<?php

/**
 * Represents and handles all schema management for a MS SQL database
 * 
 * @package mssql
 */
class MSSQLSchemaManager extends DBSchemaManager {

	/**
	 * Stores per-request cached constraint checks that come from the database.
	 * 
	 * @var array
	 */
	protected static $cached_checks = array();
	
	/**
	 * Builds the internal MS SQL Server index name given the silverstripe table and index name
	 * 
	 * @param string $tableName
	 * @param string $indexName 
	 * @param string $prefix The optional prefix for the index. Defaults to "ix" for indexes.
	 * @return string The name of the index
	 */
	function buildMSSQLIndexName($tableName, $indexName, $prefix = 'ix') {

		// Cleanup names of namespaced tables
		$tableName = str_replace('\\', '_', $tableName);
		$indexName = str_replace('\\', '_', $indexName);

		return "{$prefix}_{$tableName}_{$indexName}";
	}


	/**
	 * This will set up the full text search capabilities.
	 * 
	 * @param string $name Name of full text catalog to use
	 */
	function createFullTextCatalog($name = 'ftCatalog') {
		$result = $this->fullTextCatalogExists();
		if(!$result) $this->query("CREATE FULLTEXT CATALOG \"$name\" AS DEFAULT;");
	}

	/**
	 * Check that a fulltext catalog has been created yet.
	 * 
	 * @param string $name Name of full text catalog to use
	 * @return boolean
	 */
	public function fullTextCatalogExists($name = 'ftCatalog') {
		return (bool) $this->preparedQuery(
			"SELECT name FROM sys.fulltext_catalogs WHERE name = ?;",
			array($name)
		)->value();
	}

	/**
	 * Sleep until the catalog has been fully rebuilt. This is a busy wait designed for situations
	 * when you need to be sure the index is up to date - for example in unit tests.
	 *
	 * TODO: move this to Database class? Can we assume this will be useful for all databases?
	 * Also see the wrapper functions "waitUntilIndexingFinished" in SearchFormTest and TranslatableSearchFormTest
	 *
	 * @param int $maxWaitingTime Time in seconds to wait for the database.
	 */
	function waitUntilIndexingFinished($maxWaitingTime = 15) {
		if(!$this->database->fullTextEnabled()) return;
		
		$this->query("EXEC sp_fulltext_catalog 'ftCatalog', 'Rebuild';");

		// Busy wait until it's done updating, but no longer than 15 seconds.
		$start = time();
		while(time() - $start < $maxWaitingTime) {
			$status = $this->query("EXEC sp_help_fulltext_catalogs 'ftCatalog';")->first();

			if (isset($status['STATUS']) && $status['STATUS'] == 0) {
				// Idle!
				break;
			}
			sleep(1);
		}
	}

	/**
	 * Check if a fulltext index exists on a particular table name.
	 * 
	 * @param string $tableName
	 * @return boolean TRUE index exists | FALSE index does not exist | NULL no support
	 */
	function fulltextIndexExists($tableName) {
		// Special case for no full text index support
		if(!$this->database->fullTextEnabled()) return null;

		return (bool) $this->preparedQuery("
			SELECT 1 FROM sys.fulltext_indexes i
			JOIN sys.objects o ON i.object_id = o.object_id
			WHERE o.name = ?",
			array($tableName)
		)->value();
	}

