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456 lines
14 KiB
456 lines
14 KiB
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class BarnesHutSolver {
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constructor(body, physicsBody, options) {
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this.body = body;
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this.physicsBody = physicsBody;
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this.barnesHutTree;
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this.setOptions(options);
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}
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setOptions(options) {
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this.options = options;
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this.thetaInversed = 1 / this.options.theta;
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this.overlapAvoidanceFactor = 1 - Math.max(0, Math.min(1,this.options.avoidOverlap)); // if 1 then min distance = 0.5, if 0.5 then min distance = 0.5 + 0.5*node.shape.radius
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}
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/**
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* This function calculates the forces the nodes apply on eachother based on a gravitational model.
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* The Barnes Hut method is used to speed up this N-body simulation.
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*
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* @private
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*/
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solve() {
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if (this.options.gravitationalConstant !== 0 && this.physicsBody.physicsNodeIndices.length > 0) {
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let node;
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let nodes = this.body.nodes;
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let nodeIndices = this.physicsBody.physicsNodeIndices;
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let nodeCount = nodeIndices.length;
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// create the tree
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let barnesHutTree = this._formBarnesHutTree(nodes, nodeIndices);
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// for debugging
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this.barnesHutTree = barnesHutTree;
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// place the nodes one by one recursively
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for (let i = 0; i < nodeCount; i++) {
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node = nodes[nodeIndices[i]];
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if (node.options.mass > 0) {
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// starting with root is irrelevant, it never passes the BarnesHutSolver condition
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this._getForceContribution(barnesHutTree.root.children.NW, node);
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this._getForceContribution(barnesHutTree.root.children.NE, node);
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this._getForceContribution(barnesHutTree.root.children.SW, node);
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this._getForceContribution(barnesHutTree.root.children.SE, node);
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}
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}
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}
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}
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/**
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* This function traverses the barnesHutTree. It checks when it can approximate distant nodes with their center of mass.
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* If a region contains a single node, we check if it is not itself, then we apply the force.
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*
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* @param parentBranch
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* @param node
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* @private
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*/
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_getForceContribution(parentBranch, node) {
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// we get no force contribution from an empty region
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if (parentBranch.childrenCount > 0) {
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let dx, dy, distance;
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// get the distance from the center of mass to the node.
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dx = parentBranch.centerOfMass.x - node.x;
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dy = parentBranch.centerOfMass.y - node.y;
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distance = Math.sqrt(dx * dx + dy * dy);
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// BarnesHutSolver condition
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// original condition : s/d < theta = passed === d/s > 1/theta = passed
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// calcSize = 1/s --> d * 1/s > 1/theta = passed
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if (distance * parentBranch.calcSize > this.thetaInversed) {
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this._calculateForces(distance, dx, dy, node, parentBranch);
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}
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else {
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// Did not pass the condition, go into children if available
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if (parentBranch.childrenCount === 4) {
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this._getForceContribution(parentBranch.children.NW, node);
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this._getForceContribution(parentBranch.children.NE, node);
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this._getForceContribution(parentBranch.children.SW, node);
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this._getForceContribution(parentBranch.children.SE, node);
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}
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else { // parentBranch must have only one node, if it was empty we wouldnt be here
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if (parentBranch.children.data.id != node.id) { // if it is not self
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this._calculateForces(distance, dx, dy, node, parentBranch);
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}
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}
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}
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}
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}
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/**
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* Calculate the forces based on the distance.
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*
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* @param distance
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* @param dx
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* @param dy
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* @param node
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* @param parentBranch
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* @private
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*/
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_calculateForces(distance, dx, dy, node, parentBranch) {
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if (distance === 0) {
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distance = 0.1 * Math.random();
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dx = distance;
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}
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if (this.overlapAvoidanceFactor < 1) {
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distance = Math.max(0.1 + (this.overlapAvoidanceFactor * node.shape.radius), distance - node.shape.radius);
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}
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// the dividing by the distance cubed instead of squared allows us to get the fx and fy components without sines and cosines
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// it is shorthand for gravityforce with distance squared and fx = dx/distance * gravityForce
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let gravityForce = this.options.gravitationalConstant * parentBranch.mass * node.options.mass / Math.pow(distance,3);
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let fx = dx * gravityForce;
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let fy = dy * gravityForce;
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this.physicsBody.forces[node.id].x += fx;
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this.physicsBody.forces[node.id].y += fy;
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}
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/**
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* This function constructs the barnesHut tree recursively. It creates the root, splits it and starts placing the nodes.
