<?php
// This file is part of Moodle - http://moodle.org/
//
// Moodle is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Moodle is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.

/**
 * Database manager instance is responsible for all database structure modifications.
 *
 * @package    core_ddl
 * @copyright  1999 onwards Martin Dougiamas     http://dougiamas.com
 *             2001-3001 Eloy Lafuente (stronk7) http://contiento.com
 *             2008 Petr Skoda                   http://skodak.org
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */

defined('MOODLE_INTERNAL') || die();

/**
 * Database manager instance is responsible for all database structure modifications.
 *
 * It is using db specific generators to find out the correct SQL syntax to do that.
 *
 * @package    core_ddl
 * @copyright  1999 onwards Martin Dougiamas     http://dougiamas.com
 *             2001-3001 Eloy Lafuente (stronk7) http://contiento.com
 *             2008 Petr Skoda                   http://skodak.org
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
class database_manager {

    /** @var moodle_database A moodle_database driver specific instance.*/
    protected $mdb;

    /** @var sql_generator A driver specific SQL generator instance. Public because XMLDB editor needs to access it.*/
    public $generator;

    /**
     * Creates a new database manager instance.
     * @param moodle_database $mdb A moodle_database driver specific instance.
     * @param sql_generator $generator A driver specific SQL generator instance.
     */
    public function __construct($mdb, $generator) {
        $this->mdb       = $mdb;
        $this->generator = $generator;
    }

    /**
     * Releases all resources
     */
    public function dispose() {
        if ($this->generator) {
            $this->generator->dispose();
            $this->generator = null;
        }
        $this->mdb = null;
    }

    /**
     * This function will execute an array of SQL commands.
     *
     * @param string[] $sqlarr Array of sql statements to execute.
     * @throws ddl_change_structure_exception This exception is thrown if any error is found.
     */
    protected function execute_sql_arr(array $sqlarr) {
        $this->mdb->change_database_structure($sqlarr);
    }

    /**
     * Execute a given sql command string.
     *
     * @param string $sql The sql string you wish to be executed.
     * @throws ddl_change_structure_exception This exception is thrown if any error is found.
     */
    protected function execute_sql($sql) {
        $this->mdb->change_database_structure($sql);
    }

    /**
     * Given one xmldb_table, check if it exists in DB (true/false).
     *
     * @param string|xmldb_table $table The table to be searched (string name or xmldb_table instance).
     * @return bool True is a table exists, false otherwise.
     */
    public function table_exists($table) {
        if (!is_string($table) and !($table instanceof xmldb_table)) {
            throw new ddl_exception('ddlunknownerror', NULL, 'incorrect table parameter!');
        }
        return $this->generator->table_exists($table);
    }

    /**
     * Reset a sequence to the id field of a table.
     * @param string|xmldb_table $table Name of table.
     * @throws ddl_exception thrown upon reset errors.
     */
    public function reset_sequence($table) {
        if (!is_string($table) and !($table instanceof xmldb_table)) {
            throw new ddl_exception('ddlunknownerror', NULL, 'incorrect table parameter!');
        }

        // Do not test if table exists because it is slow

        if (!$sqlarr = $this->generator->getResetSequenceSQL($table)) {
            throw new ddl_exception('ddlunknownerror', null, 'table reset sequence sql not generated');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * Given one xmldb_field, check if it exists in DB (true/false).
     *
     * @param string|xmldb_table $table The table to be searched (string name or xmldb_table instance).
     * @param string|xmldb_field $field The field to be searched for (string name or xmldb_field instance).
     * @return boolean true is exists false otherwise.
     * @throws ddl_table_missing_exception
     */
    public function field_exists($table, $field) {
        // Calculate the name of the table
        if (is_string($table)) {
            $tablename = $table;
        } else {
            $tablename = $table->getName();
        }

