use strict;
use warnings;

# Set block size and deviation
my $block_size = 4;
my $deviation  = 0.5;

# Initialise the candidates, maximum and minimum values
my $candidates = [[split(' ', <DATA>)]];
my $maxval     = my $minval = $candidates->[0][3];

# Loop through the rest of the DATA
while (<DATA>) {
  my $current = [split];
  if (within_range($current->[3], $maxval, $minval)) {
    push(@$candidates, $current);
    $maxval = $current->[3] if $maxval < $current->[3];
    $minval = $current->[3] if $minval > $current->[3];
  }
  elsif (@$candidates >= $block_size) {
    print_block($candidates);
    push(@$candidates, $current);
    ($candidates, $maxval, $minval) = trim_candidates($candidates);
  }
  else {
    push(@$candidates, $current);
    ($candidates, $maxval, $minval) = trim_candidates($candidates);
  }
}

# deal with stragglers
if ( @$candidates >= $block_size ) {
  print_block( $candidates );
}

sub within_range {
  my ($testval, $testmax, $testmin) = @_;
  return 0 if $testmax - $testval > $deviation;
  return 0 if $testval - $testmin > $deviation;
  return 1;
}

sub print_block {
  my ( $lines ) = @_;
  print "BLOCK\n";
  for my $line ( @$lines ) {
    print join(' ', @$line ), "\n";
  }
}

sub trim_candidates {
  my $worklist = shift;
  # drop the first entry
  shift @$worklist;
  # Check if the remaining worklist qualifies
  my $workmax = my $workmin = $worklist->[0][3];
  foreach my $item (@$worklist) {
    return trim_candidates($worklist)
      unless within_range($item->[3], $workmax, $workmin);
    $workmax = $item->[3] if $workmax < $item->[3];
    $workmin = $item->[3] if $workmin > $item->[3];
  }
  return ($worklist, $workmax, $workmin);
}

0;
__DATA__
1 10492 rs55998931 0.272727272727273 0.4375
1 13418 . 0.25 0.0625
1 13752 . 0.153846153846154 0.25
1 13813 . 0.0357142857142857 0.2
1 13838 . 0.0357142857142857 0.2
1 14907 rs79585140 0.5 0.555555555555556
1 14930 rs75454623 0.535714285714286 0.611111111111111
1 14933 rs199856693 0.0357142857142857 0.0555555555555556
1 14948 rs201855936 0.107142857142857 0
1 10492 rs55998931 1 0.4375
1 10492 rs55998931 1.5 0.4375
1 10492 rs55998931 1.9 0.4375
1 10492 rs55998931 2 0.4375
1 10492 rs55998931 2.6 0.4375
1 13418 blah 20.0 blah
1 13418 blah 20.1 blah
1 13418 blah 20.2 blah
1 13418 blah 20.3 blah
1 13418 blah 20.4 blah
1 13418 blah 20.5 blah
1 13418 blah 20.6 blah
1 13418 blah 20.7 blah
1 13418 blah 30.5 blah
1 13418 blah 30.0 blah
1 13418 blah 30.0 blah
1 13418 blah 30.5 blah
1 13418 blah 30.6 blah
1 13418 blah 30.9 blah
1 13418 blah 30.6 blah
1 13418 blah 30.9 blah

##</code><code>##

BLOCK
1 10492 rs55998931 0.272727272727273 0.4375
1 13418 . 0.25 0.0625
1 13752 . 0.153846153846154 0.25
1 13813 . 0.0357142857142857 0.2
1 13838 . 0.0357142857142857 0.2
1 14907 rs79585140 0.5 0.555555555555556
BLOCK
1 13418 blah 20.0 blah
1 13418 blah 20.1 blah
1 13418 blah 20.2 blah
1 13418 blah 20.3 blah
1 13418 blah 20.4 blah
1 13418 blah 20.5 blah
BLOCK
1 13418 blah 20.1 blah
1 13418 blah 20.2 blah
1 13418 blah 20.3 blah
1 13418 blah 20.4 blah
1 13418 blah 20.5 blah
1 13418 blah 20.6 blah
BLOCK
1 13418 blah 20.2 blah
1 13418 blah 20.3 blah
1 13418 blah 20.4 blah
1 13418 blah 20.5 blah
1 13418 blah 20.6 blah
1 13418 blah 20.7 blah
BLOCK
1 13418 blah 30.5 blah
1 13418 blah 30.0 blah
1 13418 blah 30.0 blah
1 13418 blah 30.5 blah
BLOCK
1 13418 blah 30.5 blah
1 13418 blah 30.6 blah
1 13418 blah 30.9 blah
1 13418 blah 30.6 blah
1 13418 blah 30.9 blah