PlayerController.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerController : MonoBehaviour {
    public float moveSpeed;
    private float moveSpeedOrigin;

    public float speedMutiplier;
    public float speedIncreaseMilestone;
    private float speedIncreaseMilestoneOrigin;
    private float speedMilestoneCounts;
    private float speedMilestoneCountsOrigin;

    public float jumpForce;
    public float jumpTime;
    private float jumpTimeCounter;

    // Ground checking properties
    public bool isOnGround;

    public LayerMask groundLayer;

    public Transform groundCheckPoint;
    public float groundCheckRadius;
    private bool stopJumpping;
    private bool canDoubleJumping;

    private Rigidbody2D myRigidbody;
    private Animator myAnimator;

    public GameManager gameManager;

    public AudioSource jumpSound, deathSound;

    // Use this for initialization
    void Start() {
        myRigidbody = GetComponent<Rigidbody2D>();
        myAnimator = GetComponent<Animator>();

        speedMilestoneCounts = speedIncreaseMilestone;

        moveSpeedOrigin = moveSpeed;
        speedMilestoneCountsOrigin = speedMilestoneCounts;
        speedIncreaseMilestoneOrigin = speedIncreaseMilestone;
    }

    // Update is called once per frame
    void Update() {
        // Detect the ground
        isOnGround = Physics2D.OverlapCircle(groundCheckPoint.position, groundCheckRadius, groundLayer);

        // Speed up if pass the speed milestone
        if (transform.position.x > speedMilestoneCounts) {
            speedMilestoneCounts += speedIncreaseMilestone;
            moveSpeed = moveSpeed * speedMutiplier;

            speedIncreaseMilestone += speedIncreaseMilestone * speedMutiplier;
        }

        // Moving right
        myRigidbody.velocity = new Vector2(moveSpeed, myRigidbody.velocity.y);

        // Jump (Space and left key of mouse)
        if (isActiveAndEnabled &&
            (Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButtonDown(0))) {
            jumpSound.Play();
            if (isOnGround) {
                myRigidbody.velocity = new Vector2(myRigidbody.velocity.x, jumpForce);
                stopJumpping = false;
            } else if (canDoubleJumping) {
                canDoubleJumping = false;
                jumpTimeCounter = jumpTime;
                myRigidbody.velocity = new Vector2(myRigidbody.velocity.x, jumpForce);
                stopJumpping = false;
            }
        }

        if ((Input.GetKey(KeyCode.Space) || Input.GetMouseButton(0)) && !stopJumpping) {
            if (jumpTimeCounter > 0) {
                myRigidbody.velocity = new Vector2(myRigidbody.velocity.x, jumpForce);
                jumpTimeCounter -= Time.deltaTime;
            }
        }

        if (Input.GetKeyUp(KeyCode.Space) || Input.GetMouseButtonUp(0)) {
            jumpTimeCounter = 0; // Lock user keeping jumping
            stopJumpping = true;
        }

        if (isOnGround) {
            jumpTimeCounter = jumpTime;
            canDoubleJumping = true;
        }

        // Setup animators
        myAnimator.SetFloat("Speed", myRigidbody.velocity.x);
        myAnimator.SetBool("IsOnGround", isOnGround);
    }

    private void OnCollisionEnter2D(Collision2D other) {
        if (other.gameObject.CompareTag("KillBox")) {
            deathSound.Play();
            gameManager.RestartGame();
            moveSpeed = moveSpeedOrigin;
            speedMilestoneCounts = speedMilestoneCountsOrigin;
            speedIncreaseMilestone = speedIncreaseMilestoneOrigin;
        }
    }
}