kilo commentary: Moving the cursor (steps 43-45)

In spite of a hiatus that's now more than a year long, I haven't forgotten about my kilo project. Steps 43-45 involve code to keep track of where the cursor is on the screen, and allow the user to move it around. Here's the original code that does that:

void editorMoveCursor(char key) {
  switch (key) {
    case 'a':
      E.cx--;
      break;
    case 'd':
      E.cx++;
      break;
    case 'w':
      E.cy--;
      break;
    case 's':
      E.cy++;
      break;
  }
}
void editorProcessKeypress() {
  char c = editorReadKey();
  switch (c) {
    case CTRL_KEY('q'):
      write(STDOUT_FILENO, "\x1b[2J", 4);
      write(STDOUT_FILENO, "\x1b[H", 3);
      exit(0);
      break;
    case 'w':
    case 's':
    case 'a':
    case 'd':
      editorMoveCursor(c);
      break;
  }
}

The code is basically decoding the w, s, a, and d characters twice: once to determine that the editorMoveCursor function should be called, and another to determine how the cursor should be moved. This is (IMO) another bug waiting to happen, in a large project these two points in the code could easily get out of sync. There's no reason not to do something like this (which is also my current solution):

void editor_move_cursor(int dcx, int dcy)
{
    g_editor_state.cx += dcx;
    g_editor_state.cy += dcy;
}


void editor_process_keypress(char c)
{
    string_const clear_screen = get_clear_screen_str();
    string_const home_cursor  = get_home_cursor_str();

    switch (c)
    {
    case CTRL_KEY('q'):
        write_str(clear_screen);
        write_str(home_cursor);
        exit(0);
        break;

    case 'w': editor_move_cursor( 0, -1); break;
    case 's': editor_move_cursor( 0,  1); break;
    case 'a': editor_move_cursor(-1,  0); break;
    case 'd': editor_move_cursor( 1,  0); break;
    }
}

Note that now only editor_process_keypress needs to know what keys move the cursor, and editor_move_cursor gets parameters that are directly interpretable.