils = require('../../lib/utils');
  var crypto = require('crypto');
  var CJSON = {stringify: require('canonical-json')};
  var async = require('async');
  var assert = require('assert');
  var debug = require('debug')('loopback:change');
  var deprecate = require('depd')('loopback');
  
  /**
   * Change list entry.
   *
   * @property {String} id Hash of the modelName and ID.
   * @property {String} rev The current model revision.
   * @property {String} prev The previous model revision.
   * @property {Number} checkpoint The current checkpoint at time of the change.
   * @property {String} modelName Model name.
   * @property {String} modelId Model ID.
   * @property {Object} settings Extends the `Model.settings` object.
   * @property {String} settings.hashAlgorithm Algorithm used to create cryptographic hash, used as argument
   * to [crypto.createHash](http://nodejs.org/api/crypto.html#crypto_crypto_createhash_algorithm).  Default is sha1.
   * @property {Boolean} settings.ignoreErrors By default, when changes are rectified, an error will throw an exception.
   * However, if this setting is true, then errors will not throw exceptions.
   * @class Change
   * @inherits {PersistedModel}
   */
  
  module.exports = function(Change) {
  
    /*!
     * Constants
     */
  
    Change.UPDATE = 'update';
    Change.CREATE = 'create';
    Change.DELETE = 'delete';
    Change.UNKNOWN = 'unknown';
  
    /*!
     * Conflict Class
     */
  
    Change.Conflict = Conflict;
  
    /*!
     * Setup the extended model.
     */
  
    Change.setup = function() {
      PersistedModel.setup.call(this);
      var Change = this;
  
      Change.getter.id = function() {
        var hasModel = this.modelName && this.modelId;
        if (!hasModel) return null;
  
        return Change.idForModel(this.modelName, this.modelId);
      };
    };
    Change.setup();
  
    /**
     * Track the recent change of the given modelIds.
     *
     * @param  {String}   modelName
     * @param  {Array}    modelIds
     * @callback {Function} callback
     * @param {Error} err
     * @param {Array} changes Changes that were tracked
     */
  
    Change.rectifyModelChanges = function(modelName, modelIds, callback) {
      var Change = this;
      var errors = [];
  
      callback = callback || utils.createPromiseCallback();
  
      var tasks = modelIds.map(function(id) {
        return function(cb) {
          Change.findOrCreateChange(modelName, id, function(err, change) {
            if (err) return next(err);
            change.rectify(next);
          });
  
          function next(err) {
            if (err) {
              err.modelName = modelName;
              err.modelId = id;
              errors.push(err);
            }
            cb();
          }
        };
      });
  
      async.parallel(tasks, function(err) {
        if (err) return callback(err);
        if (errors.length) {
          var desc = errors
            .map(function(e) {
              return '#' + e.modelId + ' - ' + e.toString();
            })
            .join('
  ');
  
          var msg = g.f('Cannot rectify %s changes:
  %s', modelName, desc);
          err = new Error(msg);
          err.details = { errors: errors };
          return callback(err);
        }
        callback();
      });
      return callback.promise;
    };
  
    /**
     * Get an identifier for a given model.
     *
     * @param  {String} modelName
     * @param  {String} modelId
     * @return {String}
     */
  
    Change.idForModel = function(modelName, modelId) {
      return this.hash([modelName, modelId].join('-'));
    };
  
    /**
     * Find or create a change for the given model.
     *
     * @param  {String}   modelName
     * @param  {String}   modelId
     * @callback  {Function} callback
     * @param {Error} err
     * @param {Change} change
     * @end
     */
  
    Change.findOrCreateChange = function(modelName, modelId, callback) {
      assert(this.registry.findModel(modelName), modelName + ' does not exist');
      callback = callback || utils.createPromiseCallback();
      var id = this.idForModel(modelName, modelId);
      var Change = this;
  
      this.findById(id, function(err, change) {
        if (err) return callback(err);
        if (change) {
          callback(null, change);
        } else {
          var ch = new Change({
            id: id,
            modelName: modelName,
            modelId: modelId
          });
          ch.debug('creating change');
          Change.updateOrCreate(ch, callback);
        }
      });
      return callback.promise;
    };
  
