using System;

namespace Quik.VertexGenerator
{
    /// <summary>
    /// Generates vertices from draw commands for GPU APIs like OpenGL.
    /// </summary>
    public class QuikVertexGenerator
    {
        // There is a very specific reason I am not using lists like a regular
        // person would use. It has to do with the fact that there is no way
        // to access the internal pointer of a System.Collections.Generic.List<>
        // in older versions of .NET. Avoiding a copy of an entire vertex buffer
        // would be very much appreciated by many devs. So please don't be
        // "smart" around this code.
        // - mixed.

        /// <summary>
        /// Controls the buffer granularity.
        /// </summary>
        private const int BufferGranularity = 4096;

        /// <summary>
        /// List of vertices.
        /// </summary>
        private QuikVertex[] _vertexBuffer = new QuikVertex[BufferGranularity];
        
        /// <summary>
        /// Pointer into the vertex buffer.
        /// </summary>
        private int _vertexBufferPointer = 0;

        private float _vertexBufferUsage = 0;

        /// <summary>
        /// List of element indices.
        /// </summary>
        private short[] _elementBuffer = new short[BufferGranularity];

        /// <summary>
        /// Pointer into the element buffer. 
        /// </summary>
        private int _elementBufferPointer = 0;

        private float _elementBufferUsage;
        private long _bufferUsageCounter;

        /// <summary>
        /// Get a reference to the vertex buffer.
        /// </summary>
        public QuikVertex[] VertexBuffer => _vertexBuffer;
        
        /// <summary>
        /// Number of vertices in the vertex buffer.
        /// </summary>
        public int VertexCount => _vertexBufferPointer;
        
        /// <summary>
        /// Get a reference to the element buffer.
        /// </summary>
        public short[] ElementBuffer => _elementBuffer;

        /// <summary>
        /// Number of elements in the element buffer.
        /// </summary>
        public int ElementCount => _elementBufferPointer;

        public float CurveGranularity { get; set; } = 0.5f;
        
        public QuikContext Context { get; }

        public QuikVertexGenerator(QuikContext context)
        {
            Context = context;
        }

        /// <summary>
        /// Expands the vertex buffer by the buffer granularity constant.
        /// </summary>
        private void ExpandVertexBuffer()
        {
            Array.Resize(ref _vertexBuffer, _vertexBuffer.Length + BufferGranularity);
        }
        
        /// <summary>
        /// Expands the element buffer by the buffer granularity constant.
        /// </summary>
        private void ExpandElementBuffer()
        {
            Array.Resize(ref _elementBuffer, _elementBuffer.Length + BufferGranularity);    
        }

        /// <summary>
        /// Add vertices to the list.
        /// </summary>
        /// <param name="vertices">The list of vertices to add.</param>
        private void AddVertex(params QuikVertex[] vertices)
        {
            int requiredCapacity = _vertexBufferPointer + vertices.Length; 
            while (requiredCapacity > _vertexBuffer.Length)
            {
                ExpandVertexBuffer();
            }
            
            Array.Copy(vertices, 0, _vertexBuffer, _vertexBufferPointer, vertices.Length);
            _vertexBufferPointer += vertices.Length;
        }

        /// <summary>
        /// Add element indices to the list.
        /// </summary>
        /// <param name="indices">The list of indices to add.</param>
        private void AddElement(params short[] indices)
        {
            int requiredCapacity = _elementBufferPointer + indices.Length; 
            while (requiredCapacity > _elementBuffer.Length)
            {
                ExpandElementBuffer();
            }
            
            Array.Copy(indices, 0, _elementBuffer, _elementBufferPointer, indices.Length);
            _elementBufferPointer += indices.Length;
        }

        private void MovingAverage(ref float average, long sampleCounter, int newSample)
        {
            // Thanks to stackoverflow for a neat formula.
            // https://stackoverflow.com/questions/12636613/how-to-calculate-moving-average-without-keeping-the-count-and-data-total

            const float order = 4;

            average = average + (newSample - average) / Math.Min(sampleCounter + 1, order);
        }
        
        private bool _renderStencilMask = false;

        private QuikRectangle _bounds = new QuikRectangle(
            float.PositiveInfinity, float.PositiveInfinity,
            float.NegativeInfinity, float.NegativeInfinity);
        
