squad-rain-ops-mini/Helpers/CaptchaHelper.cs

using System;
using System.Text;
using System.Security.Cryptography;

namespace RainOpsMini.Helpers
{
    public static class CaptchaHelper
    {
        private static readonly char[] Chars = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ".ToCharArray();

        public static (string Code, string Svg) GenerateCaptcha(int width = 120, int height = 40)
        {
            var code = GenerateRandomCode(4);
            var svg = GenerateSvg(code, width, height);
            return (code, svg);
        }

        private static string GenerateRandomCode(int length)
        {
            var sb = new StringBuilder(length);
            var data = new byte[length];
            using (var rng = RandomNumberGenerator.Create())
            {
                rng.GetBytes(data);
            }

            foreach (var b in data)
            {
                sb.Append(Chars[b % Chars.Length]);
            }
            return sb.ToString();
        }

        private static string GenerateSvg(string code, int width, int height)
        {
            var sb = new StringBuilder();
            var random = new Random();

            sb.Append($"<svg width=\"{width}\" height=\"{height}\" xmlns=\"http://www.w3.org/2000/svg\" viewBox=\"0 0 {width} {height}\">");
            
            // Background
            sb.Append($"<rect width=\"100%\" height=\"100%\" fill=\"#f2f2f2\"/>");

            // Noise lines
            for (int i = 0; i < 5; i++)
            {
                int x1 = random.Next(0, width);
                int y1 = random.Next(0, height);
                int x2 = random.Next(0, width);
                int y2 = random.Next(0, height);
                string color = GetRandomColor(random);
                sb.Append($"<line x1=\"{x1}\" y1=\"{y1}\" x2=\"{x2}\" y2=\"{y2}\" stroke=\"{color}\" stroke-width=\"1\" opacity=\"0.5\"/>");
            }

            // Dots
            for (int i = 0; i < 50; i++)
            {
                int cx = random.Next(0, width);
                int cy = random.Next(0, height);
                string color = GetRandomColor(random);
                sb.Append($"<circle cx=\"{cx}\" cy=\"{cy}\" r=\"1\" fill=\"{color}\" opacity=\"0.5\"/>");
            }

            // Text
            int charWidth = width / code.Length;
            for (int i = 0; i < code.Length; i++)
            {
                int x = i * charWidth + random.Next(5, 10);
                int y = height / 2 + random.Next(5, 10);
                int fontSize = random.Next(20, 28);
                string color = GetRandomDarkColor(random);
                int rotate = random.Next(-20, 20);
                
                sb.Append($"<text x=\"{x}\" y=\"{y}\" font-family=\"Arial, sans-serif\" font-size=\"{fontSize}\" font-weight=\"bold\" fill=\"{color}\" transform=\"rotate({rotate} {x} {y})\">{code[i]}</text>");
            }

            sb.Append("</svg>");
            return sb.ToString();
        }

        private static string GetRandomColor(Random random)
        {
            return $"rgb({random.Next(0, 256)},{random.Next(0, 256)},{random.Next(0, 256)})";
        }

        private static string GetRandomDarkColor(Random random)
        {
            return $"rgb({random.Next(0, 150)},{random.Next(0, 150)},{random.Next(0, 150)})";
        }
    }
}