	/**
	 * MSSQL stores the primary key column with an internal identifier,
	 * so a lookup needs to be done to determine it.
	 * 
	 * @param string $tableName Name of table with primary key column "ID"
	 * @return string Internal identifier for primary key
	 */
	function getPrimaryKey($tableName){
		$indexes = $this->query("EXEC sp_helpindex '$tableName';");
		$indexName = '';
		foreach($indexes as $index) {
			if($index['index_keys'] == 'ID') {
				$indexName = $index['index_name'];
				break;
			}
		}

		return $indexName;
	}

	/**
	 * Gets the identity column of a table
	 * 
	 * @param string $tableName
	 * @return string|null
	 */
	function getIdentityColumn($tableName) {
		return $this->preparedQuery("
			SELECT
				TABLE_NAME + '.' + COLUMN_NAME,
				TABLE_NAME
 			FROM
				INFORMATION_SCHEMA.COLUMNS
 			WHERE
				TABLE_SCHEMA = ? AND
				COLUMNPROPERTY(object_id(TABLE_NAME), COLUMN_NAME, 'IsIdentity') = 1 AND
				TABLE_NAME = ?
		", array('dbo', $tableName))->value();
	}

	public function createDatabase($name) {
		$this->query("CREATE DATABASE \"$name\"");
	}
	
	public function dropDatabase($name) {
		$this->query("DROP DATABASE \"$name\"");
	}

	public function databaseExists($name) {
		$databases = $this->databaseList();
		foreach($databases as $dbname) {
			if($dbname == $name) return true;
		}
		return false;
	}
	
	public function databaseList() {
		return $this->query('SELECT NAME FROM sys.sysdatabases')->column();	
	}

	/**
	 * Create a new table.
	 * @param $tableName The name of the table
	 * @param $fields A map of field names to field types
	 * @param $indexes A map of indexes
	 * @param $options An map of additional options.  The available keys are as follows:
	 *   - 'MSSQLDatabase'/'MySQLDatabase'/'PostgreSQLDatabase' - database-specific options such as "engine" for MySQL.
	 *   - 'temporary' - If true, then a temporary table will be created
	 * @return The table name generated.  This may be different from the table name, for example with temporary tables.
	 */
	public function createTable($tableName, $fields = null, $indexes = null, $options = null, $advancedOptions = null) {
		$fieldSchemas = $indexSchemas = "";
		if($fields) foreach($fields as $k => $v) $fieldSchemas .= "\"$k\" $v,\n";

		// Temporary tables start with "#" in MSSQL-land
		if(!empty($options['temporary'])) {
			// Randomize the temp table name to avoid conflicts in the tempdb table which derived databases share
			$tableName = "#$tableName" . '-' . rand(1000000, 9999999);
		}

		$this->query("CREATE TABLE \"$tableName\" (
			$fieldSchemas
			primary key (\"ID\")
		);");

		//we need to generate indexes like this: CREATE INDEX IX_vault_to_export ON vault (to_export);
		//This needs to be done AFTER the table creation, so we can set up the fulltext indexes correctly
		if($indexes) foreach($indexes as $k => $v) {
			$indexSchemas .= $this->getIndexSqlDefinition($tableName, $k, $v) . "\n";
		}

		if($indexSchemas) $this->query($indexSchemas);

		return $tableName;
	}

	/**
	 * Alter a table's schema.
	 * @param $table The name of the table to alter
	 * @param $newFields New fields, a map of field name => field schema
	 * @param $newIndexes New indexes, a map of index name => index type
	 * @param $alteredFields Updated fields, a map of field name => field schema
	 * @param $alteredIndexes Updated indexes, a map of index name => index type
	 */
	public function alterTable($tableName, $newFields = null, $newIndexes = null, $alteredFields = null, $alteredIndexes = null, $alteredOptions=null, $advancedOptions=null) {
		$alterList = array();

		// drop any fulltext indexes that exist on the table before altering the structure
		if($this->fullTextIndexExists($tableName)) {
			$alterList[] = "\nDROP FULLTEXT INDEX ON \"$tableName\";";
		}

		if($newFields) foreach($newFields as $k => $v) $alterList[] = "ALTER TABLE \"$tableName\" ADD \"$k\" $v";

		if($alteredFields) foreach($alteredFields as $k => $v) $alterList[] = $this->alterTableAlterColumn($tableName, $k, $v);
		if($alteredIndexes) foreach($alteredIndexes as $k => $v) $alterList[] = $this->getIndexSqlDefinition($tableName, $k, $v);
		if($newIndexes) foreach($newIndexes as $k => $v) $alterList[] = $this->getIndexSqlDefinition($tableName, $k, $v);

		if($alterList) {
			foreach($alterList as $alteration) {
				if($alteration != '') {
					$this->query($alteration);
				}
			}
		}
	}

