Here is my next try. Based on the know how from which place a particular value needs to replaced. This requires an explicit loop and some temp integers as well. Still in Perl but easy to translate to C and pointer arithmetics.

use strict;
use warnings;

my @buffer = qw[X0 X1 X2 X3 X4 Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7];

sub swapbuffers {
  my ($n1, $buf) = @_;
  my $n2 = @$buf-$n1;
  my $here = 0;
  my $tmp = $$buf[$here];
  my $istart = $here;
  for (0..@$buf-1) {
    my $from = $here < $n2 ? $here+$n1 : $here-$n2; 
    if( $from == $istart ) { # loop finished
      $$buf[$here] = $tmp;
      $here = ++$istart; 
      $tmp = $$buf[$istart]
    } else {
      $$buf[$here] = $$buf[$from];
      $here = $from;
    }
    print "@$buf\n";
  }
}

print "@buffer\n";
swapbuffers 5, \@buffer;
print "@buffer\n";
[download]

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