First: This is open-source. If you want to have a look, there is a mecurial repository hosted here @ sourceforge. Please feel free to have a look into the sources.
Integer Calculator Tool
supports 8,16,32Bit Operations
The Tool was inspired by https://sourceforge.net/projects/hextool/,
I used it daily! But I missed some functionality...
So now QTHexTool offers a different approach:
more Mathematical Operations [ myabe some more to come :-) ],
implemented with QT4 => supports Linux...