        // Check the table exists
        if (!$this->table_exists($table)) {
            throw new ddl_table_missing_exception($tablename);
        }

        if (is_string($field)) {
            $fieldname = $field;
        } else {
            // Calculate the name of the table
            $fieldname = $field->getName();
        }

        // Get list of fields in table
        $columns = $this->mdb->get_columns($tablename);

        $exists = array_key_exists($fieldname,  $columns);

        return $exists;
    }

    /**
     * Given one xmldb_index, the function returns the name of the index in DB
     * of false if it doesn't exist
     *
     * @param xmldb_table $xmldb_table table to be searched
     * @param xmldb_index $xmldb_index the index to be searched
     * @param bool $returnall true means return array of all indexes, false means first index only as string
     * @return array|string|bool Index name, array of index names or false if no indexes are found.
     * @throws ddl_table_missing_exception Thrown when table is not found.
     */
    public function find_index_name(xmldb_table $xmldb_table, xmldb_index $xmldb_index, $returnall = false) {
        // Calculate the name of the table
        $tablename = $xmldb_table->getName();

        // Check the table exists
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($tablename);
        }

        // Extract index columns
        $indcolumns = $xmldb_index->getFields();

        // Get list of indexes in table
        $indexes = $this->mdb->get_indexes($tablename);

        $return = array();

        // Iterate over them looking for columns coincidence
        foreach ($indexes as $indexname => $index) {
            $columns = $index['columns'];
            // Check if index matches queried index
            $diferences = array_merge(array_diff($columns, $indcolumns), array_diff($indcolumns, $columns));
            // If no differences, we have find the index
            if (empty($diferences)) {
                if ($returnall) {
                    $return[] = $indexname;
                } else {
                    return $indexname;
                }
            }
        }

        if ($return and $returnall) {
            return $return;
        }

        // Arriving here, index not found
        return false;
    }

    /**
     * Given one xmldb_index, check if it exists in DB (true/false).
     *
     * @param xmldb_table $xmldb_table The table to be searched.
     * @param xmldb_index $xmldb_index The index to be searched for.
     * @return boolean true id index exists, false otherwise.
     */
    public function index_exists(xmldb_table $xmldb_table, xmldb_index $xmldb_index) {
        if (!$this->table_exists($xmldb_table)) {
            return false;
        }
        return ($this->find_index_name($xmldb_table, $xmldb_index) !== false);
    }

    /**
     * This function IS NOT IMPLEMENTED. ONCE WE'LL BE USING RELATIONAL
     * INTEGRITY IT WILL BECOME MORE USEFUL. FOR NOW, JUST CALCULATE "OFFICIAL"
     * KEY NAMES WITHOUT ACCESSING TO DB AT ALL.
     * Given one xmldb_key, the function returns the name of the key in DB (if exists)
     * of false if it doesn't exist
     *
     * @param xmldb_table $xmldb_table The table to be searched.
     * @param xmldb_key $xmldb_key The key to be searched.
     * @return string key name if found
     */
    public function find_key_name(xmldb_table $xmldb_table, xmldb_key $xmldb_key) {

        $keycolumns = $xmldb_key->getFields();

        // Get list of keys in table
        // first primaries (we aren't going to use this now, because the MetaPrimaryKeys is awful)
            //TODO: To implement when we advance in relational integrity
        // then uniques (note that Moodle, for now, shouldn't have any UNIQUE KEY for now, but unique indexes)
            //TODO: To implement when we advance in relational integrity (note that AdoDB hasn't any MetaXXX for this.
        // then foreign (note that Moodle, for now, shouldn't have any FOREIGN KEY for now, but indexes)
            //TODO: To implement when we advance in relational integrity (note that AdoDB has one MetaForeignKeys()
            //but it's far from perfect.
        // TODO: To create the proper functions inside each generator to retrieve all the needed KEY info (name
        //       columns, reftable and refcolumns