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*
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* @param nodes
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* @param nodeIndices
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* @private
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*/
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_formBarnesHutTree(nodes, nodeIndices) {
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let node;
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let nodeCount = nodeIndices.length;
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let minX = nodes[nodeIndices[0]].x;
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let minY = nodes[nodeIndices[0]].y;
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let maxX = nodes[nodeIndices[0]].x;
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let maxY = nodes[nodeIndices[0]].y;
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// get the range of the nodes
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for (let i = 1; i < nodeCount; i++) {
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let x = nodes[nodeIndices[i]].x;
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let y = nodes[nodeIndices[i]].y;
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if (nodes[nodeIndices[i]].options.mass > 0) {
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if (x < minX) {
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minX = x;
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}
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if (x > maxX) {
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maxX = x;
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}
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if (y < minY) {
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minY = y;
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}
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if (y > maxY) {
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maxY = y;
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}
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}
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}
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// make the range a square
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let sizeDiff = Math.abs(maxX - minX) - Math.abs(maxY - minY); // difference between X and Y
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if (sizeDiff > 0) {
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minY -= 0.5 * sizeDiff;
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maxY += 0.5 * sizeDiff;
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} // xSize > ySize
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else {
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minX += 0.5 * sizeDiff;
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maxX -= 0.5 * sizeDiff;
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} // xSize < ySize
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let minimumTreeSize = 1e-5;
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let rootSize = Math.max(minimumTreeSize, Math.abs(maxX - minX));
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let halfRootSize = 0.5 * rootSize;
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let centerX = 0.5 * (minX + maxX), centerY = 0.5 * (minY + maxY);
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// construct the barnesHutTree
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let barnesHutTree = {
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root: {
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centerOfMass: {x: 0, y: 0},
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mass: 0,
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range: {
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minX: centerX - halfRootSize, maxX: centerX + halfRootSize,
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minY: centerY - halfRootSize, maxY: centerY + halfRootSize
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},
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size: rootSize,
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calcSize: 1 / rootSize,
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children: {data: null},
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maxWidth: 0,
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level: 0,
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childrenCount: 4
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}
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};
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this._splitBranch(barnesHutTree.root);
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// place the nodes one by one recursively
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for (let i = 0; i < nodeCount; i++) {
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node = nodes[nodeIndices[i]];
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if (node.options.mass > 0) {
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this._placeInTree(barnesHutTree.root, node);
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}
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}
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// make global
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return barnesHutTree
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}
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/**
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* this updates the mass of a branch. this is increased by adding a node.
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*
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* @param parentBranch
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* @param node
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* @private
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*/
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_updateBranchMass(parentBranch, node) {
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let totalMass = parentBranch.mass + node.options.mass;
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let totalMassInv = 1 / totalMass;
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parentBranch.centerOfMass.x = parentBranch.centerOfMass.x * parentBranch.mass + node.x * node.options.mass;
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parentBranch.centerOfMass.x *= totalMassInv;
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parentBranch.centerOfMass.y = parentBranch.centerOfMass.y * parentBranch.mass + node.y * node.options.mass;
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parentBranch.centerOfMass.y *= totalMassInv;
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parentBranch.mass = totalMass;
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let biggestSize = Math.max(Math.max(node.height, node.radius), node.width);
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parentBranch.maxWidth = (parentBranch.maxWidth < biggestSize) ? biggestSize : parentBranch.maxWidth;
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}
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/**
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* determine in which branch the node will be placed.
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*
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* @param parentBranch
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* @param node
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* @param skipMassUpdate
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* @private
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*/
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_placeInTree(parentBranch, node, skipMassUpdate) {
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if (skipMassUpdate != true || skipMassUpdate === undefined) {
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// update the mass of the branch.
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this._updateBranchMass(parentBranch, node);
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}
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if (parentBranch.children.NW.range.maxX > node.x) { // in NW or SW
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if (parentBranch.children.NW.range.maxY > node.y) { // in NW
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this._placeInRegion(parentBranch, node, "NW");
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}
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else { // in SW
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this._placeInRegion(parentBranch, node, "SW");
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}
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}
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else { // in NE or SE
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if (parentBranch.children.NW.range.maxY > node.y) { // in NE
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this._placeInRegion(parentBranch, node, "NE");
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}
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else { // in SE
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this._placeInRegion(parentBranch, node, "SE");
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}
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}
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}
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/**
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* actually place the node in a region (or branch)
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*
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* @param parentBranch
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* @param node
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* @param region
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* @private
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*/
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_placeInRegion(parentBranch, node, region) {
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switch (parentBranch.children[region].childrenCount) {
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case 0: // place node here
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parentBranch.children[region].children.data = node;
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parentBranch.children[region].childrenCount = 1;
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this._updateBranchMass(parentBranch.children[region], node);
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break;
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case 1: // convert into children
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// if there are two nodes exactly overlapping (on init, on opening of cluster etc.)
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// we move one node a pixel and we do not put it in the tree.
