#!/usr/bin/perl
# This script translates white space to tabs,
# if they come in groups of four. It takes one argument: 'text file'.

use 5.14.0;
use strict;
use warnings;
use File::Basename qw(basename);
use File::Copy qw(move);

my($infile, @content);
my $tab = ' {4}';

if (!$ARGV[0] || !-T $ARGV[0]) {
	say "Usage: " . basename($0) . " [ text file ]";
	exit;
} else { $infile = $ARGV[0]; }

my $outfile = $infile . '.' . int(rand(10000));
@content = `cat '$infile'`;

foreach my $n (0 .. (scalar(@content) - 1)) {
	my($match) = $content[$n] =~ /^( +)/;	# Find lines that start with
	if ($match) {							# spaces.
		my $write = $match;
		$write =~ s/($tab)/\t/g; # Translate individual spaces to tabs.
		$content[$n] =~ s/^$match/$write/;
	}
}

open(FILE, '>', $outfile) or die "Can't open '$outfile': $!";
print FILE @content;
close(FILE) or die "Can't close '$outfile': $!";

move($outfile, $infile) or die "Can't rename '$outfile': $!";