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Re: Coding challenge: Optimise a custom string encoding

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On 8/18/2014 3:16 PM, Alex Willmer wrote:
> A challenge, just for fun. Can you speed up this function?

You should give a specification here, with examples. You should perhaps 
be using .maketrans and .translate.

> import string
>
> charset = set(string.ascii_letters + string.digits + '@_-')
> byteseq = [chr(i) for i in xrange(256)]
> bytemap = {byte: byte if byte in charset else '+' + byte.encode('hex')
>             for byte in byteseq}
>
> def plus_encode(s):
>      """Encode a unicode string with only ascii letters, digits, _, -, @, +
>      """
>      bytemap_ = bytemap
>      s_utf8 = s.encode('utf-8')
>      return ''.join([bytemap[byte] for byte in s_utf8])
>
> On my machine (Ubuntu 14.04, CPython 2.7.6, PyPy 2.2.1) this gets
>
> alex@martha:~$ python -m timeit -s 'import plus_encode' 'plus_encode.plus_encode(u"""qwertyuiop1234567890!"£$%^&*()EURO""")'
> 100000 loops, best of 3: 2.96 usec per loop
>
> alex@martha:~$ pypy -m timeit -s 'import plus_encode' 'plus_encode.plus_encode(u"""qwertyuiop1234567890!"£$%^&*()EURO""")'
> 1000000 loops, best of 3: 1.24 usec per loop
>
> Back story:
> Last week we needed a custom encoding to store unicode usernames in a config file that only allowed mixed case ascii, digits, underscore, dash, at-sign and plus sign. We also wanted to keeping the encoded usernames somewhat human readable.
>
> My design was utf-8 and a variant of %-escaping, using the plus symbol. So u'alic EURO 123' would be encoded as b'alic+e2+82+ac123'. This evening as a learning exercise I've tried to make it fast. This is the result.
>
> This challenge is just for fun. The chosen solution ended up being
>
> def name_encode(s):
>      return %s_%s' % (s.encode('utf-8').encode('hex'),
>                       re.replace('[A-Za-z0-9]', '', s))
>
> Regards, Alex
>


-- 
Terry Jan Reedy

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Thread

Coding challenge: Optimise a custom string encoding Alex Willmer <alex@moreati.org.uk> - 2014-08-18 12:16 -0700
  Re: Coding challenge: Optimise a custom string encoding Terry Reedy <tjreedy@udel.edu> - 2014-08-18 16:16 -0400
    Re: Coding challenge: Optimise a custom string encoding Alex Willmer <alex@moreati.org.uk> - 2014-08-18 14:27 -0700
      Re: Coding challenge: Optimise a custom string encoding Peter Otten <__peter__@web.de> - 2014-08-19 01:35 +0200
  Re: Coding challenge: Optimise a custom string encoding Chris Angelico <rosuav@gmail.com> - 2014-08-19 09:28 +1000
  Re: Coding challenge: Optimise a custom string encoding Lele Gaifax <lele@metapensiero.it> - 2014-08-19 12:00 +0200

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