//http://nesdev.parodius.com/bbs/viewtopic.php?p=4571&sid=db4c7e35316cc5d734606dd02f11dccb

//todo - read http://wiki.nesdev.com/w/index.php/PPU_sprite_priority

//TODO - correctly emulate PPU OFF state

using BizHawk.Common;

namespace BizHawk.Emulation.Cores.Nintendo.NES
{
	sealed partial class PPU
	{
		const int kFetchTime = 2;

		struct BGDataRecord {
			public byte nt, at;
			public byte pt_0, pt_1;
		};

		public short[] xbuf = new short[256*240];

		int ppu_addr_temp;
		void Read_bgdata(ref BGDataRecord bgdata)
		{
			for (int i = 0; i < 8; i++)
				Read_bgdata(i,ref bgdata);
		}
		void Read_bgdata(int cycle, ref BGDataRecord bgdata)
		{
			switch (cycle)
			{
				case 0:
					ppu_addr_temp = ppur.get_ntread();
					bgdata.nt = ppubus_read(ppu_addr_temp, true);
					runppu(1);
					break;
				case 1:
					runppu(1);
					break;
				case 2:
					{
						ppu_addr_temp = ppur.get_atread();
						byte at = ppubus_read(ppu_addr_temp, true);

						//modify at to get appropriate palette shift
						if ((ppur.vt & 2) != 0) at >>= 4;
						if ((ppur.ht & 2) != 0) at >>= 2;
						at &= 0x03;
						at <<= 2;
						bgdata.at = at;

						//horizontal scroll clocked at cycle 3 and then
						//vertical scroll at 251
						runppu(1);
						if (reg_2001.PPUON)
						{
							ppur.increment_hsc();
							if (ppur.status.cycle == 251)
								ppur.increment_vs();
						}
						break;
					}
				case 3:
					runppu(1);
					break;
				case 4:
					ppu_addr_temp = ppur.get_ptread(bgdata.nt);
					bgdata.pt_0 = ppubus_read(ppu_addr_temp, true);
					runppu(1);
					break;
				case 5:
					runppu(1);
					break;
				case 6:
					ppu_addr_temp |= 8;
					bgdata.pt_1 = ppubus_read(ppu_addr_temp, true);
					runppu(1);
					break;
				case 7:
					runppu(1);
					break;
			} //switch(cycle)
		}

		unsafe struct TempOAM
		{
			public fixed byte oam[4];
			public fixed byte patterns[2];
			public byte index;
			public byte present;
		}

		//TODO - check flashing sirens in werewolf
		short PaletteAdjustPixel(int pixel)
		{
			//tack on the deemph bits. THESE MAY BE ORDERED WRONG. PLEASE CHECK IN THE PALETTE CODE
			return (short)(pixel | reg_2001.intensity_lsl_6);
		}

		const int kLineTime = 341;
		public unsafe void FrameAdvance()
		{
			BGDataRecord *bgdata = stackalloc BGDataRecord[34]; //one at the end is junk, it can never be rendered

			//262 scanlines
			if (ppudead != 0)
			{
				FrameAdvance_ppudead();
				return;
			}

			Reg2002_vblank_active_pending = true;
			ppuphase = PPUPHASE.VBL;
			ppur.status.sl = 241;

			//Not sure if this is correct.  According to Matt Conte and my own tests, it is. Timing is probably off, though.
			//NOTE:  Not having this here breaks a Super Donkey Kong game.
			reg_2003 = 0;

			//this was repeatedly finetuned from the fceux days thrugh the old cpu core and into the new one to pass 05-nmi_timing.nes
			//note that there is still some leniency. for instance, 4,2 will pass in addition to 3,3
			const int delay = 6;
			runppu(3);
			bool nmi_destiny = reg_2000.vblank_nmi_gen && Reg2002_vblank_active;
			runppu(3);
			if (nmi_destiny) TriggerNMI();
			nes.Board.AtVsyncNMI();
			runppu(postNMIlines * kLineTime - delay);

			//this seems to run just before the dummy scanline begins
			clear_2002();

