Difftastic spends a remarkable 49% of its runtime just reading and writing to a FxHashMap of ~1 million items. This hashmap stores the predecessor vertex when calculating the shortest route.

I'm running out of ideas. I can't use perfect hashmaps, are there other speed tricks?

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Update: I should have noticed that calling .get() and .insert() on a hashmap is slower than using .entry(), it saves a lookup!

Fixing this saves up to 6% of runtime -- not huge, but doesn't hurt.

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