SpringFloat.js

//http://codeflow.org/entries/2010/aug/28/integration-by-example-euler-vs-verlet-vs-runge-kutta/

//https://jsfiddle.net/bkanber/pDngH/
//https://github.com/pqml/spring
//http://lolengine.net/blog/2015/05/03/damping-with-delta-time
//https://github.com/keijiro/SmoothingTest
//https://gafferongames.com/post/spring_physics/
//https://stackoverflow.com/questions/5100811/algorithm-to-control-acceleration-until-a-position-is-reached
//
//http://lolengine.net/blog/2015/05/03/damping-with-delta-time
/*
http://www.rorydriscoll.com/2016/03/07/frame-rate-independent-damping-using-lerp/
Damping from A to B.
Smoothing rate dictates the proportion of source remaining after one second
public static float Damp(float source, float target, float smoothing, float dt)
{
    return Mathf.Lerp(source, target, 1 - Mathf.Pow(smoothing, dt))
}

Both functions should produce about the same value.
public static float Damp(float a, float b, float lambda, float dt)
{
    return Mathf.Lerp(a, b, 1 - Mathf.Exp(-lambda * dt))
}

This damps v value to zero
public static float Damp(float source, float smoothing, float dt)
{
    return source * Mathf.Pow(smoothing, dt);
}
*/

class SpringFloat{
	constructor( stiff=0.1, damp=0.3 ){
		this.to			= 0;
		this.at			= 0;

		this.vel 		= 0;
		this.stiffness	= stiff;	//tension
		this.damping 	= damp;		
		this.rate 		= 10 / 16.67; // 60 FPS

		//this.damping 	= Math.pow( 1-damp, 10 / 16 );
	}

	set( to, at=null ){
		this.to = to;
		if( at != null ) this.at = at;
		return this;
	}

	dif(){ return this.at - this.to; }

	setAt( v ){ this.at = v; return this; }

	update( dt ){
		//let accel 	= ( -this.stiffness * ( this.at - this.to ) ) + (-this.damping * this.vel);
		//this.vel	+= accel * this.rate;
		//this.at		+= this.vel * this.rate;
	
		this.stiffness	= 0.1;	//tension
		this.damping 	= 0.35;
		
		//GOOD, very springy and fast
		//let accel = -this.stiffness * ( this.at - this.to );
		//this.vel = ( this.vel + accel * this.rate ) * Math.pow( 1 - this.damping, this.rate ); 
		//this.at = this.at + this.vel * this.rate;

		dt *= 40;

		//let accel = -this.stiffness * ( this.at - this.to );
		//this.vel = ( this.vel + accel * dt ) * Math.pow( 1 - this.damping, dt ); 
		//this.at = this.at + this.vel * dt;

		//http://lolengine.net/blog/2015/05/03/damping-with-delta-time
		let frameRate = 60;
		let accel = -this.stiffness * ( this.at - this.to );
		this.vel = ( this.vel + accel * dt ) * Math.pow( 1 - this.damping / frameRate, frameRate * dt ); 
		this.at = this.at + this.vel * dt;

		//GOOD, Slow accell and decell.
		//let accel = -this.stiffness * ( this.at - this.to ); // Accell = Force / Mass, but not using mass.
    	//this.vel += ( accel - this.damping * this.vel ) * dt;
    	//this.at = this.at + this.vel * dt;


		//if( Math.abs( this.at - this.to ) < 0.0001 ) console.log( "done" );
		//
		//test for Velocity and Distance to be sure we're done.
		//if( Math.abs( this.vel ) < 0.00005 ){
			//console.log( "done" );
			//this.at = this.to;
		//}

		//console.log( this.at, this.to );
		//console.log("accel", accel, "velocity", this.vel, "dif", ( this.at - this.to ), -this.stiffness * ( this.at - this.to )   );

		return this.at;
	}
}