var FILE_SYSTEM_NAME = require('../constants.js').FILE_SYSTEM_NAME;
var FILE_STORE_NAME = require('../constants.js').FILE_STORE_NAME;
var WSQL_VERSION = require('../constants.js').WSQL_VERSION;
var WSQL_SIZE = require('../constants.js').WSQL_SIZE;
var WSQL_DESC = require('../constants.js').WSQL_DESC;
var Errors = require('../errors.js');
var base64ArrayBuffer = require('base64-arraybuffer');

function WebSQLContext(db, isReadOnly) {
  var that = this;
  this.getTransaction = function(callback) {
    if(that.transaction) {
      callback(that.transaction);
      return;
    }
    // Either do readTransaction() (read-only) or transaction() (read/write)
    db[isReadOnly ? 'readTransaction' : 'transaction'](function(transaction) {
      that.transaction = transaction;
      callback(transaction);
    });
  };
}

WebSQLContext.prototype.clear = function(callback) {
  function onError(transaction, error) {
    callback(error);
  }
  function onSuccess() {
    callback(null);
  }
  this.getTransaction(function(transaction) {
    transaction.executeSql('DELETE FROM ' + FILE_STORE_NAME + ';',
      [], onSuccess, onError);
  });
};

function _get(getTransaction, key, callback) {
  function onSuccess(transaction, result) {
    // If the key isn't found, return null
    var value = result.rows.length === 0 ? null : result.rows.item(0).data;
    callback(null, value);
  }
  function onError(transaction, error) {
    callback(error);
  }
  getTransaction(function(transaction) {
    transaction.executeSql('SELECT data FROM ' + FILE_STORE_NAME + ' WHERE id = ? LIMIT 1;',
      [key], onSuccess, onError);
  });
}
WebSQLContext.prototype.getObject = function(key, callback) {
  _get(this.getTransaction, key, function(err, result) {
    if(err) {
      return callback(err);
    }

    try {
      if(result) {
        result = JSON.parse(result);
      }
    } catch(e) {
      return callback(e);
    }

    callback(null, result);
  });
};
WebSQLContext.prototype.getBuffer = function(key, callback) {
  _get(this.getTransaction, key, function(err, result) {
    if(err) {
      return callback(err);
    }

    // Deal with zero-length ArrayBuffers, which will be encoded as ''
    if(result || result === '') {
      var arrayBuffer = base64ArrayBuffer.decode(result);
      result = Buffer.from(arrayBuffer);
    }

    callback(null, result);
  });
};

function _put(getTransaction, key, value, callback) {
  function onSuccess() {
    callback(null);
  }
  function onError(transaction, error) {
    callback(error);
  }
  getTransaction(function(transaction) {
    transaction.executeSql('INSERT OR REPLACE INTO ' + FILE_STORE_NAME + ' (id, data) VALUES (?, ?);',
      [key, value], onSuccess, onError);
  });
}
WebSQLContext.prototype.putObject = function(key, value, callback) {
  var json = JSON.stringify(value);
  _put(this.getTransaction, key, json, callback);
};
WebSQLContext.prototype.putBuffer = function(key, uint8BackedBuffer, callback) {
  var base64 = base64ArrayBuffer.encode(uint8BackedBuffer.buffer);
  _put(this.getTransaction, key, base64, callback);
};

WebSQLContext.prototype.delete = function(key, callback) {
  function onSuccess() {
    callback(null);
  }
  function onError(transaction, error) {
    callback(error);
  }
  this.getTransaction(function(transaction) {
    transaction.executeSql('DELETE FROM ' + FILE_STORE_NAME + ' WHERE id = ?;',
      [key], onSuccess, onError);
  });
};


function WebSQL(name) {
  this.name = name || FILE_SYSTEM_NAME;
  this.db = null;
}
WebSQL.isSupported = function() {
  return !!global.openDatabase;
};

WebSQL.prototype.open = function(callback) {
  var that = this;

  // Bail if we already have a db open
  if(that.db) {
    return callback();
  }

  var db = global.openDatabase(that.name, WSQL_VERSION, WSQL_DESC, WSQL_SIZE);
  if(!db) {
    callback('[WebSQL] Unable to open database.');
    return;
  }

  function onError(transaction, error) {
    if (error.code === 5) {
      callback(new Errors.EINVAL('WebSQL cannot be accessed. If private browsing is enabled, disable it.'));
    }
    callback(error);
  }
  function onSuccess() {
    that.db = db;
    callback();
  }

  // Create the table and index we'll need to store the fs data.
  db.transaction(function(transaction) {
    function createIndex(transaction) {
      transaction.executeSql('CREATE INDEX IF NOT EXISTS idx_' + FILE_STORE_NAME + '_id' +
                             ' on ' + FILE_STORE_NAME + ' (id);',
      [], onSuccess, onError);
    }
    transaction.executeSql('CREATE TABLE IF NOT EXISTS ' + FILE_STORE_NAME + ' (id unique, data TEXT);',
      [], createIndex, onError);
  });
};
WebSQL.prototype.getReadOnlyContext = function() {
  return new WebSQLContext(this.db, true);
};
WebSQL.prototype.getReadWriteContext = function() {
  return new WebSQLContext(this.db, false);
};

module.exports = WebSQL;