        /// <summary>
        /// Clear the drawing buffers.
        /// </summary>
        public void Clear()
        {
            int newVertexSize;
            int newElementSize;

            _bufferUsageCounter++;
            MovingAverage(ref _vertexBufferUsage, _bufferUsageCounter, _elementBufferPointer);
            MovingAverage(ref _elementBufferUsage, _bufferUsageCounter, _elementBufferPointer);

            newVertexSize = (int)(Math.Ceiling(_vertexBufferUsage / BufferGranularity) * BufferGranularity);
            newElementSize = (int)(Math.Ceiling(_elementBufferUsage / BufferGranularity) * BufferGranularity);
            
            Array.Resize(ref _vertexBuffer, newVertexSize);
            Array.Resize(ref _elementBuffer, newElementSize);

            _vertexBufferPointer = 0;
            _elementBufferPointer = 0;
        }

        public QuikDrawCall? ConsumeCommand(QuikCommand command)
        {
            QuikDrawCall call = new QuikDrawCall()
            {
                Target = _renderStencilMask ? QuikRenderTarget.Stencil : QuikRenderTarget.Color
            };

            switch (command.Type)
            {
            case QuikCommandType.StencilMaskClear:
                call.ClearStencil = true;
                break;
            case QuikCommandType.StencilMaskBegin:
                _renderStencilMask = true;
                call.Target = QuikRenderTarget.Stencil;
                break;
            case QuikCommandType.StencilMaskEnd:
                _renderStencilMask = false;
                call.Target = QuikRenderTarget.Color;
                break;
            case QuikCommandType.Line:
                RenderLine(ref call, command as QuikCommandLine);
                return call;
            case QuikCommandType.Lines:
                RenderLine(ref call, command as QuikCommandLines);
                return call;
            }
            return null;
        }

        /// <summary>
        /// Generates a basic line segment.
        /// </summary>
        /// <param name="baseVertex">The vertex base to modify when generating vertices.</param>
        /// <param name="line">The line segment.</param>
        /// <param name="width">The width of the line.</param>
        /// <param name="lineStartIndex">Start index of the generated line.</param>
        /// <param name="normal">The line normal.</param>
        private void GenerateLineSegment(
            QuikVertex baseVertex,
            QuikLine line,
            float width,
            out short lineStartIndex,
            out QuikVec2 normal)
        {
            QuikVec2 tangent = (line.End - line.Start).Normalize();
            normal = new QuikVec2() {X = -tangent.Y, Y = tangent.X};
            lineStartIndex = (short)_vertexBufferPointer;

            QuikVertex startA = baseVertex, startB = baseVertex;
            QuikVertex endA = baseVertex, endB = baseVertex;

            startA.Position = line.Start + width / 2 * normal;
            startB.Position = line.Start - width / 2 * normal;
            endA.Position = line.End + width / 2 * normal;
            endB.Position = line.End - width / 2 * normal;

            // Add the major line vertices.
            AddVertex(startA, startB, endA, endB);
            
            // Add the line indices.
            AddElement(
                (short) (lineStartIndex + 1),
                (short) (lineStartIndex + 2),
                (short) (lineStartIndex + 0),
                (short) (lineStartIndex + 1),
                (short) (lineStartIndex + 3),
                (short) (lineStartIndex + 2)
            );
        }
        
        /// <summary>
        /// Gets the rounding resolution for a line segment.
        /// </summary>
        /// <param name="width">The width of the line.</param>
        /// <param name="arc">The angle of the cap or joint arc.</param>
        /// <returns>The rounding resolution.</returns>
        private int GetRoundingResolution(float width, float arc)
        {
            int endCapResolution = (int) Math.Ceiling(arc * width * CurveGranularity);
            return endCapResolution;
        }
        
        /// <summary>
        /// Generate a round start cap on a line.
        /// </summary>
        /// <param name="baseVertex">The base vertex to modify for generation.</param>
        /// <param name="start">The start point.</param>
        /// <param name="normal">The line normal.</param>
        /// <param name="width">The width of line to generate.</param>
        /// <param name="resolution">End cap resolution.</param>
        /// <param name="lineStartPositiveIndex">The positive vertex index of the line start.</param>
        /// <param name="lineStartNegativeIndex">The negative vertex index of the line start.</param>
        public void GenerateStartCap(
            QuikVertex baseVertex,
            QuikVec2 start,
            QuikVec2 normal,
            float width,
            int resolution,
            short lineStartPositiveIndex,
            short lineStartNegativeIndex
        )
        {
            QuikVertex circlePoint = baseVertex;
            short lastIndex = lineStartPositiveIndex;
            for (int i = 0; i < resolution; i++)
            {
                float angle = (float) (i + 1) / (resolution + 1) * MathF.PI;
                float cosT = MathF.Cos(angle);
                float sinT = MathF.Sin(angle);

