/**
 * Module Dependencies.
 */

var PersistedModel = require('./persisted-model')
  , loopback = require('../loopback')
  , crypto = require('crypto')
  , CJSON = {stringify: require('canonical-json')}
  , async = require('async')
  , assert = require('assert')
  , debug = require('debug')('loopback:change');

/**
 * Properties
 */

var properties = {
  id: {type: String, generated: true, id: true},
  rev: {type: String},
  prev: {type: String},
  checkpoint: {type: Number},
  modelName: {type: String},
  modelId: {type: String}
};

/**
 * Options
 */

var options = {
  trackChanges: false
};

/**
 * Change list entry.
 *
 * @property id {String} Hash of the modelName and id
 * @property rev {String} the current model revision
 * @property prev {String} the previous model revision
 * @property checkpoint {Number} the current checkpoint at time of the change
 * @property modelName {String}  the model name
 * @property modelId {String} the model id
 * 
 * @class
 * @inherits {Model}
 */

var Change = module.exports = PersistedModel.extend('Change', properties, options);

/*!
 * 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 tasks = [];
  var Change = this;

  modelIds.forEach(function(id) {
    tasks.push(function(cb) {
      Change.findOrCreateChange(modelName, id, function(err, change) {
        if(err) return Change.handleError(err, cb);
        change.rectify(cb);
      });
    });
  });
  async.parallel(tasks, callback);
}

/**
 * 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(loopback.getModel(modelName), modelName + ' does not exist');
  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');
      ch.save(callback);
    }
  });
}

/**
 * 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 tasks = [
    updateRevision,
    updateCheckpoint
  ];
  var currentRev = this.rev;

  change.debug('rectify change');

  cb = cb || function(err) {
    if(err) throw new Error(err);
  }

  async.parallel(tasks, function(err) {
    if(err) return cb(err);
    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);
    }
  });

  function updateRevision(cb) {
    // get the current revision
    change.currentRevision(function(err, rev) {
      if(err) return Change.handleError(err, cb);
      if(rev) {
        // avoid setting rev and prev to the same value
        if(currentRev !== rev) {
          change.rev = rev;
          change.prev = currentRev;
        } else {
          change.debug('rev and prev are equal (not updating rev)');
        }
      } else {
        change.rev = null;
        if(currentRev) {
          change.prev = currentRev;
        } else if(!change.prev) {
          change.debug('ERROR - could not determing prev');
          change.prev = Change.UNKNOWN;
        }
      }
      change.debug('updated revision (was ' + currentRev + ')');
      cb();
    });
  }

  function updateCheckpoint(cb) {
    change.constructor.getCheckpointModel().current(function(err, checkpoint) {
      if(err) return Change.handleError(err);
      change.checkpoint = checkpoint;
      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) {
  var model = this.getModelCtor();
  var id = this.getModelId();
  model.findById(id, function(err, inst) {
    if(err) return Change.handleError(err, cb);
    if(inst) {
      cb(null, Change.revisionForInst(inst));
    } else {
      cb(null, null);
    }
  });
}

/**
 * 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) {
  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) {
  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},
      checkpoint: {gte: since}
    }
  }, function(err, localChanges) {
    if(err) return callback(err);
    var deltas = [];
    var conflicts = [];
    var localModelIds = [];

    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) {
        deltas.push(remoteChangeIndex[id]);
      }
    });

    callback(null, {
      deltas: deltas,
      conflicts: conflicts
    });
  });
}

/**
 * Correct all change list entries.
 * @param  {Function} callback
 */

Change.rectifyAll = function(cb) {
  debug('rectify all');
  var Change = this;
  // this should be optimized
  this.find(function(err, changes) {
    if(err) return cb(err);
    changes.forEach(function(change) {
      change = new Change(change);
      change.rectify();
    });
  });
}

/**
 * Get the checkpoint model.
 * @return {Checkpoint}
 */

Change.getCheckpointModel = function() {
  var checkpointModel = this.Checkpoint;
  if(checkpointModel) return checkpointModel;
  this.checkpoint = checkpointModel = require('./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) {
  if(!this.settings.ignoreErrors) {
    throw err;
  }
}

Change.prototype.debug = function() {
  if(debug.enabled) {
    var args = Array.prototype.slice.call(arguments);
    debug.apply(this, args);
    debug('\tid', this.id);
    debug('\trev', this.rev);
    debug('\tprev', this.prev);
    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
 */

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}
 * @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) {
    conflict.SourceChange.findOne({where: {
      modelId: conflict.modelId
    }}, function(err, change) {
      if(err) return cb(err);
      sourceChange = change;
      cb();
    });
  }

  function getTargetChange(cb) {
    conflict.TargetChange.findOne({where: {
      modelId: 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.
 *
 * @callback {Function} callback
 * @param {Error} err
 */

Conflict.prototype.resolve = function(cb) {
  var conflict = this;
  conflict.changes(function(err, sourceChange, targetChange) {
    if(err) return callback(err);
    sourceChange.prev = targetChange.rev;
    sourceChange.save(cb);
  });
}

/**
 * Determine the conflict type. 
 * 
 * ```js
 * // possible results are
 * Change.UPDATE // => source and target models were updated
 * Change.DELETE // => the 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);
  });
}