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[map] Implement Robinhood hashing

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Numbers are not improved though. :(
This commit is contained in:
Behdad Esfahbod 2023-05-09 22:44:22 -06:00
parent 2133aa2407
commit 18c3ba5018
2 changed files with 35 additions and 7 deletions
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2
src/hb-algs.hh
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@ -314,7 +314,7 @@ struct
template <typename T> constexpr auto
impl (const T& v, hb_priority<2>) const HB_RETURN (uint32_t, hb_deref (v).hash ())
#if 0
#if 1
// The following, unfortunately, while keeps the probing chains short, slows
// down the overall hash table performance. Not because of the extra operation
// itself in my opinion, but something else going on that we have not been able

40
src/hb-map.hh
View file

@ -200,22 +200,46 @@ struct hb_hashmap_t
return true;
}
unsigned
probe_distance (unsigned i, unsigned hash) const
{
return (i + mask + 1 - (hash % prime)) & mask;
}
template <typename KK, typename VV>
bool set_with_hash (KK&& key, uint32_t hash, VV&& value)
{
if (unlikely (!successful)) return false;
if (unlikely ((occupancy + occupancy / 2) >= mask && !resize ())) return false;
K k = std::forward<KK> (key);
V v = std::forward<VV> (value);
hash &= 0x3FFFFFFF; // We only store lower 30bit of hash
unsigned int i = hash % prime;
unsigned dist = 0;
while (items[i].is_used ())
{
if ((hb_is_same (K, hb_codepoint_t) || items[i].hash == hash) &&
items[i] == key)
break;
if (items[i].is_tombstone ())
break;
if ((hb_is_same (K, hb_codepoint_t) || items[i].hash == hash) &&
items[i] == k)
break;
// Robinhood hashing
// https://www.sebastiansylvan.com/post/robin-hood-hashing-should-be-your-default-hash-table-implementation/
unsigned existing_dist = probe_distance (i, items[i].hash);
if (existing_dist < dist)
{
hb_swap (items[i].key, k);
//hb_swap (items[i].hash, hash);
uint32_t tmp = items[i].hash; items[i].hash = hash; hash = tmp;
hb_swap (items[i].value, v);
dist = existing_dist;
}
i = (i + 1) & mask;
dist++;
}
item_t &item = items[i];
@ -227,8 +251,8 @@ struct hb_hashmap_t
population--;
}
item.key = std::forward<KK> (key);
item.value = std::forward<VV> (value);
item.key = std::move (k);
item.value = std::move (v);
item.hash = hash;
item.set_used (true);
item.set_tombstone (false);
@ -284,9 +308,12 @@ struct hb_hashmap_t
if (unlikely (!items)) return nullptr;
hash &= 0x3FFFFFFF; // We only store lower 30bit of hash
unsigned int i = hash % prime;
unsigned i = hash % prime;
unsigned dist = 0;
while (items[i].is_used ())
{
if (dist > probe_distance (i, items[i].hash))
return nullptr;
if ((hb_is_same (K, hb_codepoint_t) || items[i].hash == hash) &&
items[i] == key)
{
@ -296,6 +323,7 @@ struct hb_hashmap_t
return nullptr;
}
i = (i + 1) & mask;
dist++;
}
return nullptr;
}