LarpixServer/LarpixServer/Account/Captcha.cs

using SkiaSharp;
using System;
using System.Runtime.InteropServices;

namespace LarpixServer.Account;

public class Captcha
{
    private const string Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
    private static readonly SKTypeface CaptchaTypeface = SKTypeface.FromFamilyName("Google Sans Code", SKFontStyle.Bold);

    public static (byte[] ImageBytes, string CaptchaText) GenerateCaptcha()
    {
        int width = 1120;
        int height = 320;
        int charCount = 8;

        Span<char> captchaSpan = stackalloc char[charCount];
        Span<SKColor> charColors = stackalloc SKColor[charCount];

        for (int i = 0; i < charCount; i++)
        {
            captchaSpan[i] = Chars[Random.Shared.Next(Chars.Length)];
            charColors[i] = new SKColor(
                (byte)Random.Shared.Next(100, 256),
                (byte)Random.Shared.Next(100, 256),
                (byte)Random.Shared.Next(100, 256)
            );
        }
        string captchaText = new string(captchaSpan);

        var imageInfo = new SKImageInfo(width, height, SKColorType.Bgra8888, SKAlphaType.Premul);
        using var sourceBitmap = new SKBitmap(imageInfo);
        using var canvas = new SKCanvas(sourceBitmap);
        canvas.Clear(SKColors.Transparent);
        
        using var dotPaint = new SKPaint { IsAntialias = true };
        float spacing = 12f;

        Span<float> randOffsets = stackalloc float[256];
        Span<float> randRadii = stackalloc float[256];
        Span<SKColor> randColors = stackalloc SKColor[256];

        for (int i = 0; i < 256; i++)
        {
            randOffsets[i] = (float)(Random.Shared.NextDouble() - 0.5) * 8f;
            randRadii[i] = (float)Random.Shared.NextDouble() * 3.5f + 1f;
            randColors[i] = new SKColor(
                (byte)Random.Shared.Next(120, 200),
                (byte)Random.Shared.Next(120, 200),
                (byte)Random.Shared.Next(120, 200)
            );
        }

        int noiseIdx = 0;
        for (float y = 0; y < height; y += spacing)
        {
            for (float x = 0; x < width; x += spacing)
            {
                noiseIdx = (noiseIdx + 1) & 255;
                dotPaint.Color = randColors[noiseIdx];
                canvas.DrawCircle(x + randOffsets[noiseIdx], y + randOffsets[255 - noiseIdx], randRadii[noiseIdx], dotPaint);
            }
        }
        
        using var textPaint = new SKPaint
        {
            IsAntialias = true,
            TextSize = 158,
            Typeface = CaptchaTypeface
        };

        float cellWidth = width / (float)charCount;

        for (int i = 0; i < charCount; i++)
        {
            string charStr = captchaText[i].ToString();
            SKRect bounds = new SKRect();
            textPaint.MeasureText(charStr, ref bounds);
            textPaint.Color = charColors[i];

            canvas.Save();

            float maxRadius = (float)Math.Sqrt((bounds.Width * bounds.Width) + (bounds.Height * bounds.Height)) / 2f;
            float margin = 5f; 
            
            float minX = (cellWidth * i) + maxRadius + margin;
            float maxX = (cellWidth * (i + 1)) - maxRadius - margin;
            float xCenter = (minX >= maxX) ? (cellWidth * i) + (cellWidth / 2f) : minX + (float)Random.Shared.NextDouble() * (maxX - minX);

            float minY = maxRadius + margin;
            float maxY = height - maxRadius - margin;
            float yCenter = (minY >= maxY) ? height / 2f : minY + (float)Random.Shared.NextDouble() * (maxY - minY);

            canvas.Translate(xCenter, yCenter);
            canvas.RotateDegrees(Random.Shared.Next(-45, 46));
            canvas.DrawText(charStr, -bounds.MidX, -bounds.MidY, textPaint);
            canvas.Restore();
        }

        using var linePaint = new SKPaint { StrokeWidth = 8, IsAntialias = true };

        for (int i = 0; i < charCount; i++)
        {
            linePaint.Color = charColors[i];
            float cellStartX = cellWidth * i;
            float x1 = cellStartX + (float)Random.Shared.NextDouble() * cellWidth;
            float x2 = cellStartX + (float)Random.Shared.NextDouble() * cellWidth;
            canvas.DrawLine(x1, 0, x2, height, linePaint);
        }

        float bandHeight = height / 4f;
        for (int i = 0; i < 4; i++)
        {
            linePaint.Color = charColors[Random.Shared.Next(charColors.Length)];
            float bandStartY = bandHeight * i;
            float y1 = bandStartY + (float)Random.Shared.NextDouble() * bandHeight;
            float y2 = bandStartY + (float)Random.Shared.NextDouble() * bandHeight;
            canvas.DrawLine(0, y1, width, y2, linePaint);
        }

        canvas.Flush();

        float amplitude = 10.5f; 
        float frequency = 0.03f;

        Span<int> sinY = stackalloc int[height];
        for (int y = 0; y < height; y++)
            sinY[y] = (int)(Math.Sin(y * frequency) * amplitude);

        Span<int> cosX = stackalloc int[width];
        for (int x = 0; x < width; x++)
            cosX[x] = (int)(Math.Cos(x * frequency) * amplitude);

        using var warpedBitmap = new SKBitmap(imageInfo);

        Span<uint> srcPixels = MemoryMarshal.Cast<byte, uint>(sourceBitmap.GetPixelSpan());
        Span<uint> dstPixels = MemoryMarshal.Cast<byte, uint>(warpedBitmap.GetPixelSpan());

        for (int y = 0; y < height; y++)
        {
            int offsetX = sinY[y];
            int yOffsetBase = y * width;

            for (int x = 0; x < width; x++)
            {
                int srcX = x + offsetX;
                int srcY = y + cosX[x];

                if ((uint)srcX < (uint)width && (uint)srcY < (uint)height)
                {
                    dstPixels[yOffsetBase + x] = srcPixels[srcY * width + srcX];
                }
                else
                {
                    dstPixels[yOffsetBase + x] = 0; 
                }
            }
        }
        
        using var warpedImage = SKImage.FromBitmap(warpedBitmap);
        
        using var data = warpedImage.Encode(SKEncodedImageFormat.Webp, 90);

        return (data.ToArray(), captchaText);
    }
}