On Fri, 18 Apr 2008 22:30:45 -0500, Grant Edwards wrote: > On 2008-04-18, Bob Greschke <[EMAIL PROTECTED]> wrote: > >> However, in playing around with your suggestion and Grant's code I've >> found that the struct stuff is WAY slower than doing something like >> this >> >> Value = (ord(Buf[s])*65536)+(ord(Buf[s+1])*256)+ord(Buf[s+2]) if Value >> >= 0x800000: >> Value -= 0x1000000 >> >> This is almost twice as fast just sitting here grinding through a few >> hundred thousand conversions (like 3sec vs. ~5secs just counting on my >> fingers - on an old Sun...it's a bit slow). Replacing *65536 with <<16 >> and *256 with <<8 might even be a little faster, but it's too close to >> call without really profiling it. > > I didn't know speed was important. This might be a little faster > (depending on hardware): > > Value = (ord(Buf[s])<<16) | (ord(Buf[s+1])<<8) | ord(Buf[s+2]) > > It also makes the intention a bit more obvious (at least to me). > > A decent C compiler will recognize that <<16 and <<8 are special and > just move bytes around rather than actually doing shifts. I doubt the > Python compiler does optimizations like that, but shifts are still > usually faster than multiplies (though, again, a good compiler will > recognize that multiplying by 65536 is the same as shifting by 16 and > just move bytes around).
So why not put it in C extension? It's easier than most people think: <code> from3bytes.c ============ #include <Python.h> PyObject* from3bytes(PyObject* self, PyObject* args) { const char * s; int len; if (!PyArg_ParseTuple(args, "s#", &s, &len)) return NULL; long n = (s[0]<<16) | (s[1]<<8) | s[2]; if (n >= 0x800000) n -= 0x1000000; return PyInt_FromLong(n); } static PyMethodDef functions[] = { {"from3bytes", (PyCFunction)from3bytes, METH_VARARGS}, {NULL, NULL, 0, NULL}, }; DL_EXPORT(void) init_from3bytes(void) { Py_InitModule("_from3bytes", functions); } buildme.py ========== import os import sys from distutils.core import Extension, setup os.chdir(os.path.dirname(os.path.abspath(__file__))) sys.argv = [sys.argv[0], 'build_ext', '-i'] setup(ext_modules = [Extension('_from3bytes', ['from3bytes.c'])]) </code> 'python buildme.py' will create '_from3bytes.so' file 'from _from3bytes import from3bytes' will import C-optimized function Hope this helps. -- Ivan Illarionov -- http://mail.python.org/mailman/listinfo/python-list