Negative/positive numbers in object inverting themselves?

Question

So I'm sure the answer to this will be something simple and I'll read it and immediately commence epic facepalming, but at the moment I've spent an entire day wondering why the heck this isn't working and I just can't find the problem...

I'm trying to build a simple (supposed to be anyway) system to rotate 2d points around a central axis, but I've found 2 problems so far that make no sense to me whatsoever.

Problem 1: The positive/negative numbers assigned to the input object are being flipflopped in a seemingly random fashion. Problem 2: The rotation math is correct, but the results are anything but.

Here's the code to reproduce this problem:

function doRotate(pos){
    //convert angle to radians
    var ang = 90 * Math.PI / 180;

    //rotate points
    pos.left = Math.round(pos.left * Math.cos(ang) + pos.top * Math.sin(ang));
    pos.top = Math.round(pos.top * Math.cos(ang) - pos.left * Math.sin(ang));

    return pos;
}

var points = {
    'a':{left:32,top:32},
    'b':{left:-32,top:32},
    'c':{left:-32,top:-32},
    'd':{left:32,top:-32}
};

for( var k in points ){
    var msg = 'Start: X:'+points[k].left+' Y:'+points[k].top+'<br />';
    points[k] = doRotate(points[k]);
    var msg = msg+'End: X:'+points[k].left+' Y:'+points[k].top+'<br />';
    document.write( '<p>'+k+':<br />'+msg+'</p>' );
}

Now you'd expect to see: a: Start: X:32 Y:32 End: X:32 Y:-32

b: Start: X:-32 Y:32 End: X:-32 Y:-32

c: Start: X:-32 Y:-32 End: X:-32 Y:32

d: Start: X:-32 Y:32 End: X:32 Y:32

But instead you get this:

a: Start: X:32 Y:32 End: X:32 Y:32

b: Start: X:-32 Y:-32 End: X:32 Y:32

c: Start: X:-32 Y:-32 End: X:-32 Y:-32

d: Start: X:32 Y:32 End: X:-32 Y:-32

I've triple checked the math and that is 100% accurate to rotate points around origin in 2D. I've even recreated this test in PHP and it works perfectly. What I can't figure out is why the heck JS is a) screwing up the variable assignments and b) not coming up with the correct results?

Anyone who can help may rest assured that I will salute their insight with a facepalm Sir Patrick would be proud of ;)

Answer 1

Wait a sec - shouldn't you save pos.left before you compute pos.top so that you can use the old pos.left value in that calculation?