coverage2cytosine

#!/usr/bin/env perl
use warnings;
use strict;
$|++;
use Getopt::Long;
use Cwd;
use Carp;

## This program is Copyright (C) 2010-17, Felix Krueger (felix.krueger@babraham.ac.uk)

## This program is free software: you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation, either version 3 of the License, or
## (at your option) any later version.

## This program is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
## GNU General Public License for more details.

## You should have received a copy of the GNU General Public License
## along with this program. If not, see <http://www.gnu.org/licenses/>.

my %chromosomes; # storing sequence information of all chromosomes/scaffolds
my %processed;   # keeping a record of which chromosomes have been processed
my $coverage2cytosine_version = 'v0.17.0_dev';

my ($output_dir,$genome_folder,$zero,$CpG_only,$CX_context,$split_by_chromosome,$parent_dir,$coverage_infile,$cytosine_out,$merge_CpGs,$gc_context,$gzip,$tetra,$nome) = process_commandline();

warn "Summary of parameters for genome-wide cytosine report:\n";
warn '='x78,"\n";
warn "Coverage infile:\t\t\t$coverage_infile\n";
warn "Output directory:\t\t\t>$output_dir<\n";
warn "Parent directory:\t\t\t>$parent_dir<\n";
warn "Genome directory:\t\t\t>$genome_folder<\n";

if ($CX_context){
  warn "CX context:\t\t\t\tyes\n";
}
else{
  warn "CX context:\t\t\t\tno (CpG context only, default)\n";
}

if ($merge_CpGs){
  warn "Pooling CpG top/bottom strand evidence:\tyes\n";
}

if($nome){
    warn "Sample specified as NOMe-Seq\t\tyes (only reporting ACG and TCG context)\n";
}

if($gc_context){
    if($nome){
	warn "Optional GC context track:\t\tyes (NOMe-Seq; only reporting GCA, GCC and GCT context)\n";
    }
    else{
	warn "Optional GC context track:\t\tyes\n";
    }
}

if ($tetra){
    warn "Tetra/Penta nucleotide context:\t\tyes\n";
}

if ($zero){
  warn "Genome coordinates used:\t\t0-based (user specified)\n";
}
else{
  warn "Genome coordinates used:\t\t1-based (default)\n";
}

if ($gzip){
    warn "GZIP compression:\t\t\tyes\n";
}
else{
    warn "GZIP compression:\t\t\tno\n";
}

if ($split_by_chromosome){
  warn "Split by chromosome:\t\t\tyes\n\n\n";
}
else{
  warn "Split by chromosome:\t\t\tno\n\n\n";
}
sleep (3);

read_genome_into_memory();
warn "Stored sequence information of ",scalar keys %chromosomes," chromosomes/scaffolds in total\n\n";

generate_genome_wide_cytosine_report($coverage_infile);

### 11 December 2014

# The following optional section re-reads the genome-wide report and merges methylation evidence of both top and bottom strand
# into a single CpG dinucleotide entity. This significantly simplifies downstream processing, e.g. by the bsseq R-/Bioconductor package
# which recommends this merging process to increase coverage per CpG and reduce the memory burden for its processing
# Merge CpGs does not work in conjuction with --nome-seq (since some positions are filtered out).

if ($merge_CpGs){
    # we only allow this operation if the report is limited to CpG context, and for a single report for the entire genome for the time being
    combine_CpGs_to_single_CG_entity($cytosine_out);
}

### 18 August 2015. Update 12 Jan 2017: Additional restrictions for NOMe-Seq
# The following section reprocessed the genome to generate cytosine methylation output in GC context (e.g. when a GpC methylase had been deployed
if ($gc_context){
    generate_GC_context_report($coverage_infile);
}


sub combine_CpGs_to_single_CG_entity{
  my $CpG_report_file = shift;
  warn "Now merging top and bottom strand CpGs into a single CG dinucleotide entity\n";

  open (IN,$CpG_report_file) or die "Failed to open file $CpG_report_file: $!\n\n";
  my $pooled_CG = $CpG_report_file;
  $pooled_CG =~ s/$/.merged_CpG_evidence.cov/;
  open (OUT,'>',$pooled_CG) or die "Failed to write to file '$pooled_CG': $!\n\n";
  warn ">>> Writing a new coverage file with top and bottom strand CpG methylation evidence merged to $pooled_CG <<<\n\n";
  sleep(1);

  while (1){
    my $line1 = <IN>;
    my $line2 = <IN>;
    last unless ($line1 and $line2);

    chomp $line1;
    chomp $line2;

    my ($chr1,$pos1,$strand1,$m1,$u1,$context1) = (split /\t/,$line1);
    my ($chr2,$pos2,$strand2,$m2,$u2,$context2) = (split /\t/,$line2);
    # print join ("\t",$chr1,$pos1,$strand1,$m1,$u1,$context1),"\n";
    # print join ("\t",$chr2,$pos2,$strand2,$m2,$u2,$context2),"\n"; sleep(1);

    # If there is a CpG right at the start of the reference chromosome it will be present for the top strand,
    # but will be missing for the reverse strand because it is impossible to determine a trinucleotide context
    if ($zero){ # if the CpG report was written out with zero-based coordinates 
   	if ($pos1 < 1){ # the position has to be adjusted to by -1
	    $line1 = $line2;
	    $line2 = <IN>;
	    chomp $line2;
	    ($chr1,$pos1,$strand1,$m1,$u1,$context1) = (split /\t/,$line1);
	    ($chr2,$pos2,$strand2,$m2,$u2,$context2) = (split /\t/,$line2);
	}
    }
    else{ # default
	if ($pos1 < 2){
	    $line1 = $line2;
	    $line2 = <IN>;
	    chomp $line2;
	    ($chr1,$pos1,$strand1,$m1,$u1,$context1) = (split /\t/,$line1);
	    ($chr2,$pos2,$strand2,$m2,$u2,$context2) = (split /\t/,$line2);
	}
    }

    # a few sanity checks
    die "The context of the first line was not CG:\t$line1"  unless ($context1 eq 'CG');
    die "The context of the second line was not CG:\t$line2" unless ($context2 eq 'CG');

    unless ($strand1 eq '+' and $strand2 eq'-'){
      die "The strand of line 1 and line 2 were not + and -:\nline1:\t$line1\nline2:\t$line2\n";
    }
    unless ($pos2 eq ($pos1 + 1)){
      die "The reported position were not spaced 1 bp apart:\nline1:\t$line1\nline2:\t$line2\n";
    }
    unless($chr1 eq $chr2){
      die "The chromsome information for line 1 and 2 did not match:\nline1:\t$line1\nline2:\t$line2\n";
    }

