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add test, assert JumpHash is close enough to ideally consistent #2

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80 changes: 80 additions & 0 deletions t/02_rebucket.t
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Original file line number Diff line number Diff line change
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use strict;
use warnings;
use Test::More tests => 12;
use Algorithm::ConsistentHash::JumpHash;

my $DEFAULT_NUM_KEYS = 100_000;

# default threshold could be scaled to the number of keys
my $DEFAULT_THRESHOLD = 0.001;

my $is_consistent = sub {
my ( $from_buckets, $to_buckets, $num_keys, $first_key, $threshold ) = @_;

$from_buckets ||= 4;
$to_buckets ||= $from_buckets + 1;
$num_keys ||= $DEFAULT_NUM_KEYS;
$threshold ||= $DEFAULT_THRESHOLD;
$first_key ||= 0;

my $key_limit = ( $first_key + $num_keys );

my $ideal;
if ( $from_buckets < $to_buckets ) {
$ideal = ( $from_buckets * 1.0 ) / ( $to_buckets * 1.0 );
}
else {
$ideal = ( $to_buckets * 1.0 ) / ( $from_buckets * 1.0 );
}
my $stayed = 0;
my $moved = 0;
my ( $key, $first, $next );
for ( $key = $first_key ; $key < $key_limit ; ++$key ) {
$first = Algorithm::ConsistentHash::JumpHash::jumphash_numeric( $key,
$from_buckets );
$next = Algorithm::ConsistentHash::JumpHash::jumphash_numeric( $key,
$to_buckets );
if ( $first == $next ) {
++$stayed;
}
else {
++$moved;
}
}
my $ratio = ( $stayed * 1.0 ) / ( ( $moved + $stayed ) * 1.0 );
my $diff = ( $ratio >= $ideal ) ? $ratio - $ideal : $ideal - $ratio;
my $fail = ( $diff < $threshold ) ? 0 : 1;

my $msg = <<"EOL";

tested $num_keys keys ($first_key <= $key_limit)
with $from_buckets buckets and $to_buckets buckets
$stayed stayed in the same bucket
$moved moved buckets
for a ratio of $ratio, ideal would be $ideal
ratio diff from ideal: $diff, threshold: $threshold

EOL

is( $fail, 0, "FAIL: diff == $diff, expected < $threshold\n$msg" );
};

# $is_consistent->(
# $from_buckets, $to_buckets, $num_keys, $first_key, $threshold
# );
$is_consistent->( 1, 2 );
$is_consistent->( 2, 3 );
$is_consistent->( 3, 7 );
$is_consistent->( 4, 8 );
$is_consistent->( 5, 11 );
$is_consistent->( 6, 12 );
$is_consistent->( 7, 13 );
$is_consistent->( 8, 16 );

# going to fewer buckets
$is_consistent->( 16, 8 );
$is_consistent->( 5, 4 );
$is_consistent->( 3, 2 );

# lager number of keys, a higher threshold, over keys in a range above 2^32
$is_consistent->( 4, 5, 10_000_000, 2**34, 0.00001 );