using System;
using UnityEngine;

namespace Unity.XRTools.Rendering
{
    /// <summary>
    /// An XR-Focused drop-in replacement for the Trail Renderer
    /// This renderer draws fixed-width lines with simulated volume and glow.
    /// This has many of the advantages of the traditional Line Renderer, old-school system-level line rendering functions,
    /// and volumetric (a linked series of capsules or cubes) rendering
    /// </summary>
    [RequireComponent(typeof(MeshRenderer))]
    [RequireComponent(typeof(MeshFilter))]
    [ExecuteInEditMode]
    public class XRTrailRenderer : MeshChainRenderer
    {
        const float k_AbsoluteMinVertexDistance = 0.01f;

        // Stored Trail Data
        [SerializeField]
        [Tooltip("How many points to store for tracing.")]
        int m_MaxTrailPoints = 20;

        [SerializeField]
        [Tooltip("Whether to use the last point or the first point of the trail when more are needed and none are available.")]
        bool m_StealLastPointWhenEmpty = true;

        [SerializeField]
        [Tooltip("How long the tail should be (second) [ 0, infinity ].")]
        float m_Time = 5.0f;

        [SerializeField]
        [Tooltip("The minimum distance to spawn a new point on the trail [ 0, infinity ].")]
        float m_MinVertexDistance = 0.1f;

        [SerializeField]
        [Tooltip("Destroy GameObject when there is no trail?")]
        bool m_Autodestruct = false;

        [SerializeField]
        [Tooltip("With this enabled, the last point will smooth lerp between the last recorded anchor point and the one after it")]
        bool m_SmoothInterpolation = false;

        // Circular array support for trail point recording
        Vector3[] m_Points;
        float[] m_PointTimes;
        int m_PointIndexStart = 0;
        int m_PointIndexEnd = 0;

        // Cached Data
        Vector3 m_LastRecordedPoint = Vector3.zero;
        float m_LastPointTime;

        float m_EditorDeltaHelper; // This lets us have access to a time data while not in play mode


        /// <summary>
        /// How long does the trail take to fade out.
        /// </summary>
        public float time
        {
            get { return m_Time; }
            set { m_Time = Mathf.Max(value, 0); }
        }

        /// <summary>
        /// Set the minimum distance the trail can travel before a new vertex is added to it.
        /// </summary>
        public float minVertexDistance
        {
            get { return m_MinVertexDistance; }
            set { m_MinVertexDistance = Mathf.Max(value, k_AbsoluteMinVertexDistance); }
        }

        /// <summary>
        /// Get the number of line segments in the trail
        /// </summary>
        public int positionCount { get; private set; }

        /// <summary>
        /// Destroy GameObject when there is no trail?
        /// </summary>
        public bool autodestruct
        {
            get { return m_Autodestruct; }
            set { m_Autodestruct = value; }
        }

        /// <summary>
        /// Set if the last point will smooth lerp between the last recorded anchor point and the one after it
        /// </summary>
        public bool smoothInterpolation
        {
            get { return m_SmoothInterpolation; }
            set { m_SmoothInterpolation = value; }
        }

        /// <summary>
        /// Updates the built-in mesh data for each control point of the trail
        /// </summary>
        protected override void LateUpdate()
        {
            // We do  the actual internal mesh updating as late as possible so nothing ends up a frame behind
            var deltaTime = Time.deltaTime;

            // We give the editor a little help with handling delta time in edit mode
            if (Application.isPlaying == false)
            {
                deltaTime = Time.realtimeSinceStartup - m_EditorDeltaHelper;
                m_EditorDeltaHelper = Time.realtimeSinceStartup;
            }

            // Get the current position of the renderer
            var currentPoint = transform.position;
            var pointDistance = (currentPoint - m_LastRecordedPoint).sqrMagnitude;
            var shrunkThisFrame = false;

            // Is it more than minVertexDistance from the last position?
            if (pointDistance > (m_MinVertexDistance * m_MinVertexDistance))
            {
                // In the situation we have no points, we need to record the start point as well
                if (m_PointIndexStart == m_PointIndexEnd)
                {
                    m_Points[m_PointIndexStart] = m_LastRecordedPoint;
                    m_PointTimes[m_PointIndexStart] = m_Time;
                }

                // Make space for a new point
                var newEndIndex = (m_PointIndexEnd + 1) % m_MaxTrailPoints;