    # This looks alright from what I can tell, lets pool the 2 strands

    my $pooled_m = $m1 + $m2;
    my $pooled_u = $u1 + $u2;

    # since this is a new coverage file we only write it out if the coverage is at least 1
    next unless ($pooled_m + $pooled_u) > 0;

    my $pooled_percentage = sprintf ("%.6f",$pooled_m/ ($pooled_m + $pooled_u) *100);
    # print join ("\t",$chr1,$pos1,$strand1,$m1,$u1,$context1),"\n";
    # print join ("\t",$chr2,$pos2,$strand2,$m2,$u2,$context2),"\n";
    # print join ("\t",$chr1,$pos1,$pos2,$pooled_percentage,$pooled_m,$pooled_u),"\n";
    # sleep(1);
    if ($zero){ # printing a half-open format (0-based starting coordinate and 1-based end coordinate like in a bedGraph file
	print OUT join ("\t",$chr1,$pos1,$pos2+1,$pooled_percentage,$pooled_m,$pooled_u),"\n";
    }
    else{
	print OUT join ("\t",$chr1,$pos1,$pos2,$pooled_percentage,$pooled_m,$pooled_u),"\n";
    }

  }

  warn "CpG merging complete. Good luck finding DMRs with bsseq!\n\n";

}


sub process_commandline{
  my $help;
  my $output_dir;
  my $genome_folder;
  my $zero;
  my $CpG_only;
  my $CX_context;
  my $gzip;

  my $split_by_chromosome;
  my $cytosine_out;
  my $parent_dir;
  my $version;
  my $merge_CpGs;
  my $gc_context;
  my $tetra;
  my $nome; 

  my $command_line = GetOptions ('help|man'               => \$help,
				 'dir=s'                  => \$output_dir,
				 'g|genome_folder=s'      => \$genome_folder,
				 "zero_based"             => \$zero,	
				 "CX|CX_context"          => \$CX_context,
				 "split_by_chromosome"    => \$split_by_chromosome,
				 'o|output=s'             => \$cytosine_out,
				 'parent_dir=s'           => \$parent_dir,
				 'version'                => \$version,
				 'merge_CpGs'             => \$merge_CpGs,
				 'GC|GC_context'          => \$gc_context,
				 'ff|qq'                  => \$tetra,
				 'gzip'                   => \$gzip,
				 'nome-seq'               => \$nome,
      );

  ### EXIT ON ERROR if there were errors with any of the supplied options
  unless ($command_line){
      die "Please respecify command line options\n";
  }

  ### HELPFILE
  if ($help){
    print_helpfile();
    exit;
  }

  if ($version){
    print << "VERSION";


                      Bismark Methylation Extractor Module -
                               coverage2cytosine

                    Bismark coverage2cytosine Version: $coverage2cytosine_version
              Copyright 2010-17 Felix Krueger, Babraham Bioinformatics
                www.bioinformatics.babraham.ac.uk/projects/bismark/
                    https://github.com/FelixKrueger/Bismark


VERSION
    exit;
  }

  ### no files provided
  unless (@ARGV){
    warn "You need to provide a Bismark coverage file (with counts methylated/unmethylated cytosines) to create an individual C methylation output. Please respecify!\n";
    sleep(2);

    print_helpfile();
    exit;
  }

  my $coverage_infile = shift @ARGV;

  unless ($parent_dir){
    $parent_dir = getcwd();
  }
  unless ($parent_dir =~ /\/$/){
    $parent_dir =~ s/$/\//;
  }

  unless (defined $cytosine_out){
    die "Please provide the name of the output file using the option -o/--output filename\n";
  }

  ### OUTPUT DIR PATH
  if (defined $output_dir){
    unless ($output_dir eq ''){ # if the output dir has been passed on by the methylation extractor and is an empty string we don't want to change it
      unless ($output_dir =~ /\/$/){
	$output_dir =~ s/$/\//;
      }
    }
  }
  else{
    $output_dir = '';
  }

  unless ($CX_context){
    $CX_context = 0;
    $CpG_only = 1;
  }

  ### GENOME folder
  if ($genome_folder){
    unless ($genome_folder =~/\/$/){
      $genome_folder =~ s/$/\//;
    }
  }
  else{
    die "Please specify a genome folder to proceed (full path only)\n";
  }

  if ($merge_CpGs){
    if ($CX_context){
      die "Merging individual CpG calls into a single CpG dinucleotide entity is currently only supported if CpG-context is selected only (lose the option --CX)\n";
    }
    if ($split_by_chromosome){
      die "Merging individual CpG calls into a single CpG dinucleotide entity is currently only supported if a single CpG report is written out (lose the option --split_by_chromosome)\n";
    }
  }
  
  if ($nome){
      die "NOMe-Seq filtering only works for CpG context, please drop the '--CX' option\n" if ($CX_context);
      die "NOMe-Seq filtering does not work in conjunction with --merge_CpG (because some postions may be filtered out). Please respecify\ns" if ($merge_CpGs);
      unless ($gc_context){
	  warn "Sample specified as NOMe-Seq. Also setting `--gc` context\n\n";
	  $gc_context = 1;
      }
  }

  
  return ($output_dir,$genome_folder,$zero,$CpG_only,$CX_context,$split_by_chromosome,$parent_dir,$coverage_infile,$cytosine_out,$merge_CpGs,$gc_context,$gzip,$tetra,$nome);
}



sub generate_genome_wide_cytosine_report {
    
    warn  "="x78,"\n";
    warn "Methylation information will now be written into a genome-wide cytosine report\n";
    warn  "="x78,"\n\n";
    sleep (2);
    
    my $number_processed = 0;
    
    ### changing to the output directory again
    unless ($output_dir eq ''){ # default
	chdir $output_dir or die "Failed to change directory to $output_dir\n";
	# warn "Changed directory to $output_dir\n";
    }
    
    my $in = shift;
    # infiles handed over by the methylation extractor will be just the filename on their own. The directory should have been handed over with --dir
    if ($in =~ /gz$/){
	open (IN,"gunzip -c $in |") or die "Failed to read from gzipped file $in: $!\n"; # changed from gunzip -c to gunzip -c 08 04 16
    }
    else{
	open (IN,"$in") or die "Failed to read from file $in: $!\n";
    }

