using System;
using System.Threading;
using System.Threading.Tasks;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;

namespace XP.Common.Controls.ImageHistogram
{
    /// <summary>
    /// 直方图后台计算引擎 | Histogram background computation engine
    /// 负责在后台线程中执行灰度值遍历和统计计算
    /// </summary>
    internal sealed class HistogramEngine : IDisposable
    {
        /// <summary>
        /// 单帧计算超时时间（毫秒）| Single frame computation timeout (ms)
        /// </summary>
        private const int ComputeTimeoutMs = 5000;

        private CancellationTokenSource? _timeoutCts;
        private readonly object _lock = new();
        private bool _disposed;

        /// <summary>
        /// 从 Image&lt;Rgba32&gt; 计算灰度直方图 | Compute histogram from Image&lt;Rgba32&gt;
        /// 使用 ITU-R BT.601 亮度公式：Gray = 0.299R + 0.587G + 0.114B
        /// </summary>
        /// <param name="image">输入图像 | Input image</param>
        /// <param name="ct">取消令牌 | Cancellation token</param>
        /// <returns>256 长度的频次数组，失败返回 null | 256-length frequency array, null on failure</returns>
        public Task<long[]?> ComputeAsync(Image<Rgba32> image, CancellationToken ct)
        {
            if (image == null)
                return Task.FromResult<long[]?>(null);

            // 创建超时令牌 | Create timeout token
            var linkedCts = CreateLinkedTimeoutToken(ct);
            var linkedToken = linkedCts.Token;

            return Task.Run(() =>
            {
                try
                {
                    var width = image.Width;
                    var height = image.Height;
                    var histogram = new long[256];

                    // 遍历像素，使用亮度公式计算灰度值 | Iterate pixels, compute grayscale using luminance formula
                    for (int y = 0; y < height; y++)
                    {
                        linkedToken.ThrowIfCancellationRequested();

                        for (int x = 0; x < width; x++)
                        {
                            var pixel = image[x, y];
                            // ITU-R BT.601 亮度公式 | ITU-R BT.601 luminance formula
                            var gray = (int)(0.299 * pixel.R + 0.587 * pixel.G + 0.114 * pixel.B);
                            // 钳位到 0-255 范围 | Clamp to 0-255 range
                            gray = Math.Clamp(gray, 0, 255);
                            histogram[gray]++;
                        }
                    }

                    return (long[]?)histogram;
                }
                catch (OperationCanceledException)
                {
                    // 超时或取消，返回 null | Timeout or cancelled, return null
                    return null;
                }
                catch
                {
                    // 所有异常内部捕获，不向外抛出 | Catch all exceptions internally
                    return null;
                }
                finally
                {
                    linkedCts.Dispose();
                }
            }, linkedToken);
        }

        /// <summary>
        /// 从原始字节数组计算灰度直方图 | Compute histogram from raw byte array
        /// </summary>
        /// <param name="rawData">原始像素数据 | Raw pixel data</param>
        /// <param name="width">图像宽度 | Image width</param>
        /// <param name="height">图像高度 | Image height</param>
        /// <param name="bitDepth">位深度（8 或 16）| Bit depth (8 or 16)</param>
        /// <param name="ct">取消令牌 | Cancellation token</param>
        /// <returns>频次数组（8位:256长度，16位:65536长度），失败返回 null | Frequency array, null on failure</returns>
        public Task<long[]?> ComputeAsync(byte[] rawData, int width, int height, int bitDepth, CancellationToken ct)
        {
            // 参数有效性验证 | Parameter validation
            if (rawData == null || width <= 0 || height <= 0)
                return Task.FromResult<long[]?>(null);

            if (bitDepth != 8 && bitDepth != 16)
                return Task.FromResult<long[]?>(null);

            int expectedLength = bitDepth == 8 ? width * height : width * height * 2;
            if (rawData.Length != expectedLength)
                return Task.FromResult<long[]?>(null);

            // 创建超时令牌 | Create timeout token
            var linkedCts = CreateLinkedTimeoutToken(ct);
            var linkedToken = linkedCts.Token;

            return Task.Run(() =>
            {
                try
                {
                    if (bitDepth == 8)
                    {
                        return ComputeHistogram8Bit(rawData, width, height, linkedToken);
                    }
                    else
                    {
                        return ComputeHistogram16Bit(rawData, width, height, linkedToken);
                    }
                }
                catch (OperationCanceledException)
                {
                    return null;
                }
                catch
                {
                    return null;
                }
                finally
                {
                    linkedCts.Dispose();
                }
            }, linkedToken);
        }

        /// <summary>
        /// 计算 8 位灰度直方图 | Compute 8-bit grayscale histogram
        /// </summary>
        private static long[]? ComputeHistogram8Bit(byte[] rawData, int width, int height, CancellationToken ct)
        {
            var histogram = new long[256];
            int totalPixels = width * height;

            for (int i = 0; i < totalPixels; i++)
            {
                if (i % 65536 == 0)
                    ct.ThrowIfCancellationRequested();

                histogram[rawData[i]]++;
            }

            return histogram;
        }

        /// <summary>
        /// 计算 16 位灰度直方图 | Compute 16-bit grayscale histogram
        /// </summary>
        private static long[]? ComputeHistogram16Bit(byte[] rawData, int width, int height, CancellationToken ct)
        {
            var histogram = new long[65536];
            int totalPixels = width * height;

            for (int i = 0; i < totalPixels; i++)
            {
                if (i % 65536 == 0)
                    ct.ThrowIfCancellationRequested();

                // 小端序读取 16 位值 | Read 16-bit value in little-endian
                int offset = i * 2;
                ushort value = (ushort)(rawData[offset] | (rawData[offset + 1] << 8));
                histogram[value]++;
            }

            return histogram;
        }

        /// <summary>
        /// 创建带超时的链接取消令牌 | Create linked cancellation token with timeout
        /// </summary>
        private CancellationTokenSource CreateLinkedTimeoutToken(CancellationToken externalToken)
        {
            var timeoutCts = new CancellationTokenSource(ComputeTimeoutMs);
            var linkedCts = CancellationTokenSource.CreateLinkedTokenSource(externalToken, timeoutCts.Token);

            lock (_lock)
            {
                _timeoutCts?.Dispose();
                _timeoutCts = timeoutCts;
            }

            return linkedCts;
        }

        /// <summary>
        /// 释放资源 | Dispose resources
        /// </summary>
        public void Dispose()
        {
            if (_disposed) return;
            _disposed = true;

            lock (_lock)
            {
                _timeoutCts?.Cancel();
                _timeoutCts?.Dispose();
                _timeoutCts = null;
            }
        }
    }
}