Understanding Fixes - Debugging

As I work trough the text effects I make test files on the PC version so I can test on the OSX version. The test files are reasonably straight forward - I test the range of settings, use different styling and a mixture of paragraph and character styling. In this I came across a problem. The issue was a word based effect with a change of style within the word resulted in some characters being badly positioned. The screen shot below is an example is the output from the simplest case I could create that demonstrated the problem - the "r" is kicked down below the base-line.
This bug had me floored - it was just very hard to work out why it was not working. Running the PC and OSX versions back to back can be very helpful but the in the case of text and fonts there are a lot of numbers many of which are different on the two platforms. CoreText , for example, gives different ascent and descents for fonts. Much of the math in Cello is integer and CoreText is fully floating. As an example of some or the sorts of differences you can get [even] Arial is different on the PC than then Mac. As an example below is a comparison between the same glyph in Arial on the two different platforms.

Getting nowhere for a while I wrote a list of all the possibilities that might bet the cause problem and worked through them. This got me as far as the point where the offset for positioning a glyph within an effect block was calculated. I discovered that if I abandoned the offset calculation, for the glyph that was failing, and set the offset to 0 (so it was the same offset the other glyph) everything worked!

OK- so this was a natural time to give up the chase and move on - but it really niggled me. I had no idea why forcing the offset to zero got things to work. I could kind of imagine that the offset might be zero in the PC version, or that CoreText was giving me values that were different enough for somehow this to be right or something. Checking on the PC version there was an offset that was different from mine but definitely not zero and not that far away.

Moving on and saying "well sure it works now" was tempting. I have worked for many years with a really excellent programmer who is careful, methodical, and very good. When things happen and start mysteriously to work wants to know why. He also, incidentally, writes some of the best code I have seen. I have learnt a lot from him - I carried on the chase. This morning before work I found the problem - misunderstanding some of the code I was not aggregating separate styles within a common word into the same effect block. This was an important bug to find and my setting the offset magically to zero would have been a bodge that would just have masked the problem.