gistfile1.cs

using System;
using System.Diagnostics;
using System.Drawing;
using System.Drawing.Imaging;
using System.Windows.Forms;

namespace Ulam {
	public partial class Form1 : Form {
		public Form1() {
			InitializeComponent();
		}

		bool[] NotPrimes = new bool[2];

		void RecalculatePrimesUpTo( int limit )
		{
			var n = limit+1;
			NotPrimes = new bool[n];
			for ( int i=2 ; i<=n/2 ; ++i ) {
				if (!NotPrimes[i]) {
					for (int j = 2 * i; j < n; j += i) {
						NotPrimes[j] = true;
					}
				}
			}
		}

		bool IsPrime( int n )
		{
			if ( NotPrimes.Length <= n )
				RecalculatePrimesUpTo(Math.Max(n,2*NotPrimes.Length));
			return !NotPrimes[n];
		}

		int ToSpiralN( int x, int y )
		{
			var ring = Math.Max(Math.Abs(x),Math.Abs(y));
			int n = ring * (ring+1) * 4;

			if      ( y == +ring ) n = n - 0*ring + x - ring;
			else if ( x == -ring ) n = n - 2*ring + y - ring;
			else if ( y == -ring ) n = n - 4*ring - x + ring;
			else if ( x == +ring ) n = n - 6*ring - y + ring;
			else Debug.Fail("Impossible... the balls aren't there!");

			return n;
		}

		long LongestPaint = 0;

		unsafe protected override void OnPaint( PaintEventArgs e ) {
			var w = ClientSize.Width;
			var h = ClientSize.Height;

			var stopwatch = new Stopwatch();
			stopwatch.Start();

			using ( var bitmap = new Bitmap(w,h,PixelFormat.Format32bppArgb) )
			{
				var bits = bitmap.LockBits( ClientRectangle, ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb );

				for ( int y=0 ; y<h ; ++y )
				{
					uint* scanline = (uint*)(bits.Scan0.ToInt64() + y*bits.Stride);
					for ( int x=0 ; x<w ; ++x )
					{
						scanline[x] = IsPrime( ToSpiralN( x-w/2, h/2-y )+1 ) ? 0xFF0000FFu : 0xFFB0B0B0u;
					}
				}

				bitmap.UnlockBits(bits);
				e.Graphics.DrawImage( bitmap, ClientRectangle );
			}

			stopwatch.Stop();
			LongestPaint = Math.Max(LongestPaint, stopwatch.ElapsedMilliseconds);

			Text = "Longest paint took "+LongestPaint+" ms";
			base.OnPaint(e);
		}

		protected override void OnResize( EventArgs e ) {
			Invalidate();
			base.OnResize(e);
		}
	}
}