NGram issue with strings shorter than N

[adamko147]

NGram distance returns wrong value for strings shorter than N.

from strsimpy.ngram import NGram
ng = NGram()
ng.distance("abc", "abc") == 0.0
ng.distance("a", "b") == 0.0 # should be 1.0

Distance between a and b should be 1.0 as the strings are completely different. if N is two, then the code at https://github.com/luozhouyang/python-string-similarity/blob/master/strsimpy/ngram.py#L45 calculates cost of 0 and returns 1.0 * cost / max(sl, tl). This returns similarity (which actually is 0, because the strings are completely different). However the code is returning normalized distance, which should be maximum possible here.

This issue seems to be at https://github.com/luozhouyang/python-string-similarity/blob/master/strsimpy/ngram.py#L49 where it does

return 1.0 * cost / max(sl, tl)

however I think in this case it should return

return 1.0 - cost / max(sl, tl)

I'll create PR to address this.

Thank you

[adamko147]

@luozhouyang any chance you can package a publish new version to pypi.org including this fix? Thanks a lot!

[luozhouyang]

@adamko147 just released v0.2.1! Thanks for you contribution!