LeafList.hashCode() fixed

[ruslansennov]

@danieldietrich @orionll #1818
I believe that only LeafList.hashCode() should be changed.

[codecov-io]

Current coverage is 97.08% (diff: 100%)

Merging #1823 into master will increase coverage by <.01%

@@             master      #1823   diff @@
==========================================
  Files            87         87          
  Lines         11277      11283     +6   
  Methods           0          0          
  Messages          0          0          
  Branches       1869       1870     +1   
==========================================
+ Hits          10948      10954     +6   
  Misses          207        207          
  Partials        122        122          

Powered by Codecov. Last update be0b078...c4d9580

[orionll]

return Objects.hash(hash, value) + tail.hashCode();

I think this can fail with StackOverflowError if the map contains large number of keys with the same hash codes.

Try something like this:

class A {
    @Override
    public int hashCode() {
        return 1;
    }

    public static void main(String[] args) {
        HashMap<A, Integer> map = HashMap.empty();
        for (int i = 0; i < 10000; i++) {
            map = map.put(new A(), i);
        }
        System.out.println(map.hashCode());
    }
}

I suggest rewriting hashCode() the same way it is done in java.util.AbstractMap. This is simple and safe: http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/37a05a11f281/src/share/classes/java/util/AbstractMap.java#l490

[danieldietrich]

I also think adding hashcodes is a viable solution. I created #1818 to ensure all Hash* impls work properly.
The StackOverflow problem is real (using Objects.hash(val) ... + tail.hashCode()). I fixed it recently for List.hashCode...

[danieldietrich]

@ruslansennov Looks good to me! Please merge it if no one has any further objections :)