Problem: Currently select-keys uses conj to add entries. If the map is editable, conj! could be used instead to improve select-keys performance.
Additionally keyseq is traversed as a seq but could be traversed via reduce instead, which might be faster.
Approach 1: Use a transient map and conj!, keeping loop/recur
Approach 2: Reimplement select-keys to use reduce instead of loop/recur
Approach 3: Combine approach one and two
|selected key size | loop/recur | transient | reduce | transient + reduce |
| :-- | :-- | :-- | :-- | :-- | :-- |
|1 | 243 ns | 256 ns | 161 ns | 188 ns |
|7 | 1.1 ms | - | 885 ns | 454 ns |
From these numbers, approach 3 was chosen.
Note: In order to implement select-keys in terms of reduce, select-keys needed to be moved until after the definition of reduce. This forced a (declare select-keys) since it's used before the definition of reduce
Patch: (link: https://dev.clojure.org/jira/secure/attachment/17392/0001-CLJ-1789-Use-transients-and-reduce-with-select-keys.patch text: 0001-CLJ-1789-Use-transients-and-reduce-with-select-keys.patch)
