import { len, toLength, translate, fromTo as fromToVec } from "flo-vector2d";


/**
 * For the given bernstein basis cubic bezier curve return a new cubic bezier
 * curve with its initial and terminal speeds modified.
 * 
 * * only the 2nd and 3rd control points are modified
 * * call the original curve `A` and the returned curve `B` then it will be 
 * true that `A[0] === B[0]` and `A[3] === B[3]`
 * 
 * @param ps an order 3 (cubic) bezier curve given as an ordered array of its
 * control point coordinates, e.g. `[[0,0], [1,1], [2,1], [2,0]]`
 */
 function setCubicSpeeds(
        ps: number[][],
        s0: number,
        s1: number): number[][] {

    const p0 = ps[0];
    const p1 = ps[1];
    const p2 = ps[2];
    const p3 = ps[3];

    const v = [p3[0] - p0[0], p3[1] - p0[1]];  // vector from 1st to last point
    const L = len(v);
    
    const v01 = fromToVec(p0,p1);
    const v32 = fromToVec(p3,p2);
    
    const v01_ = toLength(v01, s0/3*L);
    const p1_ = translate(p0,v01_);
    const v32_ = toLength(v32, s1/3*L);
    const p2_ = translate(p3,v32_);

    return [p0,p1_,p2_,p3];  // keep reference to first and last points
}


export { setCubicSpeeds }