	/**
	 * Given the table and column name, retrieve the constraint name for that column
	 * in the table.
	 * 
	 * @param string $tableName Table name column resides in
	 * @param string $columnName Column name the constraint is for
	 * @return string|null
	 */
	public function getConstraintName($tableName, $columnName) {
		return $this->preparedQuery("
			SELECT CONSTRAINT_NAME
			FROM INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE
			WHERE TABLE_NAME = ? AND COLUMN_NAME = ?",
			array($tableName, $columnName)
		)->value();
	}

	/**
	 * Given a table and column name, return a check constraint clause for that column in
	 * the table.
	 * 
	 * This is an expensive query, so it is cached per-request and stored by table. The initial
	 * call for a table that has not been cached will query all columns and store that
	 * so subsequent calls are fast.
	 * 
	 * @param string $tableName Table name column resides in
	 * @param string $columnName Column name the constraint is for
	 * @return string The check string
	 */
	public function getConstraintCheckClause($tableName, $columnName) {
		// Check already processed table columns
		if(isset(self::$cached_checks[$tableName])) {
			if(!isset(self::$cached_checks[$tableName][$columnName])) {
				return null;
			}
			return self::$cached_checks[$tableName][$columnName];
		}

		// Regenerate cehcks for this table
		$checks = array();
		foreach($this->preparedQuery("
			SELECT CAST(CHECK_CLAUSE AS TEXT) AS CHECK_CLAUSE, COLUMN_NAME
			FROM INFORMATION_SCHEMA.CHECK_CONSTRAINTS AS CC
			INNER JOIN INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE AS CCU ON CCU.CONSTRAINT_NAME = CC.CONSTRAINT_NAME
			WHERE TABLE_NAME = ?",
			array($tableName)
		) as $record) {
			$checks[$record['COLUMN_NAME']] = $record['CHECK_CLAUSE'];
		}
		self::$cached_checks[$tableName] = $checks;
		
		// Return via cached records
		return $this->getConstraintCheckClause($tableName, $columnName);
	}

	/**
	 * Return the name of the default constraint applied to $tableName.$colName.
	 * Will return null if no such constraint exists
	 * 
	 * @param string $tableName Name of the table
	 * @param string $colName Name of the column
	 * @return string|null
	 */
	protected function defaultConstraintName($tableName, $colName) {
		return $this->preparedQuery("
			SELECT s.name --default name
			FROM sys.sysobjects s
			join sys.syscolumns c ON s.parent_obj = c.id
			WHERE s.xtype = 'd'
			and c.cdefault = s.id
			and parent_obj = OBJECT_ID(?)
			and c.name = ?",
			array($tableName, $colName)
		)->value();
	}

	/**
	 * Get enum values from a constraint check clause.
	 * 
	 * @param string $clause Check clause to parse values from
	 * @return array Enum values
	 */
	protected function enumValuesFromCheckClause($clause) {
		$segments = preg_split('/ +OR *\[/i', $clause);
		$constraints = array();
		foreach($segments as $segment) {
			$bits = preg_split('/ *= */', $segment);
			for($i = 1; $i < sizeof($bits); $i += 2) {
				array_unshift($constraints, substr(rtrim($bits[$i], ')'), 1, -1));
			}
		}
		return $constraints;
	}

	/*
	 * Creates an ALTER expression for a column in MS SQL
	 *
	 * @param string $tableName Name of the table to be altered
	 * @param string $colName   Name of the column to be altered
	 * @param string $colSpec   String which contains conditions for a column
	 * @return string
	 */
	protected function alterTableAlterColumn($tableName, $colName, $colSpec){