    ### note: we are still in the folder: $output_dir, so we do not have to include this into the open commands
    unless ($split_by_chromosome){ ### writing all output to a single file (default)
	if ($gzip){
	    unless ($cytosine_out =~ /\.gz$/){
		$cytosine_out .= '.gz';
	    }
	    open (CYT,"| gzip -c - > $cytosine_out") or die "Failed to write to file $cytosine_out: $!\n";
	}
	else{
	    open (CYT,'>',$cytosine_out) or die "Failed to write to file $cytosine_out: $!\n";
	}
	warn ">>> Writing genome-wide cytosine report to: $cytosine_out <<<\n\n";
	sleep (1);
    }

    my $last_chr;
    my %chr; # storing reads for one chromosome at a time
    
    my $count = 0;
    while (<IN>){
	chomp;
	++$count;
	my ($chr,$start,$end,undef,$meth,$nonmeth) = (split /\t/);
      
	# defining the first chromosome
	unless (defined $last_chr){
	    $last_chr = $chr;
	    ++$number_processed;
	    # warn "Storing all covered cytosine positions for chromosome: $chr\n";
	}
	
	### As of version 0.9.1 the start positions are 1-based!
	if ($chr eq $last_chr){
	    $chr{$chr}->{$start}->{meth} = $meth;
	    $chr{$chr}->{$start}->{nonmeth} = $nonmeth;
	}
	else{
	    warn "Writing cytosine report for chromosome $last_chr (stored ",scalar keys %{$chr{$last_chr}}," different covered positions)\n";
	    ++$number_processed;
	    
	    if ($split_by_chromosome){ ## writing output to 1 file per chromosome
		my $chromosome_out = $cytosine_out;
		if ($chromosome_out =~ /\.txt$/){
		    $chromosome_out =~ s/\.txt$/chr${last_chr}.txt/;
		}
		else{
		    $chromosome_out =~ s/$/.chr${last_chr}.txt/;
		}
		open (CYT,'>',$chromosome_out) or die $!;
		# warn "Writing output for $last_chr to $chromosome_out\n";
	    }
	    
	    my $tri_nt;
	    my $tetra_nt;
	    my $penta_nt;
	    my $context;
	    my $upstream_context; # for NOMe-Seq
	    
	    $processed{$last_chr} = 1;
	    while ( $chromosomes{$last_chr} =~ /([CG])/g){ # C or G
		
		$tri_nt = '';
		$context = '';

		if ($tetra){
		    $tetra_nt = ''; # clearing
		    $penta_nt = '';
		}
		
		if ($nome){
		    $upstream_context = '';
		}
		
		my $pos = pos$chromosomes{$last_chr};
		
		my $strand;
		my $meth = 0;
		my $nonmeth = 0;
		
		if ($1 eq 'C'){    # C on forward strand
		    $tri_nt = substr ($chromosomes{$last_chr},($pos-1),3);   # positions are 0-based!
		    if ($tetra){
			if ( length($chromosomes{$last_chr}) >= ($pos - 1 + 4) ){
			    $tetra_nt = substr ($chromosomes{$last_chr},($pos-1),4);
			}
			else{
			    $tetra_nt = '';
			}
			if ( length($chromosomes{$last_chr}) >= ($pos - 1 + 5) ){
			    $penta_nt = substr ($chromosomes{$last_chr},($pos-1),5);
			}
			else{
			    $penta_nt = '';
			}
		    }
		    if ($nome){
			$upstream_context = substr ($chromosomes{$last_chr},($pos-2),3);
		 	# warn "$1\t$upstream_context\n"; sleep(1);
		    }
		    $strand = '+';
		}
		elsif ($1 eq 'G'){ # C on reverse strand
		    
		    $tri_nt = substr ($chromosomes{$last_chr},($pos-3),3);   # positions are 0-based!
		    $tri_nt = reverse $tri_nt;
		    $tri_nt =~ tr/ACTG/TGAC/;
		    
		    if ($tetra){
			if ( $pos - 4 >= 0 ){
			    $tetra_nt = substr ($chromosomes{$last_chr},($pos-4),4);
			    $tetra_nt = reverse $tetra_nt;
			    $tetra_nt =~ tr/ACTG/TGAC/; 
			}
			else{
			    $tetra_nt = '';
			}
			if ( $pos - 5 >= 0 ){
			    $penta_nt = substr ($chromosomes{$last_chr},($pos-5),5);
			    $penta_nt = reverse $penta_nt;
			    $penta_nt =~ tr/ACTG/TGAC/;
			} 
			else{
			    $penta_nt = '';
			}
		    }
		    
		    if ($nome){
			$upstream_context = substr ($chromosomes{$last_chr},($pos-2),3);
			$upstream_context = reverse $upstream_context;
			$upstream_context =~ tr/ACTG/TGAC/;
			# warn "$1\t$upstream_context\n"; sleep(1);
		    }
		    $strand = '-';
		}
		
		next if (length$tri_nt < 3); # trinucleotide sequence could not be extracted
		
		### If the very last position is a C in CpG context on the bottom strand we need to exclude it because its top strand equivalent
		### will have been rejected due to insufficient length for $tri_nt
		if ( (length($chromosomes{$last_chr}) - $pos) == 0){
		    next;
		}

		# if (length$penta_nt < 5){
		# warn "$tri_nt\t$tetra_nt\t$penta_nt\n"; sleep(1);		  
		# }
	      
		if (exists $chr{$last_chr}->{($pos)}){ # stored positions are 1-based! (as of v0.9.1)
		    $meth =  $chr{$last_chr}->{$pos}->{meth};
		    $nonmeth = $chr{$last_chr}->{$pos}->{nonmeth};
		}
		
		
		### determining cytosine context	
		if ($tri_nt =~ /^CG/){
		    $context = 'CG';
		}
		elsif ($tri_nt =~ /^C.{1}G$/){
		    $context = 'CHG';
		}
		elsif ($tri_nt =~ /^C.{2}$/){
		    $context = 'CHH';
		}
		else{ # if the context can't be determined the positions will not be printed (it will equally not have been reported by Bismark)
		    warn "The sequence context could not be determined (found: '$tri_nt'). Skipping.\n";
		    next;
		}

		if ($CpG_only){
		    if ($tri_nt =~ /^CG/){ # CpG context is the default
			if ($nome){ # NOMe-Seq is CpG context only
			    if ( ($upstream_context eq 'ACG') or ($upstream_context eq 'TCG') ){
				# fine
			    }
			    else{
				next; # skipping this base
			    }
			}
		      
			if ($zero){ # zero based coordinates
			    $pos -= 1;
			    if ($tetra){
				print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
			    }
			    else{
				print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
			    }
			}
			else{ # default
			    if ($tetra){
				print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
			    }
			    else{
				print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
			    }
			}
		    }
		}
		else{ ## all cytosines, specified with --CX
		    if ($zero){ # zero based coordinates
			$pos -= 1;
			if ($tetra){
			    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
			}
			else{
			    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n"
			}
		    }
		    else{ # default
			if ($tetra){
			    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
			}
			else{
			    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
			}
		    }
		}
	    }
	    
