From 4e51d019fd0c1d21aa8e965fd68dbacec39a2576 Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Mon, 26 Apr 2021 12:14:45 +0200
Subject: Test screenblock entry demo

---
 src/common.h  | 23 ++++++++++++++++++----
 src/main.c    | 62 ++++++++++++++++++++++++++++++++++++++++++++++++++---------
 src/sprites.h |  1 +
 3 files changed, 73 insertions(+), 13 deletions(-)

diff --git a/src/common.h b/src/common.h
index 35b4619..6047e5f 100644
--- a/src/common.h
+++ b/src/common.h
@@ -76,6 +76,20 @@
 #define BG_V_SCROLL_2 *((vu16*)(0x04000012 + 0x0004 * 2))
 #define BG_V_SCROLL_3 *((vu16*)(0x04000012 + 0x0004 * 3))
 
+// Screenblocks for BGs.
+typedef u16 ScreenBlock[1024];
+#define SCREENBLOCK_MEM ((ScreenBlock*)MEM_VRAM)
+
+// Screenblock entry bits.
+#define SCREENBLOCK_ENTRY_H_FLIP (1 << 0xA)
+#define SCREENBLOCK_ENTRY_V_FLIP (1 << 0xB)
+#define SCREENBLOCK_ENTRY_PAL(N) ((N) << 0xC)
+
+size_t se_index(size_t tile_x, size_t tile_y, size_t map_width) {
+    size_t sbb = ((tile_x >> 5) + (tile_y >> 5) * (map_width >> 5));
+    return sbb * 1024 + ((tile_x & 31) + (tile_y & 31) * 32);
+}
+
 // Screen settings.
 #define SCREEN_WIDTH 240
 #define SCREEN_HEIGHT 160
@@ -84,6 +98,9 @@
 // (RGB) have a 0--31 range. For example, pure red would be rgb15(31, 0, 0).
 typedef u16 Color;
 
+// A palette is composed of 16 colors, with color at index 0 being transparent.
+typedef Color Palette[16];
+
 //
 // Tile memory access.
 //
@@ -96,9 +113,6 @@ typedef struct Tile {
 typedef Tile TileBlock[512];
 #define TILE_MEM ((TileBlock*) MEM_VRAM)
 
-typedef u16 ScreenBlock[1024];
-#define SCREENBLOCK_MEM ((ScreenBlock*)MEM_VRAM)
-
 // We can treat the screen as a HxW matrix. With the following macro we can
 // write a pixel to the screen at the (x, y) position using:
 //
@@ -109,6 +123,8 @@ typedef Color Scanline[SCREEN_WIDTH];
 #define SCREEN_BUFFER      ((u16*) MEM_VRAM)
 #define PAL_BUFFER_BG      ((u16*) MEM_PAL)
 #define PAL_BUFFER_SPRITES ((u16*) 0x05000200)
+#define PAL_BANK_BG        ((Palette*) MEM_PAL)
+#define PAL_BANK_SPRITES   ((Palette*) 0x05000200)
 
 //
 // Colors.
@@ -277,5 +293,4 @@ key_hold(u32 key) {
 // Check if the given key/button is currently pressed.
 #define KEY_PRESSED(key) (~(KEY_INPUTS) & key)
 
-
 #endif // GBAEXP_COMMON_H
diff --git a/src/main.c b/src/main.c
index b8f2334..793baa8 100644
--- a/src/main.c
+++ b/src/main.c
@@ -10,23 +10,58 @@
 //
 
 // TODO: Cleanup OBJ/OAM memory copying and access.
+//
+#define CBB_0  0
+#define SBB_0 28
 
-int main(void) {
-    // Load background palette.
-    memcpy(&PAL_BUFFER_BG[0], &bg_palette, 512);
-    memcpy(&TILE_MEM[0][0], bg_data, 3168);
-    memcpy(&SCREENBLOCK_MEM[30][0], bg_map, 2048);
-
-    // Configure BG0 to use 4bpp, 64x32 tile map in charblock 0 and screenblock
-    // 31.
-    BG_CTRL_0 = BG_CHARBLOCK(0) | BG_SCREENBLOCK(30) | BG_SIZE(0);
+#define CROSS_TX 15
+#define CROSS_TY 10
+
+u16 *bg0_map = SCREENBLOCK_MEM[SBB_0];
+
+void
+init_map() {
+    // initialize a background
+    BG_CTRL_0 = BG_CHARBLOCK(CBB_0) | BG_SCREENBLOCK(SBB_0) | BG_SIZE(3);
     BG_H_SCROLL_0 = 0;
     BG_V_SCROLL_0 = 0;
 
+    // (1) create the tiles: basic tile and a cross
+    const Tile tiles[2] = {
+        {{0x11111111, 0x01111111, 0x01111111, 0x01111111,
+          0x01111111, 0x01111111, 0x01111111, 0x00000001}},
+        {{0x00000000, 0x00100100, 0x01100110, 0x00011000,
+          0x00011000, 0x01100110, 0x00100100, 0x00000000}},
+    };
+    TILE_MEM[CBB_0][0] = tiles[0];
+    TILE_MEM[CBB_0][1] = tiles[1];
+
+    // (2) create a palette
+    PAL_BANK_BG[0][1] = rgb15(31,  0,  0);
+    PAL_BANK_BG[1][1] = rgb15( 0, 31,  0);
+    PAL_BANK_BG[2][1] = rgb15( 0,  0, 31);
+    PAL_BANK_BG[3][1] = rgb15(16, 16, 16);
+
+    // (3) Create a map: four contingent blocks of
+    //   0x0000, 0x1000, 0x2000, 0x3000.
+    u16 *pse = bg0_map;
+    for(int i = 0; i < 4; i++) {
+        for(int j = 0; j < 32 * 32; j++) {
+            *pse++ = SCREENBLOCK_ENTRY_PAL(i) | 0;
+        }
+    }
+}
+
+int main(void) {
+    init_map();
     // Configure the display in mode 0 to show OBJs, where tile memory is
     // sequential.
     DISP_CTRL = DISP_ENABLE_SPRITES | DISP_MODE_0 | DISP_BG_0;
 
+    u32 tx, ty, se_curr, se_prev= CROSS_TY * 32 + CROSS_TX;
+
+    bg0_map[se_prev]++; // initial position of cross
+
     // Initialize sprite button overlay.
     init_sprite_pal(0, COLOR_WHITE);
     init_sprites(0);
@@ -51,6 +86,15 @@ int main(void) {
         if (key_hold(KEY_RIGHT)) {
             x += 3;
         }
+        tx = ((x >> 3) + CROSS_TX) & 0x3F;
+        ty = ((y >> 3) + CROSS_TY) & 0x3F;
+
+        se_curr = se_index(tx, ty, 64);
+        if(se_curr != se_prev) {
+            bg0_map[se_prev]--;
+            bg0_map[se_curr]++;
+            se_prev= se_curr;
+        }
 
         frame_counter++;
         BG_H_SCROLL_0 = x;
diff --git a/src/sprites.h b/src/sprites.h
index e7439f1..e249e95 100644
--- a/src/sprites.h
+++ b/src/sprites.h
@@ -95,4 +95,5 @@ init_sprite_pal(size_t starting_index, Color col) {
     }
 
 }
+
 #endif // GBAEXP_SPRITES_H
-- 
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