			TempOAM* oams = stackalloc TempOAM[128];
			int* oamcounts = stackalloc int[2];
			int oamslot=0;
			int oamcount=0;

			idleSynch ^= true;

			//render 241 scanlines (including 1 dummy at beginning)
			for (int sl = 0; sl < 241; sl++)
			{
				ppur.status.cycle = 0;

				ppur.status.sl = sl;

				int yp = sl - 1;
				ppuphase = PPUPHASE.BG;

				if (NTViewCallback != null && yp == NTViewCallback.Scanline) NTViewCallback.Callback();
				if (PPUViewCallback != null && yp == PPUViewCallback.Scanline) PPUViewCallback.Callback();

				//twiddle the oam buffers
				int scanslot = oamslot ^ 1;
				int renderslot = oamslot;
				oamslot ^= 1;

				oamcount = oamcounts[renderslot];

				//ok, we're also going to draw here.
				//unless we're on the first dummy scanline
				if (sl != 0)
				{
					//the main scanline rendering loop:
					//32 times, we will fetch a tile and then render 8 pixels.
					//two of those tiles were read in the last scanline.
					int yp_shift = yp << 8;
					for (int xt = 0; xt < 32; xt++)
					{
						int xstart = xt << 3;
						oamcount = oamcounts[renderslot];
						int target = yp_shift + xstart;
						int rasterpos = xstart;

						//check all the conditions that can cause things to render in these 8px
						bool renderspritenow = reg_2001.show_obj && (xt > 0 || reg_2001.show_obj_leftmost);
						bool renderbgnow = reg_2001.show_bg && (xt > 0 || reg_2001.show_bg_leftmost);

						for (int xp = 0; xp < 8; xp++, rasterpos++)
						{
							//process the current clock's worth of bg data fetching
							//this needs to be split into 8 pieces or else exact sprite 0 hitting wont work due to the cpu not running while the sprite renders below
							Read_bgdata(xp, ref bgdata[xt + 2]);

							//bg pos is different from raster pos due to its offsetability.
							//so adjust for that here
							int bgpos = rasterpos + ppur.fh;
							int bgpx = bgpos & 7;
							int bgtile = bgpos >> 3;

							int pixel = 0, pixelcolor;

							//generate the BG data
							if (renderbgnow)
							{
								byte pt_0 = bgdata[bgtile].pt_0;
								byte pt_1 = bgdata[bgtile].pt_1;
								int sel = 7 - bgpx;
								pixel = ((pt_0 >> sel) & 1) | (((pt_1 >> sel) & 1) << 1);
								if (pixel != 0)
									pixel |= bgdata[bgtile].at;
								pixelcolor = PALRAM[pixel];
							}
							else
							{
								if (!renderspritenow)
								{
									//according to qeed's doc, use palette 0 or $2006's value if it is & 0x3Fxx
									//EDIT - this requires corect emulation of PPU OFF state, and seems only to apply when the PPU is OFF
									// not sure why this was off, but having it on fixes full_nes_palette, and it's a behavior that's been
									// verified on the decapped PPU

									// if there's anything wrong with how we're doing this, someone please chime in
									int addr = ppur.get_2007access();
									if ((addr & 0x3F00) == 0x3F00)
									{
									//  System.Console.WriteLine("{0:X4}", addr);
										pixel = addr & 0x1F;
									}
								}
								pixelcolor = PALRAM[pixel];
								pixelcolor |= 0x8000; //whats this? i think its a flag to indicate a hidden background to be used by the canvas filling logic later
							}

							if (!nes.Settings.DispBackground)
								pixelcolor = 0x8000; //whats this? i think its a flag to indicate a hidden background to be used by the canvas filling logic later

							//look for a sprite to be drawn
							bool havepixel = false;
							int renderslot_shift = renderslot << 6;
							for (int s = 0; s < oamcount; s++)
							{
								TempOAM* oam = &oams[renderslot_shift + s];
								int x = oam->oam[3];
								if (rasterpos >= x && rasterpos < x + 8)
								{
									//build the pixel.
									//fetch the LSB of the patterns
									int spixel = oam->patterns[0] & 1;
									spixel |= (oam->patterns[1] & 1) << 1;

