/*
**  Voxel rendering
**  Copyright (c) 1998-2016 Randy Heit
**  Copyright (c) 2016 Magnus Norddahl
**
**  This software is provided 'as-is', without any express or implied
**  warranty.  In no event will the authors be held liable for any damages
**  arising from the use of this software.
**
**  Permission is granted to anyone to use this software for any purpose,
**  including commercial applications, and to alter it and redistribute it
**  freely, subject to the following restrictions:
**
**  1. The origin of this software must not be misrepresented; you must not
**     claim that you wrote the original software. If you use this software
**     in a product, an acknowledgment in the product documentation would be
**     appreciated but is not required.
**  2. Altered source versions must be plainly marked as such, and must not be
**     misrepresented as being the original software.
**  3. This notice may not be removed or altered from any source distribution.
**
*/

#include <stdlib.h>
#include "templates.h"
#include "doomdef.h"
#include "sbar.h"
#include "r_data/r_translate.h"
#include "r_data/colormaps.h"
#include "r_data/voxels.h"
#include "r_data/sprites.h"
#include "d_net.h"
#include "po_man.h"
#include "r_utility.h"
#include "swrenderer/drawers/r_draw.h"
#include "swrenderer/drawers/r_thread.h"
#include "swrenderer/things/r_visiblesprite.h"
#include "swrenderer/things/r_voxel.h"
#include "swrenderer/r_main.h"

namespace swrenderer
{
	namespace
	{
		FCoverageBuffer *OffscreenCoverageBuffer;
		int OffscreenBufferWidth, OffscreenBufferHeight;
		uint8_t *OffscreenColorBuffer;
	}

	void R_DrawVisVoxel(vissprite_t *sprite, int minZ, int maxZ, short *cliptop, short *clipbottom)
	{
		R_SetColorMapLight(sprite->Style.BaseColormap, 0, sprite->Style.ColormapNum << FRACBITS);
		bool visible = R_SetPatchStyle(sprite->Style.RenderStyle, sprite->Style.Alpha, sprite->Translation, sprite->FillColor);
		if (!visible)
			return;

		DVector3 view_origin = { sprite->pa.vpos.X, sprite->pa.vpos.Y, sprite->pa.vpos.Z };
		FAngle view_angle = sprite->pa.vang;
		DVector3 sprite_origin = { sprite->gpos.X, sprite->gpos.Y, sprite->gpos.Z };
		DAngle sprite_angle = sprite->Angle;
		double sprite_xscale = FIXED2DBL(sprite->xscale);
		double sprite_yscale = sprite->yscale;
		FVoxel *voxel = sprite->voxel;
		
		// Select mipmap level:
		
		double viewSin = view_angle.Cos();
		double viewCos = view_angle.Sin();
		double logmip = fabs((view_origin.X - sprite_origin.X) * viewCos - (view_origin.Y - sprite_origin.Y) * viewSin);
		int miplevel = 0;
		while (miplevel < voxel->NumMips - 1 && logmip >= FocalLengthX)
		{
			logmip *= 0.5;
			miplevel++;
		}
		
		const FVoxelMipLevel &mip = voxel->Mips[miplevel];
		if (mip.SlabData == nullptr)
			return;

		minZ >>= miplevel;
		maxZ >>= miplevel;
		sprite_xscale *= (1 << miplevel);
		sprite_yscale *= (1 << miplevel);
		
		// Find voxel cube eigenvectors and origin in world space:

		double spriteSin = sprite_angle.Sin();
		double spriteCos = sprite_angle.Cos();
		
		DVector2 dirX(spriteSin * sprite_xscale, -spriteCos * sprite_xscale);
		DVector2 dirY(spriteCos * sprite_xscale, spriteSin * sprite_xscale);
		double dirZ = -sprite_yscale;
		
		DVector3 voxel_origin = sprite_origin;
		voxel_origin.X -= dirX.X * mip.Pivot.X + dirX.Y * mip.Pivot.Y;
		voxel_origin.Y -= dirY.X * mip.Pivot.X + dirY.Y * mip.Pivot.Y;
		voxel_origin.Z -= dirZ * mip.Pivot.Z;
		
