using System;
using System.Drawing;
using Blacklight.Core;

namespace Konis.Testbed
{
    /// <summary>
    /// The sprite every particle draws, made in code so the testbed needs no art.
    /// </summary>
    public static class SoftDisc
    {
        public const int Side = 64;

        // The highlight sits off center so a particle's rotation can be seen.
        private const double HighlightOffset = 0.45;
        private const double HighlightRadius = 0.14;

        private static double SmoothStep(double edge0, double edge1, double value)
        {
            double fraction = Math.Max(0.0, Math.Min(1.0, (value - edge0) / (edge1 - edge0)));

            return fraction * fraction * (3.0 - 2.0 * fraction);
        }

        /// <summary>
        /// Returns white that fades from the middle to nothing at the edge, with a brighter spot toward the right.
        /// </summary>
        public static DecodedImage Build()
        {
            var rgba = new byte[Side * Side * DecodedImage.BytesPerPixel];
            double middle = (Side - 1) / 2.0;

            for (int y = 0; y < Side; y++)
            {
                for (int x = 0; x < Side; x++)
                {
                    double across = (x - middle) / middle;
                    double down = (y - middle) / middle;
                    double fromMiddle = Math.Sqrt(across * across + down * down);
                    double spotX = across - HighlightOffset;
                    double fromSpot = Math.Sqrt(spotX * spotX + down * down);

                    double alpha = 0.7 * (1.0 - SmoothStep(0.0, 1.0, fromMiddle));
                    alpha = Math.Max(alpha, 1.0 - SmoothStep(0.0, HighlightRadius, fromSpot));

                    int index = (y * Side + x) * DecodedImage.BytesPerPixel;
                    rgba[index + 0] = 255;
                    rgba[index + 1] = 255;
                    rgba[index + 2] = 255;
                    rgba[index + 3] = (byte)(alpha * 255.0 + 0.5);
                }
            }

            return new DecodedImage(rgba, new Size(Side, Side));
        }
    }
}