my @spectrum = qw/174.0 102.99 71.62 70.53 82.49 69.05 18.94 102.99 71.62 70.53 82.49 69.05 18.94 102.99 71.62 70.53 82.49 69.05 18.94 102.99 71.62 70.53 82.49 69.05 18.94/;
my @subsp = qw/102.0 18.94 71.61 82.0 173.0 68.7/;


##</code><code>##

REQUEST    = 18.94 68.7 71.61 82.0 102.0 173.0
BEST_SUBSP = 18.94 69.05 71.62 82.49 102.99 174.0
SIMILARITY = 2.11

##</code><code>##

#!/usr/bin/perl

use warnings;
use strict;

sub similarity {
    my ($request, $spectrum, $accuracy, $limit) = @_[0..3];
    my $fmt = "%.${accuracy}f";

    my @spectrum = @$spectrum;
    my @request  = sort {$a <=> $b} @$request;

    return 0, [] unless @request;
    return 0, [] if @spectrum < @request;
    
    my %good;
    foreach my $signal (@request) {
        my @good;
        return 0 unless (@good = grep { abs($_-$signal) <= $limit } @spectrum);
        %good = (%good, map { ($_ => 1) } @good);
    }
    @spectrum = sort {$a <=> $b} grep { $good{$_} } @spectrum;
    
    my $alpha;
    my $best_subsp;
    my $alphabeta;
    $alphabeta = sub {
        my ($beta, $spectrum, $subsp, $found) = @_;
        unless (@$subsp) {
            $alpha = $beta;
            $best_subsp = $found;
            return;
        }

        my $compared = $subsp->[0];
        foreach my $sign_id (0..$#{$spectrum}) {
            my $diff = abs($spectrum->[$sign_id] - $compared);
            next if $diff > $limit;
            next if defined($alpha) && ($diff + $beta) >= $alpha;
            $alphabeta->($diff + $beta, 
                [@{$spectrum}[0..$sign_id-1, $sign_id+1..$#{$spectrum}]],
                [@{$subsp}[1..$#{$subsp}]],
                [@$found, $spectrum->[$sign_id]],
            );
        }
        return;
    };
    $alphabeta->(0, \@spectrum, \@request, []);
    return 0, [] unless defined $alpha;

    my $max_similarity = sprintf $fmt, $alpha ? @request/$alpha : 1000;
    return $max_similarity, $best_subsp;    
}

my $limit = 3;

my @spectrum = qw/174.0 102.99 71.62 70.53 82.49 69.05 18.94 102.99 71.62 70.53 82.49 69.05 18.94 102.99 71.62 70.53 82.49 69.05 18.94 102.99 71.62 70.53 82.49 69.05 18.94/;
my @subsp = qw/102.0 18.94 71.61 82.0 173.0 68.7/;

my ($sim, $best) = similarity(\@subsp, \@spectrum, 2, $limit);

@subsp = sort {$a <=> $b} @subsp;

print <<DEBUG;
REQUEST     = @subsp
BEST_SUBSP  = @{$best}
SIMILARITY  = $sim
DEBUG