idzio/shifter.c: Factor out shifter sim logic

idzio/shifter.c

@@ -0,0 +1,32 @@
+#include <stdbool.h>
+#include <stdint.h>
+
+#include "idzio/shifter.h"
+
+static bool idz_shifter_shifting;
+static uint8_t idz_shifter_gear;
+
+void idz_shifter_reset(void)
+{
+    idz_shifter_gear = 0;
+}
+
+void idz_shifter_update(bool shift_dn, bool shift_up)
+{
+    if (!idz_shifter_shifting) {
+        if (shift_dn && idz_shifter_gear > 0) {
+            idz_shifter_gear--;
+        }
+
+        if (shift_up && idz_shifter_gear < 6) {
+            idz_shifter_gear++;
+        }
+    }
+
+    idz_shifter_shifting = shift_dn || shift_up;
+}
+
+uint8_t idz_shifter_current_gear(void)
+{
+    return idz_shifter_gear;
+}