constraint_graph.h

// SPDX-FileCopyrightText: 2023 Erin Catto
// SPDX-License-Identifier: MIT

#pragma once

#include "bitset.h"
#include "contact.h"
#include "joint.h"
#include "box2d/constants.h"

#include "box2d/types.h"

typedef struct b2Body b2Body;
typedef struct b2Contact b2Contact;
typedef struct b2StepContext b2StepContext;
typedef struct b2World b2World;

// This holds constraints that cannot fit the graph color limit. This happens when a single dynamic body
// is touching many other bodies.
#define B2_OVERFLOW_INDEX ( B2_GRAPH_COLOR_COUNT - 1 )

// This keeps constraints involving two dynamic bodies at a lower solver priority than constraints
// involving a dynamic and static bodies. This reduces tunneling due to push through.
#define B2_DYNAMIC_COLOR_COUNT ( B2_GRAPH_COLOR_COUNT - 4 )

typedef struct b2GraphColor
{
	// This bitset is indexed by bodyId so this is over-sized to encompass static bodies
	// however I never traverse these bits or use the bit count for anything
	// This bitset is unused on the overflow color.
	//
	// Dirk suggested having a uint64_t per body that tracks the graph color membership
	// but I think this would make debugging harder and be less flexible. With the bitset
	// I can trivially increase the number of graph colors beyond 64. See usage of b2CountSetBits
	// for validation.
	b2BitSet bodySet;

	// cache friendly arrays
	b2Array( b2ContactSim ) contactSims;
	b2Array( b2JointSim ) jointSims;

	// transient
	union
	{
		void* wideConstraints;
		struct b2ContactConstraint* overflowConstraints;
	};

	int wideConstraintCount;

} b2GraphColor;

typedef struct b2ConstraintGraph
{
	// including overflow at the end
	b2GraphColor colors[B2_GRAPH_COLOR_COUNT];
} b2ConstraintGraph;

void b2CreateGraph( b2ConstraintGraph* graph, const b2Capacity* capacity );
void b2DestroyGraph( b2ConstraintGraph* graph );

void b2AddContactToGraph( b2World* world, b2ContactSim* contactSim, b2Contact* contact );
void b2RemoveContactFromGraph( b2World* world, int bodyIdA, int bodyIdB, int colorIndex, int localIndex );

b2JointSim* b2CreateJointInGraph( b2World* world, b2Joint* joint );
void b2AddJointToGraph( b2World* world, b2JointSim* jointSim, b2Joint* joint );
void b2RemoveJointFromGraph( b2World* world, int bodyIdA, int bodyIdB, int colorIndex, int localIndex );