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Improve set of candidate multipliers for perfect hash function generation

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The previous set of multipliers was not adapted for large sets of short
keys, and this new set of multipliers allows to generate perfect hash
functions for larger sets without having an impact for existing callers
of those functions, as experimentation has showed.  A future commit will
make use of that to improve the performance of unicode normalization.

All multipliers compile to shift-and-add instructions on most platforms.
This has been tested as far back as gcc 4.1 and clang 3.8.

Author: John Naylor
Reviewed-by: Mark Dilger, Michael Paquier
Discussion: https://postgr.es/m/CACPNZCt4fbJ0_bGrN5QPt34N4whv=mszM0LMVQdoa2rC9UMRXA@mail.gmail.com
This commit is contained in:
Michael Paquier 2020-10-08 13:16:43 +09:00
parent 9868167500
commit 2a73164581
1 changed files with 2 additions and 2 deletions
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4
src/tools/PerfectHash.pm
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@ -81,13 +81,13 @@ sub generate_hash_function
# to calculate via shift-and-add, so don't change them without care.
# (Commonly, random seeds are tried, but we want reproducible results
# from this program so we don't do that.)
my $hash_mult1 = 31;
my $hash_mult1 = 257;
my $hash_mult2;
my $hash_seed1;
my $hash_seed2;
my @subresult;
FIND_PARAMS:
foreach (127, 257, 521, 1033, 2053)
foreach (17, 31, 127, 8191)
{
$hash_mult2 = $_; # "foreach $hash_mult2" doesn't work
for ($hash_seed1 = 0; $hash_seed1 < 10; $hash_seed1++)