####################################
               #         SNAPSHOT CREATION        #
               ####################################

sub take_new_snapshot {

  my $snap_name = create_snapshot_name();

  system('btrfs subvolume snapshot '
         . $subvol_to_snapshot_arg . ' '
         . $snap_root_dir_arg . '/'
         . $yabsm_subvol_name_arg . '/'
         . $timeframe_arg . '/'
         . $snap_name);
}


sub create_snapshot_name { 

  my ($min, $hr, $day, $mon, $yr) = (localtime())[1..5];

  $mon++;      # month count starts at zero. 
  $yr += 1900; # year represents years since 1900. 

  if ($date_format_arg eq 'mm/dd/yyyy')
    { return
      'day='.pad($mon).'_'.pad($day).'_'.$yr.',time='.pad($hr).':'.pad($min);
    }
  elsif ($date_format_arg eq 'dd/mm/yyyy')
    { return
      'day='.pad($day).'_'.pad($mon).'_'.$yr.',time='.pad($hr).':'.pad($min);
    }
  else {
    die "$date_format_arg is not a valid date format";
  }
}

sub pad { 
  if ($_[0] < 10) {
    return '0' . $_[0];
  } else {
    return $_[0];
  }
}

               ####################################
               #        SNAPSHOT DELETION         #
               ####################################

sub delete_earliest_snapshot {

  opendir(DIR,"${snap_root_dir_arg}/${yabsm_subvol_name_arg}/$timeframe_arg");

  my @snaps = grep(/^[^\.]/, readdir DIR); # exclude dot files

  closedir DIR;

  return if scalar(@snaps) < $snaps_to_keep_arg; 

  my $earliest_snap = earliest_snapshot(\@snaps);

  system('btrfs subvolume delete '
         . $snap_root_dir_arg . '/'
         . $yabsm_subvol_name_arg . '/'
         . $timeframe_arg . '/'
         . $earliest_snap);
}

sub earliest_snapshot {

  my @snaps = @{$_[0]};

  my $earliest_snap = $snaps[0];
  foreach my $snap (@snaps) {
    $earliest_snap = $snap if snapshot_lt($snap,$earliest_snap);
  }
    
  return $earliest_snap;
}

sub snapshot_lt {
  return lexically_lt(snap_name_to_lex_ord_nums($_[0]),
                      snap_name_to_lex_ord_nums($_[1]));
}

sub lexically_lt {

  my ($head1, @tail1) = @{$_[0]};
  my ($head2, @tail2) = @{$_[1]};

  if ($head1 > $head2) {
    return 0;
  }
  elsif ($head1 < $head2) {
    return 1;
  }
  elsif (@tail1 == 0) {
    return 1;
  }
  elsif (@tail2 == 0) {
    return 0;
  }
  else {
    return lexically_lt(\@tail1,\@tail2);
  }
}

sub snap_name_to_lex_ord_nums {

  my ($yr, $mon, $day, $hr, $min);

  if ($date_format_arg eq 'mm/dd/yyyy') {
    ($mon,$day,$yr,$hr,$min) = snap_name_nums($_[0]);
    return [$yr,$mon,$day,$hr,$min];
  }
  elsif ($date_format_arg eq 'dd/mm/yyyy') {
    ($day,$mon,$yr,$hr,$min) = snap_name_nums($_[0]);
    return [$yr,$mon,$day,$hr,$min];
  }
  else {
    die "$date_format_arg is not a valid date format";
  }
}

sub snap_name_nums { 
  return $_[0] =~ m/([0-9]+)/g;
}