January 16, 2021, 06:05:13 AM

### Author Topic: [bmx] Reverse bytes and bits by N [ 1+ years ago ]  (Read 420 times)

#### BlitzBot

• Jr. Member
• Posts: 1
##### [bmx] Reverse bytes and bits by N [ 1+ years ago ]
« on: June 29, 2017, 12:28:39 AM »
Title : Reverse bytes and bits
Author : N
Posted : 1+ years ago

Description : Pretty simple, it just reverse the byte and bit order.

For an example:
Code: [Select]
`Local p@ Ptr = "blah".ToWString( )ReverseBytes( p, 8, False )Print Bin( Int Ptr(p)[0] )Print String.FromShorts( Short Ptr(p), 8 )ReverseBytes( p, 8, False )Print Bin( Int Ptr(p)[0] )ReverseBytes( p, 8, True )Print Bin( Int Ptr(p)[0] )Print String.FromShorts( Short Ptr(p), 8 )ReverseBytes( p, 8, True )Print Bin( Int Ptr(p)[0] )Print String.FromShorts( Short Ptr(p), 8 )ReverseBytes( p, 8, True )Print Bin( Int Ptr(p)[0] )ReverseBytes( p, 8, False )Print Bin( Int Ptr(p)[0] )Print String.FromShorts( Short Ptr(p), 8 )ReverseBytes( p, 8, False )Print Bin( Int Ptr(p)[0] )ReverseBytes( p, 8, True )Print Bin( Int Ptr(p)[0] )Print String.FromShorts( Short Ptr(p), 8 )Local t% = Millisecs( )For Local i% = 0 To 99999    ReverseBytes(p,8,True)Nextt = Millisecs( ) - tPrint "Time taken with rbits as True: "+t+"ms"Print "Time taken per call with rbits as True average: "+(t*0.00001)+"ms"t = Millisecs( )For Local i% = 0 To 199999    ReverseBytes(p,8,False)Nextt = Millisecs( ) - tPrint "Time taken with rbits as False: "+t+"ms"Print "Time taken per call with rbits as False average: "+(t*0.00001)+"ms"MemFree( p ) ' Ok, we're done with the memoryPrint Bin(%10000000)[24..]Print Bin(ReverseBits( %10000000 ))[24..]Print Bin(%11000000)[24..]Print Bin(ReverseBits( %11000000 ))[24..]Print Bin(%10100000)[24..]Print Bin(ReverseBits( %10100000 ))[24..]Print Bin(%00000011)[24..]Print Bin(ReverseBits( %00000011 ))[24..]Input( )`

It's fun to use. [/i]

Code :
Code: BlitzMax
1. Function ReverseBytes( p@ Ptr, sz%, rbits%=0 )
2.     Local szh% = Int(Floor(sz*.5))
3.     sz :- 1
4.     If rbits>0 Then
5.         For Local i:Int = 0 To szh-1
6.             szh = p[i] ' Because szh is only used once, we can reuse it here
7.             p[i] = ReverseBits(p[sz-i])
8.             p[sz-i] = ReverseBits(szh)
9.         Next
10.         sz :+ 1
11.         If Int(sz*.5) <> Ceil((sz*.5)-.1) Then
12.             sz = Int(Ceil(sz*.5))
13.             p[sz] = ReverseBits(p[sz])
14.         EndIf
15.     Else
16.         For Local i:Int = 0 To szh-1
17.             szh = p[i]
18.             p[i] = p[sz-i]
19.             p[sz-i] = szh
20.         Next
21.     EndIf
22. End Function
23.
24. Function ReverseBitsA@( p@ )
25.     ?Debug
26.     Return (p & %1) Shl 7..
27.     | ((p&%10000000) Shr 7)..
28.     | (p & %10) Shl 5..
29.     | ((p&%1000000) Shr 5)..
30.     | (p & %100) Shl 3..
31.     | ((p&%100000) Shr 3)..
32.     | (p & %1000) Shl 1..
33.     | ((p&%10000) Shr 1)
34.     ?
35.     Local p2:Int = ((p & %11110000) Shr 4) | ((p & %1111) Shl 4)
36.     p =((p & %11001100) Shr 2) | ((p & %110011) Shl 2)
37.     Return ((p & %10101010) Shr 1) | ((p & %1010101) Shl 1)
38. End Function

ImaginaryHuman(Posted 1+ years ago)

Code: [Select]
`Function ReverseBits@( p@ )   Local p2:Int=((p & %11110000) Shr 4) | ((p & %1111) Shl 4)   p=((p2 & %11001100) Shr 2) | ((p2 & %110011) Shl 2)   Return ((p & %10101010) Shr 1) | ((p & %1010101) Shl 1)End Function`'6 shifts 6 and's 3 or's instead of 8 , 8 and 7 - should be up to 25% faster.The only downside is having to store the interim values into a variable so you can then read in the changes to continue the operation.You can probably optimize your byte sort routine in much the same way and it should be a lot faster. It's a binary flip, so if you're flipping an Int it should only take 5 passes of swapping the sets of bits. Much better than doing it a bit at a time.I have no idea what your example program is trying to do. :-)

N(Posted 1+ years ago)

For some reason that's only faster in release mode, so I'll use this:
Code: [Select]
`Function ReverseBitsA@( p@ )    ?Debug    Return (p & %1) Shl 7..    | ((p&%10000000) Shr 7)..    | (p & %10) Shl 5..    | ((p&%1000000) Shr 5)..    | (p & %100) Shl 3..    | ((p&%100000) Shr 3)..    | (p & %1000) Shl 1..    | ((p&%10000) Shr 1)    ?    Local p2:Int = ((p & %11110000) Shr 4) | ((p & %1111) Shl 4)    p =((p & %11001100) Shr 2) | ((p & %110011) Shl 2)    Return ((p & %10101010) Shr 1) | ((p & %1010101) Shl 1)End Function`Bit bloated, but it's fast in both situations.Example program is just trying to show that it's fun to use.  It's pretty useless for a lot of things, but it's fun to use it for completely bizarre things too. [/i]

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