VB6如何获得类似C的整数溢出 [英] VB6 how to get C-like integer overflow

问题描述

我想将这个简单的散列算法移植到VB6中。

 公共函数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 语句会抑制错误6：溢出异常，如果发生溢出，变量 hash 不会再更新。 p>



我该如何解决这个问题或者以不同的方式实施这个算法？

会质疑以这种方式哈希UTF-16LE（Unicode）是否合理。将VB字符串转换为UTF-8 然后对其进行哈希处理可能会更有意义。

虽然我找不到任何 djb2 的测试向量来验证我自己的实现，它似乎运行得非常快：

 私有类型CURRENCY_CURRENCY 
值作为货币
结束类型
 
私有类型CURRENCY_2LONGS 
 ValueLo As Long 
 ValueHi As Long 
结束类型
 
 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 
下一个I 
 
 djb2 = C2L.ValueLo 
结束函数
  

I want to port this simple hash algorithm to VB6.

I have come up with:

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

The problem is, despite my On Error statement which suppresses the Error 6: Overflow exceptions, the variable hash is not updated any more if an overflow occurs.

How can I fix that or implement this algorithm differently?

解决方案

I would question whether it makes sense to hash UTF-16LE ("Unicode") this way. It might make a lot more sense to convert your VB String to UTF-8 and then hash that.

While I can't find any test vectors for djb2 to validate my own implementation it seems to run quite quickly:

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

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