		// Voxel cube walking directions:
		
		int startX[4] = { 0, mip.SizeX - 1,             0, mip.SizeX - 1 };
		int startY[4] = { 0,             0, mip.SizeY - 1, mip.SizeY - 1 };
		int stepX[4] = { 1, -1,  1, -1 };
		int stepY[4] = { 1,  1, -1, -1 };
		
		// The point in cube mipmap local space where voxel sides change from front to backfacing:
		
		double dx = (view_origin.X - sprite_origin.X) / sprite_xscale;
		double dy = (view_origin.Y - sprite_origin.Y) / sprite_xscale;
		int backX = (int)(dx * spriteCos - dy * spriteSin + mip.Pivot.X);
		int backY = (int)(dy * spriteCos + dx * spriteSin + mip.Pivot.Y);
		int endX = clamp(backX, 0, mip.SizeX - 1);
		int endY = clamp(backY, 0, mip.SizeY - 1);
		
		// Draw the voxel cube:
		
		for (int index = 0; index < 4; index++)
		{
			if ((stepX[index] < 0 && endX >= startX[index]) ||
			    (stepX[index] > 0 && endX <= startX[index]) ||
			    (stepY[index] < 0 && endY >= startY[index]) ||
			    (stepY[index] > 0 && endY <= startY[index])) continue;
		
			for (int x = startX[index]; x != endX; x += stepX[index])
			{
				for (int y = startY[index]; y != endY; y += stepY[index])
				{
					kvxslab_t *slab_start = R_GetSlabStart(mip, x, y);
					kvxslab_t *slab_end = R_GetSlabEnd(mip, x, y);
				
					for (kvxslab_t *slab = slab_start; slab != slab_end; slab = R_NextSlab(slab))
					{
						// To do: check slab->backfacecull
						
						int ztop = slab->ztop;
						int zbottom = ztop + slab->zleng;
						
						//ztop = MAX(ztop, minZ);
						//zbottom = MIN(zbottom, maxZ);
						
						for (int z = ztop; z < zbottom; z++)
						{
							uint8_t color = slab->col[z - slab->ztop];
						
							DVector3 voxel_pos = voxel_origin;
							voxel_pos.X += dirX.X * x + dirX.Y * y;
							voxel_pos.Y += dirY.X * x + dirY.Y * y;
							voxel_pos.Z += dirZ * z;
						
							R_FillBox(voxel_pos, sprite_xscale, sprite_yscale, color, cliptop, clipbottom, false, false);
						}
					}
				}
			}
		}
	}
	
	kvxslab_t *R_GetSlabStart(const FVoxelMipLevel &mip, int x, int y)
	{
		return (kvxslab_t *)&mip.SlabData[mip.OffsetX[x] + (int)mip.OffsetXY[x * (mip.SizeY + 1) + y]];
	}

	kvxslab_t *R_GetSlabEnd(const FVoxelMipLevel &mip, int x, int y)
	{
		return R_GetSlabStart(mip, x, y + 1);
	}
	
	kvxslab_t *R_NextSlab(kvxslab_t *slab)
	{
		return (kvxslab_t*)(((uint8_t*)slab) + 3 + slab->zleng);
	}

	void R_FillBox(DVector3 origin, double extentX, double extentY, int color, short *cliptop, short *clipbottom, bool viewspace, bool pixelstretch)
	{
		double viewX, viewY, viewZ;
		if (viewspace)
		{
			viewX = origin.X;
			viewY = origin.Y;
			viewZ = origin.Z;
		}
		else // world space
		{
			double translatedX = origin.X - ViewPos.X;
			double translatedY = origin.Y - ViewPos.Y;
			double translatedZ = origin.Z - ViewPos.Z;
			viewX = translatedX * ViewSin - translatedY * ViewCos;
			viewY = translatedZ;
			viewZ = translatedX * ViewTanCos + translatedY * ViewTanSin;
		}

		if (viewZ < 0.01f)
			return;

		double screenX = CenterX + viewX / viewZ * CenterX;
		double screenY = CenterY - viewY / viewZ * InvZtoScale;
		double screenExtentX = extentX / viewZ * CenterX;
		double screenExtentY = pixelstretch ? screenExtentX * YaspectMul : screenExtentX;