                // In the situation that we are rendering all available vertices
                // We can either keep using the current point, or take the last point, depending on the user's preference
                if (newEndIndex != m_PointIndexStart)
                {
                    m_PointIndexEnd = newEndIndex;
                    m_PointTimes[m_PointIndexEnd] = 0;
                    positionCount++;
                }
                else
                {
                    if (m_StealLastPointWhenEmpty)
                    {
                        m_XRMeshData.SetElementSize(m_PointIndexStart * 2, 0);
                        m_XRMeshData.SetElementSize((m_PointIndexStart * 2) + 1, 0);
                        m_PointIndexStart = (m_PointIndexStart + 1) % m_MaxTrailPoints;
                        m_PointIndexEnd = newEndIndex;
                        m_PointTimes[m_PointIndexEnd] = 0;
                        m_LastPointTime = m_PointTimes[m_PointIndexStart];
                    }
                }

                m_Points[m_PointIndexEnd] = currentPoint;

                // Update the last recorded point
                m_LastRecordedPoint = currentPoint;
            }

            // Do time processing
            // The end point counts up to a maximum of 'time'
            m_PointTimes[m_PointIndexEnd] = Mathf.Min(m_PointTimes[m_PointIndexEnd] + deltaTime, m_Time);

            if (m_PointIndexStart != m_PointIndexEnd)
            {
                // Run down the counter on the start point
                m_PointTimes[m_PointIndexStart] -= deltaTime;

                // If we've hit 0, this point is done for
                if (m_PointTimes[m_PointIndexStart] <= 0.0f)
                {
                    m_XRMeshData.SetElementSize(m_PointIndexStart * 2, 0);
                    m_XRMeshData.SetElementSize((m_PointIndexStart * 2) + 1, 0);
                    m_PointIndexStart = (m_PointIndexStart + 1) % m_MaxTrailPoints;
                    m_LastPointTime = m_PointTimes[m_PointIndexStart];
                    positionCount--;
                    shrunkThisFrame = true;
                }
            }

            if (m_PointIndexStart != m_PointIndexEnd)
            {
                m_MeshNeedsRefreshing = true;
                m_MeshRenderer.enabled = true;
            }
            else
            {
                m_MeshNeedsRefreshing = false;
                m_MeshRenderer.enabled = false;
                if (m_Autodestruct && Application.isPlaying && shrunkThisFrame)
                {
                    Destroy(gameObject);
                }
            }

            if (m_MeshNeedsRefreshing)
            {
                m_MeshRenderer.enabled = true;

                // Update first and last points position-wise
                var nextIndex = (m_PointIndexStart + 1) % m_MaxTrailPoints;
                if (m_SmoothInterpolation)
                {
                    var toNextPoint = 1.0f - (m_PointTimes[m_PointIndexStart] / m_LastPointTime);
                    var lerpPoint = Vector3.Lerp(m_Points[m_PointIndexStart], m_Points[nextIndex], toNextPoint);
                    m_XRMeshData.SetElementPosition((m_PointIndexStart * 2), ref lerpPoint);
                    m_XRMeshData.SetElementPipe((m_PointIndexStart * 2) + 1, ref lerpPoint, ref m_Points[nextIndex]);
                }
                else
                {
                    m_XRMeshData.SetElementPosition((m_PointIndexStart * 2), ref m_Points[m_PointIndexStart]);
                    m_XRMeshData.SetElementPipe((m_PointIndexStart * 2) + 1, ref m_Points[m_PointIndexStart], ref m_Points[nextIndex]);
                }

                var prevIndex = m_PointIndexEnd - 1;
                if (prevIndex < 0)
                {
                    prevIndex = m_MaxTrailPoints - 1;
                }

                m_XRMeshData.SetElementPipe((prevIndex * 2) + 1, ref m_Points[prevIndex], ref m_Points[m_PointIndexEnd]);
                m_XRMeshData.SetElementPosition((m_PointIndexEnd * 2), ref m_Points[m_PointIndexEnd]);


                // Go through all points and update size and color
                var pointUpdateCounter = m_PointIndexStart;
                var pointCount = 0;
                m_StepSize = (positionCount > 0) ? (1.0f / positionCount) : 1.0f;

                var percent = 0.0f;
                var lastWidth = m_WidthCurve.Evaluate(percent) * m_Width;
                var lastColor = m_Color.Evaluate(percent);
                percent += m_StepSize;

                while (pointUpdateCounter != m_PointIndexEnd)
                {
                    var nextWidth = m_WidthCurve.Evaluate(percent) * m_Width;
                    m_XRMeshData.SetElementSize(pointUpdateCounter * 2, lastWidth);
                    m_XRMeshData.SetElementSize((pointUpdateCounter * 2) + 1, lastWidth, nextWidth);
                    lastWidth = nextWidth;

                    var nextColor = m_Color.Evaluate(percent);
                    m_XRMeshData.SetElementColor(pointUpdateCounter * 2, ref lastColor);
                    m_XRMeshData.SetElementColor((pointUpdateCounter * 2) + 1, ref lastColor, ref nextColor);
                    lastColor = nextColor;

                    pointUpdateCounter = (pointUpdateCounter + 1) % m_MaxTrailPoints;
                    pointCount++;
                    percent += m_StepSize;
                }

                lastWidth = m_WidthCurve.Evaluate(1) * m_Width;
                m_XRMeshData.SetElementSize((m_PointIndexEnd * 2), lastWidth);
                lastColor = m_Color.Evaluate(1);
                m_XRMeshData.SetElementColor((m_PointIndexEnd * 2), ref lastColor);
                m_XRMeshData.SetMeshDataDirty(XRMeshChain.MeshRefreshFlag.All);
                m_XRMeshData.RefreshMesh();
            }
        }