									//shift down the patterns so the next pixel is in the LSB
									oam->patterns[0] >>= 1;
									oam->patterns[1] >>= 1;

									//bail out if we already have a pixel from a higher priority sprite.
									//notice that we continue looping anyway, so that we can shift down the patterns
									//transparent pixel bailout
									if (!renderspritenow || havepixel || spixel == 0) continue;

									havepixel = true;

									//TODO - make sure we dont trigger spritehit if the edges are masked for either BG or OBJ
									//spritehit:
									//1. is it sprite#0?
									//2. is the bg pixel nonzero?
									//then, it is spritehit.
									Reg2002_objhit |= (oam->index == 0 && pixel != 0 && rasterpos < 255);
									//priority handling, if in front of BG:
									bool drawsprite = !(((oam->oam[2] & 0x20) != 0) && ((pixel & 3) != 0));
									if (drawsprite && nes.Settings.DispSprites)
									{
										//bring in the palette bits and palettize
										spixel |= (oam->oam[2] & 3) << 2;
										//save it for use in the framebuffer
										pixelcolor = PALRAM[0x10 + spixel];
									}
								} //rasterpos in sprite range
							} //oamcount loop
							if (reg_2001.color_disable)
								pixelcolor &= 0x30;

							xbuf[target] = PaletteAdjustPixel(pixelcolor);

							target++;
						} //loop across 8 pixels
					} //loop across 32 tiles
				}
				else
					for (int xt = 0; xt < 32; xt++)
						Read_bgdata(ref bgdata[xt + 2]);

				//look for sprites (was supposed to run concurrent with bg rendering)
				oamcounts[scanslot] = 0;
				oamcount = 0;
				int spriteHeight = reg_2000.obj_size_16 ? 16 : 8;

				int scanslot_lshift = scanslot << 6;

				for (int i = 0; i < 64; i++)
				{
					oams[scanslot_lshift + i].present = 0;
					int spr = i * 4;
					if (yp >= OAM[spr] && yp < OAM[spr] + spriteHeight)
					{
						//if we already have maxsprites, then this new one causes an overflow,
						//set the flag and bail out.
						//should we set this flag anyway??
						if (oamcount >= 8 && reg_2001.PPUON)
						{
							Reg2002_objoverflow = true;
							if(!nes.Settings.AllowMoreThanEightSprites)
								break;
						}
						//just copy some bytes into the internal sprite buffer
						TempOAM* oam = &oams[scanslot_lshift + oamcount];
						for (int j = 0; j < 4; j++)
							oam->oam[j] = OAM[spr + j];
						oam->present = 1;
						//note that we stuff the oam index into [6].
						//i need to turn this into a struct so we can have fewer magic numbers
						oams[scanslot_lshift + oamcount].index = (byte)i;
						oamcount++;
					}
				}
				oamcounts[scanslot] = oamcount;

				ppuphase = PPUPHASE.OBJ;

				//fetch sprite patterns
				int oam_todo = oamcount;
				if (oam_todo < 8)
					oam_todo = 8;
				for (int s = 0; s < oam_todo; s++)
				{
					//if this is a real sprite sprite, then it is not above the 8 sprite limit.
					//this is how we support the no 8 sprite limit feature.
					//not that at some point we may need a virtual CALL_PPUREAD which just peeks and doesnt increment any counters
					//this could be handy for the debugging tools also
					bool realSprite = (s < 8);
					bool junksprite = (s >= oamcount || !reg_2001.PPUON);

					TempOAM* oam = &oams[scanslot_lshift + s];
					int line = yp - oam->oam[0];
					if ((oam->oam[2] & 0x80) != 0) //vflip
						line = spriteHeight - line - 1;

					int patternNumber = oam->oam[1];
					int patternAddress;

