On 22 апр, 00:10, Ivan Illarionov <[EMAIL PROTECTED]> wrote:
> On 20 ÁÐÒ, 04:10, George Sakkis <[EMAIL PROTECTED]> wrote:
>
>
>
> > On Apr 18, 9:36 pm, Ross Ridge <[EMAIL PROTECTED]>
> > wrote:
>
> > > Ross Ridge <[EMAIL PROTECTED]> said:
>
> > > > If you have Python 2.5, here's a faster version:
>
> > > > from struct import *
> > > > unpack_i32be = Struct(">l").unpack
>
> > > > def from3Bytes_ross2(s):
> > > > return unpack_i32be(s + "\0")[0] >> 8
>
> > > Bob Greschke <[EMAIL PROTECTED]> wrote:
>
> > > > That's not even intelligible. I wanna go back to COBOL. :)
>
> > > It's the same as the previous version except that it "precompiles"
> > > the struct.unpack() format string. It works similar to the way Python
> > > handles regular expressions.
>
> > I didn't know about the Struct class; pretty neat. It's amazing that
> > this version without Psyco is as fast Bob's version with Psyco! Adding
> > Psyco to it though makes it *slower*, not faster. So here's how I'd
> > write it (if I wanted or had to stay in pure Python):
>
> > try: import psyco
> > except ImportError:
> > from struct import Struct
> > unpack_i32be = Struct(">l").unpack
> > def from3Bytes(s):
> > return unpack_i32be(s + "\0")[0] >> 8
> > else:
> > def from3Bytes(s):
> > Value = (ord(s[0])<<16) + (ord(s[1])<<8) + ord(s[2])
> > if Value >= 0x800000:
> > Value -= 0x1000000
> > return Value
> > psyco.bind(from3Bytes)
>
> > HTH,
> > George
>
> I was able to get even faster pure-python version using array module:
>
> a = array.array('l', ''.join(('\0' + s[i:i+3] for i in xrange(0,
> len(s), 3))))
> if sys.byteorder == 'little':
> a.byteswap()
>
> It actually moves bytes around on C level.
>
> test code:
> import struct
> import array
> import sys
>
> unpack_i32be = struct.Struct(">l").unpack
> s = ''.join(struct.pack('>i', 1234567)[1:]*1000)
>
> def from3bytes_ord(s):
> values = []
> for i in xrange(0, len(s), 3):
> Value = (ord(s[i])<<16) | (ord(s[i+1])<<8) | ord(s[i+2])
> if Value >= 0x800000:
> Value -= 0x1000000
> values.append(Value)
> return values
>
> def from3bytes_struct(s):
> return [unpack_i32be(s[i:i+3] + "\0")[0] >> 8 for i in xrange(0,
> len(s), 3)]
>
> def from3bytes_array(s):
> a = array.array('l', ''.join(('\0' + s[i:i+3] for i in xrange(0,
> len(s), 3))))
> if sys.byteorder == 'little':
> a.byteswap()
> return a.tolist()
>
> from timeit import Timer
>
> t1 = Timer("from3bytes_ord(s)", "from __main__ import s,
> from3bytes_ord")
> t2 = Timer("from3bytes_struct(s)", "from __main__ import s,
> from3bytes_struct")
> t3 = Timer("from3bytes_array(s)", "from __main__ import s,
> from3bytes_array")
>
> print 'ord:\t', t1.timeit(1000)
> print 'struct:\t', t2.timeit(1000)
> print 'array:\t', t3.timeit(1000)
>
> Output:
> ord: 7.08213110884
> struct: 3.7689164405
> array: 2.62995268952
>
> Inspired by Guido's essayhttp://www.python.org/doc/essays/list2str/
And even faster:
a = array.array('i', '\0' + '\0'.join((s[i:i+3] for i in xrange(0,
len(s), 3))))
if sys.byteorder == 'little':
a.byteswap()
I think it's a fastest possible implementation in pure python
--
http://mail.python.org/mailman/listinfo/python-list
