1 files changed, 97 insertions, 0 deletions
diff --git a/ppu/sprites.go b/ppu/sprites.go
new file mode 100644
index 0000000..83d1703
--- /dev/null
+++ b/ppu/sprites.go
@@ -0,0 +1,97 @@
+package ppu
+
+import "math/bits"
+
+const (
+	spriteAttrPalette = 0x03
+	spriteAttrBehind  = 0x20 // draw behind opaque background pixels
+	spriteAttrFlipH   = 0x40
+	spriteAttrFlipV   = 0x80
+)
+
+type sprite struct {
+	patternLo, patternHi uint8
+	attr                 uint8
+	x                    uint8
+	isSprite0            bool
+}
+
+// ill add comments later im tired
+// https://www.nesdev.org/wiki/PPU_sprite_evaluation
+func (p *PPU) evaluateSprites() {
+	p.spriteCount = 0
+	// nothing is drawn above line 1, so the pre-render line and the last line leave it empty
+	if !p.renderingEnabled() || p.Scanline >= ScreenHeight-1 {
+		return
+	}
+
+	height := 8
+	if p.Ctrl&ctrlSpriteSize16 != 0 {
+		height = 16
+	}
+
+	for i := 0; i < 64; i++ {
+		row := p.Scanline - int(p.OAM[i*4])
+		if row < 0 || row >= height {
+			continue
+		}
+		if p.spriteCount == 8 {
+			p.spriteOverflow = true
+			break
+		}
+		p.spriteLine[p.spriteCount] = p.fetchSprite(i, row, height)
+		p.spriteCount++
+	}
+}
+
+func (p *PPU) fetchSprite(i, row, height int) sprite {
+	tile := p.OAM[i*4+1]
+	attr := p.OAM[i*4+2]
+	if attr&spriteAttrFlipV != 0 {
+		row = height - 1 - row
+	}
+
+	var addr uint16
+	if height == 16 {
+		table := uint16(tile&1) << 12
+		tile &^= 1
+		if row >= 8 {
+			tile++
+			row -= 8
+		}
+		addr = table | uint16(tile)<<4 | uint16(row)
+	} else {
+		var base uint16
+		if p.Ctrl&ctrlSpritePatternHigh != 0 {
+			base = 0x1000
+		}
+		addr = base | uint16(tile)<<4 | uint16(row)
+	}
+
+	lo := p.readVRAM(addr)
+	hi := p.readVRAM(addr + 8)
+	if attr&spriteAttrFlipH != 0 {
+		lo, hi = bits.Reverse8(lo), bits.Reverse8(hi)
+	}
+	return sprite{patternLo: lo, patternHi: hi, attr: attr, x: p.OAM[i*4+3], isSprite0: i == 0}
+}
+
+func (p *PPU) spritePixel(x int) (pixel, palette uint8, behind, isSprite0 bool) {
+	if p.Mask&maskShowSprites == 0 || (x < 8 && p.Mask&maskShowSpritesLeft == 0) {
+		return 0, 0, false, false
+	}
+	for i := 0; i < p.spriteCount; i++ {
+		s := &p.spriteLine[i]
+		offset := x - int(s.x)
+		if offset < 0 || offset > 7 {
+			continue
+		}
+		shift := 7 - offset
+		pixel = (s.patternHi>>shift&1)<<1 | (s.patternLo >> shift & 1)
+		if pixel == 0 {
+			continue
+		}
+		return pixel, s.attr & spriteAttrPalette, s.attr&spriteAttrBehind != 0, s.isSprite0
+	}
+	return 0, 0, false, false
+}