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New hash function for integer keys 

When integer keys do not form a sequence, it is better to use all their
bits to compute their hashes. (The previous implementation was quite bad
for integer keys with common lower bits, and disastrous for integer keys
changing only in their upper 32 bits.)
This commit is contained in:
Roberto Ierusalimschy 2021-03-29 15:47:18 -03:00
parent bf10593a3a
commit 7fbe215808
1 changed files with 14 additions and 2 deletions
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16
ltable.c
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@ -84,8 +84,6 @@
#define hashstr(t,str) hashpow2(t, (str)->hash) #define hashstr(t,str) hashpow2(t, (str)->hash)
#define hashboolean(t,p) hashpow2(t, p) #define hashboolean(t,p) hashpow2(t, p)
#define hashint(t,i) hashpow2(t, i)
#define hashpointer(t,p) hashmod(t, point2uint(p)) #define hashpointer(t,p) hashmod(t, point2uint(p))
@ -101,6 +99,20 @@ static const Node dummynode_ = {
static const TValue absentkey = {ABSTKEYCONSTANT}; static const TValue absentkey = {ABSTKEYCONSTANT};
/*
** Hash for integers. To allow a good hash, use the remainder operator
** ('%'). If integer fits as a non-negative int, compute an int
** remainder, which is faster. Otherwise, use an unsigned-integer
** remainder, which uses all bits and ensures a non-negative result.
*/
static Node *hashint (const Table *t, lua_Integer i) {
lua_Unsigned ui = l_castS2U(i);
if (ui <= (unsigned int)INT_MAX)
return hashmod(t, cast_int(ui));
else
return hashmod(t, ui);
}
/* /*
** Hash for floating-point numbers. ** Hash for floating-point numbers.