flow like the river

VISUALIZACION/node_modules/ngraph.forcelayout/test/eulerIntegrator.js

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+var test = require('tap').test;
+var dimensions = 2;
+var Body = require('../lib/codeGenerators/generateCreateBody')(dimensions);
+var integrate = require('../lib/codeGenerators/generateIntegrator')(dimensions);
+
+test('Body preserves velocity without forces', function (t) {
+  var body = new Body();
+  var timeStep = 1;
+  body.mass = 1; body.velocity.x = 1;
+
+  integrate([body], timeStep);
+  t.equal(body.pos.x, 1, 'Should move by 1 pixel on first iteration');
+
+  timeStep = 2; // let's increase time step:
+  integrate([body], timeStep);
+  t.equal(body.pos.x, 3, 'Should move by 2 pixel on second iteration');
+  t.end();
+});
+
+test('Body gains velocity under force', function (t) {
+  var body = new Body();
+  var timeStep = 1;
+  body.mass = 1; body.force.x = 0.1;
+
+  // F = m * a;
+  // since mass = 1 =>  F = a = y';
+  integrate([body], timeStep);
+  t.equal(body.velocity.x, 0.1, 'Should increase velocity');
+
+  integrate([body], timeStep);
+  t.equal(body.velocity.x, 0.2, 'Should increase velocity');
+  // floating point math:
+  t.ok(0.29 < body.pos.x && body.pos.x < 0.31, 'Position should be at 0.3 now');
+
+  t.end();
+});
+
+test('No bodies yield 0 movement', function (t) {
+  var movement = integrate([], 2);
+  t.equal(movement, 0, 'Nothing has moved');
+  t.end();
+});
+
+test('Body does not move faster than 1px', function (t) {
+  var body = new Body();
+  var timeStep = 1;
+  body.mass = 1; body.force.x = 2;
+
+  integrate([body], timeStep);
+  t.ok(body.velocity.x <= 1, 'Velocity should be within speed limit');
+
+  integrate([body], timeStep);
+  t.ok(body.velocity.x <= 1, 'Velocity should be within speed limit');
+
+  t.end();
+});
+
+test('Can get total system movement', function (t) {
+  var body = new Body();
+  var timeStep = 1;
+  body.mass = 1; body.velocity.x = 0.2;
+
+  var movement = integrate([body], timeStep);
+  // to improve performance, integrator does not take square root, thus
+  // total movement is .2 * .2 = 0.04;
+  t.ok(0.04 <= movement && movement <= 0.041, 'System should travel by 0.2 pixels');
+  t.end();
+});