	    %chr = (); # resetting the hash
	    
	    # new first entry
	    $last_chr = $chr;
	    $chr{$chr}->{$start}->{meth} = $meth;
	    $chr{$chr}->{$start}->{nonmeth} = $nonmeth;
	}
    }
    
    # Last found chromosome
    # If there never was a last chromosome then something must have gone wrong with reading the data in
    unless (defined $last_chr){
	die "No last chromosome was defined, something must have gone wrong while reading the data in (e.g. specified wrong file path for a gzipped coverage file?). Please check your command!\n\n";
    }
    
    warn "Writing cytosine report for last chromosome $last_chr (stored ",scalar keys %{$chr{$last_chr}}," different covered positions)\n";
    $processed{$last_chr} = 1;
    
    if ($split_by_chromosome){ ## writing output to 1 file per chromosome
	my $chromosome_out = $cytosine_out;
	if ($chromosome_out =~ /\.txt$/){ # files passed on by the methylation extractor end in _report.txt
	    $chromosome_out =~ s/\.txt$/chr${last_chr}.txt/;
	}
	else{ # user specified output file name
	    $chromosome_out =~ s/$/.chr${last_chr}.txt/;
	}
	open (CYT,'>',$chromosome_out) or die $!;
	# warn "Writing output for $last_chr to $chromosome_out\n";
    }
    
    my $tri_nt;
    my $tetra_nt;
    my $penta_nt;
    my $context;
    my $upstream_context;
    
    while ( $chromosomes{$last_chr} =~ /([CG])/g){
      
	$tri_nt = ''; # clearing
	$context = '';
	
	if ($tetra){
	    $tetra_nt = '';
	    $penta_nt = '';
	}
	if ($nome){
	    $upstream_context = ''; # clearing
	}
	
	my $pos = pos$chromosomes{$last_chr};
	
	my $strand;
	my $meth = 0;
	my $nonmeth = 0;
      
	if ($1 eq 'C'){    # C on forward strand
	    $tri_nt = substr ($chromosomes{$last_chr},($pos-1),3);   # positions are 0-based!
	    $strand = '+';
	    
	    if ($tetra){
		if ( length($chromosomes{$last_chr}) >= ($pos - 1 + 4) ){
		    $tetra_nt = substr ($chromosomes{$last_chr},($pos-1),4);
		}
		else{
		    $tetra_nt = '';
		}
		if ( length($chromosomes{$last_chr}) >= ($pos - 1 + 5) ){
		    $penta_nt = substr ($chromosomes{$last_chr},($pos-1),5);
		}
		else{
		    $penta_nt = '';
		}
	    }
	    
	    if ($nome){
		$upstream_context = substr ($chromosomes{$last_chr},($pos-2),3);
		# warn "$1\t$upstream_context\n"; sleep(1);
	    }
	}
	elsif ($1 eq 'G'){ # C on reverse strand
	    $tri_nt = substr ($chromosomes{$last_chr},($pos-3),3);   # positions are 0-based!
	    $tri_nt = reverse $tri_nt;
	    $tri_nt =~ tr/ACTG/TGAC/;
	    $strand = '-';
	    
	    if ($tetra){
		if ( $pos - 4 >= 0 ){
		    $tetra_nt = substr ($chromosomes{$last_chr},($pos-4),4);
		    $tetra_nt = reverse $tetra_nt;
		    $tetra_nt =~ tr/ACTG/TGAC/; 
		}
		else{
		    $tetra_nt = '';
		}
		
		if ( $pos - 5 >= 0 ){
		    $penta_nt = substr ($chromosomes{$last_chr},($pos-5),5);
		    $penta_nt = reverse $penta_nt;
		    $penta_nt =~ tr/ACTG/TGAC/;
		} 
		else{
		    $penta_nt = '';
		}
	    } 
	    if ($nome){
		$upstream_context = substr ($chromosomes{$last_chr},($pos-2),3);
		$upstream_context = reverse $upstream_context;
		$upstream_context =~ tr/ACTG/TGAC/;
		# warn "$1\t$upstream_context\n"; sleep(1);
	    }
	}
	
	if (exists $chr{$last_chr}->{($pos)}){ # stored positions are 1-based! as of v0.9.1
	    $meth =  $chr{$last_chr}->{$pos}->{meth};
	    $nonmeth = $chr{$last_chr}->{$pos}->{nonmeth};
	}
	
	next if (length$tri_nt < 3); # trinucleotide sequence could not be extracted
	### If the very last position is a C in CpG context on the bottom strand we need to exclude it because its top strand equivalent
	### will have been rejected due to insufficient length for $tri_nt
	if ( (length($chromosomes{$last_chr}) - $pos) == 0){
	    next;
	}
	
	# if (length$penta_nt < 5){
	# warn "$tri_nt\t$tetra_nt\t$penta_nt\n"; sleep(1);		  
	# }
	
	### determining cytosine context 
	if ($tri_nt =~ /^CG/){
	    $context = 'CG';
	}
	elsif ($tri_nt =~ /^C.{1}G$/){
	    $context = 'CHG';
	}
	elsif ($tri_nt =~ /^C.{2}$/){
	    $context = 'CHH';
	}
	else{ # if the context can't be determined the positions will not be printed (it will equally not have been reported by Bismark)
	    warn "The cytosine context could not be determined (found: '$tri_nt'). Skipping.\n";
	    next;
	}
	
	if ($CpG_only){
	    if ($tri_nt =~ /^CG/){ # CpG context is the default
		if ($nome){ 
		    if ( ($upstream_context eq 'ACG') or ($upstream_context eq 'TCG') ){
			# fine
		    }
		    else{
			next; # skipping this base
		    }
		}
		if ($zero){ # zero-based coordinates
		    $pos -= 1;
		    if ($tetra){
			print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
		    }
		    else{
			print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
		    }
		}
		else{ # default
		    if ($tetra){
			print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
		    }
		    else{
			print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
		    }
		}
	    }
	}
	else{ ## all cytosines, specified with --CX
	    if ($zero){ # zero based coordinates
		$pos -= 1;
		if ($tetra){
		    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
		}
		else{
		    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
		}
	    }
	    else{ # default
		if ($tetra){
		    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
		}
		else{
		    print CYT join ("\t",$last_chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
		}
	    }
	}
    }

    if ($nome){
	warn "Finished writing out NOMe-Seq specific cytosine report for covered chromosomes (only reporting CGs in ACG and TCG context; processed $number_processed chromosomes/scaffolds in total)\n\n";
    }
    else{
	warn "Finished writing out cytosine report for covered chromosomes (processed $number_processed chromosomes/scaffolds in total)\n\n";
    }
    
