Coding challenge: Optimise a custom string encoding

[Alex Willmer]

A challenge, just for fun. Can you speed up this function?

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 at 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 at 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