1 files changed, 70 insertions, 0 deletions
diff --git a/engine/flag_predef.asm b/engine/flag_predef.asm
new file mode 100644
index 000000000..bd4f37882
--- /dev/null
+++ b/engine/flag_predef.asm
@@ -0,0 +1,70 @@
+FlagPredef: ; 4d7c1
+; Perform action b on flag c in flag array hl.
+; If checking a flag, check flag array d:hl unless d is 0.
+
+; For longer flag arrays, see FlagAction.
+
+	push hl
+	push bc
+
+; Divide by 8 to get the byte we want.
+	push bc
+	srl c
+	srl c
+	srl c
+	ld b, 0
+	add hl, bc
+	pop bc
+
+; Which bit we want from the byte
+	ld a, c
+	and 7
+	ld c, a
+
+; Shift left until we can mask the bit
+	ld a, 1
+	jr z, .shifted
+.shift
+	add a
+	dec c
+	jr nz, .shift
+.shifted
+	ld c, a
+
+; What are we doing to this flag?
+	dec b
+	jr z, .set ; 1
+	dec b
+	jr z, .check ; 2
+
+.reset
+	ld a, c
+	cpl
+	and [hl]
+	ld [hl], a
+	jr .done
+
+.set
+	ld a, [hl]
+	or c
+	ld [hl], a
+	jr .done
+
+.check
+	ld a, d
+	cp 0
+	jr nz, .farcheck
+
+	ld a, [hl]
+	and c
+	jr .done
+
+.farcheck
+	call GetFarByte
+	and c
+
+.done
+	pop bc
+	pop hl
+	ld c, a
+	ret