Keystroke internals

First of all, just like to say - I love this.

In the spirit of _we're going to try to answer this question **in as much detail as possible**_ though I think there are are some missing pieces in the initial 'keystroke' parts, which I have wondered if I would use if I was ever asked this in an interview.

> The "g" key is pressed

- Has a modifier key been pressed as well?
- Is this part of a dead-key combination?
- Is this part of a diacritic?
- Has a [compose key](https://en.wikipedia.org/wiki/Compose_key) been pressed?

With Windows, I used to have a handle on how some of these worked internally, but that project was a long, long time ago, and all I am left with is the vestigial knowledge that such things exist. Nevertheless, all major OSs will have support for these.

The one I can still remember well enough - on Windows - is a check that would be made:

- Has the letter 'g' been mapped to another key?

This is the concept of 'scancode mappings' - where a keypress is interpreted as another key.

On Windows these are saved in the registry, apply to all users [0], and would be checked on each keystroke. Here's an example of a mapping from `f` to `h`:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout]
"Scancode Map"=hex:00,00,00,00,00,00,00,00,02,00,00,00,23,00,21,00,00,00,00,00

In this case, the check would be made and the answer would be _'no, `g` has not been mapped to another key'_

On Linux,[`xmodmap`](https://man.linuxreviews.org/man1/xmodmap.1.html) and [XKE](https://wiki.archlinux.org/title/X_keyboard_extension) look like they do the same, and on OSX there are [third-party programs](https://karabiner-elements.pqrs.org/docs/manual/configuration/configure-simple-modifications/) which perform the same function.

[0] Windows XP had per-user scancode mappings, but it was undocumented and removed in Vista..