Pager For Lots o’ Data

/**
 * Display a pager. It constructs URLs with a "skip" query parameter.
 *
 * The pager shows the left arrow, first page, right arrow, last page, and
 * 3 pages around the middle, with a half-way page towards the start and the
 * end. If the pager is too close to the start/end for a half-way page, then
 * the half-way page is not shown.
 *
 * $skip    Index of first item on page, 0-based.
 * $count   Count of total items.
 * $length  Count of items on a page.
 * Returns nothing. The HTML is echo'd out.
 */
function EchoPager ($skip, $count, $length)
{
    $prevStr = 'Prev';
    $nextStr = 'Next';
    $elideStr = '... '; // Include trailing space.
    $pageSpan = 3;      // How many pages to the left and right of current.
    $getParam = 'skip'; // Query parameter for URL.
    
    // The mod expression accounts for whether there needs to be another page
    // to pick up the remaining elements. 
    $pageCount = (int)floor ($count / $length) + ($count % $length != 0);
    $lastPage = $pageCount - 1;
    $page = (int)floor ($skip / $length);
    
    // These define the 7 pages in the middle, including the current page.
    $rangeFirst = ($page - $pageSpan);
    $rangeLast = ($page + $pageSpan);
    if ($rangeFirst < 0) $rangeFirst = 0;
    if ($rangeLast > $lastPage) $rangeLast = $lastPage;
    
    // These define the halfway pages.
    $prevHalfway = (int)floor ($rangeFirst / 2);
    $nextHalfway = (int)ceil (($rangeLast + $lastPage) / 2);
    
    // Can move forward or backward?
    $canPrev = ($page > 0);
    $canNext = ($page < $lastPage);
    
    // Range does not include first or last pages?
    $canFirst = ($rangeFirst > 0);
    $canLast = ($rangeLast < $lastPage);
    
    // Range does not include half-way pages?
    $canPrevHalfway = ($prevHalfway > 0 && $prevHalfway < $rangeFirst);
    $canNextHalfway = ($nextHalfway < $lastPage && $nextHalfway > $rangeLast);
    
    // Pages missing before/after half-way pages?
    $elideBeforePrevHalf = ($canPrevHalfway && $prevHalfway > 1);
    $elideAfterPrevHalf = ($canPrevHalfway && ($rangeFirst - $prevHalfway) > 1);
    $elideBeforeNextHalf = ($canNextHalfway && ($nextHalfway - $rangeLast) > 1);
    $elideAfterNextHalf = ($canNextHalfway && $nextHalfway < ($lastPage - 1));
    
    // No half-way pages, but pages missing before/after range?
    $elideBeforeRange = (!$canPrevHalfway && $rangeFirst > 1);
    $elideAfterRange = (!$canNextHalfway && ($lastPage - $rangeLast) > 1);
    
    // Pre-figure part of the new links.
    $baseLink = '<a href="' . $_SERVER['PHP_SELF'] . '?';
    $query = $_SERVER['QUERY_STRING'];

    // Do prev arrow.
    if ($canPrev) echo $baseLink .
        htmlspecialchars(ReplaceQueryValue ($query, $getParam, $skip - $length)) .
        '">';
    echo $prevStr;
    if ($canPrev) echo '</a>';
    echo ' ';
    
    // Do first page.
    if ($canFirst) echo $baseLink .
        htmlspecialchars(ReplaceQueryValue ($query, $getParam, 0)) .
        '">1</a> ';
    
    // Do prev halfway point.
    if ($elideBeforePrevHalf) echo $elideStr;
    if ($canPrevHalfway) echo $baseLink .
        htmlspecialchars(ReplaceQueryValue ($query, $getParam, $prevHalfway * $length)) .
        '">' . ($prevHalfway + 1) . '</a> ';
    if ($elideAfterPrevHalf) echo $elideStr;
    
    // Do range.
    if ($elideBeforeRange) echo $elideStr;
    for ($i = $rangeFirst; $i <= $rangeLast; ++$i)
    {
        if ($i != $page) echo $baseLink .
            htmlspecialchars(ReplaceQueryValue ($query, $getParam, $i * $length)) .
            '">';
        echo ($i + 1);
        if ($i != $page) echo '</a>';
        echo ' ';
    }
    if ($elideAfterRange) echo $elideStr;
    
    // Do next halfway point.
    if ($elideBeforeNextHalf) echo $elideStr;
    if ($canNextHalfway) echo $baseLink .
        htmlspecialchars(ReplaceQueryValue ($query, $getParam, $nextHalfway * $length)) .
        '">' . ($nextHalfway + 1) . '</a> ';
    if ($elideAfterNextHalf) echo $elideStr;
    
    // Do last page.
    if ($canLast) echo $baseLink .
        htmlspecialchars(ReplaceQueryValue ($query, $getParam, $lastPage * $length)) .
        '">' . ($lastPage + 1) . '</a> ';
        
    // Do next arrow.
    if ($canNext) echo $baseLink .
        htmlspecialchars(ReplaceQueryValue ($query, $getParam, $skip + $length)) .
        '">';
    echo $nextStr;
    if ($canNext) echo '</a>';
}