        // So all we do is to return the official name of the requested key without any confirmation!)
        // One exception, hardcoded primary constraint names
        if ($this->generator->primary_key_name && $xmldb_key->getType() == XMLDB_KEY_PRIMARY) {
            return $this->generator->primary_key_name;
        } else {
            // Calculate the name suffix
            switch ($xmldb_key->getType()) {
                case XMLDB_KEY_PRIMARY:
                    $suffix = 'pk';
                    break;
                case XMLDB_KEY_UNIQUE:
                    $suffix = 'uk';
                    break;
                case XMLDB_KEY_FOREIGN_UNIQUE:
                case XMLDB_KEY_FOREIGN:
                    $suffix = 'fk';
                    break;
            }
            // And simply, return the official name
            return $this->generator->getNameForObject($xmldb_table->getName(), implode(', ', $xmldb_key->getFields()), $suffix);
        }
    }

    /**
     * This function will delete all tables found in XMLDB file from db
     *
     * @param string $file Full path to the XML file to be used.
     * @return void
     */
    public function delete_tables_from_xmldb_file($file) {

        $xmldb_file = new xmldb_file($file);

        if (!$xmldb_file->fileExists()) {
            throw new ddl_exception('ddlxmlfileerror', null, 'File does not exist');
        }

        $loaded    = $xmldb_file->loadXMLStructure();
        $structure = $xmldb_file->getStructure();

        if (!$loaded || !$xmldb_file->isLoaded()) {
            // Show info about the error if we can find it
            if ($structure) {
                if ($errors = $structure->getAllErrors()) {
                    throw new ddl_exception('ddlxmlfileerror', null, 'Errors found in XMLDB file: '. implode (', ', $errors));
                }
            }
            throw new ddl_exception('ddlxmlfileerror', null, 'not loaded??');
        }

        if ($xmldb_tables = $structure->getTables()) {
            // Delete in opposite order, this should help with foreign keys in the future.
            $xmldb_tables = array_reverse($xmldb_tables);
            foreach($xmldb_tables as $table) {
                if ($this->table_exists($table)) {
                    $this->drop_table($table);
                }
            }
        }
    }

    /**
     * This function will drop the table passed as argument
     * and all the associated objects (keys, indexes, constraints, sequences, triggers)
     * will be dropped too.
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @return void
     */
    public function drop_table(xmldb_table $xmldb_table) {
        // Check table exists
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }

        if (!$sqlarr = $this->generator->getDropTableSQL($xmldb_table)) {
            throw new ddl_exception('ddlunknownerror', null, 'table drop sql not generated');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * Load an install.xml file, checking that it exists, and that the structure is OK.
     * @param string $file the full path to the XMLDB file.
     * @return xmldb_file the loaded file.
     */
    private function load_xmldb_file($file) {
        $xmldb_file = new xmldb_file($file);

        if (!$xmldb_file->fileExists()) {
            throw new ddl_exception('ddlxmlfileerror', null, 'File does not exist');
        }

        $loaded = $xmldb_file->loadXMLStructure();
        if (!$loaded || !$xmldb_file->isLoaded()) {
            // Show info about the error if we can find it
            if ($structure = $xmldb_file->getStructure()) {
                if ($errors = $structure->getAllErrors()) {
                    throw new ddl_exception('ddlxmlfileerror', null, 'Errors found in XMLDB file: '. implode (', ', $errors));
                }
            }
            throw new ddl_exception('ddlxmlfileerror', null, 'not loaded??');
        }

        return $xmldb_file;
    }

    /**
     * This function will load one entire XMLDB file and call install_from_xmldb_structure.
     *
     * @param string $file full path to the XML file to be used
     * @return void
     */
    public function install_from_xmldb_file($file) {
        $xmldb_file = $this->load_xmldb_file($file);
        $xmldb_structure = $xmldb_file->getStructure();
        $this->install_from_xmldb_structure($xmldb_structure);
    }

