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phash.ph: use a bipartite graph to reduce the storage requirements

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Since we fold the f- and g-functions together, if we guarantee that g is
bipartite, we can make g twice the size of f without cost.  This greatly
improves the odds of generating a smaller hash.
This commit is contained in:
H. Peter Anvin 2007-08-30 23:42:39 +00:00
parent 74cc5e569c
commit fb5a599c8a
2 changed files with 20 additions and 14 deletions
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2
Makefile.in
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@ -113,7 +113,7 @@ regs.h: regs.dat regs.pl
$(PERL) $(srcdir)/regs.pl h $(srcdir)/regs.dat > regs.h
# Token hash
tokhash.c: insns.dat regs.dat tokens.dat tokhash.pl
tokhash.c: insns.dat regs.dat tokens.dat tokhash.pl perllib/phash.ph
$(PERL) $(srcdir)/tokhash.pl $(srcdir)/insns.dat $(srcdir)/regs.dat \
$(srcdir)/tokens.dat > tokhash.c

32
perllib/phash.ph
View file

@ -3,7 +3,7 @@
# Perfect Minimal Hash Generator written in Perl, which produces
# C output.
#
# Requires the CPAN Graph module (tested against 0.83)
# Requires the CPAN Graph module (tested against 0.81, 0.83, 0.84)
use Graph::Undirected;
@ -65,10 +65,15 @@ sub shuffle(@) {
#
# ffunc(N)
#
sub ffunc($) {
my($n) = @_;
sub ffunc($$$) {
my($n,$s,$i) = @_;
my(@l) = ();
return shuffle(0..$n-1);
while ($n--) {
push(@l, $i);
$i += $s;
}
return shuffle(@l);
}
#
@ -107,12 +112,13 @@ sub gen_hash_n($$$) {
my $i, $sv, @f1, @f2, @g;
my $gr;
my $k, $v;
my $gsize = 2*$n;
@f1 = ffunc($n);
@f2 = ffunc($n);
@f1 = ffunc($n, 2, 0);
@f2 = ffunc($n, 2, 1);
$gr = Graph::Undirected->new;
for ($i = 0; $i < $n; $i++) {
for ($i = 0; $i < $gsize; $i++) {
$gr->add_vertex($i);
}
@ -149,7 +155,7 @@ sub gen_hash_n($$$) {
# edge, the sum of the values for the two vertices give the value
# for the edge (which is our hash index.) Since the graph is
# acyclic, this is always doable.
for ($i = 0; $i < $n; $i++) {
for ($i = 0; $i < $gsize; $i++) {
if (!$gr->has_vertex_attribute($i, "val")) {
walk_graph($gr,$i,0); # First vertex in a cluster
}
@ -160,7 +166,7 @@ sub gen_hash_n($$$) {
# print STDERR "Vertex ", $i, ": ", $g[$i], "\n";
# }
print STDERR "Done: n = $n, sv = $sv\n";
print STDERR "Done: n = $n, sv = [", join(',', @$sv), "]\n";
return ($n, $sv, \@f1, \@f2, \@g);
}
@ -184,8 +190,10 @@ sub gen_perfect_hash($) {
my @hashinfo;
my $n, $i, $j, $sv, $maxj;
# Minimal power of 2 value for N with enough wiggle room
my $room = scalar(@keys)*1.3;
# Minimal power of 2 value for N with enough wiggle room.
# The scaling constant must be larger than 0.5 in order for the
# algorithm to ever terminate.
my $room = scalar(@keys)*0.8;
$n = 1;
while ($n < $room) {
$n <<= 1;
@ -243,8 +251,6 @@ sub verify_hash_table($$)
my $k;
my $err = 0;
print STDERR "Verify: n = $n, sv = $sv\n";
foreach $k (keys(%$href)) {
my ($p1, $p2) = prehash($k, $n, $sv);
my $pf1 = ${$f1}[$p1];