#extension GL_ARB_gpu_shader5 : enable

#ifdef GL_ARB_gpu_shader5
#define USE_INVOC_EXT
#endif

#ifdef DOUBLE_MANIFOLD
#  ifdef USE_INVOC_EXT
#    define invoc_len 2
#  else
#    define vert_len 8
#  endif
#else
#  ifdef USE_INVOC_EXT
#    define invoc_len 1
#  else
#    define vert_len 4
#  endif
#endif

#ifdef USE_INVOC_EXT
layout(lines_adjacency, invocations = invoc_len) in;
layout(triangle_strip, max_vertices = 4) out;
#else
layout(lines_adjacency) in;
layout(triangle_strip, max_vertices = vert_len) out;
#endif

uniform vec3 lightDirection = vec3(0.57, 0.57, -0.57);

in VertexData {
	vec3 pos;           /* local position */
	vec4 frontPosition; /* final ndc position */
	vec4 backPosition;
} vData[];

#define DEGENERATE_THRESHOLD 1e-12

#define len_sqr(a) dot(a, a)

void extrude_edge(bool invert)
{
	/* Reverse order if backfacing the light. */
	ivec2 idx = (invert) ? ivec2(1, 2) : ivec2(2, 1);
	gl_Position = vData[idx.x].frontPosition; EmitVertex();
	gl_Position = vData[idx.y].frontPosition; EmitVertex();
	gl_Position = vData[idx.x].backPosition; EmitVertex();
	gl_Position = vData[idx.y].backPosition; EmitVertex();
	EndPrimitive();
}

void main()
{
	vec3 v10 = vData[0].pos - vData[1].pos;
	vec3 v12 = vData[2].pos - vData[1].pos;
	vec3 v13 = vData[3].pos - vData[1].pos;

	vec3 n1 = cross(v12, v10);
	vec3 n2 = cross(v13, v12);

#ifdef DEGENERATE_THRESHOLD
	/* Check if area is null */
	vec2 faces_area = vec2(len_sqr(n1), len_sqr(n2));
	bvec2 degen_faces = lessThan(abs(faces_area), vec2(DEGENERATE_THRESHOLD));

	/* Both triangles are degenerate, abort. */
	if (all(degen_faces)) {
		return;
	}
#endif

	vec2 facing = vec2(dot(n1, lightDirection),
	                   dot(n2, lightDirection));

	/* WATCH: maybe unpredictable in some cases. */
	bool is_manifold = any(notEqual(vData[0].pos, vData[3].pos));

	bvec2 backface = greaterThan(facing, vec2(0.0));

#ifdef DEGENERATE_THRESHOLD
#  ifndef DOUBLE_MANIFOLD
	/* If the mesh is known to be manifold and we don't use double count,
	 * only create an quad if the we encounter a facing geom. */
	if ((degen_faces.x && backface.y) ||
	    (degen_faces.y && backface.x))
		return;
#  endif

	/* If one of the 2 triangles is degenerate, replace edge by a non-manifold one. */
	backface.x = (degen_faces.x) ? !backface.y : backface.x;
	backface.y = (degen_faces.y) ? !backface.x : backface.y;
	is_manifold = (any(degen_faces)) ? false : is_manifold;
#endif

	/* If both faces face the same direction it's not an outline edge. */
	if (backface.x == backface.y) {
		return;
	}

#ifdef USE_INVOC_EXT
	if (gl_InvocationID == 0) {
		extrude_edge(backface.x);
	}
	else if (is_manifold) {
#  ifdef DOUBLE_MANIFOLD
		/* Increment/Decrement twice for manifold edges. */
		extrude_edge(backface.x);
#  endif
	}
#else
	extrude_edge(backface.x);
	if (is_manifold) {
#  ifdef DOUBLE_MANIFOLD
		/* Increment/Decrement twice for manifold edges. */
		extrude_edge(backface.x);
#  endif
	}
#endif
}