                QuikVec2 displacement = new QuikVec2()
                {
                    X = normal.X * cosT - normal.Y * sinT,
                    Y = normal.X * sinT + normal.Y * cosT
                } * (width / 2);

                circlePoint.Position = start + displacement;

                short segmentIndex = (short)_vertexBufferPointer;

                AddVertex(circlePoint);
                AddElement(
                    lineStartNegativeIndex,
                    lastIndex,
                    segmentIndex);

                lastIndex = segmentIndex;
            }
        }

        /// <summary>
        /// Generate a round line end cap.
        /// </summary>
        /// <param name="baseVertex">Base vertex to modify for generation.</param>
        /// <param name="end">The line end.</param>
        /// <param name="normal">The line normal.</param>
        /// <param name="width">The line width.</param>
        /// <param name="resolution">Cap generation resolution.</param>
        /// <param name="lineEndPositiveIndex">Vertex index of the positive line end.</param>
        /// <param name="lineEndNegativeIndex">Vertex index of the negative line end.</param>
        public void GenerateEndCap(
            QuikVertex baseVertex,
            QuikVec2 end,
            QuikVec2 normal,
            float width,
            int resolution,
            short lineEndPositiveIndex,
            short lineEndNegativeIndex
        )
        {
            QuikVertex circlePoint = baseVertex;
            short lastIndex = lineEndPositiveIndex;
            for (int i = 0; i < resolution; i++)
            {
                float angle = -(float) (i + 1) / (resolution + 1) * MathF.PI;
                float cosT = MathF.Cos(angle);
                float sinT = MathF.Sin(angle);

                QuikVec2 displacement = new QuikVec2()
                {
                    X = normal.X * cosT - normal.Y * sinT,
                    Y = normal.X * sinT + normal.Y * cosT
                } * (width / 2);

                circlePoint.Position = end + displacement;

                AddVertex(circlePoint);
                AddElement(
                    lineEndNegativeIndex,
                    lastIndex,
                    (short) (_vertexBufferPointer - 1));

                lastIndex = (short) (_vertexBufferPointer - 1);
            }
        }
        
        /// <summary>
        /// Generate a joint on the line negative edge.
        /// </summary>
        /// <param name="baseVertex">Base vertex to modify for generation.</param>
        /// <param name="focus">The focus of the joint.</param>
        /// <param name="width">With of the lines.</param>
        /// <param name="prevNegativeLineIndex">Index of the negative end vertex.</param>
        /// <param name="nextNegativeLineIndex">Index of the negative start vertex.</param>
        /// <param name="resolution">Joint resolution.</param>
        /// <param name="arc">Arc length of the joint.</param>
        /// <param name="prevNormal">Normal of the previous line.</param>
        private void GenerateNegativeJoint(
            QuikVertex baseVertex,
            QuikVec2 focus,
            float width,
            short prevNegativeLineIndex,
            short nextNegativeLineIndex,
            int resolution,
            float arc,
            QuikVec2 prevNormal)
        {
            QuikVertex focusVertex = baseVertex;
            short focusIndex = (short) _vertexBufferPointer;
            short lastIndex = prevNegativeLineIndex;

            focusVertex.Position = focus;
            AddVertex(focusVertex);

            for (int i = 0; i < resolution; i++)
            {
                float angle = (float) (i + 1) / (resolution + 1) * arc;
                float cosT = MathF.Cos(angle);
                float sinT = MathF.Sin(angle);

                QuikVec2 displacement = new QuikVec2()
                {
                    X = -prevNormal.X * cosT + prevNormal.Y * sinT,
                    Y = -prevNormal.X * sinT - prevNormal.Y * cosT
                } * (width / 2);

                QuikVertex segmentVertex = focusVertex;
                segmentVertex.Position += displacement;

                short segmentIndex = (short) _vertexBufferPointer;
                AddVertex(segmentVertex);

