ダニエルの冒険

In 2006 I wrote some code to work out the coordinates of a point on the circumference of a circle based on an angle. This was so I could draw this nice clock in PHP:

It should display the current time in Ireland (though my server’s clock seems about 4 minutes behind).

The following code calculates the point at angle $angle on the circumference of the circle of radius $r centred on (0,0).

function findposc($angle, $r)
{
    if ($angle > 180) $ang = $angle - 180;
    $ang = 90-$angle;
 
    $m = tan(deg2rad($ang));
    $x = sqrt(($r*$r) / (1+($m*$m)));
    $y = $m*$x;
 
    if ($angle>180) {
        $x = $x*(-1);
        $y = $y*(-1);
    }
 
    return $res;
}

Look at it! It’s embarrassing! I don’t even know what “$m” is supposed to be! I remember sitting down and working it out on paper, but I don’t know how my understanding of trigonometry was so horrible considering I had just done the leaving cert. 90-$angle? Why?? $y ends up being this (pseudo-code, and $angle is in radians):

$y = tan($angle) * (sqrt(($radius^2) / (1+(tan^2($angle)))

Looks badass, I know, but what Y really should be is:

$y = $radius * cos($angle)

I honestly couldn’t sleep the night I saw this code. I went to bed and after going over and over the trig in my head for about 20 minutes, I had to get up and rewrite it. Here’s the proper version in Java:

Point2D.Double calcPointOnCircle( double angle, double radius )
{
       double angleAsRad = angle / (180/Math.PI);
 
       Double x = radius * Math.sin( angleAsRad );
       Double y = -radius * Math.cos( angleAsRad );
 
       return new Point2D.Double( x, y );
}

… and then I could sleep.