        /// <summary>
        /// Editor helper function to ensure changes are reflected in edit-mode
        /// </summary>
        public void EditorCheckForUpdate()
        {
            // If we did not initialize, refresh all the properties instead
            Initialize();
        }

        /// <summary>
        /// Removes all points from the TrailRenderer. Useful for restarting a trail from a new position.
        /// </summary>
        public void Clear()
        {
            var zeroVec = Vector3.zero;
            var zeroColor = Color.clear;

            var elementCounter = 0;
            var pointCounter = 0;
            while (pointCounter < m_Points.Length)
            {
                // Start point
                m_XRMeshData.SetElementSize(elementCounter, 0);
                m_XRMeshData.SetElementPosition(elementCounter, ref zeroVec);
                m_XRMeshData.SetElementColor(elementCounter, ref zeroColor);
                elementCounter++;

                // Pipe to the next point
                m_XRMeshData.SetElementSize(elementCounter, 0);
                m_XRMeshData.SetElementPipe(elementCounter, ref zeroVec, ref zeroVec);
                m_XRMeshData.SetElementColor(elementCounter, ref zeroColor);

                // Go onto the next point while retaining previous values we might need to lerp between
                elementCounter++;
                pointCounter++;
            }

            m_PointIndexStart = 0;
            m_PointIndexEnd = 0;
            positionCount = 0;
            m_LastRecordedPoint = transform.position;
        }

        /// <summary>
        /// Creates or updates the underlying mesh data
        /// </summary>
        protected override void Initialize(bool setMesh = true)
        {
            base.Initialize(setMesh);

            m_MaxTrailPoints = Mathf.Max(m_MaxTrailPoints, 3);

            // If we already have the right amount of points and mesh, then we can get away with just clearing the curve out
            if (m_Points != null && m_MaxTrailPoints == m_Points.Length && m_XRMeshData != null)
            {
                Clear();
                return;
            }

            m_Points = new Vector3[m_MaxTrailPoints];
            m_PointTimes = new float[m_MaxTrailPoints];

            // For a trail renderer we assume one big chain
            // We need a control point for each billboard and a control point for each pipe connecting them together
            // We make this a circular trail so the update logic is easier.  This gives us (position * 2)
            var neededPoints = Mathf.Max((m_MaxTrailPoints * 2), 0);

            if (m_XRMeshData == null)
            {
                m_XRMeshData = new XRMeshChain();
            }

            if (m_XRMeshData.reservedElements != neededPoints)
            {
                m_XRMeshData.worldSpaceData = true;
                m_XRMeshData.centerAtRoot = true;
                m_XRMeshData.GenerateMesh(gameObject, true, neededPoints, setMesh);

                if (neededPoints == 0)
                {
                    return;
                }

                // Dirty all the VRMeshChain flags so everything gets refreshed
                m_MeshRenderer.enabled = false;
                m_XRMeshData.SetMeshDataDirty(XRMeshChain.MeshRefreshFlag.All);
                m_MeshNeedsRefreshing = true;
            }

            Clear();
        }

        /// <summary>
        /// Tests if the mesh data needs to be created or rebuilt
        /// </summary>
        /// <returns>true if the mesh data needs recreation, false if it is already set up properly</returns>
        protected override bool NeedsReinitialize()
        {
            // No mesh data means we definitely need to reinitialize
            if (m_XRMeshData == null)
            {
                return true;
            }

            // Mismatched point data means we definitely need to reinitialize
            if (m_Points == null || m_MaxTrailPoints != m_Points.Length)
            {
                return true;
            }

            m_MaxTrailPoints = Mathf.Max(m_MaxTrailPoints, 3);
            var neededPoints = Mathf.Max((m_MaxTrailPoints * 2), 0);

            return (m_XRMeshData.reservedElements != neededPoints);
        }

        /// <summary>
        /// Enables the internal mesh representing the line
        /// </summary>
        protected override void OnEnable()
        {
            m_MeshRenderer.enabled = (m_PointIndexStart != m_PointIndexEnd);
        }
    }
}