Reversing every character in a file - language-agnostic

Question

I'm in a little trouble here.
Can anyone help me implement a solution that reverses every byte so 0xAB becomes 0xBA but not so "abcd" becomes "dcba". I need it so AB CD EF becomes BA DC FE.
Preferably in C or C++ but it doesn't really matter provided it can run.
So far, I've implemented a UBER CRAPPY solution that doesn't even work (and yes, I know that converting to string and back to binary is a crappy solution) in PureBasic.
OpenConsole()
filename$ = OpenFileRequester("Open File","","All types | *.*",0)
If filename$ = ""
End
EndIf
OpenFile(0,filename$)
*Byte = AllocateMemory(1)
ProcessedBytes = 0
Loc=Loc(0)
Repeat
FileSeek(0,Loc(0)+1)
PokeB(*Byte,ReadByte(0))
BitStr$ = RSet(Bin(Asc(PeekS(*Byte))),16,"0")
FirstStr$ = Left(BitStr$,8)
SecondStr$ = Right(BitStr$,8)
BitStr$ = SecondStr$ + FirstStr$
Bit.b = Val(BitStr$)
WriteByte(0,Bit)
ProcessedBytes = ProcessedBytes + 1
ClearConsole()
Print("Processed Bytes: ")
Print(Str(ProcessedBytes))
Loc=Loc(0)
Until Loc = Lof(0)
Delay(10000)
Thanks for reading.

Answer 1

Reading your PureBasic code (I skipped it at first), it does seem you want to swap endian, even though it's not what your text is asking—0xAB practically always means a byte with decimal value 171, not two bytes, and it's extremely common to display a byte as two hex digits, where you use A-F in your example.
#include <iostream>
int main() {
using namespace std;
for (char a; cin.get(a);) {
char b;
if (!cin.get(b)) {
cout.put(a); // better to write it than lose it
cerr << "Damn it, input ends with an odd byte, is it in "
"the right format?\n";
return 1;
}
cout.put(b);
cout.put(a);
}
return 0;
}
// C version is a similar easy translation from the original code
import numpy
import sys
numpy.fromfile(sys.stdin, numpy.int16).byteswap(True).tofile(sys.stdout)
Original answer:
I'm not sure why you want this (it doesn't convert endian, for example, if you want that), but here you go:
#include <stdio.h>
int main() {
for (char c; (c == getchar()) != EOF;) {
putchar((c & 0xF << 4) | ((int)c & 0xF0 >> 4));
}
return 0;
}
#include <iostream>
int main() {
for (char c; std::cin.get(c);) {
std::cout.put((c & 0xF << 4) | ((int)c & 0xF0 >> 4));
}
return 0;
}
import sys
for line in sys.stdin:
sys.stdout.write("".join(
chr((ord(c) & 0xF << 4) | (ord(c) & 0xF0 >> 4))
for c in line
))
All assume that text translations don't occur (such as \n to \r\n and vice versa); you'll have to change them to opening files in binary mode if that's the case. They read from stdin and write to stdout, if you're unfamiliar with that, so just use programname < inputfile > outputfile to run them.

Answer 2

Reversing the high and low half-byte is possible through a simple arithmetic formula (assuming you operate on unsigned bytes):
reversed = (original % 16) * 16 + (original / 16);

Answer 3

A Haskell solution:
module ReverseBytes where
import qualified Data.ByteString as B
import Data.Bits
import Data.Word
-----------------------------------------------------------
main :: IO ()
main = B.getContents >>= B.putStr . B.map reverseByte
reverseByte :: Word8 -> Word8
reverseByte = flip rotate 4
runghc ReverseBytes.hs < inputfile > outputfile