		// First, we split the column specifications into parts
		// TODO: this returns an empty array for the following string: int(11) not null auto_increment
		//		 on second thoughts, why is an auto_increment field being passed through?
		$pattern = '/^(?<definition>[\w()]+)\s?(?<null>(?:not\s)?null)?\s?(?<default>default\s[\w\']+)?\s?(?<check>check\s?[\w()\'",\s]+)?$/i';
		$matches = array();
		preg_match($pattern, $colSpec, $matches);

		// drop the index if it exists
		$alterQueries = array();

		// drop *ALL* indexes on a table before proceeding
		// this won't drop primary keys, though
		$indexes = $this->indexNames($tableName);
		foreach($indexes as $indexName) {
			$alterQueries[] = "DROP INDEX \"$indexName\" ON \"$tableName\";";
		}

		$prefix = "ALTER TABLE \"$tableName\" ";

		// Remove the old default prior to adjusting the column.
		if($defaultConstraintName = $this->defaultConstraintName($tableName, $colName)) {
			$alterQueries[] = "$prefix DROP CONSTRAINT \"$defaultConstraintName\";";
		}

		if(isset($matches['definition'])) {
			//We will prevent any changes being made to the ID column.  Primary key indexes will have a fit if we do anything here.
			if($colName != 'ID'){

				// SET null / not null
				$nullFragment = empty($matches['null']) ? '' : " {$matches['null']}";
				$alterQueries[] = "$prefix ALTER COLUMN \"$colName\" {$matches['definition']}$nullFragment;";
				
				// Add a default back
				if(!empty($matches['default'])) {
					$alterQueries[] = "$prefix ADD {$matches['default']} FOR \"$colName\";";
				}

				// SET check constraint (The constraint HAS to be dropped)
				if(!empty($matches['check'])) {
					$constraint = $this->getConstraintName($tableName, $colName);
					if($constraint) {
						$alterQueries[] = "$prefix DROP CONSTRAINT {$constraint};";
					}

					//NOTE: 'with nocheck' seems to solve a few problems I've been having for modifying existing tables.
					$alterQueries[] = "$prefix WITH NOCHECK ADD CONSTRAINT \"{$tableName}_{$colName}_check\" {$matches['check']};";
				}
			}
		}

		return implode("\n", $alterQueries);
	}

	public function renameTable($oldTableName, $newTableName) {
		$this->query("EXEC sp_rename \"$oldTableName\", \"$newTableName\"");
	}

	/**
	 * Checks a table's integrity and repairs it if necessary.
	 * NOTE: MSSQL does not appear to support any vacuum or optimise commands
	 *
	 * @var string $tableName The name of the table.
	 * @return boolean Return true if the table has integrity after the method is complete.
	 */
	public function checkAndRepairTable($tableName) {
		return true;
	}

	public function createField($tableName, $fieldName, $fieldSpec) {
		$this->query("ALTER TABLE \"$tableName\" ADD \"$fieldName\" $fieldSpec");
	}

	/**
	 * Change the database type of the given field.
	 * @param string $tableName The name of the tbale the field is in.
	 * @param string $fieldName The name of the field to change.
	 * @param string $fieldSpec The new field specification
	 */
	public function alterField($tableName, $fieldName, $fieldSpec) {
		$this->query("ALTER TABLE \"$tableName\" CHANGE \"$fieldName\" \"$fieldName\" $fieldSpec");
	}

	public function renameField($tableName, $oldName, $newName) {
		$this->query("EXEC sp_rename @objname = '$tableName.$oldName', @newname = '$newName', @objtype = 'COLUMN'");
	}

	public function fieldList($table) {
		//This gets us more information than we need, but I've included it all for the moment....
		$fieldRecords = $this->preparedQuery("SELECT ordinal_position, column_name, data_type, column_default,
			is_nullable, character_maximum_length, numeric_precision, numeric_scale, collation_name
			FROM information_schema.columns WHERE table_name = ?
			ORDER BY ordinal_position;",
			array($table)
		);