    /**
     * Update (or create) the change with the current revision.
     *
     * @callback {Function} callback
     * @param {Error} err
     * @param {Change} change
     */
  
    Change.prototype.rectify = function(cb) {
      var change = this;
      var currentRev = this.rev;
  
      change.debug('rectify change');
  
      cb = cb || utils.createPromiseCallback();
  
      change.currentRevision(function(err, rev) {
        if (err) return cb(err);
  
        // avoid setting rev and prev to the same value
        if (currentRev === rev) {
          change.debug('rev and prev are equal (not updating anything)');
          return cb(null, change);
        }
  
        // FIXME(@bajtos) Allo callers to pass in the checkpoint value
        // (or even better - a memoized async function to get the cp value)
        // That will enable `rectifyAll` to cache the checkpoint value
        change.constructor.getCheckpointModel().current(
          function(err, checkpoint) {
            if (err) return cb(err);
            doRectify(checkpoint, rev);
          }
        );
      });
      return cb.promise;
  
      function doRectify(checkpoint, rev) {
        if (rev) {
          if (currentRev === rev) {
            change.debug('ASSERTION FAILED: Change currentRev==rev ' +
              'should have been already handled');
            return cb(null, change);
          } else {
            change.rev = rev;
            change.debug('updated revision (was ' + currentRev + ')');
            if (change.checkpoint !== checkpoint) {
              // previous revision is updated only across checkpoints
              change.prev = currentRev;
              change.debug('updated prev');
            }
          }
        } else {
          change.rev = null;
          change.debug('updated revision (was ' + currentRev + ')');
          if (change.checkpoint !== checkpoint) {
            // previous revision is updated only across checkpoints
            if (currentRev) {
              change.prev = currentRev;
            } else if (!change.prev) {
              change.debug('ERROR - could not determing prev');
              change.prev = Change.UNKNOWN;
            }
            change.debug('updated prev');
          }
        }
  
        if (change.checkpoint != checkpoint) {
          debug('update checkpoint to', checkpoint);
          change.checkpoint = checkpoint;
        }
  
        if (change.prev === Change.UNKNOWN) {
          // this occurs when a record of a change doesn't exist
          // and its current revision is null (not found)
          change.remove(cb);
        } else {
          change.save(cb);
        }
      }
    };
  
    /**
     * Get a change's current revision based on current data.
     * @callback  {Function} callback
     * @param {Error} err
     * @param {String} rev The current revision
     */
  
    Change.prototype.currentRevision = function(cb) {
      cb = cb || utils.createPromiseCallback();
      var model = this.getModelCtor();
      var id = this.getModelId();
      model.findById(id, function(err, inst) {
        if (err) return cb(err);
        if (inst) {
          cb(null, Change.revisionForInst(inst));
        } else {
          cb(null, null);
        }
      });
      return cb.promise;
    };
  
    /**
     * Create a hash of the given `string` with the `options.hashAlgorithm`.
     * **Default: `sha1`**
     *
     * @param  {String} str The string to be hashed
     * @return {String}     The hashed string
     */
  
    Change.hash = function(str) {
      return crypto
        .createHash(Change.settings.hashAlgorithm || 'sha1')
        .update(str)
        .digest('hex');
    };
  
    /**
     * Get the revision string for the given object
     * @param  {Object} inst The data to get the revision string for
     * @return {String}      The revision string
     */
  
    Change.revisionForInst = function(inst) {
      assert(inst, 'Change.revisionForInst() requires an instance object.');
      return this.hash(CJSON.stringify(inst));
    };
  
    /**
     * Get a change's type. Returns one of:
     *
     * - `Change.UPDATE`
     * - `Change.CREATE`
     * - `Change.DELETE`
     * - `Change.UNKNOWN`
     *
     * @return {String} the type of change
     */
  
    Change.prototype.type = function() {
      if (this.rev && this.prev) {
        return Change.UPDATE;
      }
      if (this.rev && !this.prev) {
        return Change.CREATE;
      }
      if (!this.rev && this.prev) {
        return Change.DELETE;
      }
      return Change.UNKNOWN;
    };
  
    /**
     * Compare two changes.
     * @param  {Change} change
     * @return {Boolean}
     */
  
