If you've ever been involved in chess tournaments, you would know they come in various formats, one of which is the round robin format. In a round robin, each player plays every other player, and the difference in colours is kept to a minimum, with half the field having white for one more game than the other half. I couldnt come up with a good e…

Round Robin Generator

<?php 
/************************************************ 
 * Round Robin Pairing Generator 
 * Author: Eugene Wee 
 * Date: 23 May 2005 
 * Last updated: 23 May 2005 
 * Copyright (c) 2005 Eugene Wee 
 * Licensed under the Academic Free License 2.1 
 * Based on algorithm by Tibor Simko 
 ************************************************/ 

function generateRoundRobinPairings($num_players) { 
    //do we have a positive number of players? otherwise default to 4 
    $num_players = ($num_players > 0) ? (int)$num_players : 4; 
    //set number of players to even number 
    $num_players = ($num_players % 2 == 0) ? $num_players : $num_players + 1; 
    //format for pretty alignment of pairings across rounds 
    $format = "%0" . ceil(log10($num_players)) . "d"; 
    $pairing = "$format-$format "; 
    //set the return value 
    $ret = $num_players . " Player Round Robin:\n-----------------------"; 
    //print the rounds 
    for ($round = 1; $round < $num_players; $round++) { 
        $ret .= sprintf("\nRound #$format : ", $round); 
        $players_done = array(); 
        //print the pairings 
        for ($player = 1; $player < $num_players; $player++) { 
            if (!in_array($player, $players_done)) { 
                //select opponent 
                $opponent = $round - $player; 
                $opponent += ($opponent < 0) ? $num_players : 1; 
                //ensure opponent is not the current player 
                if ($opponent != $player) { 
                    //choose colours 
                    if ($player % 2 == $opponent % 2) { 
                        if ($player < $opponent) { 
                            //player plays black 
                            $ret .= sprintf($pairing, $opponent, $player); 
                        } else { 
                            //player plays white 
                            $ret .= sprintf($pairing, $player, $opponent); 
                        } 
                    } else { 
                        if ($player < $opponent) { 
                            //player plays white 
                            $ret .= sprintf($pairing, $player, $opponent); 
                        } else { 
                            //player plays black 
                            $ret .= sprintf($pairing, $opponent, $player); 
                        } 
                    } 
                    //these players are done for this round 
                    $players_done[] = $player; 
                    $players_done[] = $opponent; 
                } 
            } 
        } 
        //print the last pairing (i.e. for the last player) 
        if ($round % 2 == 0) { 
            $opponent = ($round + $num_players) / 2; 
            //last player plays white 
            $ret .= sprintf($pairing, $num_players, $opponent); 
        } else { 
            $opponent = ($round + 1) / 2; 
            //last player plays black 
            $ret .= sprintf($pairing, $opponent, $num_players); 
        } 
    } 
    return $ret; 
} 
?>