		// Cache the records from the query - otherwise a lack of multiple active result sets
		// will cause subsequent queries to fail in this method
		$fields = array();
		$output = array();
		foreach($fieldRecords as $record) {
			$fields[] = $record;
		}

		foreach($fields as $field) {
			// Update the data_type field to be a complete column definition string for use by
			// SS_Database::requireField()
			switch($field['data_type']){
				case 'int':
				case 'bigint':
				case 'numeric':
				case 'float':
				case 'bit':
					if($field['data_type'] != 'bigint' && $field['data_type'] != 'int' && $sizeSuffix = $field['numeric_precision']) {
						$field['data_type'] .= "($sizeSuffix)";
					}

					if($field['is_nullable'] == 'YES') {
						$field['data_type'] .= ' null';
					} else {
						$field['data_type'] .= ' not null';
					}
					if($field['column_default']) {
						$default=substr($field['column_default'], 2, -2);
						$field['data_type'] .= " default $default";
					}
					break;

				case 'decimal':
					if($field['numeric_precision']) {
						$sizeSuffix = $field['numeric_precision'] . ',' . $field['numeric_scale'];
							$field['data_type'] .= "($sizeSuffix)";
					}

					if($field['is_nullable'] == 'YES') {
						$field['data_type'] .= ' null';
					} else {
						$field['data_type'] .= ' not null';
					}
					if($field['column_default']) {
						$default=substr($field['column_default'], 2, -2);
						$field['data_type'] .= " default $default";
					}
					break;

				case 'nvarchar':
				case 'varchar':
					//Check to see if there's a constraint attached to this column:
					$clause = $this->getConstraintCheckClause($table, $field['column_name']);
					if($clause) {
						$constraints = $this->enumValuesFromCheckClause($clause);
						$default=substr($field['column_default'], 2, -2);
						$field['data_type'] = $this->enum(array(
							'default' => $default,
							'name' => $field['column_name'],
							'enums' => $constraints,
							'table' => $table
						));
						break;
					}

				default:
					$sizeSuffix = $field['character_maximum_length'];
					if($sizeSuffix == '-1') $sizeSuffix = 'max';
					if($sizeSuffix) {
							$field['data_type'] .= "($sizeSuffix)";
					}

					if($field['is_nullable'] == 'YES') {
						$field['data_type'] .= ' null';
					} else {
						$field['data_type'] .= ' not null';
					}
					if($field['column_default']) {
						$default=substr($field['column_default'], 2, -2);
						$field['data_type'] .= " default '$default'";
					}
			}
			$output[$field['column_name']] = $field;

		}

		return $output;
	}

	/**
	 * Create an index on a table.
	 * @param string $tableName The name of the table.
	 * @param string $indexName The name of the index.
	 * @param string $indexSpec The specification of the index, see SS_Database::requireIndex() for more details.
	 */
	public function createIndex($tableName, $indexName, $indexSpec) {
		$this->query($this->getIndexSqlDefinition($tableName, $indexName, $indexSpec));
	}
	
	/**
	 * Return SQL for dropping and recreating an index
	 * 
	 * @param string $tableName Name of table to create this index against
	 * @param string $indexName Name of this index
	 * @param array|string $indexSpec Index specification, either as a raw string
	 * or parsed array form
	 * @return string The SQL required to generate this index
	 */
	protected function getIndexSqlDefinition($tableName, $indexName, $indexSpec) {

		// Determine index name
		$index = $this->buildMSSQLIndexName($tableName, $indexName);

		// Consolidate/Cleanup spec into array format
		$indexSpec = $this->parseIndexSpec($indexName, $indexSpec);

