Add reinsert to RawOccupiedEntryMut #450
Conversation
It is sometimes useful to modify the key associated with an entry without changing the value. For example, when building a cache where keys can change. It is currently possible to implement this using hashbrown::HashMap by removing the entry and adding a new one with the same value. However, to prevent conflicts with the new key, the code has to: 1. Lookup the entry with the new key, and return an error if it occupied. 2. Delete the entry with the old key. 3. Move the value into the entry with the new key. This is not possible with the raw entry API. A RawOccupiedEntryMut mutably borrows the map so only one can exist at a time. A workaround is to compute the hash of the new key, do the lookup, destroy the raw entry and rebuild one with the hash later, but this is not ideal as it still does two lookups with the hash when one is sufficient. RawOccupiedEntryMut already has insert_key that allows changing the key associated with an entry, but it does not actually reinserts the entry into the map, so if the new key has a different hash then the entry will no longer be accessible. Add a reinsert method to reinsert the entry into the map after its key has been changed.
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The API is not ideal because when an entry cannot be reinserted because the map already contains an entry with the same key, it does not return a reference to that existing entry. This means that if the user wants to look at that existing entry, they have to do another lookup anyway later, which kinds of defeats the purpose of this API. At least for my use case of using a hash map to implement a cache, it does not matter because when I modify the key for an entry, I expect the map not to have another entry with the same key. I don't know how to fix it easily. |
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I would like to deprecate the raw entry API in favor of |
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☔ The latest upstream changes (presumably #468) made this pull request unmergeable. Please resolve the merge conflicts. |
It is sometimes useful to modify the key associated with an entry without changing the value. For example, when building a cache where keys can change.
It is currently possible to implement this using hashbrown::HashMap by removing the entry and adding a new one with the same value. However, to prevent conflicts with the new key, the code has to:
This is not possible with the raw entry API. A RawOccupiedEntryMut mutably borrows the map so only one can exist at a time. A workaround is to compute the hash of the new key, do the lookup, destroy the raw entry and rebuild one with the hash later, but this is not ideal as it still does two lookups with the hash when one is sufficient.
RawOccupiedEntryMut already has insert_key that allows changing the key associated with an entry, but it does not actually reinserts the entry into the map, so if the new key has a different hash then the entry will no longer be accessible.
Add a reinsert method to reinsert the entry into the map after its key has been changed.