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phash.ph: more powerful prehashing

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This commit is contained in:
H. Peter Anvin 2007-08-30 21:39:37 +00:00
parent b44d7a76a2
commit 215c1a3781
1 changed files with 20 additions and 14 deletions
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34
perllib/phash.ph
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@ -29,10 +29,11 @@ sub prehash($$$) {
my($key, $n, $sv) = @_;
my $c;
my $k1 = 0, $k2 = 0;
my($s0, $s1, $s2, $s3) = @{$sv};
foreach $c (unpack("C*", $key)) {
$k1 = (rot($k1,$sv+2) + rot($k1, 5) + $c) & 0xffffffff;
$k2 = (rot($k2,$sv) + rot($k2, 7) + $c) & 0xffffffff;
$k1 = (rot($k1,$s0)-rot($k2, $s1)+$c) & 0xffffffff;
$k2 = (rot($k2,$s2)-rot($k1, $s3)+$c) & 0xffffffff;
}
return ($k1 % $n, $k2 % $n);
@ -108,8 +109,6 @@ sub gen_hash_n($$$) {
@f1 = ffunc($n);
@f2 = ffunc($n);
print STDERR "*** New graph ***\n";
$gr = Graph::Undirected->new;
for ($i = 0; $i < $n; $i++) {
$gr->add_vertex($i);
@ -164,6 +163,13 @@ sub gen_hash_n($$$) {
return ($n, $sv, \@f1, \@f2, \@g);
}
#
# Generate a random prehash vector
#
sub prehash_vector()
{
return [int(rand(32)), int(rand(32)), int(rand(32)), int(rand(32))];
}
#
# Driver for generating the function
@ -176,21 +182,21 @@ sub gen_perfect_hash($) {
my @hashinfo;
my $n, $i, $j, $sv, $maxj;
# Minimal power of 2 value for N
# Minimal power of 2 value for N with enough wiggle room
my $room = scalar(@keys)*1.3;
$n = 1;
while ($n < scalar(@keys)) {
while ($n < $room) {
$n <<= 1;
}
$maxj = 256; # Number of times to try
$maxj = 512; # Number of times to try
for ($i = 0; $i < 4; $i++) {
print STDERR "Trying n = $n...\n";
for ($j = 0; $j < $maxj; $j++) {
for ($sv = 0; $sv < 32; $sv++) {
@hashinfo = gen_hash_n($n, $sv, $href);
return @hashinfo if (defined(@hashinfo));
}
$sv = prehash_vector();
@hashinfo = gen_hash_n($n, $sv, $href);
return @hashinfo if (defined(@hashinfo));
}
$n <<= 1;
$maxj >>= 1;
@ -249,8 +255,8 @@ sub verify_hash_table($$)
$k, $p1, $p2, $pf1, $pf2, $g1, $g2, $g1+$g2, ${$href}{$k};
$err = 1;
} else {
printf STDERR "%s: %d+%d = %d ok\n",
$k, $g1, $g2, $g1+$g2;
# printf STDERR "%s: %d+%d = %d ok\n",
# $k, $g1, $g2, $g1+$g2;
}
}