Resolvelib

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Bug Fixes
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- Fix a state management bug that causes the resolver to enter an infinite loop
in certain backtracking cases. `62 <https://github.com/sarugaku/resolvelib/issues/62>`_

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Bug Fixes
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- Fix a performance regression if ``find_matches()`` returns a non-built-in sequence instance.

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Features
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- ``find_matches()`` now may return a ``Callable[[], Iterator[Candidate]]`` to avoid needing to provide all candidates eagerly for the resolver. This improves performance when fetching candidates is costly, but not always required. `57 <https://github.com/sarugaku/resolvelib/issues/57>`_

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Features
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- Add ``parent`` argument to the ``add_requirement()`` reporter hook. `46 <https://github.com/sarugaku/resolvelib/issues/46>`_

- Redesign ``find_matches()`` to support a requirement "adding" candidates to the set, and nudge the provider away from implementing ``find_matches()`` and ``is_satisfied_by()`` with incorrect set properties. `49 <https://github.com/sarugaku/resolvelib/issues/49>`_

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Features
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- Provide both the requirements and their parents as exceptiondata for the ``ResolutionImpossible`` exception, via a ``causes`` attribute that replaces the previous ``requirements`` attribute. `42 <https://github.com/sarugaku/resolvelib/issues/42>`_


Bug Fixes
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- Make resolver backtrack when none of the candidates requested for a package are able to resolve due to them requesting unworkable requirements, or a package has no candidates at all. Previously the resolver would give up on the spot. `18 <https://github.com/sarugaku/resolvelib/issues/18>`_

- Ensure the result returned by the resolver only contains candidates that are actually needed. This is done by tracing the graph after resolution completes, snipping nodes that don’t have a route to the root. `4 <https://github.com/sarugaku/resolvelib/issues/4>`_

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Features
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- Remove reporting callbacks `adding_requirements`, `adding_candidate`, and `replacing_candidate` added in 0.2.1. These are not useful, and it’s better to remove them while we can. `6 <https://github.com/sarugaku/resolvelib/issues/6>`_