sprites!!!!!!

4 files changed, 134 insertions, 20 deletions

diff --git a/ppu/memory.go b/ppu/memory.go
index 3379242..d2b0a54 100644
--- a/ppu/memory.go
+++ b/ppu/memory.go
@@ -2,12 +2,6 @@ package ppu
 
 import "crecher.net/gones/v2/cart"
 
-// maps a $2000-$3EFF address onto an offset into the 2KB of nametable RAM,
-// according to the cartridge's mirroring mode. four-screen carts will be
-// expected to use their own extra 2KB of RAM via the mapper, but since no
-// implemented mapper does that yet, four-screen just falls back to the vertical
-// mirroring as a placeholder
-
 func (p *PPU) nametableOffset(addr uint16) int {
 	addr &= 0x0FFF
 	table := addr / 0x400
diff --git a/ppu/ppu.go b/ppu/ppu.go
index 322cf3d..42b3b4c 100644
--- a/ppu/ppu.go
+++ b/ppu/ppu.go
@@ -3,13 +3,16 @@ package ppu
 import "crecher.net/gones/v2/cart"
 
 const (
-	ctrlNametableMask = 0x03
-	ctrlIncrement32   = 0x04
-	ctrlBGPatternHigh = 0x10
-	ctrlNMIEnable     = 0x80
+	ctrlNametableMask     = 0x03
+	ctrlIncrement32       = 0x04
+	ctrlSpritePatternHigh = 0x08
+	ctrlBGPatternHigh     = 0x10
+	ctrlSpriteSize16      = 0x20
+	ctrlNMIEnable         = 0x80
 
 	maskGreyscale          = 0x01
 	maskShowBackgroundLeft = 0x02
+	maskShowSpritesLeft    = 0x04
 	maskShowBackground     = 0x08
 	maskShowSprites        = 0x10
 
@@ -59,6 +62,10 @@ type PPU struct {
 	patternLo, patternHi uint16
 	attrLo, attrHi       uint16
 
+	// sprites for the line being drawn, picked at the end of the previous line
+	spriteLine  [8]sprite
+	spriteCount int
+
 	nmiOutput  bool // PPUCTRL bit 7
 	nmiLine    bool // vblank && nmiOutput, latched each tick for edge detection
 	nmiPending bool // set on nmiLine's rising edge, consumed by PollNMI
diff --git a/ppu/render.go b/ppu/render.go
index bfde9e4..ef3ad9a 100644
--- a/ppu/render.go
+++ b/ppu/render.go
@@ -31,14 +31,15 @@ func (p *PPU) renderTick() {
 		}
 	}
 
+	if dot == 257 && (visibleLine || line == preRenderLine) {
+		p.evaluateSprites()
+	}
+
 	if visibleLine && dot >= 1 && dot <= ScreenWidth {
 		p.drawPixel(dot-1, line)
 	}
 }
 
-// the fetch cycle, every tile takes 8 dots to fetch (nametable byte, attribute
-// byte, pattern low, pattern high), 2 dots each. dots 321-336 fetch the first two
-// tiles of the next line ahead of time
 func (p *PPU) backgroundFetch(dot int) {
 	if !((dot >= 2 && dot <= 257) || (dot >= 321 && dot <= 337)) {
 		return
@@ -144,18 +145,33 @@ func (p *PPU) copyVertical() {
 	p.v = (p.v &^ 0x7BE0) | (p.t & 0x7BE0)
 }
 
+func (p *PPU) backgroundPixel(x int) (pixel, palette uint8) {
+	if p.Mask&maskShowBackground == 0 || (x < 8 && p.Mask&maskShowBackgroundLeft == 0) {
+		return 0, 0
+	}
+	bit := uint16(0x8000) >> p.x
+	pixel = bitAt(p.patternHi, bit)<<1 | bitAt(p.patternLo, bit)
+	palette = bitAt(p.attrHi, bit)<<1 | bitAt(p.attrLo, bit)
+	return pixel, palette
+}
+
+// mixes the background and sprite pixel at (x, y) into the framebuffer
 func (p *PPU) drawPixel(x, y int) {
-	var pixel, palette uint8
-	if p.Mask&maskShowBackground != 0 && (x >= 8 || p.Mask&maskShowBackgroundLeft != 0) {
-		bit := uint16(0x8000) >> p.x
-		pixel = bitAt(p.patternHi, bit)<<1 | bitAt(p.patternLo, bit)
-		palette = bitAt(p.attrHi, bit)<<1 | bitAt(p.attrLo, bit)
+	bgPixel, bgPalette := p.backgroundPixel(x)
+	spritePixel, spritePalette, behind, isSprite0 := p.spritePixel(x)
+
+	// opaque sprite 0 pixel over an opaque background pixel, never at x = 255
+	if isSprite0 && bgPixel != 0 && x != 255 {
+		p.sprite0Hit = true
 	}
 
 	// pixel 0 of every palette is the shared backdrop color at $3F00
 	addr := uint16(0x3F00)
-	if pixel != 0 {
-		addr += uint16(palette)<<2 | uint16(pixel)
+	switch {
+	case spritePixel != 0 && (bgPixel == 0 || !behind):
+		addr = 0x3F10 | uint16(spritePalette)<<2 | uint16(spritePixel)
+	case bgPixel != 0:
+		addr |= uint16(bgPalette)<<2 | uint16(bgPixel)
 	}
 	color := p.readPalette(addr)
 	if p.Mask&maskGreyscale != 0 {
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
+}