Quick script to calculate the change in Scrabble rating between yourself and a field of opponents

rating.php

<?php

if ($argc < 3) {
	echo <<< EOB
USAGE: {$argv[0]} myRating opp1rating [opp2rating...]

EOB;
	exit(1);
}

array_shift($argv);

$rating = array_shift($argv);
$lessThanFiftyGames = false;

$k = calculateK($rating, $lessThanFiftyGames);

$winKeys = [
	'Win'  => 1.0,
	'Tie'  => 0.5,
	'Loss' => 0.0,
];

echo <<< EOB
Rating: {$rating}     K: {$k}

EOB;

printf('%-10s', 'OppR');
printf('%-10s', 'Win%');

foreach(array_keys($winKeys) as $desc) {
	printf('%-10s', $desc);
}
echo "\n";


while($oppRating = array_shift($argv)) {

	$expectation = winExpectation($rating, $oppRating);

	printf('%5d     ', $oppRating);
	printf('%0.5f     ', $expectation);

	foreach($winKeys as $result) {
		$excess = $result - $expectation;
		$points = convertExcessToPoints($excess, $k);
		printf('%- 10d', round($points));
	}
	echo "\n";
}

function calculateK($rating, $lessThanFiftyGames = false) {
	$k = $rating < 1800 ? 20 : ($rating < 2000 ? 16 : 10);
	if ($lessThanFiftyGames) {
		$k *= 1.5;
	}
	return $k;
}

function winExpectation($a, $b) {
	return 1 - 1/(1+exp(0.0031879*($a-$b)));
}

function convertExcessToPoints($excess, $k) {
	return $excess*$k;
}