Change reversed() to [::-1] for speed

[scragg0x]

No description provided.

[jeffwidman]

Skimming your PR made me curious... [::-1] only replaces reversed().

Why did you drop the ''.join() from your version?

I haven't dug into the surrounding code so I'm unaware of the overall calling context.

[scragg0x]

Ahh, such an old commit of mine. It would appear that I just neglected it. Good thing this wasn't pulled.

[davidism]

I timed both versions and there was no significant change, so just going to close this and leave it as is.

[scragg0x]

https://gist.github.com/scragg0x/97d2927ce20b0766710bcee43795c2a2

Interestingly, I just did a bench and reversed it much faster anyway with larger lists. 25x faster when using the example gist.

[ThiefMaster]

That benchmark is incorrect - reversed() doesn't actually reverse anything when called; it just provides an iterator that starts at the end of the collection

In [1]: lst = ['x'] * 1000
In [2]: %timeit lst[::-1]
100000 loops, best of 3: 4.37 µs per loop
In [3]: %timeit list(reversed(lst))
100000 loops, best of 3: 8.27 µs per loop

With strings:

In [1]: s = 'x' * 1000
In [2]: %timeit s[::-1]
1000000 loops, best of 3: 883 ns per loop
In [3]: %timeit ''.join(reversed(s))
10000 loops, best of 3: 20.7 µs per loop

[scragg0x]

@ThiefMaster I see, thanks for clarifying. I figured I missed something to have such a large disparity.

[jeffwidman]

Now that I understand what's happening here, I'd vote to reconsider and merge this.

BTW, thanks @ThiefMaster for taking the time to explain, I hadn't realized that reversed() only returns an iterator.

[davidism]

After a (very silly) discussion about this micro-optimization, http://chat.stackoverflow.com/transcript/message/29868921#29868921, it looks like it would be better to replace the whole thing with the following, which outperforms everything else on py2 and py3 and doesn't require compat imports.

import struct

to_int64 = struct.Struct('>Q').pack

def int_to_bytes(num):
    return to_int64(num).lstrip(b'\x00')

timeit int_to_bytes(ts)
1000000 loops, best of 3: 398 ns per loop

[davidism]

090e9e2