[PATCH] py3: make py3 compat.iterbytestr simpler and faster

[Martin von Zweigbergk]

On Tue, Mar 14, 2017 at 3:19 PM, Gregory Szorc <gregory.szorc at gmail.com> wrote:
> On Tue, Mar 14, 2017 at 2:55 PM, Martin von Zweigbergk via Mercurial-devel
> <mercurial-devel at mercurial-scm.org> wrote:
>>
>> # HG changeset patch
>> # User Martin von Zweigbergk <martinvonz at google.com>
>> # Date 1489528461 25200
>> #      Tue Mar 14 14:54:21 2017 -0700
>> # Node ID 375345ceabc9f14e780d8a8efd00ed32ffc6396c
>> # Parent  3d3109339b57341b333c1112beb41dd281fa944a
>> py3: make py3 compat.iterbytestr simpler and faster
>>
>> I don't have a py3.5 installed, so I have not run tests on this, but
>> maybe it works?
>>
>> $ python3 -m timeit -s 's=b"a"*100' 'for x in iter(s[i:i + 1] for i in
>> range(len(s))): pass'
>> 100000 loops, best of 3: 10.3 usec per loop
>>
>> $ python3 -m timeit -s 's=b"a"*100' 'for x in map(chr, s): pass'
>> 100000 loops, best of 3: 6.16 usec per loop
>>
>> (I picked the best "before" time and worst "after" time out of a few
>> runs.)
>>
>> diff -r 3d3109339b57 -r 375345ceabc9 mercurial/pycompat.py
>> --- a/mercurial/pycompat.py     Mon Mar 13 11:19:24 2017 -0700
>> +++ b/mercurial/pycompat.py     Tue Mar 14 14:54:21 2017 -0700
>> @@ -78,7 +78,7 @@
>>
>>      def iterbytestr(s):
>>          """Iterate bytes as if it were a str object of Python 2"""
>> -        return iter(s[i:i + 1] for i in range(len(s)))
>> +        return map(chr, s)
>>
>
> The change to map() makes sense to me and I can understand how it would be
> faster. However, chr returns a Python 3 str instead of bytes, which is
> wrong. Try the following:
>
> return map(struct.Struct('>B').pack, s)

Ah. That's still noticeably faster: ~7us. I'll send a v2. Thanks.

>
>>
>>      def sysstr(s):
>>          """Return a keyword str to be passed to Python functions such as
>
>