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if (parentBranch.children[region].children.data.x === node.x &&
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parentBranch.children[region].children.data.y === node.y) {
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node.x += Math.random();
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node.y += Math.random();
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}
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else {
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this._splitBranch(parentBranch.children[region]);
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this._placeInTree(parentBranch.children[region], node);
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}
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break;
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case 4: // place in branch
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this._placeInTree(parentBranch.children[region], node);
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break;
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}
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}
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/**
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* this function splits a branch into 4 sub branches. If the branch contained a node, we place it in the subbranch
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* after the split is complete.
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*
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* @param parentBranch
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* @private
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*/
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_splitBranch(parentBranch) {
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// if the branch is shaded with a node, replace the node in the new subset.
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let containedNode = null;
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if (parentBranch.childrenCount === 1) {
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containedNode = parentBranch.children.data;
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parentBranch.mass = 0;
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parentBranch.centerOfMass.x = 0;
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parentBranch.centerOfMass.y = 0;
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}
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parentBranch.childrenCount = 4;
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parentBranch.children.data = null;
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this._insertRegion(parentBranch, "NW");
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this._insertRegion(parentBranch, "NE");
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this._insertRegion(parentBranch, "SW");
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this._insertRegion(parentBranch, "SE");
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if (containedNode != null) {
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this._placeInTree(parentBranch, containedNode);
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}
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}
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/**
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* This function subdivides the region into four new segments.
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* Specifically, this inserts a single new segment.
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* It fills the children section of the parentBranch
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*
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* @param parentBranch
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* @param region
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* @param parentRange
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* @private
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*/
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_insertRegion(parentBranch, region) {
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let minX, maxX, minY, maxY;
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let childSize = 0.5 * parentBranch.size;
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switch (region) {
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case "NW":
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minX = parentBranch.range.minX;
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maxX = parentBranch.range.minX + childSize;
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minY = parentBranch.range.minY;
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maxY = parentBranch.range.minY + childSize;
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break;
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case "NE":
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minX = parentBranch.range.minX + childSize;
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maxX = parentBranch.range.maxX;
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minY = parentBranch.range.minY;
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maxY = parentBranch.range.minY + childSize;
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break;
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case "SW":
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minX = parentBranch.range.minX;
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maxX = parentBranch.range.minX + childSize;
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minY = parentBranch.range.minY + childSize;
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maxY = parentBranch.range.maxY;
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break;
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case "SE":
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minX = parentBranch.range.minX + childSize;
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maxX = parentBranch.range.maxX;
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minY = parentBranch.range.minY + childSize;
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maxY = parentBranch.range.maxY;
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break;
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}
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parentBranch.children[region] = {
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centerOfMass: {x: 0, y: 0},
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mass: 0,
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range: {minX: minX, maxX: maxX, minY: minY, maxY: maxY},
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size: 0.5 * parentBranch.size,
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calcSize: 2 * parentBranch.calcSize,
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children: {data: null},
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maxWidth: 0,
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level: parentBranch.level + 1,
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childrenCount: 0
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};
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}
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//--------------------------- DEBUGGING BELOW ---------------------------//
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/**
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* This function is for debugging purposed, it draws the tree.
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*
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* @param ctx
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* @param color
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* @private
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*/
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_debug(ctx, color) {
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if (this.barnesHutTree !== undefined) {
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ctx.lineWidth = 1;
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this._drawBranch(this.barnesHutTree.root, ctx, color);
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}
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}
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/**
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* This function is for debugging purposes. It draws the branches recursively.
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*
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* @param branch
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* @param ctx
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* @param color
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* @private
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*/
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_drawBranch(branch, ctx, color) {
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if (color === undefined) {
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color = "#FF0000";
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}
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if (branch.childrenCount === 4) {
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this._drawBranch(branch.children.NW, ctx);
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this._drawBranch(branch.children.NE, ctx);
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this._drawBranch(branch.children.SE, ctx);
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this._drawBranch(branch.children.SW, ctx);
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}
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ctx.strokeStyle = color;
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ctx.beginPath();
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ctx.moveTo(branch.range.minX, branch.range.minY);
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ctx.lineTo(branch.range.maxX, branch.range.minY);
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ctx.stroke();
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ctx.beginPath();
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ctx.moveTo(branch.range.maxX, branch.range.minY);
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ctx.lineTo(branch.range.maxX, branch.range.maxY);
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ctx.stroke();
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ctx.beginPath();
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ctx.moveTo(branch.range.maxX, branch.range.maxY);
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ctx.lineTo(branch.range.minX, branch.range.maxY);
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ctx.stroke();
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ctx.beginPath();
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ctx.moveTo(branch.range.minX, branch.range.maxY);
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ctx.lineTo(branch.range.minX, branch.range.minY);
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ctx.stroke();
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/*
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if (branch.mass > 0) {
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ctx.circle(branch.centerOfMass.x, branch.centerOfMass.y, 3*branch.mass);
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ctx.stroke();
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}
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*/
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}
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}
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export default BarnesHutSolver;
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