Another “prev 123 next” navbar

<?php
# "prev 123 next" navbar, version 2.0.1 (October 19 2002)
# Functions that can print out links to several pages in a sequence.
# You can run this script as is to see a demo.
###########################################################
# Demo code

# Sample style.
# You might want to include something similar in your style sheet.
print '<style type="text/css">
p.navigation{
	font:0.7em verdana;
	text-align:center;
}
p.navigation strong.active, p.navigation a.prev, p.navigation a.next{
	font-weight:bold;
	font-size:1.25em;
	line-height:0.8em;
	vertical-align:bottom;
	text-decoration:none;
}
</style>';

# Sample implementations.
# Try clicking on the links they generate.
$pre_href = $PHP_SELF.'?page_no=';
$post_href = '';
$page_no = $HTTP_GET_VARS['page_no'];
print navigation($pre_href, $post_href, 1548,   25, $page_no);
print navigation($pre_href, $post_href, 9465,   20, $page_no);
print navigation($pre_href, $post_href, 33487,  10, $page_no);
print navigation($pre_href, $post_href, 334870, 5,  $page_no);

# End demo code
###########################################################
# The functions

/*
	The main function:
	Returns HTML string with a navigation bar, easily styled with CSS.
	Returns false if no navigation bar was necessary (if everything 
	could fit on one page).

	$pre_href = everyhing in the links' HREF before the active page's number.
	$post_href = everyhing *after* the active page's number.
	$num_items = total number of items available.
	$items_per_page = well, d'uh!
	$active = the active page's number.
	$nearby = minimum number of nearby pages to $active to display.
	$threshold = the smaller the number, the more restrictive the 
	  selection of links will be when $total is very high. This is to 
	  conserve space.

	These default settings limit the number of links to a maximum 
	of about 20.
*/
function navigation($pre_href='?page=', $post_href='', $num_items=0, $items_per_page=25, $active=1, $nearby=5, $threshold=100)
{
	# &#8230; is the ellipse character: "..."
	$space = '<span class="spacer"> &#8230; '."nt".'</span>';

	# There's no point in printing this string if there are no items,
	# Or if they all fit on one page.
	if ($num_items > 0 && $num_items > $items_per_page)
	{
		# STEP 1:
		# Force variables into certain values.

		# $items_per_page can't be smaller than 1!
		# Also, avoid division by zero later on.
		$items_per_page = max($items_per_page, 1);

		# Calculate the number of listing pages.
		$total = ceil($num_items/$items_per_page);

		# $active can't be higher than $total or smaller than 1!
		$active = max( min($active,$total), 1);

		# STEP 2:
		# Do the rest.

		# Get the sequence of pages to show links to.
		$pages = navigationSequence($total, $active, $nearby, $threshold);
		
		# Print a descriptive string.
		$first = ($active-1)*$items_per_page + 1;
		$last  = min($first+$items_per_page-1, $num_items);
		if ($first == $last)
			$listing = $first;
		else
			# &#8211; is the EN dash, the proper hyphen to use.
			$listing = $first.'&#8211;'.$last;
		$r = '<p class="navigation">'."ntShowing $listing of $num_items<br />n";
	
		# Initialize the list of links.
		$links = array();
	
		# Add "previous" link.
		if ($active > 1 && $total > 1)
			$links[] = '<a href="'.$pre_href.($active-1).$post_href.
			           '" class="prev" title="Previous">&laquo;</a>';
	
		# Decide how the each link should be presented.
		for($i=0; $i<sizeof($pages); $i++)
		{
			# Current link.
			$curr = $pages[$i];

			# See if we should any $spacer in connection to this link.
			if ($i>0 AND $i<sizeof($pages)-1)
			{
				$prev = $pages[$i-1];
				$next = $pages[$i+1];

				# See if we should any $spacer *before* this link.
				# (Don't add one if the last link is already a spacer.)
				if ($prev < $curr-1 AND $links[sizeof($links)-1] != $space)
					$links[] = $space;
			}

			# Add the link itself!
			# If the link is not the active page, link it.
			if ($curr != $active)
				$links[] = '<a href="'.$pre_href.$curr.$post_href.'">'.$curr.'</a>';
			# Else don't link it.
			else
				$links[] = '<strong class="active">'.$active.'</strong>';

			if ($i>0 AND $i<sizeof($pages)-1)
			{
				# See if we should any $spacer *after* this link.
				# (Don't add one if the last link is already a spacer.)
				if ($next > $curr+1 AND $links[sizeof($links)-1] != $space)
					$links[] = $space;
			}
		}
	
		# Add "next" link.
		if ($active < $total && $total > 1)
			$links[] = '<a href="'.$pre_href.($active+1).$post_href.
			           '" class="next" title="Next">&raquo;</a>';
	
		# Put it all together.
		$r .= "t".implode($links, "nt")."n</p>n";
		$r = str_replace("nt".$space."nt", $space, $r);

		return $r;
	}
	else
		return false;
}

# This one is used by navigation().
function navigationSequence($total=1, $active=1, $nearby=5, $threshold=100)
{
	# STEP 1:
	# Force minimum and maximum values.
	$total     = (int)max($total, 1);
	$active    = (int)min( max($active,1), $total);
	$nearby    = (int)max($nearby, 2);
	$threshold = (int)max($threshold, 1);

	# STEP 2:
	# Initialize $pages.

	# Array where each key is a page.
	# That way we can easily overwrite duplicate keys, without
	# worrying about the order of elements.
	# Begin by adding the first, the active and the final page.
	$pages = array(1=>1, $active=>1, $total=>1);

	# STEP 3:
	# Add nearby pages.
	for ($diff=1; $diff<$nearby+1; $diff++)
	{
		$pages[min( max($active-$diff,1), $total)] = 1;
		$pages[min( max($active+$diff,1), $total)] = 1;
	}

	# STEP 4:
	# Add distant pages.

	# What are the greatest and smallest distances between page 1 
	# and active page, or between active page and final page?
	$biggest_diff  = max($total-$active, $active);
	$smallest_diff = min($total-$active, $active);

	# The lower $factor is, the bigger jumps (and ergo fewer pages) 
	# there will be between the distant pages.
	$factor = 0.75;
	# If we think there will be too many distant pages to list, 
	# reduce $factor.
	while(pow($total*max($smallest_diff,1), $factor) > $threshold)
		$factor *= 0.9;

	# Add a page between $active and the farthest end (both upwards 
	# and downwards). Then add a page between *that* page and 
	# active. And so on, until we touch the nearby pages.
	for ($diff=round($biggest_diff*$factor); $diff>$nearby; $diff=round($diff*$factor))
	{
		# Calculate the numbers.
		$lower  = $active-$diff;
		$higher = $active+$diff;

		# Round them to significant digits (for readability):
		# Significant digits should be half or less than half of 
		# the total number of digits, but at least 2.
		$lower  = round($lower,  -floor(max(strlen($lower),2)/2));
		$higher = round($higher, -floor(max(strlen($higher),2)/2));

		# Maxe sure they're within the valid range.
		$lower  = min(max($lower,  1),$total);
		$higher = min(max($higher, 1),$total);

		# Add them.
		$pages[$lower]  = 1;
		$pages[$higher] = 1;
	}

	# STEP 5:
	# Convert the keys into values and sort them.
	$return = array_keys($pages);
	sort($return);

	return $return;
}

?>