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  1. using System;
  2. using System.Linq;
  3. using System.Collections.Generic;
  4. using System.Threading.Tasks;
  5.  
  6. public static class Test
  7. {
  8. 	public static void Main()
  9. 	{
  10. 		int max = int.Parse(Console.ReadLine());
  11. 		double zeta2 = Primes(max).Aggregate(1d, (r, p) => r / (1 - 1d / p / p));
  12. 		Console.WriteLine(zeta2);
  13. 	}
  14.  
  15. 	private static IEnumerable<int> Primes(int max)
  16. 	{
  17. 		if (max < 5)
  18. 		{
  19. 			return Enumerable.Range(2, Math.Clamp(max - 1, 0, 2));
  20. 		}
  21. 		else
  22. 		{
  23. 			return new PrimesCalculator(max).PrimesAsync().Result;
  24. 		}
  25. 	}
  26. }
  27.  
  28. internal sealed class PrimesCalculator
  29. {
  30. 	const int start = 2;
  31.  
  32. 	internal PrimesCalculator(int max)
  33. 	{
  34. 		array = new bool[max + 1];
  35. 		end = ((int)Math.Sqrt(max) + 2) / 3;
  36. 		tasks = new Task[end + 1];
  37. 		removeAction = Remove;
  38. 		result = Enumerate(array);
  39. 	}
  40.  
  41. 	internal async Task<IEnumerable<int>> PrimesAsync()
  42. 	{
  43. 		if (n == start)
  44. 		{
  45. 			RunRemove();
  46. 			while (n <= end)
  47. 			{
  48. 				await Task.Delay(32);
  49. 			}
  50. 			await Task.WhenAll(tasks.Where(t => t != null));
  51. 		}
  52. 		return result;
  53. 	}
  54.  
  55. 	private static int Generator(int n) => (n & ~1) + (n << 1) - 1;
  56.  
  57. 	private static IEnumerable<int> Enumerate(bool[] array)
  58. 	{
  59. 		yield return 2;
  60. 		yield return 3;
  61. 		int value;
  62. 		for (int k = 2; (value = Generator(k)) < array.Length; k++)
  63. 		{
  64. 			if (!array[value])
  65. 			{
  66. 				yield return value;
  67. 			}
  68. 		}
  69. 	}
  70.  
  71. 	private readonly bool[] array;
  72.  
  73. 	private int n = start;
  74.  
  75. 	private readonly int end;
  76.  
  77. 	private readonly Task[] tasks;
  78.  
  79. 	private readonly Action removeAction;
  80.  
  81. 	private readonly IEnumerable<int> result;
  82.  
  83. 	private void RunRemove()
  84. 	{
  85. 		if (n <= end)
  86. 		{
  87. 			ref var task = ref tasks[n];
  88. 			task = new Task(removeAction);
  89. 			task.Start();
  90. 		}
  91. 	}
  92.  
  93. 	private void Remove()
  94. 	{
  95. 		int prime;
  96. 		while (array[prime = Generator(n++)]) ;
  97. 		int k = n - 1;
  98. 		ref var isComposite = ref array[prime];
  99. 		RunRemove();
  100. 		int composite;
  101. 		for (; !isComposite && (composite = Generator(k) * prime) < array.Length; k++)
  102. 		{
  103. 			array[composite] = true;
  104. 		}
  105. 	}
  106. }
Success #stdin #stdout 0.58s 105272KB
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https://ideone.com/MXdJvh
language:
C# (gmcs 5.20.1)
created:
2 years ago
visibility:
public

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