Make sure to test every code path

My story is not as dramatic as some, but I thought it was amusing. I’ve been using my PDF class for quite a while now, and it’s seemed to work perfectly so far in shipping code. But while working on a new app suddenly a tall skinny image was rendering too tall. When I fixed that, suddenly the image was off center. What was wrong?

Well, the PDFView class is designed to render it’s PDF’s as large as possible while preserving the aspect ratio. This means it takes the entire space in either the x- or y- direction, and centers itself in the other direction. But out of 913 images, apparently none of them filled the y-direction so that centering code had never been tested. So the code was failing to actually center properly.

It was a simple fix. Here’s the few lines to change to make the drawRect code work correctly:

        if (xRatio < yRatio) {
            ratio = xRatio;
            CGContextTranslateCTM(context, 0.0, -(self.bounds.size.height-boxRect.size.height*ratio)/2);
        } else {
            ratio = yRatio;
            CGContextTranslateCTM(context, (self.bounds.size.width-boxRect.size.width*ratio)/2, 0.0);

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