[Feature] Swap buttons on PS2 Mouse/Trackball (#9205)

* [Feature Request] Swap buttons on PS2 Mouse/Trackball

* [Feature Request] Swap buttons on PS2 Mouse/Trackball

* Added id: to the doc

* Missing space

* Solve comment
https://github.com/qmk/qmk_firmware/pull/9205#discussion_r430783182

* Solve comments https://github.com/qmk/qmk_firmware/pull/9205#discussion_r430783182 & https://github.com/qmk/qmk_firmware/pull/9205#discussion_r430783884

* Format code more according to https://docs.qmk.fm/#/coding_conventions_c

* change logic to LUT

* WIP: Clean up

* WIP: Solution with xor operators to mask the change

* delete #endif & added the missed xor operator (ahhh)

* Variable (mouse_report->buttons): avoid setting twice https://github.com/qmk/qmk_firmware/pull/9205#discussion_r430783884

* Update tmk_core/protocol/ps2_mouse.c

Co-authored-by: Nick Brassel <nick@tzarc.org>

Co-authored-by: juank <juank@fktech.net>
Co-authored-by: Nick Brassel <nick@tzarc.org>

docs/feature_ps2_mouse.md

@@ -270,6 +270,16 @@ Fine control over the scrolling is supported with the following defines:
 #define PS2_MOUSE_SCROLL_DIVISOR_V 2
 ```
 
+### Invert Mouse buttons :id=invert-buttons
+
+To invert the left & right buttons you can put:
+
+```c
+#define PS2_MOUSE_INVERT_BUTTONS
+```
+
+into config.h.
+
 ### Invert Mouse and Scroll Axes :id=invert-mouse-and-scroll-axes
 
 To invert the X and Y axes you can put:

tmk_core/protocol/ps2_mouse.c

@@ -156,8 +156,15 @@ static inline void ps2_mouse_convert_report_to_hid(report_mouse_t *mouse_report)
     mouse_report->x = X_IS_NEG ? ((!X_IS_OVF && -127 <= mouse_report->x && mouse_report->x <= -1) ? mouse_report->x : -127) : ((!X_IS_OVF && 0 <= mouse_report->x && mouse_report->x <= 127) ? mouse_report->x : 127);
     mouse_report->y = Y_IS_NEG ? ((!Y_IS_OVF && -127 <= mouse_report->y && mouse_report->y <= -1) ? mouse_report->y : -127) : ((!Y_IS_OVF && 0 <= mouse_report->y && mouse_report->y <= 127) ? mouse_report->y : 127);
 
+#ifdef PS2_MOUSE_INVERT_BUTTONS
+    // swap left & right buttons
+    uint8_t needs_left = mouse_report->buttons & PS2_MOUSE_BTN_RIGHT;
+    uint8_t needs_right = mouse_report->buttons & PS2_MOUSE_BTN_LEFT;
+    mouse_report->buttons = (mouse_report->buttons & ~(PS2_MOUSE_BTN_MASK)) | (needs_left ? PS2_MOUSE_BTN_LEFT : 0) | (needs_right ? PS2_MOUSE_BTN_RIGHT : 0);
+#else
     // remove sign and overflow flags
     mouse_report->buttons &= PS2_MOUSE_BTN_MASK;
+#endif
 
 #ifdef PS2_MOUSE_INVERT_X
     mouse_report->x = -mouse_report->x;