<?php
function get_relative_size ($src_val,$src_opposite,$target_val,$target_opposite) {
    if (!$target_opposite) {return $target_val;}
    return round($src_val/($src_opposite/$target_opposite));
}

function get_orientation ($w,$h) {
    return ($w==$h?0:($w>$h?-1:1));
}

function resize ($w=0,$h=0,$ow=0,$oh=0,$mode='max',$enlarge=false) {
    $rw = NAN;
    $rh = NAN;
    $w = intval($w);
    $h = intval($h);
    if ($w==0&&$h==0) {return [$ow,$oh];}

    $new_orientation = get_orientation($w,$h);
    $src_orientation = get_orientation($ow,$oh);

    //$mode = 'max'; // min|max|stretch

    if ($mode=='stretch') {
        return [$w,$h];
    }

    //$enlarge = true;

    $rw = get_relative_size($ow,$oh,$w,$h);
    $rh = get_relative_size($oh,$ow,$h,$w);

    $case = (!$h?'width':(!$w?'height':'both'));

    switch ($case) {
        case 'width':$rw = $w;break;
        case 'height':$rh = $h;break;
        default:
            if (($mode=='max'&&$rw<$w)||($mode=='min'&&$rw>$w)) {
                $rh = $h;
            } else {
                $rw = $w;
            }

    }

    if (!$enlarge) {
        if ($rw>$ow||$rh>$oh) {
            $rw = $ow;
            $rh = $oh;
        }
    }

    return [$rw,$rh];
}

// enlarge
$enlarges = [false,true];

// modes
$modes = ['max','min','stretch'];

// resize to
$tests = [
    [50,0],
    [0,50],
    [50,50],
    [50,150],
    [150,50],
];

// src sizes
$sizes = [
    [40,40],
    [20,40],
    [40,20],

    [150,150],
    [150,250],
    [250,150],

    [200,200],
    [200,300],
    [300,200],
];

foreach ($modes as $mode) {
    foreach ($enlarges as $enlarge) {
        $id = $mode.'_'.($enlarge?'up':'d');
        print '<div style="clear:both;padding:1em;border:1px solid blue;" onclick="var image=document.getElementById(\''.$id.'\');image.style.display=(image.style.display==\'none\')?\'block\':\'none\';">'.$mode.' '.($enlarge?'^':'').'</div>';
        print '<div id="'.$id.'">';
        foreach ($tests as $test) {
            foreach ($sizes as $size) {
                $r = call_user_func_array('resize',array_merge($test,$size,[$mode],[$enlarge]));
                print '<div style="float:left;margin:1em;">';
                print "<b>resize:".implode('x',$test).'['.get_orientation($test[0],$test[1]).']</b><br />';
                print 'src:'.implode('x',$size).'['.get_orientation($size[0],$size[1]).'] >> <b>'.implode('x',$r).'['.get_orientation($r[0],$r[1]).']</b><br />';

                print '<div style="position:relative;min-height:300px;min-width:300px;">';
                print '<div style="background-color:rgba(255,255,0,1);width:'.$size[0].'px;height:'.$size[1].'px;">';
                print '<div style="background-color:rgba(0,255,255,0.4);width:'.$r[0].'px;height:'.$r[1].'px;">';
                print '</div></div>';
                print '<div style="position:absolute;left:0;top:0;border:1px solid red;width:'.($test[0]-2).'px;height:'.($test[1]-2).'px;"></div>';
                print '</div>';
                print '</div>';
            }
            print '<hr style="clear:both" />';
        }
        print '</div>';
    }
}

?>