StackOverflow2013

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plurals

PO
primarykey
Id
13999090
data
AcceptedAnswerId
0
AnswerCount
0
ClosedDate
CommentCount
0
CommunityOwnedDate
CreationDate
2012-12-22T00:58:17.350
FavoriteCount
0
LastActivityDate
2012-12-22T01:05:20.630
LastEditDate
2012-12-22T01:05:20.630
LastEditorUserId
487064
OwnerUserId
487064
ParentId
13998960
PostTypeId
2
Score
3
ViewCount
0
LastEditorDisplayName
text
Body
The following simple script runs an infinite loop that fetches 2 random 20-byte sequences, calculates CRC16 and checks if there is a collision. Continuous evaluation of this loop de facto estimates collision percentage: <pre><code>#!/usr/bin/env perl use Digest::CRC qw(crc16); open(my $f, '<', '/dev/urandom'); my $n = 0; my $coll = 0; while (1) { read $f, $randstr1, 20; read $f, $randstr2, 20; my $crc1 = crc16($randstr1); my $crc2 = crc16($randstr2); $n++; $coll++ if $crc1 == $crc2; printf "percent of collisions = %.6f%%\n", $coll * 100.0 / $n if ($n % 100000 == 0); } </code></pre> From what I get on my computer, collision percentage seems to be around <code>0.0016%</code> (or <code>1e-5</code>, or "1 in 100_000"), which is way worse than predicted estimates based on ideal hashing distribution of a 16-bit hash (such as 2^16 / 2^160). UPDATE: I see you've clarified that 20 bytes are not just fully random bytes, but fall into range of <code>[a-z0-9]</code>. Here's the updated version that estimates collisions in that alphabet: <pre><code>#!/usr/bin/env perl use Digest::CRC qw(crc16); my $n = 0; my $coll = 0; my @chars = ('a'..'z', '0'..'9'); sub randstr() { my $res; foreach (1..20) { $res .= $chars[rand @chars]; } return $res; } while (1) { my $crc1 = crc16(randstr()); my $crc2 = crc16(randstr()); $n++; $coll++ if $crc1 == $crc2; printf "percent of collisions = %.4f%%\n", $coll * 100.0 / $n if ($n % 100000 == 0); } </code></pre> It yields approximately the same result, about <code>0.0016%</code>.
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1. POWhat is the possibility of CRC16 collisions on 20 bytes of data?
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1. POWhat is the possibility of CRC16 collisions on 20 bytes of data?
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