		$drop = "IF EXISTS (SELECT name FROM sys.indexes WHERE name = '$index') DROP INDEX $index ON \"$tableName\";";

		// create a type-specific index
		if($indexSpec['type'] == 'fulltext' && $this->database->fullTextEnabled()) {
			// enable fulltext on this table
			$this->createFullTextCatalog();
			$primary_key = $this->getPrimaryKey($tableName);

			if($primary_key) {
				return "$drop CREATE FULLTEXT INDEX ON \"$tableName\" ({$indexSpec['value']})"
					 . "KEY INDEX $primary_key WITH CHANGE_TRACKING AUTO;";
			}
		}

		if($indexSpec['type'] == 'unique') {
			return "$drop CREATE UNIQUE INDEX $index ON \"$tableName\" ({$indexSpec['value']});";
		}

		return "$drop CREATE INDEX $index ON \"$tableName\" ({$indexSpec['value']});";
	}

	public function alterIndex($tableName, $indexName, $indexSpec) {
		$this->createIndex($tableName, $indexName, $indexSpec);
	}

	/**
	 * Return the list of indexes in a table.
	 * @param string $table The table name.
	 * @return array
	 */
	public function indexList($table) {
		$indexes = $this->query("EXEC sp_helpindex '$table';");
		$indexList = array();

		// Enumerate all basic indexes
		foreach($indexes as $index) {
			if(strpos($index['index_description'], 'unique') !== false) {
				$indexType = 'unique ';
			} else {
				$indexType = 'index ';
			}

			// Extract name from index
			$baseIndexName = $this->buildMSSQLIndexName($table, '');
			$indexName = substr($index['index_name'], strlen($baseIndexName));

			// Extract columns
			$columns = $this->quoteColumnSpecString($index['index_keys']);
			$indexList[$indexName] = $this->parseIndexSpec($indexName, array(
				'name' => $indexName,
				'value' => $columns,
				'type' => $indexType
			));
		}

		// Now we need to check to see if we have any fulltext indexes attached to this table:
		if($this->database->fullTextEnabled()) {
			$result = $this->query('EXEC sp_help_fulltext_columns;');

			// Extract columns from this fulltext definition
			$columns = array();
			foreach($result as $row) {
				if($row['TABLE_NAME'] == $table) {
					$columns[] = $row['FULLTEXT_COLUMN_NAME'];
				}
			}

			if(!empty($columns)) {
				$indexList['SearchFields'] = $this->parseIndexSpec('SearchFields', array(
					'name' => 'SearchFields',
					'value' => $this->implodeColumnList($columns),
					'type' => 'fulltext'
				));
			}
		}

		return $indexList;
	}

	/**
	 * For a given table name, get all the internal index names,
	 * except for those that are primary keys and fulltext indexes.
	 *
	 * @return array
	 */
	public function indexNames($tableName) {
		return $this->preparedQuery('
			SELECT ind.name FROM sys.indexes ind
			INNER JOIN sys.tables t ON ind.object_id = t.object_id
			WHERE is_primary_key = 0 AND t.name = ?',
			array($tableName)
		)->column();
	}
	
	public function tableList() {
		$tables = array();
		foreach($this->query("EXEC sp_tables @table_owner = 'dbo';") as $record) {
			$tables[strtolower($record['TABLE_NAME'])] = $record['TABLE_NAME'];
		}
		return $tables;
	}

	/**
	 * Return a boolean type-formatted string
	 * We use 'bit' so that we can do numeric-based comparisons
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function boolean($values) {
		$default = ($values['default']) ? '1' : '0';
		return 'bit not null default ' . $default;
	}

	/**
	 * Return a date type-formatted string.
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function date($values) {
		return 'date null';
	}

	/**
	 * Return a decimal type-formatted string
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function decimal($values) {
		// Avoid empty strings being put in the db
		if($values['precision'] == '') {
			$precision = 1;
		} else {
			$precision = $values['precision'];
		}