    ### Now processing chromosomes that were not covered in the coverage file
    warn "Now processing chromosomes that were not covered by any methylation calls in the coverage file...\n";
    my $unprocessed = 0;
    
    foreach my $chr (sort keys %processed) {
	unless ( $processed{$chr} ) {
	    ++$unprocessed;
	    ++$number_processed;
	    process_unprocessed_chromosomes($chr);
	}
    }
    
    if ($unprocessed == 0) {
	warn "All chromosomes in the genome were covered by at least some reads. coverage2cytosine processing complete.\n\n";
    }
    else{
	warn "Finished writing out cytosine report (processed $number_processed chromosomes/scaffolds in total). coverage2cytosine processing complete.\n\n";
    }
    
    close CYT or warn $!;
    
}


#### GC CONTEXT - optional
####

sub generate_GC_context_report {

    warn  "="x82,"\n";
    warn "Methylation information for GC context will now be written to a GpC-context report\n";
    warn  "="x82,"\n\n";
    sleep (2);
    
    my $number_processed = 0;
    
    ### changing to the output directory again
    unless ($output_dir eq ''){ # default
	chdir $output_dir or die "Failed to change directory to $output_dir\n";
	# warn "Changed directory to $output_dir\n";
    }
    
    my $in = shift;
    if ($in =~ /gz$/){
	open (IN,"gunzip -c $in |") or die "Failed to read from gzipped file $in: $!\n";
    }
    else{
	open (IN,"$in") or die "Failed to read from file $in: $!\n";
    }

    my $gc_out;
    my $gc_cov;

    # for NOMe-Seq (nucleosome occupancy and methylome sequencing) we limit the reporting of 
    # 1. CpGs to A-C-G and T-C-G
    # 2. GpC report files / cov files that only include G-C-A, G-C-C and G-C-T

    if ($nome){
	$gc_out = $cytosine_out.'.NOMe.GpC_report.txt';
	$gc_cov = $cytosine_out.'.NOMe.GpC.cov';
    }
    else{
	$gc_out = $cytosine_out.'.GpC_report.txt';
	$gc_cov = $cytosine_out.'.GpC.cov';
    }
    
    ### note: we are still in the folder: $output_dir, so we do not have to include this into the open commands
    open (GC,'>',$gc_out) or die "Failed to write to GpC file $gc_out: !\n\n";
    warn ">>> Writing genome-wide GpC cytosine report to: $gc_out <<<\n";
    open (GCCOV,'>',$gc_cov) or die "Failed to write to GpC coverage file $gc_cov: !\n\n";
    warn ">>> Writing genome-wide GpC coverage file to: $gc_cov <<<\n\n";
    
    my $last_chr;
    my %chr; # storing reads for one chromosome at a time
    
    my $count = 0;
    while (<IN>){
	chomp;
	++$count;
	my ($chr,$start,$end,undef,$meth,$nonmeth) = (split /\t/);
	
	# defining the first chromosome
	unless (defined $last_chr){
	    $last_chr = $chr;
	    ++$number_processed;
	    # warn "Storing all covered cytosine positions for chromosome: $chr\n";
	}
	
	### start positions are 1-based
	if ($chr eq $last_chr){
	    $chr{$chr}->{$start}->{meth} = $meth;
	    $chr{$chr}->{$start}->{nonmeth} = $nonmeth;
	}
	else{
	    warn "Writing cytosine report for chromosome $last_chr (stored ",scalar keys %{$chr{$last_chr}}," different covered positions)\n";
	    ++$number_processed;
	    $processed{$last_chr} = 1;
	    my $enough = 0;
	   
	    while ( $chromosomes{$last_chr} =~ /(GC)/g){
		
		# a GC on the top strand automatically means that there is a GC on the bottom strand as well, so we can process both at the same time
		my $context_top = '';
		my $context_bottom = '';
		my $pos = pos$chromosomes{$last_chr};
		
		my $meth_top = 0;
		my $meth_bottom = 0;
		my $nonmeth_top = 0;
		my $nonmeth_bottom = 0;
		
		#warn "$1\n"; sleep(1);
		# C on forward strand
		my $tri_nt_top = substr ($chromosomes{$last_chr},($pos-1),3);   # positions are 0-based!
		my $strand_top = '+';
		
		# C on reverse strand
		my $tri_nt_bottom = substr ($chromosomes{$last_chr},($pos-4),3);   # positions are 0-based!
		$tri_nt_bottom = reverse $tri_nt_bottom;
		$tri_nt_bottom =~ tr/ACTG/TGAC/;
		my $strand_bottom = '-';
		
		next if (length $tri_nt_top < 3);     # trinucleotide sequence could not be extracted for the top strand
		next if (length $tri_nt_bottom < 3);  # trinucleotide sequence could not be extracted for the reverse strand
	
		# top strand
		if (exists $chr{$last_chr}->{($pos)}){ # stored positions are 1-based!
		    $meth_top    = $chr{$last_chr}->{$pos}->{meth};
		    $nonmeth_top = $chr{$last_chr}->{$pos}->{nonmeth};
		}
		
		# bottom strand
		if (exists $chr{$last_chr}->{($pos-1)}){ # stored positions are 1-based! -1 for bottom strand Cs
		    $meth_bottom    = $chr{$last_chr}->{$pos-1}->{meth};
		    $nonmeth_bottom = $chr{$last_chr}->{$pos-1}->{nonmeth}; 
		}
		
		### determining cytosine context top strand	
		if ($tri_nt_top =~ /^CG/){
		    $context_top = 'CG';
		}
		elsif ($tri_nt_top =~ /^C.{1}G$/){
		    $context_top = 'CHG';
		}
		elsif ($tri_nt_top =~ /^C.{2}$/){
		    $context_top = 'CHH';
		}
		else{ # if the context can't be determined the positions will not be printed (it will equally not have been reported by Bismark)
		    warn "The sequence context could not be determined for the top strand (found: '$tri_nt_top'). Skipping.\n";
		    next;
		}
		
		### determining cytosine context bottom strand
		if ($tri_nt_bottom =~ /^CG/){
		    $context_bottom = 'CG';
		}
		elsif ($tri_nt_bottom =~ /^C.{1}G$/){
		    $context_bottom = 'CHG';
		}
		elsif ($tri_nt_bottom =~ /^C.{2}$/){
		    $context_bottom = 'CHH';
		}
		else{ # if the context can't be determined the positions will not be printed
		    warn "The sequence context could not be determined for the bottom strand (found: '$tri_nt_bottom'). Skipping.\n";
		    next;
		}
		
