// <copyright file="ParallelImageProcessor.cs" company="James Jackson-South">
// Copyright (c) James Jackson-South and contributors.
// Licensed under the Apache License, Version 2.0.
// </copyright>

namespace ImageProcessorCore
{
    using System;
    using System.Threading;
    using System.Threading.Tasks;

    /// <summary>
    /// Allows the application of processors using parallel processing.
    /// </summary>
    public abstract class ParallelImageProcessor : IImageProcessor
    {
        /// <inheritdoc/>
        public event ProgressEventHandler OnProgress;

        /// <summary>
        /// Gets or sets the count of workers to run the process in parallel.
        /// </summary>
        public virtual int Parallelism { get; set; } = Environment.ProcessorCount * 2;

        /// <summary>
        /// The number of rows processed by a derived class.
        /// </summary>
        private int numRowsProcessed;

        /// <summary>
        /// The total number of rows that will be processed by a derived class.
        /// </summary>
        private int totalRows;

        /// <inheritdoc/>
        public void Apply(ImageBase target, ImageBase source, Rectangle sourceRectangle)
        {
            try
            {
                this.OnApply(source, target, target.Bounds, sourceRectangle);

                this.numRowsProcessed = 0;
                this.totalRows = sourceRectangle.Height;

                if (this.Parallelism > 1)
                {
                    int partitionCount = this.Parallelism;

                    Task[] tasks = new Task[partitionCount];

                    for (int p = 0; p < partitionCount; p++)
                    {
                        int current = p;
                        tasks[p] = Task.Run(
                            () =>
                                {
                                    int batchSize = sourceRectangle.Height / partitionCount;
                                    int yStart = sourceRectangle.Y + (current * batchSize);
                                    int yEnd = current == partitionCount - 1
                                                   ? sourceRectangle.Bottom
                                                   : yStart + batchSize;

                                    this.Apply(target, source, target.Bounds, sourceRectangle, yStart, yEnd);
                                });
                    }

                    Task.WaitAll(tasks);
                }
                else
                {
                    this.Apply(target, source, target.Bounds, sourceRectangle, sourceRectangle.Y, sourceRectangle.Bottom);
                }

                this.AfterApply(source, target, target.Bounds, sourceRectangle);
            }
            catch (Exception ex)
            {

                throw new ImageProcessingException($"An error occured when processing the image using {this.GetType().Name}. See the inner exception for more detail.", ex);
            }
        }

        /// <inheritdoc/>
        public void Apply(ImageBase target, ImageBase source, int width, int height, Rectangle targetRectangle = default(Rectangle), Rectangle sourceRectangle = default(Rectangle))
        {
            try
            {
                float[] pixels = new float[width * height * 4];
                target.SetPixels(width, height, pixels);

                if (sourceRectangle == Rectangle.Empty)
                {
                    sourceRectangle = source.Bounds;
                }

                this.OnApply(source, target, target.Bounds, sourceRectangle);

                targetRectangle = target.Bounds;
                this.numRowsProcessed = 0;
                this.totalRows = targetRectangle.Bottom;

                if (this.Parallelism > 1)
                {
                    int partitionCount = this.Parallelism;

                    Task[] tasks = new Task[partitionCount];

                    for (int p = 0; p < partitionCount; p++)
                    {
                        int current = p;
                        tasks[p] = Task.Run(() =>
                        {
                            int batchSize = targetRectangle.Bottom / partitionCount;
                            int yStart = current * batchSize;
                            int yEnd = current == partitionCount - 1 ? targetRectangle.Bottom : yStart + batchSize;

                            this.Apply(target, source, targetRectangle, sourceRectangle, yStart, yEnd);
                        });
                    }

                    Task.WaitAll(tasks);
                }
                else
                {
                    this.Apply(target, source, targetRectangle, sourceRectangle, targetRectangle.Y, targetRectangle.Bottom);
                }

                this.AfterApply(source, target, target.Bounds, sourceRectangle);
            }
            catch (Exception ex)
            {
                throw new ImageProcessingException($"An error occured when processing the image using {this.GetType().Name}. See the inner exception for more detail.", ex);
            }
        }

        /// <summary>
        /// This method is called before the process is applied to prepare the processor.
        /// </summary>
        /// <param name="source">The source image. Cannot be null.</param>
        /// <param name="target">Target image to apply the process to.</param>
        /// <param name="targetRectangle">
        /// The <see cref="Rectangle"/> structure that specifies the location and size of the drawn image.
        /// The image is scaled to fit the rectangle.
        /// </param>
        /// <param name="sourceRectangle">
        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to draw.
        /// </param>
        protected virtual void OnApply(ImageBase source, ImageBase target, Rectangle targetRectangle, Rectangle sourceRectangle)
        {
        }

        /// <summary>
        /// Applies the process to the specified portion of the specified <see cref="ImageBase"/> at the specified location
        /// and with the specified size.
        /// </summary>
        /// <param name="target">Target image to apply the process to.</param>
        /// <param name="source">The source image. Cannot be null.</param>
        /// <param name="targetRectangle">
        /// The <see cref="Rectangle"/> structure that specifies the location and size of the drawn image.
        /// The image is scaled to fit the rectangle.
        /// </param>
        /// <param name="sourceRectangle">
        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to draw.
        /// </param>
        /// <param name="startY">The index of the row within the source image to start processing.</param>
        /// <param name="endY">The index of the row within the source image to end processing.</param>
        /// <remarks>
        /// The method keeps the source image unchanged and returns the
        /// the result of image process as new image.
        /// </remarks>
        protected abstract void Apply(ImageBase target, ImageBase source, Rectangle targetRectangle, Rectangle sourceRectangle, int startY, int endY);

        /// <summary>
        /// This method is called after the process is applied to prepare the processor.
        /// </summary>
        /// <param name="source">The source image. Cannot be null.</param>
        /// <param name="target">Target image to apply the process to.</param>
        /// <param name="targetRectangle">
        /// The <see cref="Rectangle"/> structure that specifies the location and size of the drawn image.
        /// The image is scaled to fit the rectangle.
        /// </param>
        /// <param name="sourceRectangle">
        /// The <see cref="Rectangle"/> structure that specifies the portion of the image object to draw.
        /// </param>
        protected virtual void AfterApply(ImageBase source, ImageBase target, Rectangle targetRectangle, Rectangle sourceRectangle)
        {
        }

        /// <summary>
        /// Must be called by derived classes after processing a single row.
        /// </summary>
        protected void OnRowProcessed()
        {
            if (this.OnProgress != null)
            {
                int currThreadNumRows = Interlocked.Add(ref this.numRowsProcessed, 1);

                // Multi-pass filters process multiple times more rows than totalRows, so update totalRows on the fly
                if (currThreadNumRows > this.totalRows)
                {
                    this.totalRows = currThreadNumRows;
                }

                // Report progress. This may be on the client's thread, or on a Task library thread.
                this.OnProgress(this, new ProgressEventArgs { RowsProcessed = currThreadNumRows, TotalRows = this.totalRows });
            }
        }
    }
}