void b2CollidePolygons()

in Box2D/Collision/b2CollidePolygon.cpp [193:317]
95 lines of code
10 McCabe index (conditional complexity)

void b2CollidePolygons(b2Manifold* manifold,
					  const b2PolygonShape* polyA, const b2Transform& xfA,
					  const b2PolygonShape* polyB, const b2Transform& xfB)
{
	manifold->pointCount = 0;
	float32 totalRadius = polyA->m_radius + polyB->m_radius;

	int32 edgeA = 0;
	float32 separationA = b2FindMaxSeparation(&edgeA, polyA, xfA, polyB, xfB);
	if (separationA > totalRadius)
		return;

	int32 edgeB = 0;
	float32 separationB = b2FindMaxSeparation(&edgeB, polyB, xfB, polyA, xfA);
	if (separationB > totalRadius)
		return;

	const b2PolygonShape* poly1;	// reference polygon
	const b2PolygonShape* poly2;	// incident polygon
	b2Transform xf1, xf2;
	int32 edge1;		// reference edge
	uint8 flip;
	const float32 k_relativeTol = 0.98f;
	const float32 k_absoluteTol = 0.001f;

	if (separationB > k_relativeTol * separationA + k_absoluteTol)
	{
		poly1 = polyB;
		poly2 = polyA;
		xf1 = xfB;
		xf2 = xfA;
		edge1 = edgeB;
		manifold->type = b2Manifold::e_faceB;
		flip = 1;
	}
	else
	{
		poly1 = polyA;
		poly2 = polyB;
		xf1 = xfA;
		xf2 = xfB;
		edge1 = edgeA;
		manifold->type = b2Manifold::e_faceA;
		flip = 0;
	}

	b2ClipVertex incidentEdge[2];
	b2FindIncidentEdge(incidentEdge, poly1, xf1, edge1, poly2, xf2);

	int32 count1 = poly1->m_count;
	const b2Vec2* vertices1 = poly1->m_vertices;

	int32 iv1 = edge1;
	int32 iv2 = edge1 + 1 < count1 ? edge1 + 1 : 0;

	b2Vec2 v11 = vertices1[iv1];
	b2Vec2 v12 = vertices1[iv2];

	b2Vec2 localTangent = v12 - v11;
	localTangent.Normalize();
	
	b2Vec2 localNormal = b2Cross(localTangent, 1.0f);
	b2Vec2 planePoint = 0.5f * (v11 + v12);

	b2Vec2 tangent = b2Mul(xf1.q, localTangent);
	b2Vec2 normal = b2Cross(tangent, 1.0f);
	
	v11 = b2Mul(xf1, v11);
	v12 = b2Mul(xf1, v12);

	// Face offset.
	float32 frontOffset = b2Dot(normal, v11);

	// Side offsets, extended by polytope skin thickness.
	float32 sideOffset1 = -b2Dot(tangent, v11) + totalRadius;
	float32 sideOffset2 = b2Dot(tangent, v12) + totalRadius;

	// Clip incident edge against extruded edge1 side edges.
	b2ClipVertex clipPoints1[2];
	b2ClipVertex clipPoints2[2];
	int np;

	// Clip to box side 1
	np = b2ClipSegmentToLine(clipPoints1, incidentEdge, -tangent, sideOffset1, iv1);

	if (np < 2)
		return;

	// Clip to negative box side 1
	np = b2ClipSegmentToLine(clipPoints2, clipPoints1,  tangent, sideOffset2, iv2);

	if (np < 2)
	{
		return;
	}

	// Now clipPoints2 contains the clipped points.
	manifold->localNormal = localNormal;
	manifold->localPoint = planePoint;

	int32 pointCount = 0;
	for (int32 i = 0; i < b2_maxManifoldPoints; ++i)
	{
		float32 separation = b2Dot(normal, clipPoints2[i].v) - frontOffset;

		if (separation <= totalRadius)
		{
			b2ManifoldPoint* cp = manifold->points + pointCount;
			cp->localPoint = b2MulT(xf2, clipPoints2[i].v);
			cp->id = clipPoints2[i].id;
			if (flip)
			{
				// Swap features
				b2ContactFeature cf = cp->id.cf;
				cp->id.cf.indexA = cf.indexB;
				cp->id.cf.indexB = cf.indexA;
				cp->id.cf.typeA = cf.typeB;
				cp->id.cf.typeB = cf.typeA;
			}
			++pointCount;
		}
	}

	manifold->pointCount = pointCount;
}