生成质数表

作者: twd2 日期: 2013 年 08 月 06 日被围观 149 次发表评论 (0) 查看评论

Imports System.IO
 
Module Module1
 
 
    Dim fs As New FileStream("pt.bin", FileMode.OpenOrCreate)
 
    Sub Main()
        For i As Long = 1 To UInt32.MaxValue / 8
            fs.WriteByte(255)
        Next
        WriteBool(0, False)
        WriteBool(1, False)
        '13
        For i As Long = 2 To UInt32.MaxValue
            If ReadBool(i) AndAlso UInt32.MaxValue / i < i Then '防止溢出
                For j As Long = i * i To UInt32.MaxValue Step i '据说i*i比i*2快一点儿
                    WriteBool(j, False)
                Next
            End If
        Next
 
        Console.WriteLine("{0}", ReadBool(UInt32.MaxValue))
        'Console.ReadKey()
    End Sub
 
    Sub WriteBool(v As Long, b As Boolean)
        If v > UInt32.MaxValue OrElse v < 0 Then
            Return
        End If
        fs.Seek(v >> 3, SeekOrigin.Begin)
        Dim byt = fs.ReadByte()
        If byt = -1 Then
            byt = 0
        End If
        If b Then
            byt = byt Or 1 << (v Mod 8)
        Else
            byt = byt And Not (1 << (v Mod 8))
        End If
        fs.Seek(v >> 3, SeekOrigin.Begin)
        fs.WriteByte(byt)
    End Sub
 
    Function ReadBool(v As Long) As Boolean
        If v > UInt32.MaxValue OrElse v < 0 Then
            Throw New Exception()
        End If
        fs.Seek(v >> 3, SeekOrigin.Begin)
        Return (fs.ReadByte() >> (v Mod 8)) And 1
    End Function
 
End Module