xcalc at home

This is a simple calculator app drawn using the X client lib for X Server protocol.

Motivation

I felt like I haven't touched C in a long time, and there's no better way to make reviewing something old interesting by also learning something new at the same time... I don't know any GUI frameworks for native apps on any of the big three OS's frankly, but I know that my system uses X11 for its windowing system and you could probably just use a programming lib for it directly...How hard could it be?

What did I get myself into? X11 as the windowing system standard is slowly getting replaced in favor of Wayland on these Linux distros... Who cares, build the thing! Learn!

Getting Started

If you have nix, you can use the provided nix flake devshell by running nix develop, then make any code changes, then build the application with make. Or you can use the provided derivation and build the app with nix build or build and run with nix run.

Otherwise assuming Ubuntu with an X server running, you can run:

sudo apt install libx11-dev
make && ./main.out

Edge Cases

"Intuitive" handling

• 1 . x 2 = -> $1 \times 2 = 2$

trailing decimal is essentially evaluated to N.0 or ignored.

• . 2 x 1 = -> $0.2 \times 1 = 0.2$

preceding decimal is treated as having a 0 in front of it.

• x / + - 1 2 + 1 2 = -> $12 + 12 = 24$

preceding operator is ignored

• 1 2 + 1 2 = + 1 2 = -> $12 + 12 = 24 + 12 = 36$

results from previous calculations can be used as the first operand to an additional operator.

Slightly less obvious behavior, so pointing it out.

• 1 . 1 . x 2 = -> $1.1 \times 2 = 2.2$
• 1 . 1 . 3 . 4 . 2 . 1 . x 2 = -> $1.13421 \times 2 = 2.26842$

Extra decimals are ignored. This looks weird, but the GUI will visually give feedback to the user that the extra decimals are clearly not registered.

• 1 2 + 1 2 = 1 2 + 1 2 = -> $12 + 12 = 24; 12 + 12 = 24$

$12 + 12$ is evaluated twice, and we only see both results. The first evaluation is shown immediately once = is hit, and then as soon as the next 1 is inputted the previous result 24 will not be seen anymore.

• 1 2 + / * 1 2 = -> $12 \times 12 = 144$

We take the last operator inputted when waiting for the second operand to be inputted.

• 1 + 1 * 2 = -> $(1+1) * 2 = 4$
• 1 * 2 + 1 / 1 = -> $((1 * 2) + 1) / 1 = 3 / 1 = 3$

The values are always evaluated from left to right, because we eagerly evaluate the forms a op b.

• 1 + 1 * + * / - 2 -> $(1 + 1) - 2 = 0$

Follows same logic as above case but this also makes clear that previous evaluations become operand 1 for next operator and operand inputs.

I might not have catched all the weird edge cases, but this is why this is xcalc-at-home and not the xcalc you get at the store!

Resources

• https://www.youtube.com/watch?v=d2E7ryHCK08
• https://stackoverflow.com/questions/10792361/how-do-i-gracefully-exit-an-x11-event-loop
• https://tronche.com/gui/x/xlib
• https://xopendisplay.hilltopia.ca/2009/Mar/Xlib-tutorial-part-9----Buttons.html
• https://man.archlinux.org/man/extra/libx11/XEvent.3.en
• Claude 3.5 Sonnet
• https://www.cl.cam.ac.uk/~mgk25/ucs/keysymdef.h