Want way to get stale keys and values out of SecondaryMap, when new keys reuse slots #55

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1. orlp commented on Oct 7, 2021

   

   orlp

   on Oct 7, 2021

   Last edited by orlp

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   Hey, sorry for the late response. I do understand the use case but I want to think a bit more about the interface. I'm also a bit uncertain since many operations silently remove stale entries, so users might expect the stale entry to be there when it's already removed.
2. ijackson commented on Apr 30, 2022

   

   ijackson

   on Apr 30, 2022

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   Hey, sorry for the late response

   No problem.

   I'm also a bit uncertain since many operations silently remove stale entries, so users might expect the stale entry to be there when it's already removed.

   That's certainly a risk. In my application the slotmaps are encapsulated and carefully managed, so that "random code" doesn't get a chance to cause this to happen. Perhaps this issue could be dealt with in documentation.

   In the meantime I have rebased my branch for #56 onto current master.

   Thanks.
3. abonander commented on Feb 29, 2024

   

   abonander

   on Feb 29, 2024

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   @orlp I have a similar, but much simpler use-case. I just need a variant of insert() that returns the stale value if one was replaced, instead of silently dropping it.

   Would you accept an API like this?

   /// Returned by [`SecondaryMap::insert_or_replace()`].
   pub enum Replaced<K, V> {
       /// The value replaced was at the same version as the given key.
       Fresh(V),
       /// The value replaced was for a previous version. The key is included for reference.
       Stale(K, V),
   }
   
   impl<K: Key, V> SecondarySlotMap<K, V> {
       /// Insert a value into the secondary map at a given key or replace the existing value.
       /// 
       /// Returns `None` if the slot was not occupied or if the key was removed from the originating slot map.
       /// Returns `Some(Fresh(value))` if the value replaced is the same version as the given key.
       /// Returns `Some(Stale(key, value))` if the key replaced an older version at the same slot. The old key is included for reference.
       pub fn insert_or_replace(&mut self, key: K, value: V) -> Option<Replaced<K, V>> {
           //...
       }
   }