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|
# -*- perl -*-
#
# Perfect Minimal Hash Generator written in Perl, which produces
# C output.
#
require 'random_sv_vectors.ph';
require 'crc64.ph';
#
# Compute the prehash for a key
#
# prehash(key, sv, N)
#
sub prehash($$$) {
my($key, $n, $sv) = @_;
my @c = crc64($sv, $key);
my $nmask = ($n << 1) - 2;
# Create a bipartite graph...
$k1 = ($c[1] & $nmask) + 0; # low word
$k2 = ($c[0] & $nmask) + 1; # high word
return ($k1, $k2);
}
#
# Walk the assignment graph, return true on success
#
sub walk_graph($$$$) {
my($nodeval,$nodeneigh,$n,$v) = @_;
my $nx;
# print STDERR "Vertex $n value $v\n";
$$nodeval[$n] = $v;
foreach $nx (@{$$nodeneigh[$n]}) {
# $nx -> [neigh, hash]
my ($o, $e) = @$nx;
# print STDERR "Edge $n,$o value $e: ";
my $ov;
if (defined($ov = $$nodeval[$o])) {
if ($v+$ov != $e) {
# Cyclic graph with collision
# print STDERR "error, should be ", $v+$ov, "\n";
return 0;
} else {
# print STDERR "ok\n";
}
} else {
return 0 unless (walk_graph($nodeval, $nodeneigh, $o, $e-$v));
}
}
return 1;
}
#
# Generate the function assuming a given N.
#
# gen_hash_n(N, sv, \%data, run)
#
sub gen_hash_n($$$$) {
my($n, $sv, $href, $run) = @_;
my @keys = keys(%{$href});
my $i;
my $gr;
my ($k, $v);
my $gsize = 2*$n;
my @nodeval;
my @nodeneigh;
my %edges;
for ($i = 0; $i < $gsize; $i++) {
$nodeneigh[$i] = [];
}
%edges = ();
foreach $k (@keys) {
my ($pf1, $pf2) = prehash($k, $n, $sv);
($pf1,$pf2) = ($pf2,$pf1) if ($pf1 > $pf2); # Canonicalize order
my $pf = "$pf1,$pf2";
my $e = ${$href}{$k};
my $xkey;
if (defined($xkey = $edges{$pf})) {
next if ($e == ${$href}{$xkey}); # Duplicate hash, safe to ignore
if (defined($run)) {
print STDERR "$run: Collision: $pf: $k with $xkey\n";
}
return;
}
# print STDERR "Edge $pf value $e from $k\n";
$edges{$pf} = $k;
push(@{$nodeneigh[$pf1]}, [$pf2, $e]);
push(@{$nodeneigh[$pf2]}, [$pf1, $e]);
}
# Now we need to assign values to each vertex, so that for each
# edge, the sum of the values for the two vertices give the value
# for the edge (which is our hash index.) If we find an impossible
# situation, the graph was cyclic.
@nodeval = (undef) x $gsize;
for ($i = 0; $i < $gsize; $i++) {
if (scalar(@{$nodeneigh[$i]})) {
# This vertex has neighbors (is used)
if (!defined($nodeval[$i])) {
# First vertex in a cluster
unless (walk_graph(\@nodeval, \@nodeneigh, $i, 0)) {
if (defined($run)) {
print STDERR "$run: Graph is cyclic\n";
}
return;
}
}
}
}
# for ($i = 0; $i < $n; $i++) {
# print STDERR "Vertex ", $i, ": ", $g[$i], "\n";
# }
if (defined($run)) {
printf STDERR "$run: Done: n = $n, sv = [0x%08x, 0x%08x]\n",
$$sv[0], $$sv[1];
}
return ($n, $sv, \@nodeval);
}
#
# Driver for generating the function
#
# gen_perfect_hash(\%data)
#
sub gen_perfect_hash($) {
my($href) = @_;
my @keys = keys(%{$href});
my @hashinfo;
my ($n, $i, $j, $sv, $maxj);
my $run = 1;
# Minimal power of 2 value for N with enough wiggle room.
# The scaling constant must be larger than 0.5 in order for the
# algorithm to ever terminate. The higher the scaling constant,
# the more space does the hash take up, but the less likely is it
# that an invalid token will require a string comparison.
my $room = int(scalar(@keys)*1.6);
$n = 1;
while ($n < $room) {
$n <<= 1;
}
# Number of times to try...
$maxj = scalar @random_sv_vectors;
for ($i = 0; $i < 4; $i++) {
printf STDERR "%d vectors, trying n = %d...\n",
scalar @keys, $n;
for ($j = 0; $j < $maxj; $j++) {
$sv = $random_sv_vectors[$j];
@hashinfo = gen_hash_n($n, $sv, $href, $run++);
return @hashinfo if (@hashinfo);
}
$n <<= 1;
}
return;
}
#
# Verify that the hash table is actually correct...
#
sub verify_hash_table($$)
{
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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