1#!./perl -w
2
3BEGIN {
4    chdir 't' if -d 't';
5    @INC = '../lib';
6    require './test.pl';
7}
8
9use strict;
10
11plan tests => 5;
12
13my %h;
14
15ok (!Internals::HvREHASH(%h), "hash doesn't start with rehash flag on");
16
17foreach (1..10) {
18  $h{"\0"x$_}++;
19}
20
21ok (!Internals::HvREHASH(%h), "10 entries doesn't trigger rehash");
22
23foreach (11..20) {
24  $h{"\0"x$_}++;
25}
26
27ok (Internals::HvREHASH(%h), "20 entries triggers rehash");
28
29
30
31
32# second part using an emulation of the PERL_HASH in perl, mounting an
33# attack on a prepopulated hash. This is also useful if you need normal
34# keys which don't contain \0 -- suitable for stashes
35
36use constant MASK_U32  => 2**32;
37use constant HASH_SEED => 0;
38use constant THRESHOLD => 14;
39use constant START     => "a";
40
41# some initial hash data
42my %h2 = map {$_ => 1} 'a'..'cc';
43
44ok (!Internals::HvREHASH(%h2), 
45    "starting with pre-populated non-pathalogical hash (rehash flag if off)");
46
47my @keys = get_keys(\%h2);
48$h2{$_}++ for @keys;
49ok (Internals::HvREHASH(%h2), 
50    scalar(@keys) . " colliding into the same bucket keys are triggerring rehash");
51
52sub get_keys {
53    my $hr = shift;
54
55    # the minimum of bits required to mount the attack on a hash
56    my $min_bits = log(THRESHOLD)/log(2);
57
58    # if the hash has already been populated with a significant amount
59    # of entries the number of mask bits can be higher
60    my $keys = scalar keys %$hr;
61    my $bits = $keys ? log($keys)/log(2) : 0;
62    $bits = $min_bits if $min_bits > $bits;
63
64    $bits = int($bits) < $bits ? int($bits) + 1 : int($bits);
65    # need to add 2 bits to cover the internal split cases
66    $bits += 2;
67    my $mask = 2**$bits-1;
68    print "# using mask: $mask ($bits)\n";
69
70    my @keys;
71    my $s = START;
72    my $c = 0;
73    # get 2 keys on top of the THRESHOLD
74    my $hash;
75    while (@keys < THRESHOLD+2) {
76        # next if exists $hash->{$s};
77        $hash = hash($s);
78        next unless ($hash & $mask) == 0;
79        $c++;
80        printf "# %2d: %5s, %10s\n", $c, $s, $hash;
81        push @keys, $s;
82    } continue {
83        $s++;
84    }
85
86    return @keys;
87}
88
89
90# trying to provide the fastest equivalent of C macro's PERL_HASH in
91# Perl - the main complication is that it uses U32 integer, which we
92# can't do it perl, without doing some tricks
93sub hash {
94    my $s = shift;
95    my @c = split //, $s;
96    my $u = HASH_SEED;
97    for (@c) {
98        # (A % M) + (B % M) == (A + B) % M
99        # This works because '+' produces a NV, which is big enough to hold
100        # the intermidiate result. We only need the % before any "^" and "&"
101        # to get the result in the range for an I32.
102        # and << doesn't work on NV, so using 1 << 10
103        $u += ord;
104        $u += $u * (1 << 10); $u %= MASK_U32;
105        $u ^= $u >> 6;
106    }
107    $u += $u << 3;  $u %= MASK_U32;
108    $u ^= $u >> 11; $u %= MASK_U32;
109    $u += $u << 15; $u %= MASK_U32;
110    $u;
111}
112