using UnityEngine;

namespace NobleMuffins.LimbHacker.Guts
{
	static class Triangulation
	{
		const float Epsilon = float.Epsilon;
		
		private static float Area(Vector2[] contour, int length)
		{
			int n = length;
			
			float A=0.0f;
			
			for(int p=n-1,q=0; q<n; p=q++)
			{
				A+= contour[p].x*contour[q].y - contour[q].x*contour[p].y;
			}
			return A*0.5f;
		}
		
		/*
		InsideTriangle decides if a point P is Inside of the triangle
		defined by A, B, C.
		*/
		private static bool InsideTriangle(float Ax, float Ay,
			float Bx, float By,
			float Cx, float Cy,
			float Px, float Py)
		{
			float ax, ay, bx, by, cx, cy, apx, apy, bpx, bpy, cpx, cpy;
			float cCROSSap, bCROSScp, aCROSSbp;
			
			ax = Cx - Bx;  ay = Cy - By;
			bx = Ax - Cx;  by = Ay - Cy;
			cx = Bx - Ax;  cy = By - Ay;
			apx= Px - Ax;  apy= Py - Ay;
			bpx= Px - Bx;  bpy= Py - By;
			cpx= Px - Cx;  cpy= Py - Cy;
			
			aCROSSbp = ax*bpy - ay*bpx;
			cCROSSap = cx*apy - cy*apx;
			bCROSScp = bx*cpy - by*cpx;
			
			return ((aCROSSbp >= 0.0f) && (bCROSScp >= 0.0f) && (cCROSSap >= 0.0f));
		}
		
		private static bool Snip(Vector2[] contour,int u,int v,int w,int n,int[] V)
		{
			int p;
			float Ax, Ay, Bx, By, Cx, Cy, Px, Py;
			
			Ax = contour[V[u]].x;
			Ay = contour[V[u]].y;
			
			Bx = contour[V[v]].x;
			By = contour[V[v]].y;
			
			Cx = contour[V[w]].x;
			Cy = contour[V[w]].y;
			
			if ( Epsilon > (((Bx-Ax)*(Cy-Ay)) - ((By-Ay)*(Cx-Ax))) )
			{				
				return false;
			}
			
			for (p=0;p<n;p++)
			{
				if( (p == u) || (p == v) || (p == w) ) continue;
				
				Px = contour[V[p]].x;
				Py = contour[V[p]].y;
				
				if (InsideTriangle(Ax,Ay,Bx,By,Cx,Cy,Px,Py))
					return false;
			}
			
			return true;
		}

		public static int GetArraySize(int length) {
			//Given that the contour features n vertices, the result is expected to feature n - 2 triangles.
			return System.Math.Max( 0, (length - 2) * 3 );
		}

		public static bool Triangulate(Vector2[] contour, int length, int[] result)
		{
			/* allocate and initialize list of Vertices in polygon */

			int n = length;
			if ( n < 3 ) {
				return false;
			}

			var expectedVertexCount = GetArraySize(length);

			if(result.Length < expectedVertexCount) {
				throw new System.ArgumentException();
			}

			int[] V = new int[n];
			
			/* we want a counter-clockwise polygon in V */
			
			if ( 0.0f < Area(contour, length) )
				for (int v=0; v<n; v++) V[v] = v;
			else
				for(int v=0; v<n; v++) V[v] = (n-1)-v;
			
			int nv = n;
			
			/*  remove nv-2 Vertices, creating 1 triangle every time */
			int count = 2*nv;   /* error detection */
			
			int resultIndex = 0;
			
			for(int m=0, v=nv-1; nv>2; )
			{
				/* if we loop, it is probably a non-simple polygon */
				if (0 >= (count--))
				{
					//** Triangulate: ERROR - probable bad polygon!
					return false;
				}
				
				/* three consecutive vertices in current polygon, <u,v,w> */
				int u = v  ; if (nv <= u) u = 0;     /* previous */
				v = u+1; if (nv <= v) v = 0;     /* new v    */
				int w = v+1; if (nv <= w) w = 0;     /* next     */
				
				if ( Snip(contour,u,v,w,nv,V) )
				{
					int a,b,c,s,t;
					
					/* true names of the vertices */
					a = V[u]; b = V[v]; c = V[w];
					
					/* output Triangle */
					result[resultIndex++] = a;
					result[resultIndex++] = b;
					result[resultIndex++] = c;
					
					m++;
					
					/* remove v from remaining polygon */
					for(s=v,t=v+1;t<nv;s++,t++) V[s] = V[t]; nv--;
					
					/* resest error detection counter */
					count = 2*nv;
				}
			}
			
			return true;
		}
	}
}