using UnityEngine;

public class Polymer2 : MonoBehaviour
{

    public float length=5f;
    public int n=200;
    float l;
    public Vector3 r0 = new Vector3(0f, 0f, 0f);
    public Vector3[] chain;
    public float[] mass;
    public float[] k;
    public Vector3[] v0;
    public Vector3[] v;
    public Vector3[] f0;
    public Vector3[] a;

    Vector3[] prev;
    public float vmax = 0.3f;
    public float springk = 80f;

    public float dt;

    public GameObject[] atoms;

    float time = 0;



    // Start is called once before the first execution of Update after the MonoBehaviour is created
    void Start()
    {

        time = 0;


        l = length/n;

        chain = new Vector3[n+2];

        Vector3 prev0 = Vector3.zero; 

        for (int i = 1; i < n+1; i++) { chain[i] = prev0 + new Vector3(l, 0f, 0f); prev0 = chain[i]; }

        chain[0] = chain[1];
        chain[n + 1] = chain[n];

        mass = new float[n + 2];
        for (int i = 0; i < n + 2; i++) { mass[i] = 1f;  }

        //mass[n] = 10000f;
        // mass[n+1] = 10000f;

        k = new float[n + 2];
        for (int i = 0; i < n + 2; i++) { k[i] = springk; }

        v0 = new Vector3[n + 2];
        v = new Vector3[n + 2];
        f0 = new Vector3[n + 2];
        a = new Vector3[n + 2];

        prev = new Vector3[n + 2];


        atoms = new GameObject[n + 0];

        dt = Time.deltaTime;
      //  dt = 1f / 20f;

        for (int i = 1; i < n + 1; i++) {

            atoms[i-1] = GameObject.CreatePrimitive(PrimitiveType.Sphere);
            atoms[i - 1].GetComponent<SphereCollider>().enabled = false;

            if (i%2==0)
            { atoms[i - 1].GetComponent<Renderer>().material.SetColor("_Color", Color.red); }
            else { atoms[i - 1].GetComponent<Renderer>().material.SetColor("_Color", Color.green); }

            atoms[i - 1].transform.localScale = new Vector3(l / 0.5f, l / 0.5f, l /0.5f);

            atoms[i - 1].transform.position = chain[i];

        }


        

    }

    // Update is called once per frame
    void FixedUpdate()
    {
        Vector3[] dr;

        dr = differences(chain);

        for (int i = 1; i < n + 1; i++) { a[i] = ( (k[i]*fdr(-dr[i-1])) + (k[i] * fdr(dr[i])) + f(time)[i])/mass[i]; }

        a[0] = a[1];
        a[n + 1] = a[n];

        for (int i = 0; i < n + 2; i++) { v[i] = a[i]*dt + prev[i]; }


       // for (int i = 1; i < n + 1; i++) { if (v[i].magnitude > vmax) { v[i].Normalize(); v[i] *= vmax; } }


        for (int i = 0; i < n + 2; i++) { if (v[i].magnitude > vmax) { v[i].Normalize(); v[i] *= vmax; } }

        for (int i = 0; i < n + 2; i++) { prev[i] = v[i]; }

        for (int i = 0; i < n + 2; i++) { chain[i] += v[i] * dt; }

        for (int i = 1; i < n + 1; i++) { atoms[i - 1].transform.position = chain[i]; }


    }


    Vector3[ ] f(float t)
    {
        Vector3[] impulse = new Vector3[n + 2];

       // for (int i = 0; i < n + 2; i++) { impulse[i] = new Vector3 (Random.Range(-1f,1f), Random.Range(-1f, 1f), Random.Range(-1f, 1f)); }

        for (int i = 0; i < n + 2; i++){ impulse[i] = Random.onUnitSphere; }

        return impulse;
    }

    Vector3 fdr(Vector3 direction)
    {
        Vector3 dr = Vector3.zero;

        if (direction.magnitude < l) { dr = -1f * direction; }
        else if (direction.magnitude == l) { dr = Vector3.zero; }
        else if (direction.magnitude > l) { dr = direction; }

        return dr;
    }

    Vector3[] differences(Vector3[] list)
    {
        Vector3[] vectors = new Vector3[n + 1];

        for (int i = 1; i < n + 2; i++) { vectors[i-1] = list[i]-list[i-1]; }

        return vectors;
    }
}