From 06f47c8d2ca39ac0a02a5878ae09ccf6958b05e6 Mon Sep 17 00:00:00 2001
From: Jeffrey Pfau <jeffrey@endrift.com>
Date: Thu, 21 Jan 2016 01:09:58 -0800
Subject: [PATCH] GB Video: Add basic scrolling support

---
 src/gb/renderers/software.c | 27 ++++++++++++++++++---------
 src/gb/renderers/software.h |  3 +++
 2 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/src/gb/renderers/software.c b/src/gb/renderers/software.c
index 3b72f1835..fc821e12b 100644
--- a/src/gb/renderers/software.c
+++ b/src/gb/renderers/software.c
@@ -61,9 +61,8 @@ static void GBVideoSoftwareRendererInit(struct GBVideoRenderer* renderer) {
 
 static void GBVideoSoftwareRendererReset(struct GBVideoRenderer* renderer) {
 	struct GBVideoSoftwareRenderer* softwareRenderer = (struct GBVideoSoftwareRenderer*) renderer;
-	int i;
-
-	// TODO
+	softwareRenderer->scy = 0;
+	softwareRenderer->scx = 0;
 }
 
 static void GBVideoSoftwareRendererDeinit(struct GBVideoRenderer* renderer) {
@@ -99,6 +98,12 @@ static uint8_t GBVideoSoftwareRendererWriteVideoRegister(struct GBVideoRenderer*
 		softwareRenderer->objPalette[1][2] = GB_PALETTE[(value >> 4) & 3];
 		softwareRenderer->objPalette[1][3] = GB_PALETTE[(value >> 6) & 3];
 		break;
+	case REG_SCY:
+		softwareRenderer->scy = value;
+		break;
+	case REG_SCX:
+		softwareRenderer->scx = value;
+		break;
 	}
 	return value;
 }
@@ -143,17 +148,21 @@ static void GBVideoSoftwareRendererDrawBackground(struct GBVideoSoftwareRenderer
 		data += 0x1000;
 	}
 	int x;
+	int topY = (((y + renderer->scy) >> 3) & 0x1F) * 0x20;
+	int bottomY = (y + renderer->scy) & 7;
 	for (x = 0; x < GB_VIDEO_HORIZONTAL_PIXELS; ++x) {
+		int topX = ((x + renderer->scx) >> 3) & 0x1F;
+		int bottomX = 7 - ((x + renderer->scx) & 7);
 		int bgTile;
 		if (GBRegisterLCDCIsTileData(renderer->lcdc)) {
-			bgTile = maps[(x >> 3) + (0x20 * (y >> 3))];
+			bgTile = maps[topX + topY];
 		} else {
-			bgTile = ((int8_t*) maps)[(x >> 3) + (0x20 * (y >> 3))];
+			bgTile = ((int8_t*) maps)[topX + topY];
 		}
-		uint8_t tileDataLower = data[(bgTile * 8 + (y & 7)) * 2];
-		uint8_t tileDataUpper = data[(bgTile * 8 + (y & 7)) * 2 + 1];
-		tileDataUpper >>= 7 - (x & 7);
-		tileDataLower >>= 7 - (x & 7);
+		uint8_t tileDataLower = data[(bgTile * 8 + bottomY) * 2];
+		uint8_t tileDataUpper = data[(bgTile * 8 + bottomY) * 2 + 1];
+		tileDataUpper >>= bottomX;
+		tileDataLower >>= bottomX;
 		renderer->row[x] = renderer->bgPalette[((tileDataUpper & 1) << 1) | (tileDataLower & 1)];
 	}
 }
diff --git a/src/gb/renderers/software.h b/src/gb/renderers/software.h
index 2fbc5ad0e..4b3b69418 100644
--- a/src/gb/renderers/software.h
+++ b/src/gb/renderers/software.h
@@ -29,6 +29,9 @@ struct GBVideoSoftwareRenderer {
 
 	uint32_t* temporaryBuffer;
 
+	uint8_t scy;
+	uint8_t scx;
+
 	GBRegisterLCDC lcdc;
 };