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Unwind just the "pseudorandom probing" part of recent sets,tables changes (#13816)

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* Unwind just the "pseudorandom probing" (whole hash-code-keyed variable
stride double hashing) part of recent sets & tables changes (which has
still been causing bugs over a month later (e.g., two days ago
https://github.com/nim-lang/Nim/issues/13794) as well as still having
several "figure this out" implementation question comments in them (see
just diffs of this PR).

This topic has been discussed in many places:
  https://github.com/nim-lang/Nim/issues/13393
  https://github.com/nim-lang/Nim/pull/13418
  https://github.com/nim-lang/Nim/pull/13440
  https://github.com/nim-lang/Nim/issues/13794

Alternative/non-mandatory stronger integer hashes (or vice-versa opt-in
identity hashes) are a better solution that is more general (no illusion
of one hard-coded sequence solving all problems) while retaining the
virtues of linear probing such as cache obliviousness and age-less tables
under delete-heavy workloads (still untested after a month of this change).

The only real solution for truly adversarial keys is a hash keyed off of
data unobservable to attackers.  That all fits better with a few families
of user-pluggable/define-switchable hashes which can be provided in a
separate PR more about `hashes.nim`.

This PR carefully preserves the better (but still hard coded!) probing
of the  `intsets` and other recent fixes like `move` annotations, hash
order invariant tests, `intsets.missingOrExcl` fixing, and the move of
`rightSize` into `hashcommon.nim`.

* Fix `data.len` -> `dataLen` problem.
This commit is contained in:
c-blake 2020-03-31 13:18:45 -04:00 • committed by GitHub
commit b1aa3b1eea
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8 changed files with 99 additions and 175 deletions
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4
lib/pure/collections/sharedtables.nim
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@ -25,7 +25,6 @@ type
SharedTable*[A, B] = object ## generic hash SharedTable
data: KeyValuePairSeq[A, B]
counter, dataLen: int
countDeleted: int
lock: Lock
template maxHash(t): untyped = t.dataLen-1
@ -50,10 +49,9 @@ proc enlarge[A, B](t: var SharedTable[A, B]) =
for i in 0..<oldSize:
let eh = n[i].hcode
if isFilled(eh):
var perturb = t.getPerturb(eh)
var j: Hash = eh and maxHash(t)
while isFilled(t.data[j].hcode):
j = nextTry(j, maxHash(t), perturb)
j = nextTry(j, maxHash(t))
rawInsert(t, t.data, n[i].key, n[i].val, eh, j)
deallocShared(n)