#! perl -slw
use strict;
use Benchmark::Timer;
use List::Util qw[ sum ];
use constant { ID => 0, DISCHARGE => 1, PRECIPITATION => 2 };

sub stdDev {
    my $ave = 0;
    $ave += $_ for @_;
    $ave /= @_;
    my $variance =  sum( map +($_ - $ave)**2, @_ ) / @_;
    return sqrt $variance;
}

our $G ||= 100;
our $N ||= 1e6;

my $T = new Benchmark::Timer;
my @data = map{ [ $_, rand 100, rand 10 ] } 1 .. $N;

my $label = "Moving Std Dev: $G of $N items";
$T->start( $label );

$G--;
my @stdDevs = map [], 1 .. $N - $G;
for my $item ( 0 .. $#data - $G ) {
    @{ $stdDevs[ $item ] } = (
        stdDev( map{ $_->[ DISCHARGE     ] } @data[ $item .. $item + $G ] ) ,
        stdDev( map{ $_->[ PRECIPITATION ] } @data[ $item .. $item + $G ] )
    );
}

$T->stop( $label );
$T->report;

#printf "id:$_ StdDev(Discharge):$stdDevs[ $_ ][ 0 ] StdDev(Precipitation):$stdDevs[ $_ ][ 1 ]\n" for 0 .. $#stdDevs;

__END__
C:\test>579883 -N=1e3
1 trial of Moving Std Dev: 100 of 1e3 items (323.326ms total)
C:\test>579883 -N=1e4
1 trial of Moving Std Dev: 100 of 1e4 items (3.750s total)
C:\test>579883 -N=1e5
1 trial of Moving Std Dev: 100 of 1e5 items (36.063s total)
C:\test>579883 -N=1e6
1 trial of Moving Std Dev: 100 of 1e6 items (372.109s total)