#!/usr/bin/perl

=pod

=head1 NAME

field - select fields in tabular files


=head1 SYNOPSIS

B<field> I<field spec> file

... | B<field> I<field spec>


=head1 DESCRIPTION

B<field> selects fields in tabular files.


=head1 EXAMPLE

Select field 2:

  field 2 file

Select field 2,3,4,5:

  field 2-5 file

Select field 2,1,3,5,7,9,13:

  field 2,1-13/2 file

Select field 2..end:

  field 2- file

Select field 2,3,4,1:

  field 2-4,1 file


=head1 AUTHOR

Copyright (C) 2017-2020 Ole Tange,
http://ole.tange.dk and Free Software Foundation, Inc.


=head1 LICENSE

Copyright (C) 2012 Free Software Foundation, Inc.

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/>.


=head1 SEE ALSO

B<cut>


=cut

use strict;

my @fields = range_to_fields(shift);
my $fields = join(",", @fields);

sub range_to_fields {
    my @ranges = split(/,/, shift);
    my @fields;
    for (@ranges) {
	# 2
	/^(\d+)$/ and push (@fields,($1-1));
	# 2-11
	/^(\d+)-(\d+)$/ and push (@fields,($1-1) .. ($2-1));
	# 3-19/2
	/^(\d+)-(\d+)\/(\d+)$/ and do {
	    for(my $t = $1; $t <= $2; $t += $3) {
		push (@fields,($t-1));
	    }
	};
	/^(\d+)-$/ and do {
	    push (@fields,($1-1)." .. \$#F");
	    $Global::must_eval = 1;
	};
    }
    # Default: field 1
    # Perl counts from 0 - not from 1
    if(not @fields) { @fields=(1-1); }
    return @fields;
}

if($Global::must_eval) {
    while(<>) {
	my @F = split/\s+/,$_;
	if(not $Calc::f{$#F}) {
	    # Eval is expensive, so only do it if we have not done before
	    # If an argument ends in '-' then we must figure out the last field
	    # which depends on the number of fields in the line
	    $Calc::f{$#F}++;
	    @{$Calc::fields->{$#F}} = eval $fields;
	}
	print join("\t",@F[@{$Calc::fields->{$#F}}]),"\n";
    }
} else {
    # awk counts from 1 not 0
    @fields = map { $_+1 } @fields;
    # Use mawk (60 MB/s)/gawk (30 MB/s)/awk (? MB/s)
    my $awk = which("mawk") || which("gawk") || which("awk");

    my $awkscript = 'BEGIN {OFS = "\t"} {print '.
			  (join ",", map { '$'.$_ } @fields).
			  '}';
    exec($awk,$awkscript);
}

sub which {
    # Input:
    #   @programs = programs to find the path to
    # Returns:
    #   @full_path = full paths to @programs. Nothing if not found
    my @which;
    for my $prg (@_) {
	push(@which, grep { not -d $_ and -x $_ }
	     map { $_."/".$prg } split(":",$ENV{'PATH'}));
	if($prg =~ m:/:) {
	    # Including path
	    push(@which, grep { not -d $_ and -x $_ } $prg);
	}
    }
    return wantarray ? @which : $which[0];
}