Pagination collapse algorithm.

pagination.php

<?php

$pages = 20;
$padding = 3;
$dedicated = 4;

function paginate($page)
{
	global $pages;
	global $padding;
	global $dedicated;
	
	// Maximum amount of elements between the < and > elements.
	$max = $dedicated + $padding * 2 + 1;

	// Will the left and/or right side collapse?
	$collapseLeft = $page - $padding - 1 > 1;
	$collapseRight = $page + $padding + 1 < $pages;

	// From which page do we start and where to we end?
	$start = $collapseLeft ? $page - $padding : 1;
	$end = $collapseRight ? $page + $padding : $pages;

	// The offset which is used to extend the amount of elements if only one
	// side is collapsed.
	$stickyOffset = $max - $padding - ($end - $start);

	// If the right side is not collapsed, we subtract the sticky offset from
	// the starting point and if the starting point is lesser than the first
	// page, we set it to 1.
	// Likewise, if the left side is not collapsed, we add the sticky offset to
	// the end point and if the end point is greater than the total amount of
	// pages, we set it to the last page.
	if (!$collapseRight && ($start -= $stickyOffset) < 1)
		$start = 1;
	else if (!$collapseLeft && ($end += $stickyOffset) > $pages)
		$end = $pages;

	// Below is just visual aid.

	echo ' << ';
	if ($collapseLeft)
	{
		echo ' _01 ';
		echo ' ... ';
	}

	for ($i = $start; $i <= $end; $i++) {
		echo ' ', $i == $page ? 'X' : '_', str_pad($i, 2, '0', STR_PAD_LEFT), ' ';
	}

	if ($collapseRight)
	{
		echo ' ... ';
		echo ' _',$pages,' ';
	}
	echo ' >> ';
	echo "\r\n";
}

echo 'From page 1 to 7.', "\r\n";
paginate(1);
paginate(2);
paginate(3);
paginate(4);
paginate(5);
paginate(6);
paginate(7);

echo "\r\n", 'From 20 to 14.', "\r\n";
paginate(20);
paginate(19);
paginate(18);
paginate(17);
paginate(16);
paginate(15);
paginate(14);