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|
# -*- perl -*-
#
# Perfect Minimal Hash Generator written in Perl, which produces
# C output.
#
# Requires the CPAN Graph module (tested against 0.81, 0.83, 0.84)
#
use Graph::Undirected;
require 'random_sv_vectors.ph';
#
# Truncate to 32-bit integer
#
sub int32($) {
my($x) = @_;
return int($x) % 4294967296;
}
#
# 32-bit rotate
#
sub rot($$) {
my($v,$s) = @_;
$v = int32($v);
return int32(($v << $s)|($v >> (32-$s)));
}
#
# Compute the prehash for a key
#
# prehash(key, sv, N)
#
sub prehash($$$) {
my($key, $n, $sv) = @_;
my $c;
my $k1 = 0, $k2 = 0;
my $ko1, $ko2;
my($s0, $s1, $s2, $s3) = @{$sv};
foreach $c (unpack("C*", $key)) {
$ko1 = $k1; $ko2 = $k2;
$k1 = int32(rot($ko1,$s0)-rot($ko2, $s1)+$c);
$k2 = int32(rot($ko2,$s2)-rot($ko1, $s3)+$c);
}
# Create a bipartite graph...
$k1 = (($k1 & ($n-1)) << 1) + 0;
$k2 = (($k2 & ($n-1)) << 1) + 1;
return ($k1, $k2);
}
#
# Walk the assignment graph
#
sub walk_graph($$$) {
my($gr,$n,$v) = @_;
my $nx;
# print STDERR "Vertex $n value $v\n";
$gr->set_vertex_attribute($n,"val",$v);
foreach $nx ($gr->neighbors($n)) {
die unless ($gr->has_edge_attribute($n, $nx, "hash"));
my $e = $gr->get_edge_attribute($n, $nx, "hash");
# print STDERR "Edge $n=$nx value $e: ";
if ($gr->has_vertex_attribute($nx, "val")) {
die if ($v+$gr->get_vertex_attribute($nx, "val") != $e);
# print STDERR "ok\n";
} else {
walk_graph($gr, $nx, $e-$v);
}
}
}
#
# Generate the function assuming a given N.
#
# gen_hash_n(N, sv, \%data)
#
sub gen_hash_n($$$) {
my($n, $sv, $href) = @_;
my @keys = keys(%{$href});
my $i, $sv, @g;
my $gr;
my $k, $v;
my $gsize = 2*$n;
$gr = Graph::Undirected->new;
for ($i = 0; $i < $gsize; $i++) {
$gr->add_vertex($i);
}
foreach $k (@keys) {
my ($pf1, $pf2) = prehash($k, $n, $sv);
my $e = ${$href}{$k};
if ($gr->has_edge($pf1, $pf2)) {
my $xkey = $gr->get_edge_attribute($pf1, $pf2, "key");
my ($xp1, $xp2) = prehash($xkey, $n, $sv);
print STDERR "Collision: $pf1=$pf2 $k with ";
print STDERR "$xkey ($xp1,$xp2)\n";
return;
}
# print STDERR "Edge $pf1=$pf2 value $e from $k\n";
$gr->add_edge($pf1, $pf2);
$gr->set_edge_attribute($pf1, $pf2, "hash", $e);
$gr->set_edge_attribute($pf1, $pf2, "key", $k);
}
# At this point, we're good if the graph is acyclic.
if ($gr->is_cyclic) {
print STDERR "Graph is cyclic\n";
return;
}
# print STDERR "Graph:\n$gr\n";
# Now we need to assign values to each vertex, so that for each
# edge, the sum of the values for the two vertices give the value
# for the edge (which is our hash index.) Since the graph is
# acyclic, this is always doable.
for ($i = 0; $i < $gsize; $i++) {
if ($gr->degree($i)) {
# This vertex has neighbors (is used)
if (!$gr->has_vertex_attribute($i, "val")) {
walk_graph($gr,$i,0); # First vertex in a cluster
}
push(@g, $gr->get_vertex_attribute($i, "val"));
} else {
# Unused vertex
push(@g, undef);
}
}
# for ($i = 0; $i < $n; $i++) {
# print STDERR "Vertex ", $i, ": ", $g[$i], "\n";
# }
print STDERR "Done: n = $n, sv = [", join(',', @$sv), "]\n";
return ($n, $sv, \@g);
}
#
# Driver for generating the function
#
# gen_perfect_hash(\%data)
#
sub gen_perfect_hash($) {
my($href) = @_;
my @keys = keys(%{$href});
my @hashinfo;
my $n, $i, $j, $sv, $maxj;
# 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;
}
# Number of times to try...
$maxj = scalar @random_sv_vectors;
for ($i = 0; $i < 4; $i++) {
print STDERR "Trying n = $n...\n";
for ($j = 0; $j < $maxj; $j++) {
$sv = $random_sv_vectors[$j];
@hashinfo = gen_hash_n($n, $sv, $href);
return @hashinfo if (defined(@hashinfo));
}
$n <<= 1;
$maxj >>= 1;
}
return;
}
#
# Read input file
#
sub read_input() {
my $key,$val;
my %out;
my $x = 0;
while (defined($l = <STDIN>)) {
chomp $l;
$l =~ s/\s*(\#.*|)$//;
next if ($l eq '');
if ($l =~ /^([^=]+)\=([^=]+)$/) {
$out{$1} = $2;
$x = $2;
} else {
$out{$l} = $x;
}
$x++;
}
return %out;
}
#
# Verify that the hash table is actually correct...
#
sub verify_hash_table($$)
{
my ($href, $hashinfo) = @_;
my ($n, $sv, $g) = @{$hashinfo};
my $k;
my $err = 0;
foreach $k (keys(%$href)) {
my ($pf1, $pf2) = prehash($k, $n, $sv);
my $g1 = ${$g}[$pf1];
my $g2 = ${$g}[$pf2];
if ($g1+$g2 != ${$href}{$k}) {
printf STDERR "%s(%d,%d): %d+%d = %d != %d\n",
$k, $pf1, $pf2, $g1, $g2, $g1+$g2, ${$href}{$k};
$err = 1;
} else {
# printf STDERR "%s: %d+%d = %d ok\n",
# $k, $g1, $g2, $g1+$g2;
}
}
die "$0: hash validation error\n" if ($err);
}
1;
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