    Change.prototype.equals = function(change) {
      if (!change) return false;
      var thisRev = this.rev || null;
      var thatRev = change.rev || null;
      return thisRev === thatRev;
    };
  
    /**
     * Does this change conflict with the given change.
     * @param  {Change} change
     * @return {Boolean}
     */
  
    Change.prototype.conflictsWith = function(change) {
      if (!change) return false;
      if (this.equals(change)) return false;
      if (Change.bothDeleted(this, change)) return false;
      if (this.isBasedOn(change)) return false;
      return true;
    };
  
    /**
     * Are both changes deletes?
     * @param  {Change} a
     * @param  {Change} b
     * @return {Boolean}
     */
  
    Change.bothDeleted = function(a, b) {
      return a.type() === Change.DELETE &&
        b.type() === Change.DELETE;
    };
  
    /**
     * Determine if the change is based on the given change.
     * @param  {Change} change
     * @return {Boolean}
     */
  
    Change.prototype.isBasedOn = function(change) {
      return this.prev === change.rev;
    };
  
    /**
     * Determine the differences for a given model since a given checkpoint.
     *
     * The callback will contain an error or `result`.
     *
     * **result**
     *
     * ```js
     * {
   *   deltas: Array,
   *   conflicts: Array
   * }
     * ```
     *
     * **deltas**
     *
     * An array of changes that differ from `remoteChanges`.
     *
     * **conflicts**
     *
     * An array of changes that conflict with `remoteChanges`.
     *
     * @param  {String}   modelName
     * @param  {Number}   since         Compare changes after this checkpoint
     * @param  {Change[]} remoteChanges A set of changes to compare
     * @callback  {Function} callback
     * @param {Error} err
     * @param {Object} result See above.
     */
  
    Change.diff = function(modelName, since, remoteChanges, callback) {
      callback = callback || utils.createPromiseCallback();
  
      if (!Array.isArray(remoteChanges) || remoteChanges.length === 0) {
        callback(null, {deltas: [], conflicts: []});
        return callback.promise;
      }
      var remoteChangeIndex = {};
      var modelIds = [];
      remoteChanges.forEach(function(ch) {
        modelIds.push(ch.modelId);
        remoteChangeIndex[ch.modelId] = new Change(ch);
      });
  
      // normalize `since`
      since = Number(since) || 0;
      this.find({
        where: {
          modelName: modelName,
          modelId: {inq: modelIds}
        }
      }, function(err, allLocalChanges) {
        if (err) return callback(err);
        var deltas = [];
        var conflicts = [];
        var localModelIds = [];
  
        var localChanges = allLocalChanges.filter(function(c) {
          return c.checkpoint >= since;
        });
  
        localChanges.forEach(function(localChange) {
          localChange = new Change(localChange);
          localModelIds.push(localChange.modelId);
          var remoteChange = remoteChangeIndex[localChange.modelId];
          if (remoteChange && !localChange.equals(remoteChange)) {
            if (remoteChange.conflictsWith(localChange)) {
              remoteChange.debug('remote conflict');
              localChange.debug('local conflict');
              conflicts.push(localChange);
            } else {
              remoteChange.debug('remote delta');
              deltas.push(remoteChange);
            }
          }
        });
  
        modelIds.forEach(function(id) {
          if (localModelIds.indexOf(id) !== -1) return;
  
          var d = remoteChangeIndex[id];
          var oldChange = allLocalChanges.filter(function(c) {
            return c.modelId === id;
          })[0];
  
          if (oldChange) {
            d.prev = oldChange.rev;
          } else {
            d.prev = null;
          }
  
          deltas.push(d);
        });
  
        callback(null, {
          deltas: deltas,
          conflicts: conflicts
        });
      });
      return callback.promise;
    };
  
    /**
     * Correct all change list entries.
     * @param {Function} cb
     */
  
    Change.rectifyAll = function(cb) {
      debug('rectify all');
      var Change = this;
      // this should be optimized
      this.find(function(err, changes) {
        if (err) return cb(err);
        async.each(
          changes,
          function(c, next) { c.rectify(next); },
          cb);
      });
    };
  
