VB6如何获取C类整数溢出

Question

我想把this simple hash algorithm移植到VB6。

我想出了：

Public Function simpleHash(hashString As String) As Long
Dim hash As Long
Dim c As Long
Dim i As Integer

    On Local Error Resume Next

    hash = 5381
    For i = 1 To Len(hashString)
        c = AscW(Mid$(hashString, i, 1))
        hash = hash * 33 + c
    Next i

    simpleHash = hash
End Function

问题是，尽管我的On Error语句禁止错误6:溢出异常，但如果发生溢出，变量hash将不再更新。

如何以不同的方式修复该算法或实现此算法？

Answer 1

我想问一下，用这种方式散列UTF-16LE ("Unicode")是否有意义。如果将VB字符串转换为UTF-8，然后进行散列，可能会更有意义。

虽然我无法为djb2找到任何测试向量来验证我自己的实现，但它似乎运行得相当快：

Private Type CURRENCY_CURRENCY
    Value As Currency
End Type

Private Type CURRENCY_2LONGS
    ValueLo As Long
    ValueHi As Long
End Type

Public Function djb2(ByVal StringToHash As String) As Long
    Dim C2L As CURRENCY_2LONGS
    Dim CC As CURRENCY_CURRENCY
    Dim I As Long

    C2L.ValueLo = 5381
    For I = 1 To Len(StringToHash)
        LSet CC = C2L
        CC.Value = CC.Value * 33@ + CCur(AscW(Mid$(StringToHash, I, 1))) / 10000@
        LSet C2L = CC
        C2L.ValueHi = 0
    Next I

    djb2 = C2L.ValueLo
End Function

Answer 2

我找到了一个解决办法，使用货币和自己做溢出运算(结果是VB6 Mod运算符也不能在Currency上工作)：

Const pow2_32 As Currency = 2@ ^ 32
Const signed32Max As Currency = 2@ ^ 31 - 1

Public Function simpleHash(hashString As String) As Long
Dim hash As Currency
Dim i As Long

    hash = 5381
    For i = 1 To Len(hashString)
        hash = hash * 33@ + CCur(AscW(Mid$(hashString, i, 1)))
        While hash >= pow2_32
            hash = hash - pow2_32
        Wend
    Next i

    If hash > signed32Max Then
        hash = hash - pow2_32
    End If

    simpleHash = CLng(hash)
End Function

但是，请参阅Bob77's answer获得的解决方案几乎是我的解决方案的两倍。