		# if Cs were not covered at all they are not written out
		unless ($meth_bottom + $nonmeth_bottom == 0){
		    my $percentage = sprintf ("%.6f",$meth_bottom/ ($meth_bottom + $nonmeth_bottom) *100);
		    if ($nome and ($context_bottom eq 'CG') ){
			# not reporting these (see point 2. above)
			# print join ("\t",$last_chr,$pos-1,$strand_bottom,$meth_bottom,$nonmeth_bottom,$context_bottom,$tri_nt_bottom)," (skipping)\n";sleep(1);
		    }
		    else{
			print GCCOV join ("\t",$last_chr,$pos-1,$pos-1,$percentage,$meth_bottom,$nonmeth_bottom),"\n";
			print GC join ("\t",$last_chr,$pos-1,$strand_bottom,$meth_bottom,$nonmeth_bottom,$context_bottom,$tri_nt_bottom),"\n";
		    }
		}
		
		unless ($meth_top + $nonmeth_top == 0){
		    my $percentage = sprintf ("%.6f",$meth_top/ ($meth_top + $nonmeth_top) *100);
		    if ($nome and ($context_top eq 'CG') ){
			# not reporting these (see point 2. above)
			# print join ("\t",$last_chr,$pos,$strand_top,$meth_top,$nonmeth_top,$context_top,$tri_nt_top),"\n"; sleep(1);
		    }
		    else{
			print GCCOV join ("\t",$last_chr,$pos,$pos,$percentage,$meth_top,$nonmeth_top),"\n";
			print GC join ("\t",$last_chr,$pos,$strand_top,$meth_top,$nonmeth_top,$context_top,$tri_nt_top),"\n";
		    }	
		}
	    }
		
	    %chr = (); # resetting the hash
	    
	    # new first entry
	    $last_chr = $chr;
	    $chr{$chr}->{$start}->{meth} = $meth;
	    $chr{$chr}->{$start}->{nonmeth} = $nonmeth;
	    
	}
    }
    
    # Last found chromosome
    warn "Writing cytosine GpC report for last chromosome $last_chr (stored ",scalar keys %{$chr{$last_chr}}," different covered positions)\n";
    $processed{$last_chr} = 1;
    while ( $chromosomes{$last_chr} =~ /(GC)/g){
	
	# a GC on the top strand automatically means that there is a GC on the bottom strand as well, so we can process both at the same time
	my $context_top = '';
	my $context_bottom = '';
	my $pos = pos$chromosomes{$last_chr};
	
	my $meth_top = 0;
	my $meth_bottom = 0;
	my $nonmeth_top = 0;
	my $nonmeth_bottom = 0;
	
	# C on forward strand
	my $tri_nt_top = substr ($chromosomes{$last_chr},($pos-1),3);   # positions are 0-based!
	my $strand_top = '+';
	
	# C on reverse strand
	my $tri_nt_bottom = substr ($chromosomes{$last_chr},($pos-4),3);   # positions are 0-based!
	$tri_nt_bottom = reverse $tri_nt_bottom;
	$tri_nt_bottom =~ tr/ACTG/TGAC/;
	
	my $strand_bottom = '-';
	
	next if (length $tri_nt_top < 3);     # trinucleotide sequence could not be extracted for the top strand
	next if (length $tri_nt_bottom < 3);  # trinucleotide sequence could not be extracted for the bottom strand

	### determining cytosine context top strand	
	if ($tri_nt_top =~ /^CG/){
	    $context_top = 'CG';
	}
	elsif ($tri_nt_top =~ /^C.{1}G$/){
	    $context_top = 'CHG';
	}
	elsif ($tri_nt_top =~ /^C.{2}$/){
	    $context_top = 'CHH';
	}
	else{ # if the context can't be determined the positions will not be printed
	    warn "The sequence context could not be determined for the top strand (found: '$tri_nt_top'). Skipping.\n";
	    next;
	}

	### determining cytosine context bottom strand
	if ($tri_nt_bottom =~ /^CG/){
	    $context_bottom = 'CG';
	}
	elsif ($tri_nt_bottom =~ /^C.{1}G$/){
	    $context_bottom = 'CHG';
	}
	elsif ($tri_nt_bottom =~ /^C.{2}$/){
	    $context_bottom = 'CHH';
	}
	else{ # if the context can't be determined the positions will not be printed
	    warn "The sequence context could not be determined for the bottom strand (found: '$tri_nt_bottom'). Skipping.\n";
	    next;
	}
	
	# top strand
	if (exists $chr{$last_chr}->{($pos)}){ # stored positions are 1-based!
	    $meth_top    = $chr{$last_chr}->{$pos}->{meth};
	    $nonmeth_top = $chr{$last_chr}->{$pos}->{nonmeth};
	}
	# bottom strand
	if (exists $chr{$last_chr}->{($pos-1)}){ # stored positions are 1-based! -1 for bottom strand Cs
	    $meth_bottom    = $chr{$last_chr}->{$pos-1}->{meth};
	    $nonmeth_bottom = $chr{$last_chr}->{$pos-1}->{nonmeth};
	}
	
	# if Cs were not covered at all they are not written out
	unless ($meth_bottom + $nonmeth_bottom == 0){
	    my $percentage = sprintf ("%.6f",$meth_bottom/ ($meth_bottom + $nonmeth_bottom) *100);
	    if ($nome and ($context_bottom eq 'CG') ){
		# not reporting these (see point 2. above)
		# print join ("\t",$last_chr,$pos-1,$strand_bottom,$meth_bottom,$nonmeth_bottom,$context_bottom,$tri_nt_bottom)," (skipping)\n";
	    }
	    else{
		print GCCOV join ("\t",$last_chr,$pos-1,$pos-1,$percentage,$meth_bottom,$nonmeth_bottom),"\n";
		print GC join ("\t",$last_chr,$pos-1,$strand_bottom,$meth_bottom,$nonmeth_bottom,$context_bottom,$tri_nt_bottom),"\n";
	    }
	}
	unless ($meth_top + $nonmeth_top == 0){
	    my $percentage = sprintf ("%.6f",$meth_top/ ($meth_top + $nonmeth_top) *100);
	    if ($nome and ($context_top eq 'CG') ){
		# not reporting these (see point 2. above)
		# print join ("\t",$last_chr,$pos,$strand_top,$meth_top,$nonmeth_top,$context_top,$tri_nt_top),"\n";
	    }
	    else{
		print GCCOV join ("\t",$last_chr,$pos,$pos,$percentage,$meth_top,$nonmeth_top),"\n";
		print GC join ("\t",$last_chr,$pos,$strand_top,$meth_top,$nonmeth_top,$context_top,$tri_nt_top),"\n";
	    }
	}
    }
    