    /**
     * This function will load one entire XMLDB file and call install_from_xmldb_structure.
     *
     * @param string $file full path to the XML file to be used
     * @param string $tablename the name of the table.
     * @param bool $cachestructures boolean to decide if loaded xmldb structures can be safely cached
     *             useful for testunits loading the enormous main xml file hundred of times (100x)
     */
    public function install_one_table_from_xmldb_file($file, $tablename, $cachestructures = false) {

        static $xmldbstructurecache = array(); // To store cached structures
        if (!empty($xmldbstructurecache) && array_key_exists($file, $xmldbstructurecache)) {
            $xmldb_structure = $xmldbstructurecache[$file];
        } else {
            $xmldb_file = $this->load_xmldb_file($file);
            $xmldb_structure = $xmldb_file->getStructure();
            if ($cachestructures) {
                $xmldbstructurecache[$file] = $xmldb_structure;
            }
        }

        $targettable = $xmldb_structure->getTable($tablename);
        if (is_null($targettable)) {
            throw new ddl_exception('ddlunknowntable', null, 'The table ' . $tablename . ' is not defined in file ' . $file);
        }
        $targettable->setNext(NULL);
        $targettable->setPrevious(NULL);

        $tempstructure = new xmldb_structure('temp');
        $tempstructure->addTable($targettable);
        $this->install_from_xmldb_structure($tempstructure);
    }

    /**
     * This function will generate all the needed SQL statements, specific for each
     * RDBMS type and, finally, it will execute all those statements against the DB.
     *
     * @param stdClass $xmldb_structure xmldb_structure object.
     * @return void
     */
    public function install_from_xmldb_structure($xmldb_structure) {

        if (!$sqlarr = $this->generator->getCreateStructureSQL($xmldb_structure)) {
            return; // nothing to do
        }
        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will create the table passed as argument with all its
     * fields/keys/indexes/sequences, everything based in the XMLDB object
     *
     * @param xmldb_table $xmldb_table Table object (full specs are required).
     * @return void
     */
    public function create_table(xmldb_table $xmldb_table) {
        // Check table doesn't exist
        if ($this->table_exists($xmldb_table)) {
            throw new ddl_exception('ddltablealreadyexists', $xmldb_table->getName());
        }

        if (!$sqlarr = $this->generator->getCreateTableSQL($xmldb_table)) {
            throw new ddl_exception('ddlunknownerror', null, 'table create sql not generated');
        }
        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will create the temporary table passed as argument with all its
     * fields/keys/indexes/sequences, everything based in the XMLDB object
     *
     * If table already exists ddl_exception will be thrown, please make sure
     * the table name does not collide with existing normal table!
     *
     * @param xmldb_table $xmldb_table Table object (full specs are required).
     * @return void
     */
    public function create_temp_table(xmldb_table $xmldb_table) {

        // Check table doesn't exist
        if ($this->table_exists($xmldb_table)) {
            throw new ddl_exception('ddltablealreadyexists', $xmldb_table->getName());
        }

        if (!$sqlarr = $this->generator->getCreateTempTableSQL($xmldb_table)) {
            throw new ddl_exception('ddlunknownerror', null, 'temp table create sql not generated');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will drop the temporary table passed as argument with all its
     * fields/keys/indexes/sequences, everything based in the XMLDB object
     *
     * It is recommended to drop temp table when not used anymore.
     *
     * @deprecated since 2.3, use drop_table() for all table types
     * @param xmldb_table $xmldb_table Table object.
     * @return void
     */
    public function drop_temp_table(xmldb_table $xmldb_table) {
        debugging('database_manager::drop_temp_table() is deprecated, use database_manager::drop_table() instead');
        $this->drop_table($xmldb_table);
    }

    /**
     * This function will rename the table passed as argument
     * Before renaming the index, the function will check it exists
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param string $newname New name of the index.
     * @return void
     */
    public function rename_table(xmldb_table $xmldb_table, $newname) {
        // Check newname isn't empty
        if (!$newname) {
            throw new ddl_exception('ddlunknownerror', null, 'newname can not be empty');
        }

