Add lightweight keyset wrapper

[bsless]

Saves on allocation and provides an immutable facade to underlying
regular set

Followup to #522
Part of #513

[bsless]

I believe the difference is significant enough to consider this given the benefit for larger schemas:

(cc/quick-bench
 ;; 1.799482 µs -> 1.694608 µs
 (m/-parse-entries [[:x int?] [:y {:optional true} boolean?] [:z string?]] nil nil))


(cc/quick-bench
 ;; 5.412949 µs -> 4.899880 µs
 (m/-parse-entries [[:a int?]
                    [:b {:optional true} boolean?]
                    [:c string?]
                    [:u int?]
                    [:v int?]
                    [:w int?]
                    [:x int?]
                    [:y int?]
                    [:z int?]]
                   nil nil))

[bsless]

@ikitommi any interest in this patch? I can rebase it.

[ikitommi]

Thanks for your work on this. Could you check the #539 , in which, I swapped the set to be a map with key->index mapping. This allows effective patching with reusing the parse-results.

[ikitommi]

... the map generation could be improved I think, just collect the key-index pairs, create a arraymap in the end?

[bsless]

The same mechanism I used for the set could be used here - use a mutable java collection and expose it later by an immutable "view"

[ikitommi]

maybe just collecting the key + values into object-array and create persistent with PersistentArrayMap/createWithCheck? looks fast, haven't tested.

[bsless]

Creating an ArrayMap is tricky. You don't want to do it for over 8 keys, and createWithCheck will be worst-case quadratic for it.

[ikitommi]

we already do object-arrays for everything else, just one with double size? looks fast and does fast duplicate key check:

(let [m {:a 1, :b 2, :c 3, :d 4, :e 5}
      a (object-array (* 2 (count m)))]
  (doseq [[i [k v]] (map-indexed vector m)]
    (aset a (* i 2) k)
    (aset a (inc (* i 2)) v))
  (assert (= m (clojure.lang.PersistentArrayMap/createWithCheck a)))
  ;; 27ns
  (cc/quick-bench
   (clojure.lang.PersistentArrayMap/createWithCheck a)))

[bsless]

static public PersistentArrayMap createWithCheck(Object[] init){
	for(int i=0;i< init.length;i += 2)
		{
		for(int j=i+2;j<init.length;j += 2)
			{
			if(equalKey(init[i],init[j]))
				throw new IllegalArgumentException("Duplicate key: " + init[i]);
			}
		}
	return new PersistentArrayMap(init);
}

It's not slow, but need to be aware it's quadratic, don't know what which size it becomes too heavy