    if ($nome){
	warn "Finished writing out NOMe-Seq specific GpC cytosine report for covered chromosomes (processed $number_processed chromosomes/scaffolds in total)\n\n";
    }
    else{
	warn "Finished writing out GpC cytosine report for covered chromosomes (processed $number_processed chromosomes/scaffolds in total)\n\n";
    }
    
    close GC or warn $!;
    close GCCOV or warn $!;
}

####
####


sub process_unprocessed_chromosomes{

  my $chr = shift;

  warn "Writing cytosine report for not covered chromosome $chr\n";
  $processed{$chr} = 1;

  if ($split_by_chromosome){ ## writing output to 1 file per chromosome
    my $chromosome_out = $cytosine_out;
    if ($chromosome_out =~ /txt$/){ # files passed on by the methylation extractor end in _report.txt
	$chromosome_out =~ s/txt$/chr${chr}.txt/;
    }
    else{ # user specified output file name
	$chromosome_out =~ s/$/.chr${chr}.txt/;
    }
    
    open (CYT,'>',$chromosome_out) or die $!;
    # warn "Writing output for $last_chr to $chromosome_out\n";
  }
  
  my $tri_nt;
  my $tetra_nt;
  my $penta_nt;
  my $context;
  
  while ( $chromosomes{$chr} =~ /([CG])/g){

      $tri_nt = '';
      $context = '';
      if ($tetra){
	  $tetra_nt = ''; # clearing
	  $penta_nt = '';
      }
      
      my $pos = pos$chromosomes{$chr};
      
      my $strand;
      my $meth = 0;
      my $nonmeth = 0;
      
      if ($1 eq 'C'){    # C on forward strand
	  $tri_nt = substr ($chromosomes{$chr},($pos-1),3);   # positions are 0-based!
	  $strand = '+';
	  if ($tetra){
	      if ( length($chromosomes{$chr}) >= ($pos - 1 + 4) ){
		  $tetra_nt = substr ($chromosomes{$chr},($pos-1),4);
	      }
	      else{
		  $tetra_nt = '';
	      }
	      if ( length($chromosomes{$chr}) >= ($pos - 1 + 5) ){
		  $penta_nt = substr ($chromosomes{$chr},($pos-1),5);
	      }
	      else{
		  $penta_nt = '';
	      }
	  }
      }
      elsif ($1 eq 'G'){ # C on reverse strand
	  $tri_nt = substr ($chromosomes{$chr},($pos-3),3);   # positions are 0-based!
	  $tri_nt = reverse $tri_nt;
	  $tri_nt =~ tr/ACTG/TGAC/;
	  $strand = '-';
	  
	  if ($tetra){
	      if ( $pos - 4 >= 0 ){
		  $tetra_nt = substr ($chromosomes{$chr},($pos-4),4);
		  $tetra_nt = reverse $tetra_nt;
		  $tetra_nt =~ tr/ACTG/TGAC/; 
	      }
	      else{
		  $tetra_nt = '';
	      }
	      if ( $pos - 5 >= 0 ){
		  $penta_nt = substr ($chromosomes{$chr},($pos-5),5);
		  $penta_nt = reverse $penta_nt;
		  $penta_nt =~ tr/ACTG/TGAC/;
	      } 
	      else{
		  $penta_nt = '';
	      }
	  }
	  $strand = '-';
      }
      
      next if (length$tri_nt < 3); # trinucleotide sequence could not be extracted

      ### determining cytosine context	
      if ($tri_nt =~ /^CG/){
	  $context = 'CG';
      }
      elsif ($tri_nt =~ /^C.{1}G$/){
	  $context = 'CHG';
      }
      elsif ($tri_nt =~ /^C.{2}$/){
	  $context = 'CHH';
      }
      else{ # if the context can't be determined the positions will not be printed (it will equally not have been reported by Bismark)
	  warn "The cytosine context could not be determined (found: '$tri_nt'). Skipping.\n";
	  next;
      }
      
      if ($CpG_only){
	  if ($tri_nt =~ /^CG/){ # CpG context is the default
	      if ($zero){ # zero-based coordinates
		  $pos -= 1;
		  if ($tetra){
		      print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
		  }
		  else{
		      print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
		  }
	      }
	      else{ # default
		  if ($tetra){
		      print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
		  }
		  else{
		      print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
		  }
	      }
	  }
      }
      else{ ## all cytosines, specified with --CX
	  if ($zero){ # zero based coordinates
	      $pos -= 1;
	      if ($tetra){
		  print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
	      }
	      else{
		  print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
	      }
	  }
	  else{ # default
	      if ($tetra){
		  print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt,$tetra_nt,$penta_nt),"\n";
	      }
	      else{
		  print CYT join ("\t",$chr,$pos,$strand,$meth,$nonmeth,$context,$tri_nt),"\n";
	      }
	  }
      }
  }

  #  close CYT or warn $!;

}


sub read_genome_into_memory{

  ## reading in and storing the specified genome in the %chromosomes hash
  chdir ($genome_folder) or die "Can't move to $genome_folder: $!";
  warn "Now reading in and storing sequence information of the genome specified in: $genome_folder\n\n";

  my @chromosome_filenames =  <*.fa>;

  ### if there aren't any genomic files with the extension .fa we will look for files with the extension .fasta
  unless (@chromosome_filenames){
    @chromosome_filenames =  <*.fasta>;
  }
  unless (@chromosome_filenames){
    die "The specified genome folder $genome_folder does not contain any sequence files in FastA format (with .fa or .fasta file extensions)\n";
  }

  foreach my $chromosome_filename (@chromosome_filenames){

    # skipping the tophat entire mouse genome fasta file
    next if ($chromosome_filename eq 'Mus_musculus.NCBIM37.fa');

    open (CHR_IN,$chromosome_filename) or die "Failed to read from sequence file $chromosome_filename $!\n";
    ### first line needs to be a fastA header
    my $first_line = <CHR_IN>;
    chomp $first_line;
    $first_line =~ s/\r//; # removing /r carriage returns