		$defaultValue = '0';
		if(isset($values['default']) && is_numeric($values['default'])) {
			$defaultValue = $values['default'];
		}

		return "decimal($precision) not null default $defaultValue";
	}

	/**
	 * Return a enum type-formatted string
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function enum($values) {
		// Enums are a bit different. We'll be creating a varchar(255) with a constraint of all the
		// usual enum options.
		// NOTE: In this one instance, we are including the table name in the values array

		$maxLength = max(array_map('strlen', $values['enums']));

		return "varchar($maxLength) not null default '" . $values['default']
			. "' check(\"" . $values['name'] . "\" in ('" . implode("','", $values['enums'])
			. "'))";
	}

	/**
	 * @todo Make this work like {@link MySQLDatabase::set()}
	 */
	public function set($values) {
		return $this->enum($values);
	}

	/**
	 * Return a float type-formatted string.
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function float($values) {
		return 'float(53) not null default ' . $values['default'];
	}

	/**
	 * Return a int type-formatted string
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function int($values) {
		return 'int not null default ' . (int) $values['default'];
	}

	/**
	 * Return a bigint type-formatted string
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function bigint($values) {
		return 'bigint not null default ' . (int) $values['default'];
	}

	/**
	 * Return a datetime type-formatted string
	 * For MS SQL, we simply return the word 'timestamp', no other parameters are necessary
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function ss_datetime($values) {
		return 'datetime null';
	}

	/**
	 * Return a text type-formatted string
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function text($values) {
		$collation = MSSQLDatabase::get_collation();
		$collationSQL = $collation ? " COLLATE $collation" : "";
		return "nvarchar(max)$collationSQL null";
	}

	/**
	 * Return a time type-formatted string.
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function time($values){
		return 'time null';
	}

	/**
	 * Return a varchar type-formatted string
	 *
	 * @params array $values Contains a tokenised list of info about this data type
	 * @return string
	 */
	public function varchar($values) {
		$collation = MSSQLDatabase::get_collation();
		$collationSQL = $collation ? " COLLATE $collation" : "";
		return "nvarchar(" . $values['precision'] . ")$collationSQL null";
	}

	/**
	 * Return a 4 digit numeric type.
	 * @return string
	 */
	public function year($values) {
		return 'numeric(4)';
	}

	/**
	 * This returns the column which is the primary key for each table
	 * @return string
	 */
	function IdColumn($asDbValue = false, $hasAutoIncPK = true){
		if($asDbValue) {
			return 'int not null';
		} else if($hasAutoIncPK) {
			return 'int identity(1,1)';
		} else {
			return 'int not null';
		}
	}

	function hasTable($tableName) {
		return (bool)$this->preparedQuery(
			"SELECT table_name FROM information_schema.tables WHERE table_name = ?",
			array($tableName)
		)->value();
	}

	/**
	 * Returns the values of the given enum field
	 * NOTE: Experimental; introduced for db-abstraction and may changed before 2.4 is released.
	 */
	public function enumValuesForField($tableName, $fieldName) {
		$classes = array();

		// Get the enum of all page types from the SiteTree table
		$clause = $this->getConstraintCheckClause($tableName, $fieldName);
		if($clause) {
			$classes = $this->enumValuesFromCheckClause($clause);
		}

		return $classes;
	}

	/**
	 * This is a lookup table for data types.
	 *
	 * For instance, MSSQL uses 'BIGINT', while MySQL uses 'UNSIGNED'
	 * and PostgreSQL uses 'INT'.
	 */
	function dbDataType($type){
		$values = array(
			'unsigned integer'=>'BIGINT'
		);
		if(isset($values[$type])) {
			return $values[$type];
		} else {
			return '';
		}
	}
	
	protected function indexKey($table, $index, $spec) {
		return $index;
	}
}