From efc7d817db01bcd267e2730f8d6f5e662342e3aa Mon Sep 17 00:00:00 2001
From: Rekai Musuka <rekai@musuka.dev>
Date: Fri, 21 Oct 2022 05:12:20 -0300
Subject: [PATCH] feat: Mode 0 MVP

---
 src/ppu.zig | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 62 insertions(+), 1 deletion(-)

diff --git a/src/ppu.zig b/src/ppu.zig
index 05d2518..bc59a80 100644
--- a/src/ppu.zig
+++ b/src/ppu.zig
@@ -4,6 +4,9 @@ const io = @import("bus/io.zig");
 const EventKind = @import("scheduler.zig").EventKind;
 const Scheduler = @import("scheduler.zig").Scheduler;
 
+const Bit = @import("bitfield").Bit;
+const Bitfield = @import("bitfield").Bitfield;
+
 const Allocator = std.mem.Allocator;
 
 pub const width = 240;
@@ -68,8 +71,50 @@ pub const Ppu = struct {
 
         switch (bg_mode) {
             0x0 => {
-                // Mode 0
+                // A Tile is always 8x8 pixels
 
+                // Mode 0 Implementation Assuming:
+                // - Scrolling isn't a thing
+                // - Bill Gates said we'll never need more than BG0
+
+                // Write to this Scanline once we're done
+                const start = framebuf_pitch * @as(usize, scanline);
+                var scanline_buf = std.mem.zeroes([framebuf_pitch]u8);
+
+                // These we can probably move to top level?
+                const charblock_len: u32 = 0x4000;
+                const screenblock_len: u32 = 0x800;
+
+                const cbb: u2 = self.bg0.cnt.char_base.read(); // Char Block Base
+                const sbb: u5 = self.bg0.cnt.screen_base.read(); // Screen Block Base
+                const is_8bpp: bool = self.bg0.cnt.palette_type.read(); // Colour Mode
+                const size: u2 = self.bg0.cnt.screen_size.read(); // Background Size
+
+                // 0x0600_000 is implied because we can access VRAM without the Bus
+                const char_base: u32 = charblock_len * @as(u32, cbb);
+                const screen_base: u32 = screenblock_len * @as(u32, sbb);
+
+                const y = @as(u32, scanline);
+                var x: u32 = 0;
+                while (x < width) : (x += 1) {
+                    const entry_addr = screen_base + tilemapIndex(size, x, y);
+                    const entry = @bitCast(ScreenEntry, @as(u16, self.vram.buf[entry_addr + 1]) << 8 | @as(u16, self.vram.buf[entry_addr]));
+
+                    const tile_id: u32 = entry.tile_id.read();
+                    const px_y = if (entry.h_flip.read()) 7 - (y % 8) else y % 8;
+                    const px_x = if (entry.v_flip.read()) 7 - (x % 8) else x % 8;
+                    const tile_addr = char_base + if (is_8bpp) 0x40 * tile_id + 0x8 * px_y else 0x20 * tile_id + 0x4 * px_y;
+
+                    var tile = self.vram.buf[tile_addr + if (is_8bpp) px_x else px_x >> 1];
+                    tile = if (px_x & 1 == 1) tile >> 4 else tile & 0xF;
+
+                    const pal_bank: u8 = @as(u8, entry.palette_bank.read()) << 4;
+                    const colour = pal_bank | tile;
+
+                    std.mem.copy(u8, scanline_buf[x * 2 ..][0..2], self.palette.buf[colour * 2 ..][0..2]);
+                }
+
+                std.mem.copy(u8, self.framebuf[start..][0..framebuf_pitch], &scanline_buf);
             },
             0x3 => {
                 const start = framebuf_pitch * @as(usize, scanline);
@@ -94,6 +139,14 @@ pub const Ppu = struct {
             else => std.debug.panic("[PPU] TODO: Implement BG Mode {}", .{bg_mode}),
         }
     }
+
+    fn tilemapIndex(size: u2, x: u32, y: u32) u32 {
+        return switch (size) {
+            0 => (((y % 256) / 8) * 64) + (((x % 256) / 8) * 2),
+            1 => (((y % 256) / 8) * 64) + (((x % 256) / 8) * 2),
+            else => std.debug.panic("tile size {}", .{size}),
+        };
+    }
 };
 
 const Palette = struct {
@@ -241,3 +294,11 @@ const Background = struct {
         };
     }
 };
+
+const ScreenEntry = extern union {
+    tile_id: Bitfield(u16, 0, 10),
+    h_flip: Bit(u16, 10),
+    v_flip: Bit(u16, 11),
+    palette_bank: Bitfield(u16, 12, 4),
+    raw: u16,
+};