    /**
     * Get the checkpoint model.
     * @return {Checkpoint}
     */
  
    Change.getCheckpointModel = function() {
      var checkpointModel = this.Checkpoint;
      if (checkpointModel) return checkpointModel;
      // FIXME(bajtos) This code creates multiple different models with the same
      // model name, which is not a valid supported usage of juggler's API.
      this.Checkpoint = checkpointModel = loopback.Checkpoint.extend('checkpoint');
      assert(this.dataSource, 'Cannot getCheckpointModel(): ' + this.modelName +
        ' is not attached to a dataSource');
      checkpointModel.attachTo(this.dataSource);
      return checkpointModel;
    };
  
    Change.handleError = function(err) {
      deprecate('Change.handleError is deprecated, ' +
        'you should pass errors to your callback instead.');
  
      if (!this.settings.ignoreErrors) {
        throw err;
      }
    };
  
    Change.prototype.debug = function() {
      if (debug.enabled) {
        var args = Array.prototype.slice.call(arguments);
        args[0] = args[0] + ' %s';
        args.push(this.modelName);
        debug.apply(this, args);
        debug('\tid', this.id);
        debug('\trev', this.rev);
        debug('\tprev', this.prev);
        debug('\tcheckpoint', this.checkpoint);
        debug('\tmodelName', this.modelName);
        debug('\tmodelId', this.modelId);
        debug('\ttype', this.type());
      }
    };
  
    /**
     * Get the `Model` class for `change.modelName`.
     * @return {Model}
     */
  
    Change.prototype.getModelCtor = function() {
      return this.constructor.settings.trackModel;
    };
  
    Change.prototype.getModelId = function() {
      // TODO(ritch) get rid of the need to create an instance
      var Model = this.getModelCtor();
      var id = this.modelId;
      var m = new Model();
      m.setId(id);
      return m.getId();
    };
  
    Change.prototype.getModel = function(callback) {
      var Model = this.constructor.settings.trackModel;
      var id = this.getModelId();
      Model.findById(id, callback);
    };
  
    /**
     * When two changes conflict a conflict is created.
     *
     * **Note**: call `conflict.fetch()` to get the `target` and `source` models.
     *
     * @param {*} modelId
     * @param {PersistedModel} SourceModel
     * @param {PersistedModel} TargetModel
     * @property {ModelClass} source The source model instance
     * @property {ModelClass} target The target model instance
     * @class Change.Conflict
     */
  
    function Conflict(modelId, SourceModel, TargetModel) {
      this.SourceModel = SourceModel;
      this.TargetModel = TargetModel;
      this.SourceChange = SourceModel.getChangeModel();
      this.TargetChange = TargetModel.getChangeModel();
      this.modelId = modelId;
    }
  
    /**
     * Fetch the conflicting models.
     *
     * @callback {Function} callback
     * @param {Error} err
     * @param {PersistedModel} source
     * @param {PersistedModel} target
     */
  
    Conflict.prototype.models = function(cb) {
      var conflict = this;
      var SourceModel = this.SourceModel;
      var TargetModel = this.TargetModel;
      var source;
      var target;
  
      async.parallel([
        getSourceModel,
        getTargetModel
      ], done);
  
      function getSourceModel(cb) {
        SourceModel.findById(conflict.modelId, function(err, model) {
          if (err) return cb(err);
          source = model;
          cb();
        });
      }
  
      function getTargetModel(cb) {
        TargetModel.findById(conflict.modelId, function(err, model) {
          if (err) return cb(err);
          target = model;
          cb();
        });
      }
  
      function done(err) {
        if (err) return cb(err);
        cb(null, source, target);
      }
    };
  
    /**
     * Get the conflicting changes.
     *
     * @callback {Function} callback
     * @param {Error} err
     * @param {Change} sourceChange
     * @param {Change} targetChange
     */
  
    Conflict.prototype.changes = function(cb) {
      var conflict = this;
      var sourceChange;
      var targetChange;
  
      async.parallel([
        getSourceChange,
        getTargetChange
      ], done);
  
      function getSourceChange(cb) {
        var SourceModel = conflict.SourceModel;
        SourceModel.findLastChange(conflict.modelId, function(err, change) {
          if (err) return cb(err);
          sourceChange = change;
          cb();
        });
      }
  
      function getTargetChange(cb) {
        var TargetModel = conflict.TargetModel;
        TargetModel.findLastChange(conflict.modelId, function(err, change) {
          if (err) return cb(err);
          targetChange = change;
          cb();
        });
      }
  
      function done(err) {
        if (err) return cb(err);
        cb(null, sourceChange, targetChange);
      }
    };
  