		int x1 = MAX((int)(screenX - screenExtentX), 0);
		int x2 = MIN((int)(screenX + screenExtentX + 0.5f), viewwidth - 1);
		int y1 = MAX((int)(screenY - screenExtentY), 0);
		int y2 = MIN((int)(screenY + screenExtentY + 0.5f), viewheight - 1);

		int pixelsize = r_swtruecolor ? 4 : 1;

		if (y1 < y2)
		{
			for (int x = x1; x < x2; x++)
			{
				int columnY1 = MAX(y1, (int)cliptop[x]);
				int columnY2 = MIN(y2, (int)clipbottom[x]);
				if (columnY1 < columnY2)
				{
					using namespace drawerargs;
					dc_dest = dc_destorg + (dc_pitch * columnY1 + x) * pixelsize;
					dc_color = color;
					dc_count = columnY2 - columnY1;
					R_Drawers()->FillColumn();
				}
			}
		}
	}

	void R_DeinitRenderVoxel()
	{
		// Free offscreen buffer
		if (OffscreenColorBuffer != nullptr)
		{
			delete[] OffscreenColorBuffer;
			OffscreenColorBuffer = nullptr;
		}
		if (OffscreenCoverageBuffer != nullptr)
		{
			delete OffscreenCoverageBuffer;
			OffscreenCoverageBuffer = nullptr;
		}
		OffscreenBufferHeight = OffscreenBufferWidth = 0;
	}

	void R_CheckOffscreenBuffer(int width, int height, bool spansonly)
	{
		// Allocates the offscreen coverage buffer and optionally the offscreen
		// color buffer. If they already exist but are the wrong size, they will
		// be reallocated.

		if (OffscreenCoverageBuffer == nullptr)
		{
			assert(OffscreenColorBuffer == nullptr && "The color buffer cannot exist without the coverage buffer");
			OffscreenCoverageBuffer = new FCoverageBuffer(width);
		}
		else if (OffscreenCoverageBuffer->NumLists != (unsigned)width)
		{
			delete OffscreenCoverageBuffer;
			OffscreenCoverageBuffer = new FCoverageBuffer(width);
			if (OffscreenColorBuffer != nullptr)
			{
				delete[] OffscreenColorBuffer;
				OffscreenColorBuffer = nullptr;
			}
		}
		else
		{
			OffscreenCoverageBuffer->Clear();
		}

		if (!spansonly)
		{
			if (OffscreenColorBuffer == nullptr)
			{
				OffscreenColorBuffer = new uint8_t[width * height * 4];
			}
			else if (OffscreenBufferWidth != width || OffscreenBufferHeight != height)
			{
				delete[] OffscreenColorBuffer;
				OffscreenColorBuffer = new uint8_t[width * height * 4];
			}
		}
		OffscreenBufferWidth = width;
		OffscreenBufferHeight = height;
	}

	////////////////////////////////////////////////////////////////////////////

	FCoverageBuffer::FCoverageBuffer(int lists)
		: Spans(nullptr), FreeSpans(nullptr)
	{
		NumLists = lists;
		Spans = new Span *[lists];
		memset(Spans, 0, sizeof(Span*)*lists);
	}

	FCoverageBuffer::~FCoverageBuffer()
	{
		if (Spans != nullptr)
		{
			delete[] Spans;
		}
	}

	void FCoverageBuffer::Clear()
	{
		SpanArena.FreeAll();
		memset(Spans, 0, sizeof(Span*)*NumLists);
		FreeSpans = nullptr;
	}

	void FCoverageBuffer::InsertSpan(int listnum, int start, int stop)
	{
		// start is inclusive.
		// stop is exclusive.

		assert(unsigned(listnum) < NumLists);
		assert(start < stop);

		Span **span_p = &Spans[listnum];
		Span *span;

		if (*span_p == nullptr || (*span_p)->Start > stop)
		{ // This list is empty or the first entry is after this one, so we can just insert the span.
			goto addspan;
		}

		// Insert the new span in order, merging with existing ones.
		while (*span_p != nullptr)
		{
			if ((*span_p)->Stop < start)							// =====		(existing span)
			{ // Span ends before this one starts.					//		  ++++	(new span)
				span_p = &(*span_p)->NextSpan;
				continue;
			}

