Endianness conversion

[Toby]

As part of a program I'm writing, I need to save to disk big amounts of
data (hundreds of MB, in 8kB chunks) swapping every couple of bytes.

I obviously cannot do it in a Python loop.

Is there a function I could use in the standard library, or do I have to
write my own C extension?


Toby

[Dan Sommers]

Re: Endianness conversion

On Sat, 24 Feb 2007 15:39:53 +0000, Toby wrote:

As part of a program I'm writing, I need to save to disk big amounts of
data (hundreds of MB, in 8kB chunks) swapping every couple of bytes.
>
I obviously cannot do it in a Python loop.
>
Is there a function I could use in the standard library, or do I have to
write my own C extension?

You could try the struct module. If your input comes in fixed sized
chunks, just call struct.unpack and struct.pack once per chunk.

HTH,
Dan

--
Dan Sommers
<http://www.tombstoneze ro.net/dan/>
Atoms are not things. -- Werner Heisenberg.

[Toby]

Re: Endianness conversion

Dan Sommers wrote:

You could try the struct module. If your input comes in fixed sized
chunks, just call struct.unpack and struct.pack once per chunk.

Thanks, but it was a bit awkward to use for big chunks.

I ended up writing my own byteswapper in Pyrex:


def swapbytes(data) :
"Swap every two bytes of a even-sized python string, in place"
cdef int i
cdef char t, *p
p = data
for i from 0 <= i < len(data) / 2:
t = p[0]
p[0] = p[1]
p[1] = t
p = p + 2


Toby

[Daniel Harding]

Re: Endianness conversion

On Feb 24, 9:39 am, Toby <etat...@gmail. comwrote:

As part of a program I'm writing, I need to save to disk big amounts of
data (hundreds of MB, in 8kB chunks) swapping every couple of bytes.
>
I obviously cannot do it in a Python loop.
>
Is there a function I could use in the standard library, or do I have to
write my own C extension?

Try the using the array module. array objects provide a byteswap
method which reverses endianness.

-Daniel

[Toby]

Re: Endianness conversion

Daniel Harding wrote:

Try the using the array module. array objects provide a byteswap
method which reverses endianness.

Thanks!


Toby

[Gabriel Genellina]

Re: Endianness conversion

En Sat, 24 Feb 2007 14:27:12 -0300, Toby <etatoby@gmail. comescribió:

I ended up writing my own byteswapper in Pyrex:

You can use the byteswap method of arrays:

>>import array
>>a = array.array('H' , 'ABcd56')
>>a.tostring( )

'ABcd56'

>>a.byteswap( )
>>a.tostring( )

'BAdc65'

--
Gabriel Genellina

[Dan Sommers]

Re: Endianness conversion

On Sat, 24 Feb 2007 17:27:12 +0000, Toby wrote:

Dan Sommers wrote:

>You could try the struct module. If your input comes in fixed sized
>chunks, just call struct.unpack and struct.pack once per chunk.

>
Thanks, but it was a bit awkward to use for big chunks.

def swapper( bytestring ):
someHs = len( bytestring ) / 2 * "H" # could check for odd-lengths?
unpackfmt = "<" + someHs
packfmt = ">" + someHs
return struct.pack( packfmt, *struct.unpack( unpackfmt, bytestring )

--
Dan Sommers
<http://www.tombstoneze ro.net/dan/>
Atoms are not things. -- Werner Heisenberg.