<?php

/**
 * Guid - class generating/verifying project-wide unique identifiers to be used as IDs in the database
 *
 * @package tools
 * @author Dave Broome <daveb@finda.co.nz>
 * @author Jakub Nesetril <jakub@finda.co.nz>
 * @todo Store guids as integers internally and only display them in URLs as base31
 * @copyright APN Finda, Ltd.,  15 December, 2006
 **/
class Guid {

    /**
     * @var Array List of vowels to be stripped from a GUID
     **/
    protected static $vowels = array('a','e','i','o','u');

    /**
     * @var Array List of consonants replacing vowels
     **/
    protected static $consonants = array('v','w','x','y','z');

    /**
     * @var int Configuration to set up the block of guids fetched at once from DB into memcache
     **/
    protected static $guid_block = 10000;

    /**
     * @var int The wait interval for concurrent synchronisation
     **/
    protected static $wait_interval = 200000;


    /**
     * @return string Base-31 number to be converted to decimal
     * @author Dave Broome <daveb@finda.co.nz>
     * @author Jakub Nesetril <jakub@finda.co.nz>
     * @param int $decimal
     **/
    protected static function decimal_to_modified_base_31($decimal) {
        return str_replace(self::$vowels, self::$consonants, base_convert($decimal, 10, 31));
    }


    /**
     * @return int
     * @author Dave Broome <daveb@finda.co.nz>
     * @author Jakub Nesetril <jakub@finda.co.nz>
     * @param string $base_31 Base-31 number to be converted to decimal
     **/
    protected static function modified_base_31_to_decimal($base_31) {
        return base_convert(str_replace(self::$consonants, self::$vowels, $base_31), 31, 10);
    }


    /**
     * Reserves a block of guids from database and sets up cache records
     *
     * @return int Returns the current valid Guid
     * @author Jakub Nesetril <jakub@finda.co.nz>
     * @todo implement a mutex on the DB fetching, so that two processes don't allocate two blocks
     * @global adodb_distributor $db
     **/
    protected static function seedCache()
    {
        global $db;

        $db->execute("START TRANSACTION");
        $guid_decimal = $db->getOne("SELECT value FROM constant WHERE name = 'next-guid-decimal' FOR UPDATE");
        $db->execute("UPDATE constant SET value = value + ".self::$guid_block." WHERE name = 'next-guid-decimal'");
        $db->execute("COMMIT");

        Cache::set('Guid_maximum', $guid_decimal+self::$guid_block-1, 'Guid');
        Cache::set('Guid_current', $guid_decimal, 'Guid');

        return $guid_decimal;
    }


    /**
     * Verify if a provided value is a valid GUID
     *
     * Checks if candidate contains only allowed alfanumeric characters (no vowels), if it's smaller
     * then the current maximum GUID and if it's bigger then the starting guid (1708)
     *
     * @return bool
     * @author Dave Broome <daveb@finda.co.nz>
     * @author Jakub Nesetril <jakub@finda.co.nz>
     * @param string $spec
     **/
    public static function isGuid($spec) {
        $start = '31';

        // invalid chars
        if (!preg_match('/[0-9b-df-hj-np-tv-z]+/', $spec)) {
            return false;
        }

        $next_guid_decimal = (Cache::exists('Guid_current')) ? Cache::get('Guid_current') : self::seedCache();

        // if proposed guid is more than highest guid or less then lowest guid
        if (self::modified_base_31_to_decimal($spec) > $next_guid_decimal || self::modified_base_31_to_decimal($spec) < $start)
        {
            return false;
        }
        else return true;
    }


    /**
     * Return a freshly generated unique ID
     *
     * @return string
     * @author Dave Broome <daveb@finda.co.nz>
     * @author Jakub Nesetril <jakub@finda.co.nz>
     **/
    public static function getGuid() {
        // find out what the current guid is
        $current = (Cache::exists('Guid_current')) ? Cache::increment('Guid_current') : self::seedCache();
        $maximum = Cache::get('Guid_maximum');


        // if current allocation block of guids has been exhausted, delete the cache
        if ($current >= $maximum) Cache::delete('Guid_current');

        if ($current > $maximum)  // if due to some bizzare concurency issue, we have exceeded our maximum and cannot return an overlapping guid
        {
            usleep(self::$wait_interval); // sleep for 0.2 second
            return self::getGuid();
        }

        return self::decimal_to_modified_base_31($current);
    }

    /**
     * Return a unique ID in decimal format
     *
     * @return int
     * @author Jakub Nesetril <jakub@finda.co.nz>
     **/
    public static function getGuidDecimal()
    {
        return self::modified_base_31_to_decimal(self::getGuid());
    }

}

?>