			// Does the new span overlap or abut the existing one?
			if ((*span_p)->Start <= start)
			{
				if ((*span_p)->Stop >= stop)						// =============
				{ // The existing span completely covers this one.	//     +++++
					return;
				}
			extend:		// Extend the existing span with the new one.		// ======
				span = *span_p;										//     +++++++
				span->Stop = stop;									// (or)  +++++

																	// Free up any spans we just covered up.
				span_p = &(*span_p)->NextSpan;
				while (*span_p != nullptr && (*span_p)->Start <= stop && (*span_p)->Stop <= stop)
				{
					Span *span = *span_p;							// ======  ======
					*span_p = span->NextSpan;						//     +++++++++++++
					span->NextSpan = FreeSpans;
					FreeSpans = span;
				}
				if (*span_p != nullptr && (*span_p)->Start <= stop)	// =======         ========
				{ // Our new span connects two existing spans.		//     ++++++++++++++
				  // They should all be collapsed into a single span.
					span->Stop = (*span_p)->Stop;
					span = *span_p;
					*span_p = span->NextSpan;
					span->NextSpan = FreeSpans;
					FreeSpans = span;
				}
				goto check;
			}
			else if ((*span_p)->Start <= stop)						//        =====
			{ // The new span extends the existing span from		//    ++++
			  // the beginning.										// (or) ++++
				(*span_p)->Start = start;
				if ((*span_p)->Stop < stop)
				{ // The new span also extends the existing span	//     ======
				  // at the bottom									// ++++++++++++++
					goto extend;
				}
				goto check;
			}
			else													//         ======
			{ // No overlap, so insert a new span.					// +++++
				goto addspan;
			}
		}
		// Append a new span to the end of the list.
	addspan:
		span = AllocSpan();
		span->NextSpan = *span_p;
		span->Start = start;
		span->Stop = stop;
		*span_p = span;
	check:
#ifdef _DEBUG
		// Validate the span list: Spans must be in order, and there must be
		// at least one pixel between spans.
		for (span = Spans[listnum]; span != nullptr; span = span->NextSpan)
		{
			assert(span->Start < span->Stop);
			if (span->NextSpan != nullptr)
			{
				assert(span->Stop < span->NextSpan->Start);
			}
		}
#endif
		;
	}

	FCoverageBuffer::Span *FCoverageBuffer::AllocSpan()
	{
		Span *span;

		if (FreeSpans != nullptr)
		{
			span = FreeSpans;
			FreeSpans = span->NextSpan;
		}
		else
		{
			span = (Span *)SpanArena.Alloc(sizeof(Span));
		}
		return span;
	}

	/////////////////////////////////////////////////////////////////////////
	// Old BUILD implementation follows:
	//
	// This file contains some code from the Build Engine.
	//
	// "Build Engine & Tools" Copyright (c) 1993-1997 Ken Silverman
	// Ken Silverman's official web site: "http://www.advsys.net/ken"
	// See the included license file "BUILDLIC.TXT" for license info.

#if 0
	void R_DrawVisVoxel(vissprite_t *spr, int minslabz, int maxslabz, short *cliptop, short *clipbot)
	{
		int flags = 0;

		// Do setup for blending.
		R_SetColorMapLight(spr->Style.BaseColormap, 0, spr->Style.ColormapNum << FRACBITS);
		bool visible = R_SetPatchStyle(spr->Style.RenderStyle, spr->Style.Alpha, spr->Translation, spr->FillColor);

		if (!visible)
		{
			return;
		}
		if (colfunc == fuzzcolfunc || colfunc == R_FillColumn)
		{
			flags = DVF_OFFSCREEN | DVF_SPANSONLY;
		}
		else if (colfunc != basecolfunc)
		{
			flags = DVF_OFFSCREEN;
		}
		if (flags != 0)
		{
			R_CheckOffscreenBuffer(RenderTarget->GetWidth(), RenderTarget->GetHeight(), !!(flags & DVF_SPANSONLY));
		}
		if (spr->bInMirror)
		{
			flags |= DVF_MIRRORED;
		}