                AddElement(lastIndex, segmentIndex, focusIndex);
                
                lastIndex = segmentIndex;
            }

            // Add final triangle.
            AddElement(lastIndex, nextNegativeLineIndex, focusIndex);
        }

        /// <summary>
        /// Generate a joint on the line negative edge.
        /// </summary>
        /// <param name="baseVertex">Base vertex to modify for generation.</param>
        /// <param name="focus">The focus of the joint.</param>
        /// <param name="width">With of the lines.</param>
        /// <param name="prevPositiveLineIndex">Index of the positive end vertex.</param>
        /// <param name="nextPositiveLineIndex">Index of the positive start vertex.</param>
        /// <param name="resolution">Joint resolution.</param>
        /// <param name="arc">Arc length of the joint.</param>
        /// <param name="nextNormal">Normal of the next line.</param>
        private void GeneratePositiveJoint(
            QuikVertex baseVertex,
            QuikVec2 focus,
            float width,
            short prevPositiveLineIndex,
            short nextPositiveLineIndex,
            int resolution,
            float arc,
            QuikVec2 nextNormal)
        {
            QuikVertex focusVertex = baseVertex;
            short focusIndex = (short) _vertexBufferPointer;
            short lastIndex = nextPositiveLineIndex;

            focusVertex.Position = focus;
            AddVertex(focusVertex);

            for (int i = 0; i < resolution; i++)
            {
                float angle = (float) (i + 1) / (resolution + 1) * arc;
                float cosT = MathF.Cos(angle);
                float sinT = MathF.Sin(angle);

                QuikVec2 displacement = new QuikVec2()
                {
                    X = nextNormal.X * cosT - nextNormal.Y * sinT,
                    Y = nextNormal.X * sinT + nextNormal.Y * cosT
                } * (width / 2);

                QuikVertex segmentVertex = focusVertex;
                segmentVertex.Position += displacement;

                short segmentIndex = (short) _vertexBufferPointer;
                AddVertex(segmentVertex);

                AddElement(lastIndex, segmentIndex, focusIndex);

                lastIndex = segmentIndex;
            }

            // Add final triangle.
            AddElement(lastIndex, prevPositiveLineIndex, focusIndex);
        }

        /// <summary>
        /// Generate a joint.
        /// </summary>
        /// <param name="baseVertex">Base vertex to modify.</param>
        /// <param name="focus">Focus of the joint.</param>
        /// <param name="prevTangent">Tangent of the previous line segment.</param>
        /// <param name="nextTangent">Tangent of the next line segment.</param>
        /// <param name="width">Width of the lines.</param>
        /// <param name="prevPositiveEndIndex">Vertex index of the positive end.</param>
        /// <param name="prevNegativeEndIndex">Vertex index of the negative end.</param>
        /// <param name="nextPositiveStartIndex">Vertex index of the positive start.</param>
        /// <param name="nextNegativeStartIndex">Vertex index of the negative start.</param>
        public void GenerateJoint(
            QuikVertex baseVertex,
            QuikVec2 focus,
            QuikVec2 prevTangent,
            QuikVec2 nextTangent,
            float width,
            short prevPositiveEndIndex,
            short prevNegativeEndIndex,
            short nextPositiveStartIndex,
            short nextNegativeStartIndex
        )
        {
            QuikVec2 prevNormal,  nextNormal;
            QuikVec2 averageTangent;

            prevNormal = new QuikVec2() { X = -prevTangent.Y, Y = prevTangent.X };
            nextNormal = new QuikVec2() { X = -nextTangent.Y, Y = nextTangent.X };

            averageTangent = 0.5f * (prevTangent + nextTangent);

