Which Endian routine is better?
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From: MM on 11 Mar 2010 12:51 I am converting big endian to little endian and have written these two functions: Function Endian32_UsingString(ByVal n As Long) As Long Dim s As String Dim d As String Dim i As Integer s = Right$("0000000" & Hex$(n), 8) For i = 1 To 7 Step 2 d = Mid$(s, i, 2) & d Next Endian32_UsingString = CLng("&H" & d) End Function Function Endian32_UsingByteArray(ByVal n As Long) As Long Dim big(3) As Byte Dim little(3) As Byte CopyMemory big(0), n, 4 little(3) = big(0) little(2) = big(1) little(1) = big(2) little(0) = big(3) CopyMemory n, little(0), 4 Endian32_UsingByteArray = n End Function Which is preferable and why? MM
From: Nobody on 11 Mar 2010 13:57 "MM" <kylix_is(a)yahoo.co.uk> wrote in message news:e7bip5tqgk2lsqgumicb9i9r3al9vp9q7j(a)4ax.com... > Which is preferable and why? Do a speed test, but the second one is probably faster. There is also an API function that does this. It's htonl(). This should be faster than both. Here is a VB6 solution that doesn't use the API and it's probably faster: Function Endian32LongSwap(ByVal n As Long) As Long Dim x As Long ' Byte 0 ' Special case for negative numbers If (n And &H80&) <> 0 Then ' Avoid overflow x = (n And &H7F&) * &H1000000 x = x Or &H80000000 Else x = (n And &HFF&) * &H1000000 End If ' Byte 1 x = x Or ((n And &HFF00&) * &H100&) ' Byte 2 x = x Or ((n And &HFF0000) \ &H100&) ' Byte 3 ' Special case for negative numbers If (n And &H80000000) <> 0 Then x = x Or ((n And &H7F000000) \ &H1000000) x = x Or &H80& Else x = x Or ((n And &HFF000000) \ &H1000000) End If Endian32LongSwap = x End Function
From: MM on 12 Mar 2010 02:22 On Thu, 11 Mar 2010 13:57:45 -0500, "Nobody" <nobody(a)nobody.com> wrote: >"MM" <kylix_is(a)yahoo.co.uk> wrote in message >news:e7bip5tqgk2lsqgumicb9i9r3al9vp9q7j(a)4ax.com... >> Which is preferable and why? > >Do a speed test, but the second one is probably faster. There is also an API >function that does this. It's htonl(). ............ Yes, I found that, but it introduces one more "dependency", namely winsock. Now, I realise that all the PCs my app is ever likely to be installed will have it, but I'd prefer something that I didn't already have reference to for other reasons. For example, I never mind about using a new routine that uses SendMessage because I already use this call. I'll write up a speed comparison test later, including the htonl one and the VB ones. That routine you provided seems quite complicated with all the ANDs and ORs. I do have access to assembler/C written shifts in vbhlp32.dll, which is a free non-COM library that I am using already. Maybe a routine could be built up using shifts, as that is the approach I've seen in C and Pascal examples. MM
From: Mike Williams on 12 Mar 2010 04:44 "MM" <kylix_is(a)yahoo.co.uk> wrote in message news:qfqjp55b82u5dlkbpihknnugu21tsdho3g(a)4ax.com... > [In response to 'Nobody's post] I'll write up a speed > comparison test later, including the htonl one and the > VB ones. That routine you provided seems quite > complicated with all the ANDs and ORs. I do have > access to assembler/C written shifts in vbhlp32.dll, > which is a free non-COM library that I am using > already. Maybe a routine could be built up using > shifts, as that is the approach I've seen in C and > Pascal examples. Shifts would obviously be much less complicated and faster than native VB code (which has to get around the fact that VB does not provide real shift methods and the fact that it does not provide unsigned Longs) but generally any code that calls a declared function to perform a very small task is slowed down by the overhead of the call itself, and so in many cases the speed difference is not quite what you might expect it to be. I've just tested a few different methods of performing your "endian swap", including a CopyMemory method and the native VB method posted by 'Nobody' and the call to htonl in "wsock32.dll" method that you mentioned. All tests were performed as a standard native code compiled exe of course, which is how your app will almost certainly be compiled. The Copymemory method used a number of calls to Copymemory and, as expected, was the slowest of the three. The call to htonl in wsock32.dll was the fastest, but the straight VB code posted by 'Nobody' came a respectable second. I'm not sure whether it can be optimized a little further (maybe, but possibly not) but it is quite useful even as it stands. Here are the results I got on my own machine (a relatively slow Celeron laptop): 0.215 microseconds for the CopyMemory method 0.056 microseconds for Nobody's VB method 0.025 microseconds for wsock32.dll method Mike
From: Donald Lessau on 12 Mar 2010 08:23
"Nobody" <nobody(a)nobody.com> schrieb im Newsbeitrag news:O4ZcAzUwKHA.4552(a)TK2MSFTNGP04.phx.gbl... > "MM" <kylix_is(a)yahoo.co.uk> wrote in message > news:e7bip5tqgk2lsqgumicb9i9r3al9vp9q7j(a)4ax.com... >> Which is preferable and why? > > Do a speed test, but the second one is probably faster. There is also an > API function that does this. It's htonl(). This should be faster than > both. > > Here is a VB6 solution that doesn't use the API and it's probably faster: > > Function Endian32LongSwap(ByVal n As Long) As Long > Dim x As Long > > ' Byte 0 > ' Special case for negative numbers > If (n And &H80&) <> 0 Then > ' Avoid overflow > x = (n And &H7F&) * &H1000000 > x = x Or &H80000000 > Else > x = (n And &HFF&) * &H1000000 > End If > ' Byte 1 > x = x Or ((n And &HFF00&) * &H100&) > ' Byte 2 > x = x Or ((n And &HFF0000) \ &H100&) > ' Byte 3 > ' Special case for negative numbers > If (n And &H80000000) <> 0 Then > x = x Or ((n And &H7F000000) \ &H1000000) > x = x Or &H80& > Else > x = x Or ((n And &HFF000000) \ &H1000000) > End If > > Endian32LongSwap = x > > End Function This one might be even faster: Public Function SwapEndian08(ByVal dw As Long) As Long SwapEndian08 = _ (((dw And &HFF000000) \ &H1000000) And &HFF&) Or _ ((dw And &HFF0000) \ &H100&) Or _ ((dw And &HFF00&) * &H100&) Or _ ((dw And &H7F&) * &H1000000) If (dw And &H80&) Then SwapEndian08 = SwapEndian08 Or &H80000000 End Function see http://www.xbeat.net/vbspeed/c_SwapEndian.htm Don |