/**
 * Handle every persistence-related task
 * The interface Datastore expects to be implemented is
 * * Persistence.loadDatabase(callback) and callback has signature err
 * * Persistence.persistNewState(newDocs, callback) where newDocs is an array of documents and callback has signature err
 */

var fs = require('fs')
  , path = require('path')
  , model = require('./model')
  , async = require('async')
  , mkdirp = require('mkdirp')
  ;


/**
 * Create a new Persistence object for database options.db
 * @param {Datastore} options.db
 * @param {Boolean} options.nodeWebkitAppName Optional, specify the name of your NW app if you want options.filename to be relative to the directory where
 *                                            Node Webkit stores application data such as cookies and local storage (the best place to store data in my opinion)
 */
function Persistence (options) {
  this.db = options.db;
  this.inMemoryOnly = this.db.inMemoryOnly;
  this.filename = this.db.filename;
  
  if (!this.inMemoryOnly && this.filename) {
    if (this.filename.charAt(this.filename.length - 1) === '~') {
      throw "The datafile name can't end with a ~, which is reserved for automatic backup files";
    } else {
      this.tempFilename = this.filename + '~';
    }
  }

  // For NW apps, store data in the same directory where NW stores application data
  if (this.filename && options.nodeWebkitAppName) {
    this.filename = Persistence.getNWAppFilename(options.nodeWebkitAppName, this.filename);
  }
};


/**
 * Check if a directory exists and create it on the fly if it is not the case
 * cb is optional, signature: err
 */
Persistence.ensureDirectoryExists = function (dir, cb) {
  var callback = cb || function () {}
    ;

  mkdirp(dir, function (err) { return callback(err); });
};


/**
 * Return the path the datafile if the given filename is relative to the directory where Node Webkit stores
 * data for this application. Probably the best place to store data
 */
Persistence.getNWAppFilename = function (appName, relativeFilename) {
  var home;

  switch (process.platform) {
    case 'win32':
    case 'win64':
      home = process.env.LOCALAPPDATA || process.env.APPDATA;
      if (!home) { throw "Couldn't find the base application data folder"; }
      home = path.join(home, appName);
      break;
    case 'darwin':
      home = process.env.HOME;
      if (!home) { throw "Couldn't find the base application data directory"; }
      home = path.join(home, 'Library', 'Application Support', appName);
      break;
    case 'linux':
      home = process.env.HOME;
      if (!home) { throw "Couldn't find the base application data directory"; }
      home = path.join(home, '.config', appName);
      break;
    default:
      throw "Can't use the Node Webkit relative path for platform " + process.platform;
      break;
  }

  return path.join(home, 'nedb-data', relativeFilename);
}


/**
 * Persist cached database
 * This serves as a compaction function since the cache always contains only the number of documents in the collection
 * while the data file is append-only so it may grow larger
 * @param {Function} cb Optional callback, signature: err
 */
Persistence.prototype.persistCachedDatabase = function (cb) {
  var callback = cb || function () {}
    , toPersist = ''
    , self = this
    ;

  if (this.inMemoryOnly) { return callback(null); } 

  this.db.getAllData().forEach(function (doc) {
    toPersist += model.serialize(doc) + '\n';
  });
  
  async.waterfall([
    function (cb) {
      fs.exists(self.tempFilename, function (exists) {
        if (exists) {   // Shouldn't happen since ensureDatafileIntegrity removes the temp datafile
          fs.unlink(self.tempFilename, function (err) { return cb(err); });
        } else {
          return cb();
        }
      });
    }
  , function (cb) {
      fs.exists(self.filename, function (exists) {
        if (exists) {
          fs.rename(self.filename, self.tempFilename, function (err) { return cb(err); });
        } else {
          return cb();
        }
      });  
  }
  , function (cb) {
      fs.writeFile(self.filename, toPersist, function (err) { return cb(err); });
    }
  , function (cb) {
      fs.exists(self.tempFilename, function (exists) {
        if (exists) {
          fs.unlink(self.tempFilename, function (err) { return cb(err); });
        } else {
          return cb();
        }
      });
    }
  ], function (err) { if (err) { return callback(err); } else { return callback(null); } })
};


/**
 * Queue a rewrite of the datafile
 */
Persistence.prototype.compactDatafile = function () {
  this.db.executor.push({ this: this, fn: this.persistCachedDatabase, arguments: [] });
};


/**
 * Set automatic compaction every interval ms
 * @param {Number} interval in milliseconds, with an enforced minimum of 5 seconds
 */
Persistence.prototype.setAutocompactionInterval = function (interval) {
  var self = this;

  if (interval < 5000) { interval = 5000; }
  this.stopAutocompaction();

  this.autocompactionIntervalId = setInterval(function () {
    self.compactDatafile();
  }, interval);
};


