`submap` sub map should maintain properties of parent map #235

[jeff303]

TODO: figure out how to handle transients (i.e. submap!)

Adding new SubMap protocol to handle the different implementations of key selection (including one that maintains order to be used for sorted-map)

Adding test to ensure order preserved for a large map

[jeff303]

I'm not sure how to handle transients here (i.e. submap!).

• There is no such thing as a transient version of PersistentTreeMap, so the problem where the previous navigator didn't preserve order doesn't apply there
• For TransientArrayMap and TransientHashMap, the order already appears preserved by select-keys (see REPL session below)

# for TransientHashMap
(select-keys (transient {:a 1 :b 2 :c 3 :d 4 :e 5 :f 6 :g 7 :h 8 :i 9 :j 10 :k 11 :l 12 :m 13}) [:a :c :e :g :i :k :m])
{:a 1, :c 3, :e 5, :g 7, :i 9, :k 11, :m 13}
# for TransientArrayMap
(select-keys (transient {:a 1 :b 2 :c 3 :d 4 :e 5 :f 6 :g 7}) [:g :f :e :d :c :b :a])
{:g 7, :f 6, :e 5, :d 4, :c 3, :b 2, :a 1}

[nathanmarz]

Couple comments:

• I think the only property that's important to maintain is order in the case of sorted-map. So I think the code can be simplified by having a case for Object which does select-keys and only having a special case for PersistentTreeMap.
• The tests should test against a sorted map with a custom comparator and verify the submap has the same custom comparator.

[nathanmarz]

Also, transients aren't supported on PersistentTreeMap, so submap! doesn't need any changes.

[jeff303]

Added the test for the actual comparator (couldn't quickly figure out if/how to make that work in cljs).

Also, transients aren't supported on PersistentTreeMap, so submap! doesn't need any changes

OK, thanks for the confirmation. The starter comment on #235 is what got me looking at it.