From 37e72d68fe6fbbdc211a6ef2b7756877e31e8061 Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Thu, 15 Apr 2021 17:24:25 +0200
Subject: Add initial Mode4 tests, copied from the TONC tutorial

---
 src/main.c | 59 +++++++++++++++++++++++++++++++----------------------------
 1 file changed, 31 insertions(+), 28 deletions(-)

diff --git a/src/main.c b/src/main.c
index 1596ba4..cbfa2a7 100644
--- a/src/main.c
+++ b/src/main.c
@@ -61,6 +61,7 @@ typedef u16 Color;
 typedef Color Scanline[SCREEN_WIDTH];
 #define FRAMEBUFFER ((Scanline*)MEM_VRAM)
 #define SCREEN_BUFFER ((vu16*) MEM_VRAM)
+#define PAL_BUFFER ((vu16*) MEM_PAL)
 
 //
 // Colors.
@@ -227,39 +228,41 @@ wait_vsync() {
 // Main functions.
 //
 
-int main(void) {
-    set_display_mode(DISP_MODE_3 | DISP_BG2);
+// In Mode4 the buffer is of 8 bytes per pixel instead of 16. We can't write the
+// color directly, instead the color is stored in the palette memory at
+// `MEM_PAL`. Note that in this mode MEM_PAL[0] is the background color. This
+// plotter takes an index to a color stored in MEM_PAL[col_index]. Because the
+// GBA needs to meet memory alignment requirements, we can't write a u8 into
+// memory, instead we need to read a u16 word, mask and or the corresponding
+// bits and wave the updated u16.
+static inline void
+put_pixel_m4(int x, int y, u8 col_index) {
+    int buffer_index = (y * SCREEN_WIDTH + x) / 2;
+    u16 *destination = &SCREEN_BUFFER[buffer_index];
+    int odd = x & 0x1;
+    if(odd) {
+        *destination= (*destination & 0xFF) | (col_index << 8);
+    } else {
+        *destination= (*destination & ~0xFF) |  col_index;
+    }
+}
 
-    // Draw a grid pattern.
-    for (int j = 30; j < SCREEN_HEIGHT - 30; j += 8) {
-        for (int i = 30; i < 50; i += 8) {
-            draw_rect(i, j, 0 + i + 8, j + 8, COLOR_RED);
+static inline void
+draw_fill_rect_m4(int x0, int y0, int x1, int y1, u8 col_index) {
+    int ix, iy;
+    for(iy = y0; iy < y1; iy++) {
+        for(ix = x0; ix < x1; ix++) {
+            put_pixel_m4(ix, iy, col_index);
         }
     }
+}
 
-    // Draw the text line.
-    for (int i = 0; i < SCREEN_WIDTH; i += 8) {
-        draw_fill_rect(i, 7, 0 + i + 7, 16, COLOR_RED);
-    }
-    put_text(8, 8, COLOR_BLACK, "Testing other patterns");
+int main(void) {
+    set_display_mode(DISP_MODE_4 | DISP_BG2);
 
-    // Test line drawings.
-    draw_fill_rect(69, 29, SCREEN_WIDTH - 69, SCREEN_HEIGHT - 29, COLOR_BLACK);
-    int step = 10;
-    for (int k = 0; k < (SCREEN_WIDTH - 70 - 70) / step; k++) {
-        int x0 = 70;
-        int y0 = 30 + k * step;
-        int x1 = x0 + k * step + step;
-        int y1 = SCREEN_HEIGHT - 30;
-        draw_line(x0, y0, x1, y1, COLOR_BLUE);
-    }
-    for (int k = 0; k < (SCREEN_WIDTH - 70 - 70) / step; k++) {
-        int x0 = 70 + k * step;
-        int y0 = 30;
-        int x1 = SCREEN_WIDTH - 70;
-        int y1 = y0 + k * step + step;
-        draw_line(x0, y0, x1, y1, COLOR_RED);
-    }
+    PAL_BUFFER[1] = COLOR_RED;
+    PAL_BUFFER[2] = COLOR_BLUE;
+    draw_fill_rect_m4(0, 0, 20, 20, 1);
 
     while(true) {
         wait_vsync();
-- 
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