/**
 * Stop autocompaction (do nothing if autocompaction was not running)
 */
Persistence.prototype.stopAutocompaction = function () {
  if (this.autocompactionIntervalId) { clearInterval(this.autocompactionIntervalId); }
};


/**
 * Persist new state for the given newDocs (can be insertion, update or removal)
 * Use an append-only format
 * @param {Array} newDocs Can be empty if no doc was updated/removed
 * @param {Function} cb Optional, signature: err
 */
Persistence.prototype.persistNewState = function (newDocs, cb) {
  var self = this
    , toPersist = ''
    , callback = cb || function () {}
    ;

  // In-memory only datastore
  if (self.inMemoryOnly) { return callback(null); }

  newDocs.forEach(function (doc) {
    toPersist += model.serialize(doc) + '\n';
  });

  if (toPersist.length === 0) { return callback(null); }

  fs.appendFile(self.filename, toPersist, 'utf8', function (err) {
    return callback(err);
  });
};


/**
 * From a database's raw data, return the corresponding
 * machine understandable collection
 */
Persistence.treatRawData = function (rawData) {
  var data = rawData.split('\n')
    , dataById = {}
    , res = []
    , i;

  for (i = 0; i < data.length; i += 1) {
    var doc;

    try {
      doc = model.deserialize(data[i]);
      if (doc._id) {
        if (doc.$$deleted === true) {
          delete dataById[doc._id];
        } else {
          dataById[doc._id] = doc;
        }
      }
    } catch (e) {
    }
  }

  Object.keys(dataById).forEach(function (k) {
    res.push(dataById[k]);
  });

  return res;
};


/**
 * Ensure that this.filename contains the most up-to-date version of the data
 * Even if a loadDatabase crashed before
 */
Persistence.prototype.ensureDatafileIntegrity = function (callback) {
  var self = this  ;
  
  fs.exists(self.filename, function (filenameExists) {
    fs.exists(self.tempFilename, function (tempFilenameExists) {
      // Normal case
      if (filenameExists && !tempFilenameExists) {
        return callback(null);
      }
    
      // Process crashed right after renaming filename
      if (!filenameExists && tempFilenameExists) {
        return fs.rename(self.tempFilename, self.filename, function (err) { return callback(err); });
      }
      
      // No file exists, create empty datafile
      if (!filenameExists && !tempFilenameExists) {
        return fs.writeFile(self.filename, '', 'utf8', function (err) { callback(err); });      
      }
      
      // Process crashed after or during write to datafile
      // If datafile is not empty, it means process crashed after the write so we use it
      // If it is empty, we don't know whether the database was emptied or we had a crash during write. The safest option is to use the temp datafile
      if (filenameExists && tempFilenameExists) {
        return fs.stat(self.filename, function (err, stats) {
          if (err) { return callback(err); }
          
          if (stats.size > 0) {
            fs.unlink(self.tempFilename, function (err) { return callback(err); });
          } else {
            fs.unlink(self.filename, function (err) {
              if (err) { return callback(err); }
              fs.rename(self.tempFilename, self.filename, function (err) { return callback(err); });
            });
          }
        });
      }
    });
  });
};


/**
 * Load the database
 * This means pulling data out of the data file or creating it if it doesn't exist
 * Also, all data is persisted right away, which has the effect of compacting the database file
 * This operation is very quick at startup for a big collection (60ms for ~10k docs)
 * @param {Function} cb Optional callback, signature: err
 */
Persistence.prototype.loadDatabase = function (cb) {
  var callback = cb || function () {}
    , self = this
    ;

  self.db.resetIndexes();

  // In-memory only datastore
  if (self.inMemoryOnly) { return callback(null); }

  async.waterfall([
    function (cb) {
      Persistence.ensureDirectoryExists(path.dirname(self.filename), function (err) {
        self.ensureDatafileIntegrity(function (exists) {
          fs.readFile(self.filename, 'utf8', function (err, rawData) {
            if (err) { return cb(err); }
            var treatedData = Persistence.treatRawData(rawData);

            try {
              self.db.resetIndexes(treatedData);
            } catch (e) {
              self.db.resetIndexes();   // Rollback any index which didn't fail
              return cb(e);
            }

            self.db.persistence.persistCachedDatabase(cb);
          });
        });
      });
    }
  ], function (err) {
       if (err) { return callback(err); }

       self.db.executor.processBuffer();
       return callback(null);
     });
};


// Interface
module.exports = Persistence;