From 0e103c6f8833266d877f190f80e28772392fd32e Mon Sep 17 00:00:00 2001
From: nmlgc <nmlgc@nmlgc.net>
Date: Fri, 24 Mar 2023 02:26:45 +0100
Subject: [PATCH] [Maintenance] Add a VRAM offset roll macro

Part of P0235, funded by Ember2528.
---
 planar.h                      | 9 +++++++++
 th02/main/bullet/pellet_r.cpp | 7 ++-----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/planar.h b/planar.h
index 59351d9d..c68b8ce0 100644
--- a/planar.h
+++ b/planar.h
@@ -84,6 +84,15 @@ typedef uint16_t uvram_offset_t;
 #define VRAM_OFFSET_SHIFT(x, y) \
 	(x >> BYTE_BITS) + (y << 6) + (y << 4)
 
+// Adds [imm] to [vo] and rolls [vo] back to the top of VRAM if it crossed the
+// bottom. Necessary with hardware scrolling.
+#define vram_offset_add_and_roll(vo, imm) { \
+	vo += imm; \
+	if(static_cast<vram_offset_t>(vo) >= PLANE_SIZE) { \
+		vo -= PLANE_SIZE; \
+	} \
+}
+
 #ifdef __cplusplus
 // MODDERS: Replace with a single function
 static inline vram_offset_t vram_offset_shift(screen_x_t x, vram_y_t y) {
diff --git a/th02/main/bullet/pellet_r.cpp b/th02/main/bullet/pellet_r.cpp
index ab00bb0a..8b8e7ed9 100644
--- a/th02/main/bullet/pellet_r.cpp
+++ b/th02/main/bullet/pellet_r.cpp
@@ -18,10 +18,7 @@ void pascal near pellet_render(screen_x_t left, vram_y_t top)
 	_CX = 8;
 	put_loop: {
 		asm { movsw; }
-		_DI += (ROW_SIZE - sizeof(dots16_t));
-		if(static_cast<int16_t>(_DI) >= PLANE_SIZE) {
-			_DI -= PLANE_SIZE;
-		}
+		vram_offset_add_and_roll(_DI, (ROW_SIZE - sizeof(dots16_t)));
+		asm { loop put_loop; }
 	}
-	asm { loop put_loop; }
 }