layout.c

#include <stdio.h>
#include <stdlib.h>

#include <unistd.h>
#include <linux/input.h>

int read_event(struct input_event *event)
{
  return fread(event, sizeof(struct input_event), 1, stdin) == 1;
}

void write_event(const struct input_event *event)
{
  if (fwrite(event, sizeof(struct input_event), 1, stdout) != 1)
    exit(EXIT_FAILURE);
}

int main(void)
{
  int use_arrow_keys = 0;

  struct input_event input;

  setbuf(stdin, NULL), setbuf(stdout, NULL);

  while (read_event(&input))
  {
    if (input.type == EV_MSC && input.code == MSC_SCAN)
      continue;

    if (input.type != EV_KEY)
    {
      write_event(&input);
      continue;
    }

   if (use_arrow_keys)
    {
      if (input.code == KEY_LEFTCTRL)
      {
        if (input.value == 0) {
          use_arrow_keys = 0;
          continue;
        }

        if (input.value == 1 || input.value == 2) {
          continue;
        }
      }
    }
    else
    {
      if (input.code == KEY_LEFTCTRL)
      {
        if (input.value == 1 || input.value == 2) {
          use_arrow_keys = 1;
          continue;
        }

        if (input.value == 0) {
          continue;
        }
      }
    }

    if (use_arrow_keys) {
      // E → Up
      if (input.code == KEY_E) {
        input.code = KEY_UP;
      }

      // D → Down
      if (input.code == KEY_D) {
        input.code = KEY_DOWN;
      }

      // S → Left
      if (input.code == KEY_S) {
        input.code = KEY_LEFT;
      }

      // F → Right
      if (input.code == KEY_F) {
        input.code = KEY_RIGHT;
      }

      // A → Home
      if (input.code == KEY_A) {
        input.code = KEY_HOME;
      }

      // G → End
      if (input.code == KEY_G) {
        input.code = KEY_END;
      }

      // C → Page Down
      if (input.code == KEY_C) {
        input.code = KEY_PAGEDOWN;
      }

      // 4 → Page Up
      if (input.code == KEY_4) {
        input.code = KEY_PAGEUP;
      }
    }

    // ALT → LEFT CTRL
    if (input.code == KEY_LEFTALT)
    {
      input.code = KEY_LEFTCTRL;
    }

    // CAPSLOCK → ESCAPE
    if (input.code == KEY_CAPSLOCK)
    {
      input.code = KEY_ESC;
    }

    // RIGHT ALT → LEFT ALT
    if (input.code == KEY_RIGHTALT)
    {
      input.code = KEY_LEFTALT;
    }

    // 102ND → COMPOSE
    if (input.code == KEY_102ND)
    {
      input.code = KEY_COMPOSE;
    }

    write_event(&input);
  }
}