        $check = new xmldb_table($newname);

        // Check table already renamed
        if (!$this->table_exists($xmldb_table)) {
            if ($this->table_exists($check)) {
                throw new ddl_exception('ddlunknownerror', null, 'table probably already renamed');
            } else {
                throw new ddl_table_missing_exception($xmldb_table->getName());
            }
        }

        // Check new table doesn't exist
        if ($this->table_exists($check)) {
            throw new ddl_exception('ddltablealreadyexists', $check->getName(), 'can not rename table');
        }

        if (!$sqlarr = $this->generator->getRenameTableSQL($xmldb_table, $newname)) {
            throw new ddl_exception('ddlunknownerror', null, 'table rename sql not generated');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will add the field to the table passed as arguments
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @return void
     */
    public function add_field(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {
         // Check the field doesn't exist
        if ($this->field_exists($xmldb_table, $xmldb_field)) {
            throw new ddl_exception('ddlfieldalreadyexists', $xmldb_field->getName());
        }

        // If NOT NULL and no default given (we ask the generator about the
        // *real* default that will be used) check the table is empty
        if ($xmldb_field->getNotNull() && $this->generator->getDefaultValue($xmldb_field) === NULL && $this->mdb->count_records($xmldb_table->getName())) {
            throw new ddl_exception('ddlunknownerror', null, 'Field ' . $xmldb_table->getName() . '->' . $xmldb_field->getName() .
                      ' cannot be added. Not null fields added to non empty tables require default value. Create skipped');
        }

        if (!$sqlarr = $this->generator->getAddFieldSQL($xmldb_table, $xmldb_field)) {
            throw new ddl_exception('ddlunknownerror', null, 'addfield sql not generated');
        }
        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will drop the field from the table passed as arguments
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @return void
     */
    public function drop_field(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }
        // Check the field exists
        if (!$this->field_exists($xmldb_table, $xmldb_field)) {
            throw new ddl_field_missing_exception($xmldb_field->getName(), $xmldb_table->getName());
        }
        // Check for dependencies in the DB before performing any action
        $this->check_field_dependencies($xmldb_table, $xmldb_field);

        if (!$sqlarr = $this->generator->getDropFieldSQL($xmldb_table, $xmldb_field)) {
            throw new ddl_exception('ddlunknownerror', null, 'drop_field sql not generated');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will change the type of the field in the table passed as arguments
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @return void
     */
    public function change_field_type(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }
        // Check the field exists
        if (!$this->field_exists($xmldb_table, $xmldb_field)) {
            throw new ddl_field_missing_exception($xmldb_field->getName(), $xmldb_table->getName());
        }
        // Check for dependencies in the DB before performing any action
        $this->check_field_dependencies($xmldb_table, $xmldb_field);

        if (!$sqlarr = $this->generator->getAlterFieldSQL($xmldb_table, $xmldb_field)) {
            return; // probably nothing to do
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will change the precision of the field in the table passed as arguments
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @return void
     */
    public function change_field_precision(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {
        // Just a wrapper over change_field_type. Does exactly the same processing
        $this->change_field_type($xmldb_table, $xmldb_field);
    }

    /**
     * This function will change the unsigned/signed of the field in the table passed as arguments
     *
     * @deprecated since 2.3, only singed numbers are allowed now, migration is automatic
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Field object (full specs are required).
     * @return void
     */
    public function change_field_unsigned(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {
        debugging('All unsigned numbers are converted to signed automatically during Moodle upgrade.');
        $this->change_field_type($xmldb_table, $xmldb_field);
    }

    /**
     * This function will change the nullability of the field in the table passed as arguments
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @return void
     */
    public function change_field_notnull(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {
        // Just a wrapper over change_field_type. Does exactly the same processing
        $this->change_field_type($xmldb_table, $xmldb_field);
    }

