use strict;
use warnings;
use diagnostics;
use Data::Dumper;

my @buffer = qw[x0 x1 x2 x3 x4 x5 x6 x7 x8 x9 y0 y1 y2 y3 y4 y5 y6];
my $bufsize = 10;
my $diff = 2 * $bufsize - scalar(@buffer);
my @smallbuf;
for my $i (0..$#buffer) {
	if ($i < $bufsize - $diff) {
		swap($i, $i + $bufsize)
	} elsif ($i < $bufsize) {
		push @smallbuf, $buffer[$i];
		mv($i+$diff, $i)
	} elsif ($i <= $#buffer - $diff) {
		mv($i+$diff, $i)
	} else {
		$buffer[$i] = shift @smallbuf;
	}
}
print Dumper @buffer;

sub swap {
	my ($i, $j) = @_;
	$buffer[$i] = $buffer[$i] ^ $buffer[$j];
	$buffer[$j] = $buffer[$i] ^ $buffer[$j];
	$buffer[$i] = $buffer[$i] ^ $buffer[$j];
}

sub mv {
	my ($from, $to) = @_;
	$buffer[$to] = $buffer[$from];
}