adding support for shortest choice

[leiyangyou]

Hi,

Hopefully this is useful for someone (A ShortestChoice variant of OrderedChoice)

[igordejanovic]

Hi. Thanks for the contribution.
Can you give an example of why this kind of expression would be useful? What is your use-case?

[igordejanovic]

This can be done more optimal. Flattening and converting to string of all resulting subtrees is slow.
It is enough to use position and track in which branch you have smaller advance.

[leiyangyou]

I will make a patch for this

[igordejanovic]

Please make a unit test also if you can. I am trying to keep test coverage as high as possible. Thanks.

[leiyangyou]

I had to use this while building a pretty complex grammar for tagging parts of a school name
(a PEG parser may not have been the best tool for this use case)

It's an equivalent of the non greedy version of regexp operations

say if you have grammar x and y
x matches 'ab'
y matches 'a'

given input 'ab', ShortestChoice([x, y]) will match 'a' while OrderedChoice([x, y]) will match 'ab'

In my case, I did not know upfront which of x y matches a shorter input, therefore something like ShortestChoice has to be used.

[igordejanovic]

I see. I think that a similar variant LongestChoice would be event more useful and very similar in implementation.