    /**
     * This function will change the default of the field in the table passed as arguments
     * One null value in the default field means delete the default
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @return void
     */
    public function change_field_default(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }
        // Check the field exists
        if (!$this->field_exists($xmldb_table, $xmldb_field)) {
            throw new ddl_field_missing_exception($xmldb_field->getName(), $xmldb_table->getName());
        }
        // Check for dependencies in the DB before performing any action
        $this->check_field_dependencies($xmldb_table, $xmldb_field);

        if (!$sqlarr = $this->generator->getModifyDefaultSQL($xmldb_table, $xmldb_field)) {
            return; //Empty array = nothing to do = no error
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will rename the field in the table passed as arguments
     * Before renaming the field, the function will check it exists
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @param string $newname New name of the field.
     * @return void
     */
    public function rename_field(xmldb_table $xmldb_table, xmldb_field $xmldb_field, $newname) {
        if (empty($newname)) {
            throw new ddl_exception('ddlunknownerror', null, 'newname can not be empty');
        }

        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }

        // Check the field exists
        if (!$this->field_exists($xmldb_table, $xmldb_field)) {
            throw new ddl_field_missing_exception($xmldb_field->getName(), $xmldb_table->getName());
        }

        // Check we have included full field specs
        if (!$xmldb_field->getType()) {
            throw new ddl_exception('ddlunknownerror', null,
                      'Field ' . $xmldb_table->getName() . '->' . $xmldb_field->getName() .
                      ' must contain full specs. Rename skipped');
        }

        // Check field isn't id. Renaming over that field is not allowed
        if ($xmldb_field->getName() == 'id') {
            throw new ddl_exception('ddlunknownerror', null,
                      'Field ' . $xmldb_table->getName() . '->' . $xmldb_field->getName() .
                      ' cannot be renamed. Rename skipped');
        }

        if (!$sqlarr = $this->generator->getRenameFieldSQL($xmldb_table, $xmldb_field, $newname)) {
            return; //Empty array = nothing to do = no error
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will check, for the given table and field, if there there is any dependency
     * preventing the field to be modified. It's used by all the public methods that perform any
     * DDL change on fields, throwing one ddl_dependency_exception if dependencies are found.
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_field $xmldb_field Index object (full specs are required).
     * @return void
     * @throws ddl_dependency_exception|ddl_field_missing_exception|ddl_table_missing_exception if dependency not met.
     */
    private function check_field_dependencies(xmldb_table $xmldb_table, xmldb_field $xmldb_field) {

        // Check the table exists
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }

        // Check the field exists
        if (!$this->field_exists($xmldb_table, $xmldb_field)) {
            throw new ddl_field_missing_exception($xmldb_field->getName(), $xmldb_table->getName());
        }

        // Check the field isn't in use by any index in the table
        if ($indexes = $this->mdb->get_indexes($xmldb_table->getName(), false)) {
            foreach ($indexes as $indexname => $index) {
                $columns = $index['columns'];
                if (in_array($xmldb_field->getName(), $columns)) {
                    throw new ddl_dependency_exception('column', $xmldb_table->getName() . '->' . $xmldb_field->getName(),
                                                       'index', $indexname . ' (' . implode(', ', $columns)  . ')');
                }
            }
        }
    }

    /**
     * This function will create the key in the table passed as arguments
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_key $xmldb_key Index object (full specs are required).
     * @return void
     */
    public function add_key(xmldb_table $xmldb_table, xmldb_key $xmldb_key) {

        if ($xmldb_key->getType() == XMLDB_KEY_PRIMARY) { // Prevent PRIMARY to be added (only in create table, being serious  :-P)
            throw new ddl_exception('ddlunknownerror', null, 'Primary Keys can be added at table create time only');
        }

