#!/opt/bin/perl -w

use strict;

# we want a valid positive integer for number of files to split the original file into
my $fn_count = $ARGV[0];
die "Bad number of files\n" unless($fn_count =~ /^\d+$/ and $fn_count > 0);

main();
exit(0);

sub main{

  # Work out how many lines per output file - there may be some additional lines in the final file
  # e.g. a 10-line file split into 3 files will be 3 lines for the first two, then 4 in the final file
  # also, if the result of the div is 0, then make it 1
  
  open(IN, $ARGV[1])||die "Cannot open $ARGV[1]:$!\n";
  my $lines = 0;
  $lines ++ while(<IN>);
  close IN;
  my $lines_per_file = int($lines/$fn_count);
  $lines_per_file = 1 if !$lines_per_file;

  # Set the current line count to 0
  # and the current output file to 1
  my $ln_count = 0;
  my $file_number = 1;

  # Re-open the input file
  open(IN, $ARGV[1])||die "Cannot open $ARGV[1]:$!\n";
  
  # Open our first output file (original filename + _<file_number>)
  open(OUT, ">$ARGV[1]_$file_number")||die "Cannot open $ARGV[1]_$file_number for write:$!\n";
  while(<IN>){

    # If we've reached our total output for this file (providing it's not the last file)...
    if($ln_count == $lines_per_file and $file_number < $fn_count){

      $file_number ++; #...bump this up for the next file...
      $ln_count = 0; # ...reset line count back to 0...

      # ... close the current file and open the next one
      close OUT;
      open(OUT, ">$ARGV[1]_$file_number")||die "Cannot open $ARGV[1]_$file_number for write:$!\n";
    }

    # print to the current output file
    print OUT $_;

    # inc our line count
    $ln_count ++;
  }
  # close the final file
  close OUT;
}