Replace state enum with a cached hash code which has the same memory overhead
and locality as the enum, but can really speed things up with non-integer-like
keys (keys for which either hash() or == take more than couple cycles, or where
the key data is "indirect" and might incur another cache miss). To function as
both empty/filled state and a hash code cache, it only needs to be ensured that
hash codes are non-zero for any real key. That is done at the one place in the
whole file hash() is called. Keep convention clear via isFilled() & isEmpty().
An isDeleted state will no longer be necessary as per below excl/inf loop fix.
Since some use sites know hc and some do not, re-factor rawGet into two forms -
one with known hash code and one with an unknown HC that returns it. Both forms
still return <0 on missing, but returns the much more informative "-1 - index".
That return can be quickly inverted by -1 - result to recover the index where
insert should happen, provided no modifications are made to the table in the
meantime. This protocol retains the prior <0 interface and also makes it easy
to avoid unnecessary duplicate search work in procs like containsOrInclImpl
(which formerly searched in the initial get and AGAIN in rawInsert). Strip the
searching part out of rawInsert to "make it even more raw". swap(s.data, n) a
bit earlier so rawGet and rawGetKnownHC can have similar parameter lists and
integrate well with rawInsert/code sharing between Set and OrderedSet impls.
This PR also fixes infinite looping upon too many deletes. [ The deleted state
(aka "tombstone") approach is vulnerable to the table filling up with deleted
items which forces giant scans for missing keys which could be anywhere. In
the version prior to this PR, table wraparound wasn't even detected yielding
infinite loops. ] This PR changes excl() from marking slots as deleted to Knuth
algo 6.4R, "local/incremental moveback rehashing" - adapted from Knuth's h->h-1
to the cache-friendlier h->h+1 probe sequence and adapted from "gotos" to a new
doWhile template. This method restores the table to a state that would have
resulted from pure inserts (in some order). Update nextTry accordingly. Since
linear probing can degrade a little faster, 50% rather than 66% may be a better
default growth threshold, but users should be able to adjust threshold anyway.
Old unit tests all pass. More extensive testing in this module is probably
warranted before taking similar enhancements over to collections.tables.
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