		// Render the voxel, either directly to the screen or offscreen.
		R_DrawVoxel(spr->pa.vpos, spr->pa.vang, spr->gpos, spr->Angle,
			spr->xscale, FLOAT2FIXED(spr->yscale), spr->voxel, spr->Style.BaseColormap, spr->Style.ColormapNum, cliptop, clipbot,
			minslabz, maxslabz, flags);

		// Blend the voxel, if that's what we need to do.
		if ((flags & ~DVF_MIRRORED) != 0)
		{
			int pixelsize = r_swtruecolor ? 4 : 1;
			for (int x = 0; x < viewwidth; ++x)
			{
				if (!(flags & DVF_SPANSONLY) && (x & 3) == 0)
				{
					rt_initcols(OffscreenColorBuffer + x * OffscreenBufferHeight);
				}
				for (FCoverageBuffer::Span *span = OffscreenCoverageBuffer->Spans[x]; span != NULL; span = span->NextSpan)
				{
					if (flags & DVF_SPANSONLY)
					{
						dc_x = x;
						dc_yl = span->Start;
						dc_yh = span->Stop - 1;
						dc_count = span->Stop - span->Start;
						dc_dest = (ylookup[span->Start] + x) * pixelsize + dc_destorg;
						colfunc();
					}
					else
					{
						rt_span_coverage(x, span->Start, span->Stop - 1);
					}
				}
				if (!(flags & DVF_SPANSONLY) && (x & 3) == 3)
				{
					rt_draw4cols(x - 3);
				}
			}
		}

		R_FinishSetPatchStyle();
		NetUpdate();
	}

	void R_DrawVoxel(const FVector3 &globalpos, FAngle viewangle,
		const FVector3 &dasprpos, DAngle dasprang,
		fixed_t daxscale, fixed_t dayscale, FVoxel *voxobj,
		FSWColormap *colormap, int colormapnum, short *daumost, short *dadmost, int minslabz, int maxslabz, int flags)
	{
		int i, j, k, x, y, syoff, ggxstart, ggystart, nxoff;
		fixed_t cosang, sinang, sprcosang, sprsinang;
		int backx, backy, gxinc, gyinc;
		int daxscalerecip, dayscalerecip, cnt, gxstart, gystart, dazscale;
		int lx, rx, nx, ny, x1=0, y1=0, x2=0, y2=0, yinc=0;
		int yoff, xs=0, ys=0, xe, ye, xi=0, yi=0, cbackx, cbacky, dagxinc, dagyinc;
		kvxslab_t *voxptr, *voxend;
		FVoxelMipLevel *mip;
		int z1a[64], z2a[64], yplc[64];

		const int nytooclose = centerxwide * 2100, nytoofar = 32768*32768 - 1048576;
		const int xdimenscale = FLOAT2FIXED(centerxwide * YaspectMul / 160);
		const double centerxwide_f = centerxwide;
		const double centerxwidebig_f = centerxwide_f * 65536*65536*8;

		// Convert to Build's coordinate system.
		fixed_t globalposx = xs_Fix<4>::ToFix(globalpos.X);
		fixed_t globalposy = xs_Fix<4>::ToFix(-globalpos.Y);
		fixed_t globalposz = xs_Fix<8>::ToFix(-globalpos.Z);

		fixed_t dasprx = xs_Fix<4>::ToFix(dasprpos.X);
		fixed_t daspry = xs_Fix<4>::ToFix(-dasprpos.Y);
		fixed_t dasprz = xs_Fix<8>::ToFix(-dasprpos.Z);

		// Shift the scales from 16 bits of fractional precision to 6.
		// Also do some magic voodoo scaling to make them the right size.
		daxscale = daxscale / (0xC000 >> 6);
		dayscale = dayscale / (0xC000 >> 6);
		if (daxscale <= 0 || dayscale <= 0)
		{
			// won't be visible.
			return;
		}

		angle_t viewang = viewangle.BAMs();
		cosang = FLOAT2FIXED(viewangle.Cos()) >> 2;
		sinang = FLOAT2FIXED(-viewangle.Sin()) >> 2;
		sprcosang = FLOAT2FIXED(dasprang.Cos()) >> 2;
		sprsinang = FLOAT2FIXED(-dasprang.Sin()) >> 2;

