Stop greedy choice after full input match

[cdamours]

Background

While investigating parser performance in an internal application, we noticed that a successful parse can still perform significant unnecessary work. Even when an early Choice element matches and consumes the entire input, the parser continues to walk the remaining elements. Our profiling confirmed numerous _walk calls occurring at the same input position after a complete match had already been established.

Issue

A most-greedy Choice selects the valid element that advances furthest through the input. Once the current best match consumes the full input, no later element can advance further. Continuing the scan in these cases causes additional computation with no effect on the final result.

Proposed Change

I suggest modifying Choice._most_greedy_result to stop walking elements once the current best match reaches end-of-input. This preserves existing behavior while avoiding redundant work. In our internal tests, this change successfully reduced the fixed cost of parsing short, complete statements.

Steps to Reproduce

You can reproduce this behavior with a grammar containing a large most-greedy choice where an early element matches the full string:

from pyleri import Choice, Grammar, Keyword, Regex, Sequence, Token

class ManyAlternativesGrammar(Grammar):
    comparison = Choice(
        Sequence(Keyword("field_a"), Token("="), Regex(r"\d+")),
        Sequence(Keyword("field_b"), Token("="), Regex(r"\d+")),
        Sequence(Keyword("field_c"), Token("="), Regex(r"\d+")),
        # ... additional elements ...
    )
    START = comparison

grammar = ManyAlternativesGrammar()
grammar.parse("field_a=123")

Currently, the parser evaluates every element even after "field_a=123" is matched. I have also verified that this change continues to correctly select the longest choice when multiple valid parsing solutions exist.

[joente]

Thanks for the pull request! I agree that it makes sense to break once _len_string is reached. I'll take a closer look next week and merge it once I've verified it doesn't break anything.