        if (!$sqlarr = $this->generator->getAddKeySQL($xmldb_table, $xmldb_key)) {
            return; //Empty array = nothing to do = no error
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will drop the key in the table passed as arguments
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_key $xmldb_key Key object (full specs are required).
     * @return void
     */
    public function drop_key(xmldb_table $xmldb_table, xmldb_key $xmldb_key) {
        if ($xmldb_key->getType() == XMLDB_KEY_PRIMARY) { // Prevent PRIMARY to be dropped (only in drop table, being serious  :-P)
            throw new ddl_exception('ddlunknownerror', null, 'Primary Keys can be deleted at table drop time only');
        }

        if (!$sqlarr = $this->generator->getDropKeySQL($xmldb_table, $xmldb_key)) {
            return; //Empty array = nothing to do = no error
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will rename the key in the table passed as arguments
     * Experimental. Shouldn't be used at all in normal installation/upgrade!
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_key $xmldb_key key object (full specs are required).
     * @param string $newname New name of the key.
     * @return void
     */
    public function rename_key(xmldb_table $xmldb_table, xmldb_key $xmldb_key, $newname) {
        debugging('rename_key() is one experimental feature. You must not use it in production!', DEBUG_DEVELOPER);

        // Check newname isn't empty
        if (!$newname) {
            throw new ddl_exception('ddlunknownerror', null, 'newname can not be empty');
        }

        if (!$sqlarr = $this->generator->getRenameKeySQL($xmldb_table, $xmldb_key, $newname)) {
            throw new ddl_exception('ddlunknownerror', null, 'Some DBs do not support key renaming (MySQL, PostgreSQL, MsSQL). Rename skipped');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will create the index in the table passed as arguments
     * Before creating the index, the function will check it doesn't exists
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_index $xmldb_intex Index object (full specs are required).
     * @return void
     */
    public function add_index($xmldb_table, $xmldb_intex) {
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }

        // Check index doesn't exist
        if ($this->index_exists($xmldb_table, $xmldb_intex)) {
            throw new ddl_exception('ddlunknownerror', null,
                      'Index ' . $xmldb_table->getName() . '->' . $xmldb_intex->getName() .
                      ' already exists. Create skipped');
        }

        if (!$sqlarr = $this->generator->getAddIndexSQL($xmldb_table, $xmldb_intex)) {
            throw new ddl_exception('ddlunknownerror', null, 'add_index sql not generated');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will drop the index in the table passed as arguments
     * Before dropping the index, the function will check it exists
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_index $xmldb_intex Index object (full specs are required).
     * @return void
     */
    public function drop_index($xmldb_table, $xmldb_intex) {
        if (!$this->table_exists($xmldb_table)) {
            throw new ddl_table_missing_exception($xmldb_table->getName());
        }

        // Check index exists
        if (!$this->index_exists($xmldb_table, $xmldb_intex)) {
            throw new ddl_exception('ddlunknownerror', null,
                      'Index ' . $xmldb_table->getName() . '->' . $xmldb_intex->getName() .
                      ' does not exist. Drop skipped');
        }

        if (!$sqlarr = $this->generator->getDropIndexSQL($xmldb_table, $xmldb_intex)) {
            throw new ddl_exception('ddlunknownerror', null, 'drop_index sql not generated');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * This function will rename the index in the table passed as arguments
     * Before renaming the index, the function will check it exists
     * Experimental. Shouldn't be used at all!
     *
     * @param xmldb_table $xmldb_table Table object (just the name is mandatory).
     * @param xmldb_index $xmldb_intex Index object (full specs are required).
     * @param string $newname New name of the index.
     * @return void
     */
    public function rename_index($xmldb_table, $xmldb_intex, $newname) {
        debugging('rename_index() is one experimental feature. You must not use it in production!', DEBUG_DEVELOPER);

        // Check newname isn't empty
        if (!$newname) {
            throw new ddl_exception('ddlunknownerror', null, 'newname can not be empty');
        }