            // Figure out which side needs the joint.
            QuikVec2 positiveEdge = ((focus + nextNormal) - (focus + prevNormal)).Normalize();
            QuikVec2 negativeEdge = ((focus - nextNormal) - (focus - prevNormal)).Normalize();

            float positiveDot, negativeDot;
            positiveDot = QuikVec2.Dot(averageTangent, positiveEdge);
            negativeDot = QuikVec2.Dot(averageTangent, negativeEdge);

            float arc = MathF.Acos(QuikVec2.Dot(prevNormal, nextNormal));
            int resolution = GetRoundingResolution(width, arc);

            if (positiveDot < negativeDot)
            {
                // Negative edge rounding.
                GenerateNegativeJoint(baseVertex, focus, width, prevNegativeEndIndex, nextNegativeStartIndex, resolution, arc, prevNormal);
            }
            else if (negativeDot < positiveDot)
            {
                // Positive edge rounding.
                GeneratePositiveJoint(baseVertex, focus, width, prevPositiveEndIndex, nextPositiveStartIndex, resolution, arc, nextNormal);
            }
            else
            {
                // There is no need to generate anything because the lines are the same???
            }
        }

        /// <summary>
        /// Renders a line.
        /// </summary>
        /// <param name="call">The draw call to generate.</param>
        /// <param name="line">The line to draw.</param>
        private void RenderLine(ref QuikDrawCall call, QuikCommandLine line)
        {
            // Skip over stipple patterns for now.
            QuikStrokeStyle style = line.Style ?? Context.DefaultStroke;
            QuikVertex baseVertex = new QuikVertex() { Color = style.Color };
            
            GenerateLineSegment(baseVertex, line.Line, style.Width, out short startOffset, out QuikVec2 normal);

            // Now calculate the end caps.
            int endCapResolution = GetRoundingResolution(style.Width, MathF.PI);

            // Construct start cap.
            GenerateStartCap(baseVertex, line.Line.Start, normal, style.Width, endCapResolution, startOffset, (short)
                (startOffset + 1));
            
            // Construct end cap.
            GenerateEndCap(baseVertex, line.Line.End, normal, style.Width, endCapResolution, (short) (startOffset + 2),(short)
                (startOffset + 3));

            call.Offset = (short) (startOffset * 2);
            call.Count = (short) (_elementBufferPointer - startOffset);
        }

        /// <summary>
        /// Render lines.
        /// </summary>
        /// <param name="call">The draw call to generate.</param>
        /// <param name="lines">The lines command.</param>
        private void RenderLine(ref QuikDrawCall call, QuikCommandLines lines)
        {
            // Skip over stipple patterns for now.
            QuikStrokeStyle style = lines.Style ?? Context.DefaultStroke;
            QuikVertex baseVertex = new QuikVertex() { Color = style.Color };
            bool isStart;
            bool isEnd;
            short startOffset = (short)_elementBufferPointer;
            short lastStartIndex = 0;
            short lineStartIndex;
            QuikVec2 normal;
            int resolution = GetRoundingResolution(style.Width, MathF.PI);

            for (int i = 0; i < lines.Lines.Length; i++)
            {
                QuikLine line = lines.Lines[i];

                isStart = i == 0 || line.Start != lines.Lines[i - 1].End;
                isEnd = i == lines.Lines.Length - 1 || line.End != lines.Lines[i + 1].Start;
                
                GenerateLineSegment(baseVertex, line, style.Width, out lineStartIndex, out normal);

                if (isStart)
                {
                    GenerateStartCap(baseVertex, line.Start, normal, style.Width, resolution, lineStartIndex,
                        (short)(lineStartIndex + 1));
                }
                else
                {
                    QuikLine prev = lines.Lines[i - 1];
                    QuikVec2 prevTangent = (prev.End - prev.Start).Normalize();
                    QuikVec2 nextTangent = (line.End - line.Start).Normalize();

                    GenerateJoint(
                        baseVertex, 
                        line.Start,
                        prevTangent,
                        nextTangent,
                        style.Width,
                        (short) (lastStartIndex + 2),
                        (short) (lastStartIndex + 3),
                        lineStartIndex,
                        (short)(lineStartIndex + 1));
                }
                
                if (isEnd)
                {
                    GenerateEndCap(baseVertex, line.End, normal, style.Width, resolution, (short)(lineStartIndex + 2), (short)(lineStartIndex + 3));
                }

                lastStartIndex = lineStartIndex;
            }

            call.Offset = (short) (startOffset * 2);
            call.Count = (short) (_elementBufferPointer - startOffset);
        }
    }

    public enum QuikRenderTarget
    {
        Color,
        Stencil
    }
    
    public struct QuikDrawCall
    {
        public QuikRenderTarget Target;
        public short Offset;
        public short Count;
        public QuikRectangle Bounds;
        public bool ClearStencil;
    }
}