Bugs and Mutations

It took me hours to find this bug, and it turned out to be pretty surprising. I had good check-ins before and after the change. I was refactoring my code to make the rectangle class immutable. After the change text was no longer positioned correctly:

correct position

incorrect position

I changed one line in my text-layout code to be compatible with immutable rectangles:

- metrics.area.location = metrics.area.location.neg.add offset
+ metrics.area = rect metrics.area.location.neg.add(offset), metrics.area.size

I can tell you there were no bugs in the new immutable Rectangle class.

Can you spot the bug?

---

There is no bug. The new line is correct, and yet that's all I changed.

It turns out I originally had a bug due to a mutation I wasn't taking into account, but that mutation made the layout algorithm work. When I stopped mutating the rectangle object, the "bug" stopped making the algorithm work. The reason it took me hours to find this bug was the new code was "right", but the algorithm was wrong.

Here's what was actually wrong, in 3 code snippets (CoffeeScript):

1) The original code that worked but had an accidental mutation:

    updateTextLayout: ->
      offset = point()
      @area = @metricsList[0].area  # object assigned here
      for metrics in @metricsList
        metrics.area.location = metrics.area.location.neg.add offset  
          # gets mutated here the first time through the loop
        offset = offset.add 0, @fontSize
        @area = @area.union metrics.area
      @size = @area.size

2) Code with one line changed that seemed correct but didn't work:

    updateTextLayout: ->
      offset = point()
      @area = @metricsList[0].area
      for metrics in @metricsList
        metrics.area = rect metrics.area.location.neg.add(offset), metrics.area.size  
          # @area no longer gets mutated
        offset = offset.add 0, @fontSize
        @area = @area.union metrics.area
      @size = @area.size

3) Final working code:

    updateTextLayout: ->
      offset = point()
      @area = null  # FIX - don't capture the wrong value here
      for metrics in @metricsList
        metrics.area = rect metrics.area.location.neg.add(offset), metrics.area.size
        offset = offset.add 0, @fontSize
        @area = if @area then @area.union metrics.area else metrics.area    
          # FIX - capture the right value here
      @size = @area.size