package ppu

func (p *PPU) ReadRegister(addr uint16) uint8 {
	switch addr & 0x7 {
	case 2:
		return p.readStatus()
	case 4:
		return p.OAM[p.OAMAddr]
	case 7:
		return p.readData()
	default:
		return p.busLatch // $2000,2001,2003,2005,2006 are write only: open bus
	}
}

// handles cpu write to $2000-2007
func (p *PPU) WriteRegister(addr uint16, v uint8) {
	p.busLatch = v
	switch addr & 0x7 {
	case 0:
		p.Ctrl = v
		p.t = (p.t &^ 0x0C00) | (uint16(v&ctrlNametableMask) << 10)
		p.nmiOutput = v&ctrlNMIEnable != 0
		p.updateNMILine()
	case 1:
		p.Mask = v
	case 3:
		p.OAMAddr = v
	case 4:
		p.OAM[p.OAMAddr] = v
		p.OAMAddr++
	case 5:
		p.writeScroll(v)
	case 6:
		p.writeAddr(v)
	case 7:
		p.writeData(v)
	}
}

// implements the $2002 read, including the hardware race around the dot
// vblank gets set (see https://www.nesdev.org/wiki/NMI)
func (p *PPU) readStatus() uint8 {
	result := p.busLatch & 0x1F
	if p.vblank {
		result |= statusVBlank
	}
	if p.sprite0Hit {
		result |= statusSprite0
	}
	if p.spriteOverflow {
		result |= statusOverflow
	}

	if p.Scanline == 241 {
		switch p.Dot {
		case 0:
			p.suppressVBlankSet = true
			p.suppressNMIForFrame = true
		case 1, 2:
			p.suppressNMIForFrame = true
			p.nmiPending = false
		}
	}

	p.vblank = false
	p.w = false
	p.updateNMILine()
	return result
}

func (p *PPU) writeScroll(v uint8) {
	if !p.w {
		p.t = (p.t &^ 0x001F) | uint16(v>>3)
		p.x = v & 0x07
	} else {
		p.t = (p.t &^ 0x73E0) | (uint16(v&0x07) << 12) | (uint16(v&0xF8) << 2)
	}
	p.w = !p.w
}

func (p *PPU) writeAddr(v uint8) {
	if !p.w {
		p.t = (p.t & 0x00FF) | (uint16(v&0x3F) << 8)
	} else {
		p.t = (p.t & 0xFF00) | uint16(v)
		p.v = p.t
	}
	p.w = !p.w
}

// implements buffered $2007 and a quirk which i am too lazy to write down
// right now :D
// https://www.nesdev.org/wiki/PPU_registers#The_PPUDATA_data_port
func (p *PPU) readData() uint8 {
	addr := p.v & 0x3FFF
	var result uint8
	if addr >= 0x3F00 {
		result = p.readPalette(addr)
		p.readBuffer = p.readVRAM(addr & 0x2FFF)
	} else {
		result = p.readBuffer
		p.readBuffer = p.readVRAM(addr)
	}
	p.incrementV()
	return result
}

func (p *PPU) writeData(v uint8) {
	addr := p.v & 0x3FFF
	if addr >= 0x3F00 {
		p.writePalette(addr, v)
	} else {
		p.writeVRAM(addr, v)
	}
	p.incrementV()
}

func (p *PPU) incrementV() {
	if p.Ctrl&ctrlIncrement32 != 0 {
		p.v += 32
	} else {
		p.v++
	}
	p.v &= 0x7FFF
}

func (p *PPU) readVRAM(addr uint16) uint8 {
	if addr < 0x2000 {
		return p.mapper.PPURead(addr)
	}
	return p.vram[p.nametableOffset(addr)]
}

func (p *PPU) writeVRAM(addr uint16, v uint8) {
	if addr < 0x2000 {
		p.mapper.PPUWrite(addr, v)
		return
	}
	p.vram[p.nametableOffset(addr)] = v
}