RequirementMachine: Use trie for left hand side simplification #38794
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In a confluent rewrite system, if the left hand side of a rule X => Y can be reduced by some other rule X' => Y', then it is permissible to delete the original rule X => Y altogether. Confluence means that rewrite rules can be applied in any order, so it is always valid to apply X' => Y' first, thus X => Y is obsolete. This was previously done in the completion procedure via a quadratic algorithm that attempted to reduce each existing rule via the newly-added rule obtained by resolving a critical pair. Instead, we can do this in the post-processing pass where we reduce right hand sides using a trie to speed up the lookup. This increases the amount of work performed by the completion procedure, but eliminates the quadratic algorithm, saving time overall.
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In a confluent rewrite system, if the left hand side of a rule
X => Y can be reduced by some other rule X' => Y', then it is
permissible to delete the original rule X => Y altogether.
Confluence means that rewrite rules can be applied in any
order, so it is always valid to apply X' => Y' first, thus
X => Y is obsolete.
This was previously done in the completion procedure via a
quadratic algorithm that attempted to reduce each existing
rule via the newly-added rule obtained by resolving a critical
pair. Instead, we can do this in the post-processing pass
where we reduce right hand sides using a trie to speed up
the lookup.
This increases the amount of work performed by the
completion procedure, but eliminates the quadratic algorithm,
saving time overall.