		R_SetupDrawSlab(colormap, 0.0f, colormapnum << FRACBITS);

		int pixelsize = r_swtruecolor ? 4 : 1;

		// Select mip level
		i = abs(DMulScale6(dasprx - globalposx, cosang, daspry - globalposy, sinang));
		i = DivScale6(i, MIN(daxscale, dayscale));
		j = xs_Fix<13>::ToFix(FocalLengthX);
		for (k = 0; i >= j && k < voxobj->NumMips; ++k)
		{
			i >>= 1;
		}
		if (k >= voxobj->NumMips) k = voxobj->NumMips - 1;

		mip = &voxobj->Mips[k];		if (mip->SlabData == NULL) return;

		minslabz >>= k;
		maxslabz >>= k;

		daxscale <<= (k+8); dayscale <<= (k+8);
		dazscale = FixedDiv(dayscale, FLOAT2FIXED(BaseYaspectMul));
		daxscale = fixed_t(daxscale / YaspectMul);
		daxscale = Scale(daxscale, xdimenscale, centerxwide << 9);
		dayscale = Scale(dayscale, FixedMul(xdimenscale, viewingrangerecip), centerxwide << 9);

		daxscalerecip = (1<<30) / daxscale;
		dayscalerecip = (1<<30) / dayscale;

		fixed_t piv_x = fixed_t(mip->Pivot.X*256.);
		fixed_t piv_y = fixed_t(mip->Pivot.Y*256.);
		fixed_t piv_z = fixed_t(mip->Pivot.Z*256.);

		x = FixedMul(globalposx - dasprx, daxscalerecip);
		y = FixedMul(globalposy - daspry, daxscalerecip);
		backx = (DMulScale10(x, sprcosang, y,  sprsinang) + piv_x) >> 8;
		backy = (DMulScale10(y, sprcosang, x, -sprsinang) + piv_y) >> 8;
		cbackx = clamp(backx, 0, mip->SizeX - 1);
		cbacky = clamp(backy, 0, mip->SizeY - 1);

		sprcosang = MulScale14(daxscale, sprcosang);
		sprsinang = MulScale14(daxscale, sprsinang);

		x = (dasprx - globalposx) - DMulScale18(piv_x, sprcosang, piv_y, -sprsinang);
		y = (daspry - globalposy) - DMulScale18(piv_y, sprcosang, piv_x,  sprsinang);

		cosang = FixedMul(cosang, dayscalerecip);
		sinang = FixedMul(sinang, dayscalerecip);

		gxstart = y*cosang - x*sinang;
		gystart = x*cosang + y*sinang;
		gxinc = DMulScale10(sprsinang, cosang, sprcosang, -sinang);
		gyinc = DMulScale10(sprcosang, cosang, sprsinang,  sinang);
		if ((abs(globalposz - dasprz) >> 10) >= abs(dazscale)) return;

		x = 0; y = 0; j = MAX(mip->SizeX, mip->SizeY);
		fixed_t *ggxinc = (fixed_t *)alloca((j + 1) * sizeof(fixed_t) * 2);
		fixed_t *ggyinc = ggxinc + (j + 1);
		for (i = 0; i <= j; i++)
		{
			ggxinc[i] = x; x += gxinc;
			ggyinc[i] = y; y += gyinc;
		}

		syoff = DivScale21(globalposz - dasprz, FixedMul(dazscale, 0xE800)) + (piv_z << 7);
		yoff = (abs(gxinc) + abs(gyinc)) >> 1;

