OtterPlayGround/Otter/Components/Lerper.cs

using System;

namespace Otter {
    /// <summary>
    /// Component that will slowly interpolate a value toward a target using a speed and acceleration.
    /// This component can move the value and does not know about time at all.
    /// </summary>
    public class Lerper : Component {

        #region Private Fields

        float initValue, targetSpeed, speed, distance;

        bool endPhase = false;

        #endregion

        #region Public Fields

        /// <summary>
        /// The acceleration for moving toward the target.
        /// </summary>
        public float Acceleration;

        /// <summary>
        /// The maximum speed for moving toward the target.
        /// </summary>
        public float MaxSpeed;

        #endregion

        #region Public Properties

        /// <summary>
        /// If the Lerper has completed its movement.
        /// </summary>
        public bool Completed { get; private set; }

        /// <summary>
        /// The current target to move toward.
        /// </summary>
        public float Target { get; private set; }

        /// <summary>
        /// The current value.
        /// </summary>
        public float Value { get; private set; }

        #endregion

        #region Constructors

        /// <summary>
        /// Create a new Lerper.
        /// </summary>
        /// <param name="value">The initial value.</param>
        /// <param name="accel">The acceleration for moving toward the target.</param>
        /// <param name="maxSpeed">The max speed for moving toward the target.</param>
        public Lerper(float value, float accel, float maxSpeed) {
            initValue = value;
            Value = value;
            Acceleration = accel;
            MaxSpeed = maxSpeed;
            Completed = false;
        }

        #endregion

        #region Public Methods

        /// <summary>
        /// Set the target.
        /// </summary>
        /// <param name="value"></param>
        public void SetTarget(float value) {
            if (Target == value) return;

            Target = value;

            initValue = Value;
            endPhase = false;
            Completed = false;
            distance = Math.Abs(initValue - Target);
        }

        /// <summary>
        /// Force the current value.
        /// </summary>
        /// <param name="value"></param>
        public void SetValue(float value) {
            Value = value;
        }

        /// <summary>
        /// Update the Lerper.
        /// </summary>
        public override void Update() {
            base.Update();

            if (Completed) {
                Value = Target;
                return;
            }

            var currentDistance = Math.Abs(Target - Value);
            var stoppingDistance = (MaxSpeed * MaxSpeed) / (2 * Acceleration);
            var earlyStopDistance = (speed * speed) / (2 * Acceleration);

            if (!endPhase) {
                targetSpeed = MaxSpeed * Math.Sign(Target - Value);

                if (currentDistance <= stoppingDistance) {
                    if (speed == MaxSpeed) {
                        targetSpeed = 0;
                        endPhase = true;
                    }
                    if (speed < MaxSpeed) {
                        if (currentDistance <= earlyStopDistance) {
                            targetSpeed = 0;
                            endPhase = true;
                        }
                    }
                }
            }

            speed = Util.Approach(speed, targetSpeed, Acceleration);

            if (endPhase) {
                if (Target > Value) {
                    if (speed < Acceleration) speed = Acceleration;
                    Value = Util.Approach(Value, Target, speed);
                }
                if (Target < Value) {
                    if (speed > -Acceleration) speed = -Acceleration;
                    Value = Util.Approach(Value, Target, -speed);
                }
            }
            else {
                Value += speed;
            }

            if (currentDistance <= Acceleration * 5) {
                speed = 0;
                Completed = true;
            }
        }

        #endregion

        #region Operators

        public static implicit operator float(Lerper lerper) {
            return lerper.Value;
        }
        public static implicit operator int(Lerper lerper) {
            return (int)lerper.Value;
        }

        #endregion
        
    }
}