Add hcode. Re-factor rawGet. Fix infinite loop.

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.

1 files changed, 94 insertions, 41 deletions

diff --git a/lib/pure/collections/sets.nim b/lib/pure/collections/sets.nim
index 4cc46149e..72fd5cd81 100644
--- a/lib/pure/collections/sets.nim
+++ b/lib/pure/collections/sets.nim
@@ -24,9 +24,12 @@ import
 when not defined(nimhygiene):
   {.pragma: dirty.}
 
+# For "integer-like A" that are too big for intsets/bit-vectors to be practical,
+# it would be best to shrink hcode to the same size as the integer.  Larger
+# codes should never be needed, and this can pack more entries per cache-line.
+# Losing hcode entirely is also possible - if some element value is forbidden.
 type
-  SlotEnum = enum seEmpty, seFilled, seDeleted
-  KeyValuePair[A] = tuple[slot: SlotEnum, key: A]
+  KeyValuePair[A] = tuple[hcode: THash, key: A]
   KeyValuePairSeq[A] = seq[KeyValuePair[A]]
   HashSet* {.myShallow.}[A] = object ## \
     ## A generic hash set.
@@ -38,6 +41,14 @@ type
 
 {.deprecated: [TSet: HashSet].}
 
+# hcode for real keys cannot be zero.  hcode==0 signifies an empty slot.  These
+# two procs retain clarity of that encoding without the space cost of an enum.
+proc isEmpty(hcode: THash): bool {.inline.} =
+  result = hcode == 0
+
+proc isFilled(hcode: THash): bool {.inline.} =
+  result = hcode != 0
+
 proc isValid*[A](s: HashSet[A]): bool =
   ## Returns `true` if the set has been initialized with `initSet <#initSet>`_.
   ##
@@ -94,7 +105,7 @@ iterator items*[A](s: HashSet[A]): A =
   ##   # --> {(a: 1, b: 3), (a: 0, b: 4)}
   assert s.isValid, "The set needs to be initialized."
   for h in 0..high(s.data):
-    if s.data[h].slot == seFilled: yield s.data[h].key
+    if isFilled(s.data[h].hcode): yield s.data[h].key
 
 const
   growthFactor = 2
@@ -104,24 +115,34 @@ proc mustRehash(length, counter: int): bool {.inline.} =
   result = (length * 2 < counter * 3) or (length - counter < 4)
 
 proc nextTry(h, maxHash: THash): THash {.inline.} =
-  result = ((5 * h) + 1) and maxHash
-
-template rawGetImpl() {.dirty.} =
-  var h: THash = hash(key) and high(s.data) # start with real hash value
-  while s.data[h].slot != seEmpty:
-    if s.data[h].key == key and s.data[h].slot == seFilled:
+  result = (h + 1) and maxHash
+
+template rawGetKnownHCImpl() {.dirty.} =
+  var h: THash = hc and high(s.data)  # start with real hash value
+  while isFilled(s.data[h].hcode):
+    # Compare hc THEN key with boolean short circuit. This makes the common case
+    # zero ==key's for missing (e.g.inserts) and exactly one ==key for present.
+    # It does slow down succeeding lookups by one extra THash cmp&and..usually
+    # just a few clock cycles, generally worth it for any non-integer-like A.
+    if s.data[h].hcode == hc and s.data[h].key == key:  # compare hc THEN key
       return h
     h = nextTry(h, high(s.data))
-  result = -1
+  result = -1 - h                   # < 0 => MISSING; insert idx = -1 - result
+
+template rawGetImpl() {.dirty.} =
+  hc = hash(key)
+  if hc == 0:       # This almost never taken branch should be very predictable.
+    hc = 314159265  # Value doesn't matter; Any non-zero favorite is fine.
+  rawGetKnownHCImpl()
 
 template rawInsertImpl() {.dirty.} =
-  var h: THash = hash(key) and high(data)
-  while data[h].slot == seFilled:
-    h = nextTry(h, high(data))
   data[h].key = key
-  data[h].slot = seFilled
+  data[h].hcode = hc
+
+proc rawGetKnownHC[A](s: HashSet[A], key: A, hc: THash): int {.inline.} =
+  rawGetKnownHCImpl()
 
