the donut.c formatted and converted to C# for learning

donut.cs

using System.Text;

float sin(float x)
{
    return MathF.Sin(x);
}

float cos(float x)
{
    return MathF.Cos(x);
}

int k;

float A = 0, B = 0, i, j;
float[] z = new float[1760];
char[] b = new char[1760];
Console.WriteLine("\x1b[2J");
StringBuilder sb;
for (; ; )
{
    Thread.Sleep((int)(1000 * (1/24f)));//force to run in 24 FPS
    sb = new();
    Array.Fill(b, ' ');
    Array.Fill(z, 0.0f);
    for (j = 0; 6.28f > j; j += 0.07f)
    {
        for (i = 0; 6.28f > i; i += 0.02f)
        {
            float c = sin(i),
                d = cos(j),
                e = sin(A),
                f = sin(j),
                g = cos(A),
                h = d + 2,
                D = 1 / (c * h * e + f * g + 5),
                l = cos(i),
                m = cos(B),
                n = sin(B),
                t = c * h * g - f * e;

            int x = (int)(40 + 30 * D * (l * h * m - t * n)),
                y = (int)(12 + 15 * D * (l * h * n + t * m)),
                o = x + 80 * y,
                N = (int)(8 * ((f * e - c * d * g) * m - c * d * e - f * g - l * d * n));

            if (22 > y && y > 0 && x > 0 && 80 > x && D > z[o])
            {
                z[o] = D;
                b[o] = ".,-~:;=!*#$@"[N > 0 ? N : 0];
            }
        }
    }
    sb.AppendLine("\x1b[H");
    for (k = 0; k < 1761; k++)
    {
        if (k % 80 == 0 && k != 0)
            sb.AppendLine("");
        else if (k < 1760) // Ensure we don't access out of bounds
            sb.Append(b[k]);
    }
    A += 0.04f;
    B += 0.02f;
    Console.Write(sb.ToString());
}

a nice page explaining the math behind it:
https://www.a1k0n.net/2011/07/20/donut-math.html