From 57cdf605f583f84190536042ebcbb96e8856da8b Mon Sep 17 00:00:00 2001
From: Nulo <git@nulo.in>
Date: Mon, 13 Dec 2021 17:13:56 -0300
Subject: [PATCH] Sticky keys

---
 keycodes.scm |  3 +++
 menelaus.scm | 26 ++++++++++++++++++++------
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/keycodes.scm b/keycodes.scm
index 0d2c957..39234d4 100644
--- a/keycodes.scm
+++ b/keycodes.scm
@@ -1,5 +1,8 @@
 ;; port of usb_keyboard.h
 
+; Actually ErrorUndefined
+(define sticky-modifier 3)
+
 (define key-a 4)
 (define key-b 5)
 (define key-c 6)
diff --git a/menelaus.scm b/menelaus.scm
index 051209d..255a030 100644
--- a/menelaus.scm
+++ b/menelaus.scm
@@ -246,6 +246,7 @@
 
 ;; Vectors to store keycodes for the USB frame we are preparing to send.
 (define modifiers (vector 0 0 0 0 0))
+(define sticky-modifiers (vector 0 0 0 0 0))
 (define keycodes-down (vector 0 0 0 0 0 0))
 
 ;; For each element of the keycodes-down or modifiers vector, which physical
@@ -263,6 +264,11 @@
   (vector-set! modifiers (- keycode 1) 1)
   (vector-set! keys-for-modifiers (- keycode 1) key))
 
+(define (press-sticky-modifier keycode)
+  (vector-set! sticky-modifiers (- keycode 1) 1))
+(define (release-sticky-modifier keycode)
+  (vector-set! sticky-modifiers (- keycode 1) 0))
+
 ;; Record that a given key resulted in a specific non-modifier press.
 (define (press-normal-key keycode key)
   (let ((slot (find keycodes-down 0)))
@@ -271,17 +277,25 @@
 
 ;; Record a key press in the modifiers/keycodes-down vectors for the layout.
 (define (press-key key)
-  (let ((keycode (lookup key)))
+  (let ((keycode (lookup key))
+        (sticky (find sticky-modifiers 1)))
     ;; Sometimes "keycodes" are procedures; in that case we call them with
     ;; true when the key is pressed and false when it's released.
     (if (procedure? keycode)
         (keycode #t)
         (if (modifier? keycode)
-            (begin (press-modifier (unmodify keycode) key)
-                   (if (uncombo keycode)
-                       (press-normal-key (uncombo keycode) key)
-                       #f))
-            (press-normal-key keycode key)))))
+            (if (uncombo keycode)
+                (if (= (uncombo keycode) sticky-modifier)
+                    (press-sticky-modifier (unmodify keycode))
+                    (begin
+                      (press-modifier (unmodify keycode) key)
+                      (press-normal-key (uncombo keycode) key)))
+                (press-modifier (unmodify keycode) key))
+            (if sticky
+                (begin (press-modifier (+ sticky 1) key)
+                       (press-normal-key keycode key)
+                       (release-sticky-modifier (+ sticky 1)))
+                (press-normal-key keycode key))))))
 
 ;; Record that a given key being released resulted in a modifier release.
 (define (release-modifier keycode key n)