		for (cnt = 0; cnt < 8; cnt++)
		{
			switch (cnt)
			{
				case 0: xs = 0;				ys = 0;				xi =  1; yi =  1; break;
				case 1: xs = mip->SizeX-1;	ys = 0;				xi = -1; yi =  1; break;
				case 2: xs = 0;				ys = mip->SizeY-1;	xi =  1; yi = -1; break;
				case 3: xs = mip->SizeX-1;	ys = mip->SizeY-1;	xi = -1; yi = -1; break;
				case 4: xs = 0;				ys = cbacky;		xi =  1; yi =  2; break;
				case 5: xs = mip->SizeX-1;	ys = cbacky;		xi = -1; yi =  2; break;
				case 6: xs = cbackx;		ys = 0;				xi =  2; yi =  1; break;
				case 7: xs = cbackx;		ys = mip->SizeY-1;	xi =  2; yi = -1; break;
			}
			xe = cbackx; ye = cbacky;
			if (cnt < 4)
			{
				if ((xi < 0) && (xe >= xs)) continue;
				if ((xi > 0) && (xe <= xs)) continue;
				if ((yi < 0) && (ye >= ys)) continue;
				if ((yi > 0) && (ye <= ys)) continue;
			}
			else
			{
				if ((xi < 0) && (xe > xs)) continue;
				if ((xi > 0) && (xe < xs)) continue;
				if ((yi < 0) && (ye > ys)) continue;
				if ((yi > 0) && (ye < ys)) continue;
				xe += xi; ye += yi;
			}

			i = sgn(ys - backy) + sgn(xs - backx) * 3 + 4;
			switch(i)
			{
				case 6: case 7: x1 = 0;				y1 = 0;				break;
				case 8: case 5: x1 = gxinc;			y1 = gyinc;			break;
				case 0: case 3: x1 = gyinc;			y1 = -gxinc;		break;
				case 2: case 1: x1 = gxinc+gyinc;	y1 = gyinc-gxinc;	break;
			}
			switch(i)
			{
				case 2: case 5: x2 = 0;				y2 = 0;				break;
				case 0: case 1: x2 = gxinc;			y2 = gyinc;			break;
				case 8: case 7: x2 = gyinc;			y2 = -gxinc;		break;
				case 6: case 3: x2 = gxinc+gyinc;	y2 = gyinc-gxinc;	break;
			}
			BYTE oand = (1 << int(xs<backx)) + (1 << (int(ys<backy)+2));
			BYTE oand16 = oand + 16;
			BYTE oand32 = oand + 32;

			if (yi > 0) { dagxinc =  gxinc; dagyinc =  FixedMul(gyinc, viewingrangerecip); }
				   else { dagxinc = -gxinc; dagyinc = -FixedMul(gyinc, viewingrangerecip); }

				/* Fix for non 90 degree viewing ranges */
			nxoff = FixedMul(x2 - x1, viewingrangerecip);
			x1 = FixedMul(x1, viewingrangerecip);

			ggxstart = gxstart + ggyinc[ys];
			ggystart = gystart - ggxinc[ys];

			for (x = xs; x != xe; x += xi)
			{
				BYTE *slabxoffs = &mip->SlabData[mip->OffsetX[x]];
				short *xyoffs = &mip->OffsetXY[x * (mip->SizeY + 1)];

				nx = FixedMul(ggxstart + ggxinc[x], viewingrangerecip) + x1;
				ny = ggystart + ggyinc[x];
				for (y = ys; y != ye; y += yi, nx += dagyinc, ny -= dagxinc)
				{
					if ((ny <= nytooclose) || (ny >= nytoofar)) continue;
					voxptr = (kvxslab_t *)(slabxoffs + xyoffs[y]);
					voxend = (kvxslab_t *)(slabxoffs + xyoffs[y+1]);
					if (voxptr >= voxend) continue;

					lx = xs_RoundToInt(nx * centerxwide_f / (ny + y1)) + centerx;
					if (lx < 0) lx = 0;
					rx = xs_RoundToInt((nx + nxoff) * centerxwide_f / (ny + y2)) + centerx;
					if (rx > viewwidth) rx = viewwidth;
					if (rx <= lx) continue;

					if (flags & DVF_MIRRORED)
					{
						int t = viewwidth - lx;
						lx = viewwidth - rx;
						rx = t;
					}

					fixed_t l1 = xs_RoundToInt(centerxwidebig_f / (ny - yoff));
					fixed_t l2 = xs_RoundToInt(centerxwidebig_f / (ny + yoff));
					for (; voxptr < voxend; voxptr = (kvxslab_t *)((BYTE *)voxptr + voxptr->zleng + 3))
					{
						const BYTE *col = voxptr->col;
						int zleng = voxptr->zleng;
						int ztop = voxptr->ztop;
						fixed_t z1, z2;

