How to ensure that a file has unique line in vb.net if the size of the file is very big

Question

Language: vb.net File size: 1GB, and stuff.

Encoding of the text file: UTF8 (so each character is represented by different numbers of b

Answer 1

Depending on how long the lines are, you may be able to compute an MD5 hash value for each line and store than in a HashMap:

Using sr As New StreamReader("myFile")
    Dim lines As New HashSet(Of String)
    Dim md5 As New Security.Cryptography.MD5Cng()

    While sr.BaseStream.Position < sr.BaseStream.Length
        Dim l As String = sr.ReadLine()
        Dim hash As String = String.Join(String.Empty, md5.ComputeHash(System.Text.Encoding.UTF8.GetBytes(l)).Select(Function(x) x.ToString("x2")))

        If lines.Contains(hash) Then
            'Lines are not unique
            Exit While
        Else
            lines.Add(hash)
        End If
    End While
End Using

Untested, but this may be fast enough for your needs. I can't think of something much faster that still maintains some semblance of conciseness :)

Answer 2

This is the contemporary answer

Public Sub makeUniqueForLargeFiles(ByVal strFileSource As String)
    Using sr As New System.IO.StreamReader(strFileSource)
        Dim changeFileName = reserveFileName(strFileSource, False, True)
        Using sw As New System.IO.StreamWriter(reserveFileName(strFileSource, False, True), False, defaultEncoding)
            sr.Peek()
            Dim lines As New Generic.Dictionary(Of Integer, System.Collections.Generic.List(Of Long))
            While sr.BaseStream.Position < sr.BaseStream.Length
                Dim offset = sr.BaseStream.Position
                Dim l As String = sr.ReadLine()
                Dim nextOffset = sr.BaseStream.Position
                Dim hash = l.GetHashCode
                Do ' a trick to put the for each in a "nest" that we can exit from
                    If lines.ContainsKey(hash) Then
                        Using sr2 = New System.IO.StreamReader(strFileSource)
                            For Each offset1 In lines.Item(hash)
                                sr2.BaseStream.Position = offset1
                                Dim l2 = sr2.ReadLine
                                If l = l2 Then
                                    Exit Do 'will sr2.dispose be called here?
                                End If
                            Next
                        End Using
                    Else
                        lines.Add(hash, New Generic.List(Of Long))
                    End If
                    lines.Item(hash).Add(offset)
                    sw.WriteLine(l)
                Loop While False
                sr.BaseStream.Position = nextOffset
            End While
        End Using
    End Using
End Sub