					//create deterministic dummy fetch pattern.
					if (oam->present == 0)
					{
						//according to nintendulator:
						//* On the first empty sprite slot, read the Y-coordinate of sprite #63 followed by $FF for the remaining 7 cycles
						//* On all subsequent empty sprite slots, read $FF for all 8 reads
						//well, we shall just read $FF and that is good enough for now to make mmc3 work
						patternNumber = 0xFF;
						line = 0;
					}

					//8x16 sprite handling:
					if (reg_2000.obj_size_16)
					{
						int bank = (patternNumber & 1) << 12;
						patternNumber = patternNumber & ~1;
						patternNumber |= (line >> 3) & 1;
						patternAddress = (patternNumber << 4) | bank;
					}
					else
						patternAddress = (patternNumber << 4) | (reg_2000.obj_pattern_hi << 12);

					//offset into the pattern for the current line.
					//tricky: tall sprites have already had lines>8 taken care of by getting a new pattern number above.
					//so we just need the line offset for the second pattern
					patternAddress += line & 7;

					//garbage nametable fetches + scroll resets
					int garbage_todo = 2;
					ppubus_read(ppur.get_ntread(), true);
					if (reg_2001.PPUON)
					{
						if (sl == 0 && ppur.status.cycle == 304)
						{
							runppu(1);
							if (reg_2001.PPUON) ppur.install_latches();
							runppu(1);
							garbage_todo = 0;
						}
						if ((sl != 0) && ppur.status.cycle == 256)
						{
							runppu(1);
							//at 257: 3d world runner is ugly if we do this at 256
							if (reg_2001.PPUON) ppur.install_h_latches();
							runppu(1);
							garbage_todo = 0;
						}
					}
					if (realSprite) runppu(garbage_todo);

					ppubus_read(ppur.get_atread(), true); //at or nt?
					if (realSprite) runppu(kFetchTime);

					// TODO - fake sprites should not come through ppubus_read but rather peek it
					// (at least, they should not probe it with AddressPPU. maybe the difference between peek and read is not necessary)

					if (junksprite)
					{
						if (realSprite)
						{
							ppubus_read(patternAddress, true);
							ppubus_read(patternAddress, true);
							runppu(kFetchTime * 2);
						}
					}
					else
					{
						int addr = patternAddress;
						oam->patterns[0] = ppubus_read(addr, true);
						if (realSprite) runppu(kFetchTime);

						addr += 8;
						oam->patterns[1] = ppubus_read(addr, true);
						if (realSprite) runppu(kFetchTime);

						// hflip
						if ((oam->oam[2] & 0x40) == 0)
						{
							oam->patterns[0] = BitReverse.Byte8[oam->patterns[0]];
							oam->patterns[1] = BitReverse.Byte8[oam->patterns[1]];
						}
					}
				} // sprite pattern fetch loop

				ppuphase = PPUPHASE.BG;

				// fetch BG: two tiles for next line
				for (int xt = 0; xt < 2; xt++)
					Read_bgdata(ref bgdata[xt]);

				// this sequence is tuned to pass 10-even_odd_timing.nes
				runppu(kFetchTime);
				bool evenOddDestiny = reg_2001.show_bg;
				runppu(kFetchTime);

				// After memory access 170, the PPU simply rests for 4 cycles (or the
				// equivelant of half a memory access cycle) before repeating the whole
				// pixel/scanline rendering process. If the scanline being rendered is the very
				// first one on every second frame, then this delay simply doesn't exist.
				if (sl == 0 && idleSynch && evenOddDestiny && chopdot)
				{ }
				else
					runppu(1);
			} // scanline loop

			ppur.status.sl = 241;

			//idle for pre NMI lines
			runppu(preNMIlines * kLineTime);

		} //FrameAdvance


		void FrameAdvance_ppudead()
		{
			//not quite emulating all the NES power up behavior
			//since it is known that the NES ignores writes to some
			//register before around a full frame, but no games
			//should write to those regs during that time, it needs
			//to wait for vblank
			ppur.status.sl = 241;
			runppu(postNMIlines * kLineTime);
			ppur.status.sl = 0;
			runppu(241 * kLineTime);
			runppu(preNMIlines * kLineTime);
			--ppudead;
		}
	}
}