    ### Extracting chromosome name from the FastA header
    my $chromosome_name = extract_chromosome_name($first_line);
	
    my $sequence;
    while (<CHR_IN>){
      chomp;
      $_ =~ s/\r//; # removing /r carriage returns

      if ($_ =~ /^>/){
	### storing the previous chromosome in the %chromosomes hash, only relevant for Multi-Fasta-Files (MFA)
	if (exists $chromosomes{$chromosome_name}){
	  warn "chr $chromosome_name (",length $sequence ," bp)\n";
	  die "Exiting because chromosome name already exists. Please make sure all chromosomes have a unique name!\n";
	}
	else {
	  if (length($sequence) == 0){
	    warn "Chromosome $chromosome_name in the multi-fasta file $chromosome_filename did not contain any sequence information!\n";
	  }
	  warn "chr $chromosome_name (",length $sequence ," bp)\n";
	  $chromosomes{$chromosome_name} = $sequence;
	  $processed{$chromosome_name} = 0; # processed chromosomes will be set to 1 later to allow a record of which chromosome has been processed
	}
	### resetting the sequence variable
	$sequence = '';
	### setting new chromosome name
	$chromosome_name = extract_chromosome_name($_);
      }
      else{
	$sequence .= uc$_;
      }
    }

    if (exists $chromosomes{$chromosome_name}){
      warn "chr $chromosome_name (",length $sequence ," bp)\t";
      die "Exiting because chromosome name already exists. Please make sure all chromosomes have a unique name.\n";
    }
    else{
      if (length($sequence) == 0){
	warn "Chromosome $chromosome_name in the file $chromosome_filename did not contain any sequence information!\n";
      }
      warn "chr $chromosome_name (",length $sequence ," bp)\n";
      $chromosomes{$chromosome_name} = $sequence;
      $processed{$chromosome_name} = 0; # processed chromosomes will be set to 1 later to allow a record of which chromosome has been processed
    }
  }
  warn "\n";
  chdir $parent_dir or die "Failed to move to directory $parent_dir\n";
}

sub extract_chromosome_name {
  ## Bowtie extracts the first string after the inition > in the FASTA file, so we are doing this as well
  my $fasta_header = shift;
  if ($fasta_header =~ s/^>//){
    my ($chromosome_name) = split (/\s+/,$fasta_header);
    return $chromosome_name;
  }
  else{
    die "The specified chromosome ($fasta_header) file doesn't seem to be in FASTA format as required!\n";
  }
}


sub print_helpfile{

  warn <<EOF

  SYNOPSIS:

  This script generates a cytosine methylation report for a genome of interest and a sorted methylation input file produced
  by the script bismark2bedGraph or the bismark_methylation_extractor when '--bedGraph' was specified. The input files
  (.cov or .cov.gz) are expected to use 1-based genomic coordinates. By default, the output uses 1-based chromosomal coordinates
  and reports CpG positions only (for both strands individually and not merged in any way). Output coordinates may be changed
  to 0-based coordinates using the option '--zero_based'.

  The input file needs to have been generated with the script bismark2bedGraph (the file is called *.cov, or .cov.gz) or
  otherwise be sorted by position and exactly in the following format:

  <chromosome>  <start position>  <end position>  <methylation percentage>  <count methylated>  <count unmethylated>

  The coordinates of the input file are expected to be 1-based throughout (do not use files ending in .zero.cov!).


  USAGE: coverage2cytosine [options] --genome_folder <path> -o <output> [input]


-o/--output <filename>   Name of the output file, mandatory.

--dir                    Output directory. Output is written to the current directory if not specified explicitly.

--genome_folder <path>   Enter the genome folder you wish to use to extract sequences from (full path only). Accepted
                         formats are FastA files ending with '.fa' or '.fasta'. Specifying a genome folder path is mandatory.

-CX/--CX_context         The output file contains information on every single cytosine in the genome irrespective of
                         its context. This applies to both forward and reverse strands. Please be aware that this will
                         generate output files with > 1.1 billion lines for a mammalian genome such as human or mouse.
                         Default: OFF (i.e. Default = CpG context only).

--merge_CpG              Using this option will post-process the genome-wide report to write out an additional coverage
                         file (see above for the coverage file format) which has the top and bottom strand methylation
                         evidence pooled into a single CpG dinucleotide entity. This may be the desirable input format
                         for some downstream processing tools such as the R-package bsseq (by K.D. Hansen). An example would be:

			   genome-wide CpG report (old)
			   gi|9626372|ref|NC_001422.1|     157     +       313     156     CG
			   gi|9626372|ref|NC_001422.1|     158     -       335     156     CG
			   merged CpG evidence coverage file (new)
			   gi|9626372|ref|NC_001422.1|     157     158     67.500000       648     312

			 This option is currently experimental, and only works if CpG context only and a single genome-wide report
                         were specified (i.e. it doesn't work with the options --CX or --split_by_chromosome).
                         If --zero_based was specified as well the output coverage file is in zero-based half-open format like
                         bedGraph files.

--gc/--gc_context        In addition to normal processing this option will reprocess the genome to find methylation in 
                         GpC context. This might be useful for specialist applications where GpC methylases had been
                         deployed. The output format is exactly the same as for the normal cytosine report, and only
                         positions covered by at least one read are reported (output file ends in .GpC_report.txt). In addition
                         this will write out a Bismark coverage file (ending in GpC.cov).

--nome-seq               Sample is NOMe-Seq (nucleosome occupancy and methylome sequencing) where accessible DNA gets methylated
                         in a GpC context (sets option '--gc' as well). The option '--nome-seq':

                            (i) filters the genome-wide CpG-report to only output cytosines in ACG and TCG context
                           (ii) filters the GC context output to only report cytosines in GCA, GCC and GCT context

                         Both of these measures aim to reduce unwanted biases, i.e. the influence of GCG and CCG on endogenous
                         CpG methylation, and the inlfluence of CpG methylation on (the NOMe-Seq specific) GC context methylation.
                         PLEASE NOTE that NOMe-Seq data requires a .cov.gz file as input which has been generated in
                         non-CG mode (--CX), else the GpC output file will be empty... Default: OFF. 

--ff                     In addition to the standard output selecting --ff will also extract a four and five (tetra/penta)
                         nucleotide context for the cytosines in question. Too short sequences (e.g. at the edges of the
                         chromosome) will be left blank; sequences containing Ns are ignored.

--zero_based             Uses 0-based coordinates instead of 1-based coordinates throughout. Default: OFF.

--split_by_chromosome    Writes the output into individual files for each chromosome instead of a single output file. Files
                         will be named to include the input filename and the chromosome number.

--gzip                   Output file will be GZIP compressed (ending in .gz). Only works for standard CpG- and CX-output.
 
--help                   Displays this help message and exits



OUTPUT FORMAT:

The genome-wide cytosine methylation output file is tab-delimited in the following format (1-based coords):
===========================================================================================================

<chromosome>  <position>  <strand>  <count methylated>  <count non-methylated>  <C-context>  <trinucleotide context>


                              Script last modified: 31 March 2017

EOF
    ;
  exit 1;
}