        // Check index exists
        if (!$this->index_exists($xmldb_table, $xmldb_intex)) {
            throw new ddl_exception('ddlunknownerror', null,
                      'Index ' . $xmldb_table->getName() . '->' . $xmldb_intex->getName() .
                      ' does not exist. Rename skipped');
        }

        if (!$sqlarr = $this->generator->getRenameIndexSQL($xmldb_table, $xmldb_intex, $newname)) {
            throw new ddl_exception('ddlunknownerror', null, 'Some DBs do not support index renaming (MySQL). Rename skipped');
        }

        $this->execute_sql_arr($sqlarr);
    }

    /**
     * Reads the install.xml files for Moodle core and modules and returns an array of
     * xmldb_structure object with xmldb_table from these files.
     * @return xmldb_structure schema from install.xml files
     */
    public function get_install_xml_schema() {
        global $CFG;
        require_once($CFG->libdir.'/adminlib.php');

        $schema = new xmldb_structure('export');
        $schema->setVersion($CFG->version);
        $dbdirs = get_db_directories();
        foreach ($dbdirs as $dbdir) {
            $xmldb_file = new xmldb_file($dbdir.'/install.xml');
            if (!$xmldb_file->fileExists() or !$xmldb_file->loadXMLStructure()) {
                continue;
            }
            $structure = $xmldb_file->getStructure();
            $tables = $structure->getTables();
            foreach ($tables as $table) {
                $table->setPrevious(null);
                $table->setNext(null);
                $schema->addTable($table);
            }
        }
        return $schema;
    }

    /**
     * Checks the database schema against a schema specified by an xmldb_structure object
     * @param xmldb_structure $schema export schema describing all known tables
     * @return array keyed by table name with array of difference messages as values
     */
    public function check_database_schema(xmldb_structure $schema) {
        $errors = array();

        $dbtables = $this->mdb->get_tables();
        $tables   = $schema->getTables();

        //TODO: maybe add several levels error/warning

        // make sure that current and schema tables match exactly
        foreach ($tables as $table) {
            $tablename = $table->getName();
            if (empty($dbtables[$tablename])) {
                if (!isset($errors[$tablename])) {
                    $errors[$tablename] = array();
                }
                $errors[$tablename][] = "Table $tablename is missing in database."; //TODO: localize
                continue;
            }

            // a) check for required fields
            $dbfields = $this->mdb->get_columns($tablename);
            $fields   = $table->getFields();
            foreach ($fields as $field) {
                $fieldname = $field->getName();
                if (empty($dbfields[$fieldname])) {
                    if (!isset($errors[$tablename])) {
                        $errors[$tablename] = array();
                    }
                    $errors[$tablename][] = "Field $fieldname is missing in table $tablename.";  //TODO: localize
                }
                unset($dbfields[$fieldname]);
            }

            // b) check for extra fields (indicates unsupported hacks) - modify install.xml if you want the script to continue ;-)
            foreach ($dbfields as $fieldname=>$info) {
                if (!isset($errors[$tablename])) {
                    $errors[$tablename] = array();
                }
                $errors[$tablename][] = "Field $fieldname is not expected in table $tablename.";  //TODO: localize
            }
            unset($dbtables[$tablename]);
        }

        // look for unsupported tables - local custom tables should be in /local/xxxx/db/install.xml ;-)
        // if there is no prefix, we can not say if tale is ours :-(
        if ($this->generator->prefix !== '') {
            foreach ($dbtables as $tablename=>$unused) {
                if (strpos($tablename, 'pma_') === 0) {
                    // ignore phpmyadmin tables for now
                    continue;
                }
                if (strpos($tablename, 'test') === 0) {
                    // ignore broken results of unit tests
                    continue;
                }
                if (!isset($errors[$tablename])) {
                    $errors[$tablename] = array();
                }
                $errors[$tablename][] = "Table $tablename is not expected.";  //TODO: localize
            }
        }

        return $errors;
    }
}
