/* Listing 2: Code to perform box filtering using the algorithm
given in the text.

First call DoPreComputation giving a source image and dimensions.
Pass the output of DoPreComputation (p) to DoBoxBlur to perform
the blur. See Listing 5 for an example */

void DoPreComputation(UBYTE *src, int src_w, int src_h, ULONG *dst)
{
	for (int y=0;y<src_h;y++)
	{
		for (int x=0;x<src_w;x++)
		{
			int tot=src[0];
			if (x>0) tot+=dst[-1];
			if (y>0) tot+=dst[-src_w];
			if (x>0 && y>0) tot-=dst[-src_w-1];
			*dst++=tot;
			src++;
		}
	}
}

// this is a utility function used by DoBoxBlur below
ULONG ReadP(ULONG *p, int w, int h, int x, int y)
{
	if (x<0) x=0; else if (x>=w) x=w-1;
	if (y<0) y=0; else if (y>=h) y=h-1;
	return p[x+y*w];
}

// the main meat of the algorithm lies here
void DoBoxBlur(UBYTE *src, int src_w, int src_h, UBYTE *dst, ULONG *p, int boxw, int boxh)
{
	if (boxw<0 || boxh<0)
	{
		memcpy(dst,src,src_w*src_h); // deal with degenerate kernel sizes
		return;
	}
	float mul=1.f/((boxw*2+1)*(boxh*2+1));
	for (int y=0;y<src_h;y++)
	{
		for (int x=0;x<src_w;x++)
		{
			int tot=ReadP(p,src_w,src_h,x+boxw,y+boxh)
				   +ReadP(p,src_w,src_h,x-boxw,y-boxh)
				   -ReadP(p,src_w,src_h,x-boxw,y+boxh)
				   -ReadP(p,src_w,src_h,x+boxw,y-boxh);
			*dst++=UBYTE(tot*mul);
			src++;
		}
	}
}