-proc rawGet[A](s: HashSet[A], key: A): int =
+proc rawGet[A](s: HashSet[A], key: A, hc: var THash): int {.inline.} =
   rawGetImpl()
 
 proc mget*[A](s: var HashSet[A], key: A): var A =
@@ -130,7 +151,8 @@ proc mget*[A](s: var HashSet[A], key: A): var A =
   ## when one overloaded 'hash' and '==' but still needs reference semantics
   ## for sharing.
   assert s.isValid, "The set needs to be initialized."
-  var index = rawGet(s, key)
+  var hc: THash
+  var index = rawGet(s, key, hc)
   if index >= 0: result = s.data[index].key
   else: raise newException(KeyError, "key not found: " & $key)
 
@@ -147,33 +169,43 @@ proc contains*[A](s: HashSet[A], key: A): bool =
   ##   values.excl(2)
   ##   assert(not values.contains(2))
   assert s.isValid, "The set needs to be initialized."
-  var index = rawGet(s, key)
+  var hc: THash
+  var index = rawGet(s, key, hc)
   result = index >= 0
 
-proc rawInsert[A](s: var HashSet[A], data: var KeyValuePairSeq[A], key: A) =
+proc rawInsert[A](s: var HashSet[A], data: var KeyValuePairSeq[A], key: A,
+                  hc: THash, h: THash) =
   rawInsertImpl()
 
 proc enlarge[A](s: var HashSet[A]) =
   var n: KeyValuePairSeq[A]
   newSeq(n, len(s.data) * growthFactor)
-  for i in countup(0, high(s.data)):
-    if s.data[i].slot == seFilled: rawInsert(s, n, s.data[i].key)
-  swap(s.data, n)
+  swap(s.data, n)                   # n is now old seq
+  for i in countup(0, high(n)):
+    if isFilled(n[i].hcode):
+      var j = -1 - rawGetKnownHC(s, n[i].key, n[i].hcode)
+      rawInsert(s, s.data, n[i].key, n[i].hcode, j)
 
 template inclImpl() {.dirty.} =
-  var index = rawGet(s, key)
+  var hc: THash
+  var index = rawGet(s, key, hc)
   if index < 0:
-    if mustRehash(len(s.data), s.counter): enlarge(s)
-    rawInsert(s, s.data, key)
+    if mustRehash(len(s.data), s.counter):
+      enlarge(s)
+      index = rawGetKnownHC(s, key, hc)
+    rawInsert(s, s.data, key, hc, -1 - index)
     inc(s.counter)
 
 template containsOrInclImpl() {.dirty.} =
-  var index = rawGet(s, key)
+  var hc: THash
+  var index = rawGet(s, key, hc)
   if index >= 0:
     result = true
   else:
-    if mustRehash(len(s.data), s.counter): enlarge(s)
-    rawInsert(s, s.data, key)
+    if mustRehash(len(s.data), s.counter):
+      enlarge(s)
+      index = rawGetKnownHC(s, key, hc)
+    rawInsert(s, s.data, key, hc, -1 - index)
     inc(s.counter)
 
 proc incl*[A](s: var HashSet[A], key: A) =
@@ -204,6 +236,10 @@ proc incl*[A](s: var HashSet[A], other: HashSet[A]) =
   assert other.isValid, "The set `other` needs to be initialized."
   for item in other: incl(s, item)
 
+template doWhile(a: expr, b: stmt): stmt =
+  b
+  while a: b
+
 proc excl*[A](s: var HashSet[A], key: A) =
   ## Excludes `key` from the set `s`.
   ##
@@ -215,10 +251,22 @@ proc excl*[A](s: var HashSet[A], key: A) =
   ##   s.excl(2)
   ##   assert s.len == 3
   assert s.isValid, "The set needs to be initialized."
-  var index = rawGet(s, key)
-  if index >= 0:
-    s.data[index].slot = seDeleted
+  var hc: THash
+  var i = rawGet(s, key, hc)
+  var msk = high(s.data)
+  if i >= 0:
+    s.data[i].hcode = 0
     dec(s.counter)
+    while true:         # KnuthV3 Algo6.4R adapted for i=i+1 instead of i=i-1
+      var j = i         # The correctness of this depends on (h+1) in nextTry,
+      var r = j         # though may be adaptable to other simple sequences.
+      s.data[i].hcode = 0              # mark current EMPTY
+      doWhile ((i >= r and r > j) or (r > j and j > i) or (j > i and i >= r)):
+        i = (i + 1) and msk            # increment mod table size
+        if isEmpty(s.data[i].hcode):   # end of collision cluster; So all done
+          return
+        r = s.data[i].hcode and msk    # "home" location of key@i
+      s.data[j] = s.data[i]            # data[j] will be marked EMPTY next loop
 
