<?php
/**
 * Handles the database connections. It uses PDO to connect to the different databases. It will use the argument of the constructor to setup a connection to the database
 * with the matching entry in the $cfg global variable.
 *
 * --> First create an object of dblayer --> $db = new DBLayer('short database name used in config')
 *
 * --> Insert --> $db->insert( $tb_name, $data )
 * 				$tb_name = table name in which we want to insert data
 * 				$data = array of data that needs to be inserted in format('fieldname' => $value) where fieldname must be a field in that table.
 *
 * --> select --> $db->select( $tb_name, $data, $where )
 *				$tb_name = table name which we want to select
 * 				$data = array of data which is then required in WHERE clause in format array('fieldname'=>$value) fieldname must be a field in that table.
 *				$where = string in format ('fieldname=:fieldname') where :fieldname takes it's value from $data array.
 *
 * --> update --> $db->update( $tb_name, $data, $where )
 * 				$tb_name = table name which we want to update
 * 				$data = array of data which contains the filelds that need to be updated with their values in the format('fieldname' => $value,...) where fieldname must be a field in that table.
 * 				$where = string contains the filename with a value at that field in the format ('fieldname = $value') where fieldname must be a field in that table and $value is value respect to that field.
 *
 * --> delete --> $db->delete( $tb_name, $data, $where )
 * 				$tb_name = table name where we want to delete.
 * 				$data = array of data which is then required in WHERE clause in format array('fieldname'=> $value) where fieldname must be a field in that table.
 * 				$where = string in format ('fieldname=:fieldname') where :fieldname takes it's value from $data array.
 *
 *
 * @author Daan Janssens, mentored by Matthew Lagoe
 *
 */

$PDOCache = array();

class DBLayer {

	private $PDO;
	/**
	* The PDO object, instantiated by the constructor
	*/

	/**
	* The constructor.
	* Instantiates the PDO object attribute by connecting to the arguments matching database(the db info is stored in the $cfg global var)
	*
	* @param $db String, the name of the databases entry in the $cfg global var.
	* @param $dbn String, the name of the databases entry in the $cfg global var if $db referenced to an action(install etc).
	*/
	function __construct($db, $dbn = null)
	{
		global $PDOCache;
		if (isset($PDOCache[$db])) {
			$this->PDO = $PDOCache[$db];
		} else {
			global $cfg;
			$dsn = "mysql:";
			$dsn .= "host=" . $cfg['db'][$db]['host'] . ";";
			$dsn .= "dbname=" . $cfg['db'][$db]['name'] . ";";
			$dsn .= "port=" . $cfg['db'][$db]['port'] . ";";

			$opt = array(
				PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
				PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC
			);
			$this->PDO = new PDO($dsn, $cfg['db'][$db]['user'], $cfg['db'][$db]['pass'], $opt);
			$PDOCache[$db] = $this->PDO;
		}
	}

	/**
	* Execute a query that doesn't have any parameters.
	*
	* @param $query the mysql query.
	* @return returns a PDOStatement object.
	*/
	public function executeWithoutParams($query) {
		$statement = $this->PDO->prepare($query);
		$statement->execute();
		return $statement;
	}

	/**
	* Execute a query that has parameters.
	*
	* @param $query the mysql query.
	* @param $params the parameters that are being used by the query.
	* @return returns a PDOStatement object.
	*/
	public function execute( $query, $params ) {
		$statement = $this -> PDO -> prepare( $query );
		$statement -> execute( $params );
		return $statement;
		}

	/**
	* Insert function which returns id of the inserting field.
	*
	* @param $tb_name table name where we want to insert data.
	* @param $data the parameters that are being inserted into table.
	* @return returns the id of the last inserted element.
	*/
	public function executeReturnId( $tb_name, $data ) {
		$field_values = ':' . implode( ',:', array_keys( $data ) );
		$field_options = implode( ',', array_keys( $data ) );
		try {
			$sth = $this -> PDO -> prepare( "INSERT INTO $tb_name ($field_options) VALUE ($field_values)" );
			foreach ( $data as $key => $value )
			{
				$sth -> bindValue( ":$key", $value );
				}
			$this -> PDO -> beginTransaction();
			$sth -> execute();
			$lastId = $this -> PDO -> lastInsertId();
			$this -> PDO -> commit();
			}
		catch ( Exception $e )
		{
			// for rolling back the changes during transaction
			$this -> PDO -> rollBack();
			throw new Exception( "error in inseting" );
			}
		return $lastId;
	}

	/**
	* Select function using prepared statement.
	* For selecting particular fields.
	*
	* @param string $param field to select, can be multiple fields.
	* @param string $tb_name Table Name to Select.
	* @param array $data array of data to be used in WHERE clause in format('fieldname'=>$value). 'fieldname' must be a field in that table.
	* @param string $where where to select.
	* @return statement object.
	*/
	public function selectWithParameter( $param, $tb_name, $data, $where )
	{
		try {
			$sth = $this -> PDO -> prepare( "SELECT $param FROM $tb_name WHERE $where" );
			$this -> PDO -> beginTransaction();
			$sth -> execute( $data );
			$this -> PDO -> commit();
		}
		catch ( Exception $e ) {
			$this -> PDO -> rollBack();
			throw new Exception( "error selection" );
			return false;
		}
		return $sth;
	}

	/**
	* Select function using prepared statement.
	* For selecting all fields in a table.
	*
	* @param string $tb_name Table Name to Select.
	* @param array $data array of data to be used with WHERE part in format('fieldname'=>$value,...). 'fieldname' must be a field in that table.
	* @param string $where where to select in format('fieldname=:fieldname' AND ...).
	* @return statement object.
	*/
	public function select( $tb_name, $data , $where )
	{
		try {
			$sth = $this -> PDO -> prepare( "SELECT * FROM $tb_name WHERE $where" );
			$this -> PDO -> beginTransaction();
			$sth -> execute( $data );
			$this -> PDO -> commit();
			}
		catch( Exception $e )
		{
			$this -> PDO -> rollBack();
			throw new Exception( "error selection" );
			return false;
			}
		return $sth;
		}

	/**
	* Update function with prepared statement.
	*
	* @param string $tb_name name of the table on which operation to be performed.
	* @param array $data array of data in format('fieldname' => $value,...).Here, only those fields must be stored which needs to be updated.
	* @param string $where where part in format ('fieldname'= $value AND ...). 'fieldname' must be a field in that table.
	* @throws Exception error in updating.
	*/
	public function update( $tb_name, $data, $where )
	{
		$field_option_values = null;
		foreach ( $data as $key => $value )
		{
			$field_option_values .= ",$key" . '=:' . $key;
			}
		$field_option_values = ltrim( $field_option_values, ',' );
		try {
			$sth = $this -> PDO -> prepare( "UPDATE $tb_name SET $field_option_values WHERE $where " );

			foreach ( $data as $key => $value )
			{
				$sth -> bindValue( ":$key", $value );
				}
			$this -> PDO -> beginTransaction();
			$sth -> execute();
			$this -> PDO -> commit();
			}
		catch ( Exception $e )
		{
			$this -> PDO -> rollBack();
			throw new Exception( 'error in updating' );
			return false;
			}
		return true;
		}

	/**
	* insert function using prepared statements.
	*
	* @param string $tb_name Name of the table on which operation to be performed.
	* @param array $data array of data to insert in format('fieldname' => $value,....). 'fieldname' must be a field in that table.
	* @throws error in inserting.
	*/
	public function insert( $tb_name, $data )
	{
		$field_values = ':' . implode( ',:', array_keys( $data ) );
		$field_options = implode( ',', array_keys( $data ) );
		try {
			$sth = $this -> PDO -> prepare( "INSERT INTO $tb_name ($field_options) VALUE ($field_values)" );
			foreach ( $data as $key => $value )
			{

				$sth -> bindValue( ":$key", $value );
				}
			$this -> PDO -> beginTransaction();
			// execution
			$sth -> execute();
			$this -> PDO -> commit();

			}
		catch ( Exception $e )
		{
			// for rolling back the changes during transaction
			$this -> PDO -> rollBack();
			throw new Exception( "error in inserting" );
			}
		}

	/**
	* Delete database entery using prepared statement.
	*
	* @param string $tb_name table name on which operations to be performed.
	* @param $data array with values in the format('fieldname'=> $value,...). 'fieldname' must be a field in that table.
	* @param string $where condition based on $data array in the format('fieldname=:fieldname' AND ...).
	* @throws error in deleting.
	*/
	public function delete( $tb_name, $data, $where )
	{
		try {
			$sth = $this -> PDO -> prepare( "DELETE FROM $tb_name WHERE $where" );
			$this -> PDO -> beginTransaction();
			$sth -> execute( $data );
			$this -> PDO -> commit();
			}
		catch ( Exception $e )
		{
			$this -> PDO -> rollBack();
			throw new Exception( "error in deleting" );
			}

		}
	}