    /**
     * Resolve the conflict.
     *
     * Set the source change's previous revision to the current revision of the
     * (conflicting) target change. Since the changes are no longer conflicting
     * and appear as if the source change was based on the target, they will be
     * replicated normally as part of the next replicate() call.
     *
     * This is effectively resolving the conflict using the source version.
     *
     * @callback {Function} callback
     * @param {Error} err
     */
  
    Conflict.prototype.resolve = function(cb) {
      var conflict = this;
      conflict.TargetModel.findLastChange(
        this.modelId,
        function(err, targetChange) {
          if (err) return cb(err);
          conflict.SourceModel.updateLastChange(
            conflict.modelId,
            { prev: targetChange.rev },
            cb);
        });
    };
  
    /**
     * Resolve the conflict using the instance data in the source model.
     *
     * @callback {Function} callback
     * @param {Error} err
     */
    Conflict.prototype.resolveUsingSource = function(cb) {
      this.resolve(function(err) {
        // don't forward any cb arguments from resolve()
        cb(err);
      });
    };
  
    /**
     * Resolve the conflict using the instance data in the target model.
     *
     * @callback {Function} callback
     * @param {Error} err
     */
    Conflict.prototype.resolveUsingTarget = function(cb) {
      var conflict = this;
  
      conflict.models(function(err, source, target) {
        if (err) return done(err);
        if (target === null) {
          return conflict.SourceModel.deleteById(conflict.modelId, done);
        }
        var inst = new conflict.SourceModel(
          target.toObject(),
          { persisted: true });
        inst.save(done);
      });
  
      function done(err) {
        // don't forward any cb arguments from internal calls
        cb(err);
      }
    };
  
    /**
     * Return a new Conflict instance with swapped Source and Target models.
     *
     * This is useful when resolving a conflict in one-way
     * replication, where the source data must not be changed:
     *
     * ```js
     * conflict.swapParties().resolveUsingTarget(cb);
     * ```
     *
     * @returns {Conflict} A new Conflict instance.
     */
    Conflict.prototype.swapParties = function() {
      var Ctor = this.constructor;
      return new Ctor(this.modelId, this.TargetModel, this.SourceModel);
    };
  
    /**
     * Resolve the conflict using the supplied instance data.
     *
     * @param {Object} data The set of changes to apply on the model
     * instance. Use `null` value to delete the source instance instead.
     * @callback {Function} callback
     * @param {Error} err
     */
  
    Conflict.prototype.resolveManually = function(data, cb) {
      var conflict = this;
      if (!data) {
        return conflict.SourceModel.deleteById(conflict.modelId, done);
      }
  
      conflict.models(function(err, source, target) {
        if (err) return done(err);
        var inst = source || new conflict.SourceModel(target);
        inst.setAttributes(data);
        inst.save(function(err) {
          if (err) return done(err);
          conflict.resolve(done);
        });
      });
  
      function done(err) {
        // don't forward any cb arguments from internal calls
        cb(err);
      }
    };
  
    /**
     * Determine the conflict type.
     *
     * Possible results are
     *
     *  - `Change.UPDATE`: Source and target models were updated.
     *  - `Change.DELETE`: Source and or target model was deleted.
     *  - `Change.UNKNOWN`: the conflict type is uknown or due to an error.
     *
     * @callback {Function} callback
     * @param {Error} err
     * @param {String} type The conflict type.
     */
  
    Conflict.prototype.type = function(cb) {
      var conflict = this;
      this.changes(function(err, sourceChange, targetChange) {
        if (err) return cb(err);
        var sourceChangeType = sourceChange.type();
        var targetChangeType = targetChange.type();
        if (sourceChangeType === Change.UPDATE && targetChangeType === Change.UPDATE) {
          return cb(null, Change.UPDATE);
        }
        if (sourceChangeType === Change.DELETE || targetChangeType === Change.DELETE) {
          return cb(null, Change.DELETE);
        }
        return cb(null, Change.UNKNOWN);
      });
    };
  };