From f3f221524e6be30217838661b4750820a7bebecf Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Fri, 4 Jun 2021 13:38:46 +0200
Subject: Add initial performance optimization for rect draw

---
 src/main.c     | 14 ++++++++----
 src/renderer.c | 72 ++++++++++++++++++++++++++++++++++++++++++++--------------
 2 files changed, 65 insertions(+), 21 deletions(-)

diff --git a/src/main.c b/src/main.c
index ab41915..8df466f 100644
--- a/src/main.c
+++ b/src/main.c
@@ -50,6 +50,15 @@ WITH REGARD TO THIS SOFTWARE.
 #define PROF_INIT()
 #endif
 
+void
+test_rects() {
+    for (size_t i = 0; i < 100; i++) {
+        draw_rect(30, 30, 45, 45, 1);
+        draw_rect(35, 35, 60, 40, 2);
+        draw_rect(1, 1, 6, 6, 3);
+    }
+}
+
 int main(void) {
     // Adjust system wait times.
     SYSTEM_WAIT = SYSTEM_WAIT_CARTRIDGE;
@@ -64,14 +73,11 @@ int main(void) {
     irq_init();
     irs_set(IRQ_VBLANK, irs_stub);
 
-
     // Main loop.
     PROF_INIT();
     while (true) {
         bios_vblank_wait();
-        txt_drawf("Hello world: %d", 4, 4, 6, 10);
-        draw_rect(30, 30, 45, 45, 1);
-        draw_rect(35, 35, 60, 40, 2);
+        PROF(test_rects(), eval_cycles);
         PROF_SHOW();
         PROF(flip_buffer(), flip_cycles);
     }
diff --git a/src/renderer.c b/src/renderer.c
index 4aa583d..51647cb 100644
--- a/src/renderer.c
+++ b/src/renderer.c
@@ -47,24 +47,62 @@ draw_pixel(u16 x, u16 y, u8 color) {
 IWRAM_CODE
 void
 draw_rect(int x0, int y0, int x1, int y1, u8 clr) {
-    if (x0 > x1) {
-        int tmp = x0;
-        x0 = x1;
-        x1 = tmp;
-    }
-    if (y0 > y1) {
-        int tmp = y0;
-        y0 = y1;
-        y1 = tmp;
-    }
-    int dx = x1 - x0;
-    int dy = y1 - y0;
-    // TODO: SLOW should be vectorized.
-    for (int i = 0; i <= dx; ++i) {
-        draw_pixel(x0 + i, y0, clr);
-        draw_pixel(x0 + i, y1, clr);
+    BOUNDCHECK_SCREEN(x0, y0);
+    BOUNDCHECK_SCREEN(x1, y1);
+
+    // Find row positions for the given x/y coordinates.
+    size_t tile_x0 = x0 / 8;
+    size_t tile_y0 = y0 / 8;
+    size_t tile_x1 = x1 / 8;
+    size_t tile_y1 = y1 / 8;
+    size_t start_col0 = x0 % 8;
+    size_t start_col1 = x1 % 8;
+    size_t start_row0 = y0 % 8;
+    size_t start_row1 = y1 % 8;
+
+    // Get a pointer to the backbuffer and the tile row.
+    u32 *backbuffer0 = &BACKBUF[start_row0 + (tile_x0 + tile_y0 * 32) * 8];
+    u32 *backbuffer1 = &BACKBUF[start_row1 + (tile_x0 + tile_y1 * 32) * 8];
+
+    u16 dx = tile_x1 - tile_x0;
+    u16 dy = y1 - y0;
+
+    // There are 3 cases:
+    //     1. Lines fit on a single tile.
+    //     2. Lines go through 2 tiles, both require partial row updates.
+    //     3. Lines go through 3 or more tiles, first and last tiles use partial
+    //        row updates, rows in the middle can write the.
+    if (dx < 1) {
+        u32 row_mask = 0xFFFFFFFF;
+        row_mask >>= (7 - start_col1 - dx) * 4;
+        row_mask &= 0xFFFFFFFF << start_col0 * 4;
+        u32 row = (0x11111111 * clr) & row_mask;
+        backbuffer0[0] = (backbuffer0[0] & ~row_mask) | row;
+        backbuffer1[0] = (backbuffer1[0] & ~row_mask) | row;
+        dirty_tiles[tile_y0] |= 1 << tile_x0;
+        dirty_tiles[tile_y1] |= 1 << tile_x0;
+    } else {
+        size_t shift_left = start_col0 * 4;
+        size_t shift_right = (7 - start_col1) * 4;
+        u32 row_mask = 0xFFFFFFFF;
+        u32 row = 0x11111111 * clr;
+        backbuffer0[0] = (backbuffer0[0] & ~(row_mask << shift_left)) | (row << shift_left);
+        backbuffer1[0] = (backbuffer1[0] & ~(row_mask << shift_left)) | (row << shift_left);
+        dirty_tiles[tile_y0] |= 1 << tile_x0;
+        dirty_tiles[tile_y1] |= 1 << tile_x0;
+        for (size_t i = 1; i < dx; i++) {
+            backbuffer0[i * 8] = row;
+            backbuffer1[i * 8] = row;
+            dirty_tiles[tile_y0] |= 1 << tile_x0 + i;
+            dirty_tiles[tile_y1] |= 1 << tile_x0 + i;
+        }
+        backbuffer0[dx * 8] = (backbuffer0[dx * 8] & ~(row_mask >> shift_right)) | (row >> shift_right);
+        backbuffer1[dx * 8] = (backbuffer1[dx * 8] & ~(row_mask >> shift_right)) | (row >> shift_right);
+        dirty_tiles[tile_y0] |= 1 << tile_x0 + dx;
+        dirty_tiles[tile_y1] |= 1 << tile_x0 + dx;
     }
-    for (int i = 0; i <= dy; ++i) {
+    // The vertical line cases are analogous to the horizontal cases.
+    for (int i = 1; i < dy; ++i) {
         draw_pixel(x0, y0 + i, clr);
         draw_pixel(x1, y0 + i, clr);
     }
-- 
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