						if (ztop < minslabz)
						{
							int diff = minslabz - ztop;
							ztop = minslabz;
							col += diff;
							zleng -= diff;
						}
						if (ztop + zleng > maxslabz)
						{
							int diff = ztop + zleng - maxslabz;
							zleng -= diff;
						}
						if (zleng <= 0) continue;

						j = (ztop << 15) - syoff;
						if (j < 0)
						{
							k = j + (zleng << 15);
							if (k < 0)
							{
								if ((voxptr->backfacecull & oand32) == 0) continue;
								z2 = MulScale32(l2, k) + centery;					/* Below slab */
							}
							else
							{
								if ((voxptr->backfacecull & oand) == 0) continue;	/* Middle of slab */
								z2 = MulScale32(l1, k) + centery;
							}
							z1 = MulScale32(l1, j) + centery;
						}
						else
						{
							if ((voxptr->backfacecull & oand16) == 0) continue;
							z1 = MulScale32(l2, j) + centery;						/* Above slab */
							z2 = MulScale32(l1, j + (zleng << 15)) + centery;
						}

						if (z2 <= z1) continue;

						if (zleng == 1)
						{
							yinc = 0;
						}
						else
						{
							if (z2-z1 >= 1024) yinc = FixedDiv(zleng, z2 - z1);
							else yinc = (((1 << 24) - 1) / (z2 - z1)) * zleng >> 8;
						}
						// [RH] Clip each column separately, not just by the first one.
						for (int stripwidth = MIN<int>(countof(z1a), rx - lx), lxt = lx;
							lxt < rx;
							(lxt += countof(z1a)), stripwidth = MIN<int>(countof(z1a), rx - lxt))
						{
							// Calculate top and bottom pixels locations
							for (int xxx = 0; xxx < stripwidth; ++xxx)
							{
								if (zleng == 1)
								{
									yplc[xxx] = 0;
									z1a[xxx] = MAX<int>(z1, daumost[lxt + xxx]);
								}
								else
								{
									if (z1 < daumost[lxt + xxx])
									{
										yplc[xxx] = yinc * (daumost[lxt + xxx] - z1);
										z1a[xxx] = daumost[lxt + xxx];
									}
									else
									{
										yplc[xxx] = 0;
										z1a[xxx] = z1;
									}
								}
								z2a[xxx] = MIN<int>(z2, dadmost[lxt + xxx]);
							}
							// Find top and bottom pixels that match and draw them as one strip
							for (int xxl = 0, xxr; xxl < stripwidth; )
							{
								if (z1a[xxl] >= z2a[xxl])
								{ // No column here
									xxl++;
									continue;
								}
								int z1 = z1a[xxl];
								int z2 = z2a[xxl];
								// How many columns share the same extents?
								for (xxr = xxl + 1; xxr < stripwidth; ++xxr)
								{
									if (z1a[xxr] != z1 || z2a[xxr] != z2)
										break;
								}

								if (!(flags & DVF_OFFSCREEN))
								{
									// Draw directly to the screen.
									R_DrawSlab(xxr - xxl, yplc[xxl], z2 - z1, yinc, col, (ylookup[z1] + lxt + xxl) * pixelsize + dc_destorg);
								}
								else
								{
									// Record the area covered and possibly draw to an offscreen buffer.
									dc_yl = z1;
									dc_yh = z2 - 1;
									dc_count = z2 - z1;
									dc_iscale = yinc;
									for (int x = xxl; x < xxr; ++x)
									{
										OffscreenCoverageBuffer->InsertSpan(lxt + x, z1, z2);
										if (!(flags & DVF_SPANSONLY))
										{
											dc_x = lxt + x;
											rt_initcols(OffscreenColorBuffer + (dc_x & ~3) * OffscreenBufferHeight);
											dc_source = col;
											dc_source2 = nullptr;
											dc_texturefrac = yplc[xxl];
											hcolfunc_pre();
										}
									}
								}
								xxl = xxr;
							}
						}
					}
				}
			}
		}
	}
#endif
}