 proc excl*[A](s: var HashSet[A], other: HashSet[A]) =
   ## Excludes everything in `other` from `s`.
@@ -494,7 +542,7 @@ proc map*[A, B](data: HashSet[A], op: proc (x: A): B {.closure.}): HashSet[B] =
 
 type
   OrderedKeyValuePair[A] = tuple[
-    slot: SlotEnum, next: int, key: A]
+    hcode: THash, next: int, key: A]
   OrderedKeyValuePairSeq[A] = seq[OrderedKeyValuePair[A]]
   OrderedSet* {.myShallow.}[A] = object ## \
     ## A generic hash set that remembers insertion order.
@@ -546,7 +594,7 @@ template forAllOrderedPairs(yieldStmt: stmt) {.dirty, immediate.} =
   var h = s.first
   while h >= 0:
     var nxt = s.data[h].next
-    if s.data[h].slot == seFilled: yieldStmt
+    if isFilled(s.data[h].hcode): yieldStmt
     h = nxt
 
 iterator items*[A](s: OrderedSet[A]): A =
@@ -571,7 +619,10 @@ iterator items*[A](s: OrderedSet[A]): A =
   forAllOrderedPairs:
     yield s.data[h].key
 
-proc rawGet[A](s: OrderedSet[A], key: A): int =
+proc rawGetKnownHC[A](s: OrderedSet[A], key: A, hc: THash): int {.inline.} =
+  rawGetKnownHCImpl()
+
+proc rawGet[A](s: OrderedSet[A], key: A, hc: var THash): int {.inline.} =
   rawGetImpl()
 
 proc contains*[A](s: OrderedSet[A], key: A): bool =
@@ -585,11 +636,12 @@ proc contains*[A](s: OrderedSet[A], key: A): bool =
   ##   values.incl(2)
   ##   assert values.contains(2)
   assert s.isValid, "The set needs to be initialized."
-  var index = rawGet(s, key)
+  var hc: THash
+  var index = rawGet(s, key, hc)
   result = index >= 0
 
-proc rawInsert[A](s: var OrderedSet[A], 
-                  data: var OrderedKeyValuePairSeq[A], key: A) =
+proc rawInsert[A](s: var OrderedSet[A], data: var OrderedKeyValuePairSeq[A],
+                  key: A, hc: THash, h: THash) =
   rawInsertImpl()
   data[h].next = -1
   if s.first < 0: s.first = h
@@ -602,12 +654,13 @@ proc enlarge[A](s: var OrderedSet[A]) =
   var h = s.first
   s.first = -1
   s.last = -1
+  swap(s.data, n)
   while h >= 0:
-    var nxt = s.data[h].next
-    if s.data[h].slot == seFilled: 
-      rawInsert(s, n, s.data[h].key)
+    var nxt = n[h].next
+    if isFilled(n[h].hcode):
+      var j = -1 - rawGetKnownHC(s, n[h].key, n[h].hcode)
+      rawInsert(s, s.data, n[h].key, n[h].hcode, j)
     h = nxt
-  swap(s.data, n)
 
 proc incl*[A](s: var OrderedSet[A], key: A) =
   ## Includes an element `key` in `s`.
@@ -726,7 +779,7 @@ proc `==`*[A](s, t: OrderedSet[A]): bool =
   while h >= 0 and g >= 0:
     var nxh = s.data[h].next
     var nxg = t.data[g].next
-    if s.data[h].slot == seFilled and s.data[g].slot == seFilled:
+    if isFilled(s.data[h].hcode) and isFilled(s.data[g].hcode):
       if s.data[h].key == s.data[g].key:
         inc compared
       else: