hook Apps key to switch IME

appime.c

/*
gcc -std=c99 -O3 -Wall -Werror \
    -DUNICODE \
    -o $@ $^ \
    -lkernel32 -luser32 \
    -Wl,--subsystem,windows,--gc-sections,--strip-all
*/
#include <windows.h>

#define WM_USER_SWITCH_IME (WM_USER + 1)

static HHOOK kbd_hook = NULL;
static int modifiers = 0;
static int shortcut_prefix = 0;

void switch_ime() {
    INPUT inputs[4] = {0};
    inputs[0].type = INPUT_KEYBOARD;
    inputs[0].ki.wVk = VK_LWIN;
    inputs[1].type = INPUT_KEYBOARD;
    inputs[1].ki.wVk = VK_SPACE;
    inputs[2].type = INPUT_KEYBOARD;
    inputs[2].ki.wVk = VK_LWIN;
    inputs[2].ki.dwFlags = KEYEVENTF_KEYUP;
    inputs[3].type = INPUT_KEYBOARD;
    inputs[3].ki.wVk = VK_SPACE;
    inputs[3].ki.dwFlags = KEYEVENTF_KEYUP;
    SendInput(4, inputs, sizeof(INPUT));
}

LRESULT CALLBACK kbd_proc(int nCode, WPARAM wParam, LPARAM lParam) {
    if (nCode == HC_ACTION) {
        KBDLLHOOKSTRUCT *hs = (KBDLLHOOKSTRUCT *)lParam;

        switch (hs->vkCode) {
        case VK_LSHIFT:
        case VK_RSHIFT:
            if (wParam == WM_KEYDOWN) {
                modifiers |= 1;
            } else {
                modifiers &= ~1;
            }
            break;
        case VK_LCONTROL:
        case VK_RCONTROL:
            if (wParam == WM_KEYDOWN) {
                modifiers |= 2;
            } else {
                modifiers &= ~2;
            }
            break;
        case VK_LMENU:
        case VK_RMENU:
            if (wParam == WM_KEYDOWN) {
                modifiers |= 4;
            } else {
                modifiers &= ~4;
            }
            break;
        case VK_LWIN:
        case VK_RWIN:
            if (wParam == WM_KEYDOWN) {
                modifiers |= 8;
            } else {
                modifiers &= ~8;
            }
            break;
        case 'Q':
            if (wParam == WM_KEYDOWN && modifiers == 7) {
                // handle ctrl+alt+shift+q
                if (!shortcut_prefix) {
                    shortcut_prefix = 1;
                } else {
                    UnhookWindowsHookEx(kbd_hook);
                    PostQuitMessage(0);
                }
            }
            break;
        case VK_APPS:
            if (wParam == WM_KEYUP && modifiers == 0) {
                PostMessage(NULL, WM_USER_SWITCH_IME, 0, 0);
            }
            if (modifiers != 8) {
                return 1;
            }
        default:
            shortcut_prefix = 0;
        }
    }
    return CallNextHookEx(kbd_hook, nCode, wParam, lParam);
}

int main() {
    kbd_hook = SetWindowsHookEx(WH_KEYBOARD_LL, kbd_proc, NULL, 0);
    if (kbd_hook == NULL) {
        return 1;
    }
    MSG msg;
    while (GetMessageW(&msg, NULL, 0, 0)) {
        switch (msg.message) {
        case WM_USER_SWITCH_IME:
            switch_ime();
            break;
        }
    }
    return (int)msg.wParam;
}