Smart Pagination Algorithm

When implementing pagination, it's often desirable to limit the number of page links displayed to avoid overwhelming the user. This can be achieved by using a "smart" pagination algorithm that only shows a few adjacent pages to the current page.

Example Algorithm

The following PHP code demonstrates a smart pagination algorithm that truncates the page list to show only two adjacent pages to the current page:

<code class="php"><?php

// Define number of adjacent pages to show
$adjacents = 2;

// Define number of items per page
$limit = 5;

// Get current page number
$page = (int)$_GET["page"] ?? 1;

// Calculate first item to display on this page
$start = ($page - 1) * $limit;

// Get data
$data = $db
    ->query("SELECT * FROM mytable LIMIT $start, $limit")
    ->fetchAll();

// Calculate total number of pages
$total_pages = count($data);

// Setup page variables
$prev = $page - 1;
$next = $page + 1;
$lastpage = ceil($total_pages / $limit);
$lpm1 = $lastpage - 1;

// Generate pagination markup
$pagination = "<nav aria-label='page navigation'><ul class='pagination'>";

// Previous button
$pagination .= "<li class='page-item" . ($page === 1 ? " disabled" : "") . "'><a class='page-link' href='?page=$prev'>? previous</a></li>";

// First pages
$pagination .= "<li class='page-item" . ($page <= 2 ? " active" : "") . "'><a class='page-link' href='?page=1'>1</a></li>";
$pagination .= "<li class='page-item" . ($page <= 3 ? " active" : "") . "'><a class='page-link' href='?page=2'>2</a></li>";

// Ellipsis
$pagination .= "<li class='page-item disabled'><span class='page-link'>...</span></li>";

// Main pagination
if ($lastpage > 5 + ($adjacents * 2)) {
    // Close to beginning
    if ($page < 1 + ($adjacents * 2)) {
        for ($i = 3; $i <= 4 + ($adjacents * 2); $i++) {
            $pagination .= "<li class='page-item" . ($i === $page ? " active" : "") . "'><a class='page-link' href='?page=$i'>$i</a></li>";
        }
    }
    // Close to end
    elseif ($lastpage - ($adjacents * 2) > $page && $page > ($adjacents * 2)) {
        for ($i = $page - $adjacents; $i <= $page + $adjacents; $i++) {
            $pagination .= "<li class='page-item" . ($i === $page ? " active" : "") . "'><a class='page-link' href='?page=$i'>$i</a></li>";
        }
    }
    // In middle
    else {
        $pagination .= "<li class='page-item" . ($lpm1 === $page ? " active" : "") . "'><a class='page-link' href='?page=$lpm1'>$lpm1</a></li>";
        $pagination .= "<li class='page-item" . ($lastpage === $page ? " active" : "") . "'><a class='page-link' href='?page=$lastpage'>$lastpage</a></li>";
    }
}
else {
    for ($i = 3; $i <= $lastpage; $i++) {
        $pagination .= "<li class='page-item" . ($i === $page ? " active" : "") . "'><a class='page-link' href='?page=$i'>$i</a></li>";
    }
}

// Ellipsis
$pagination .= "<li class='page-item disabled'><span class='page-link'>...</span></li>";

// Last pages
$pagination .= "<li class='page-item" . ($page >= $lastpage - 2 ? " active" : "") . "'><a class='page-link' href='?page=$lpm1'>$lpm1</a></li>";
$pagination .= "<li class='page-item" . ($page >= $lastpage - 1 ? " active" : "") . "'><a class='page-link' href='?page=$lastpage'>$lastpage</a></li>";

// Next button
$pagination .= "<li class='page-item" . ($page >= $lastpage ? " disabled" : "") . "'><a class='page-link' href='?page=$next'>next ?</a></li>";

$pagination .= "</ul></nav>";

if ($lastpage <= 1) {
    $pagination = "";
}

echo $pagination;

// Display data
foreach ($data as $row) {
    // Display row data
}

echo $pagination;</code>

This algorithm uses the adjacents variable to control the number of adjacent pages shown on each side of the current page. It also includes the following features:

• Ellipsis: If there are more pages than can be displayed, it uses an ellipsis to truncate the page list.
• First and last pages: It always shows the first and last pages, regardless of the current page.
• Disabled buttons: It disables the previous and next buttons when the current page is the first or last page, respectively.
• Active class: It adds the "active" class to the current page's link.

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