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());
}

original donut.c:

            k;double sin()
         ,cos();main(){float A=
       0,B=0,i,j,z[1760];char b[
     1760];printf("\x1b[2J");for(;;
  ){memset(b,32,1760);memset(z,0,7040)
  ;for(j=0;6.28>j;j+=0.07)for(i=0;6.28
 >i;i+=0.02){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=s\
in(B),t=c*h*g-f*        e;int x=40+30*D*
(l*h*m-t*n),y=            12+15*D*(l*h*n
+t*m),o=x+80*y,          N=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];}}/*#****!!-*/
  printf("\x1b[H");for(k=0;1761>k;k++)
   putchar(k%80?b[k]:10);A+=0.04;B+=
     0.02;}}/*****####*******!!=;:~
       ~::==!!!**********!!!==::-
         .,~~;;;========;;;:~-.
             ..,--------,*/

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