#!/usr/bin/perl

use strict;
use warnings;

my @pair = 
  (
      [ qw(a b) ],
      [ qw(b a) ],
      [ qw(c b) ],
      [ qw(b c) ],
  );

my @triplet = 
  (
      [ qw(a b c) ],
      [ qw(b a c) ],
      [ qw(c b a) ],
      [ qw(b c a) ],
  );

print "Individual calls to mycmp()...\n";
foreach (@pair) {
  print "Same package:      ";
  print $_->[0], ' ', $_->[1], ' => ';
  print $_->[0], ' ', $_->[1], "\n" if mycmp(@$_) == -1;
  print $_->[1], ' ', $_->[0], "\n" if mycmp(@$_) == 1;
  print $_->[1], ' ', $_->[0], "\n" if mycmp(@$_) == 0;
  print "Different package: ";
  print $_->[0], ' ', $_->[1], ' => ';
  print $_->[0], ' ', $_->[1], "\n" if Diffpackage::mycmp(@$_) == -1;
  print $_->[1], ' ', $_->[0], "\n" if Diffpackage::mycmp(@$_) == 1;
  print $_->[1], ' ', $_->[0], "\n" if Diffpackage::mycmp(@$_) == 0;
}

print "\nMultiple calls to mycmp() in sort...\n";
foreach (@triplet) {
  print "Same package:      ";
  print $_->[0], ' ', $_->[1], ' ', $_->[2], ' => ';
  print (join ' ', sort { mycmp($a, $b) } @$_ );
  print "\n";
  print "Different package: ";
  print $_->[0], ' ', $_->[1], ' ', $_->[2], ' => ';
  print (join ' ', sort { Diffpackage::mycmp($a, $b) } @$_ );
  print "\n\n";
}

exit;

sub mycmp {
  my ($first, $second) = @_;
  return 1 if $first ne ( mysort($first, $second) );
  return -1 if $first eq ( mysort($second, $first) );
  return 0;
}

sub mysort {
  my @ans =
    map { $_->[0] }
    sort { $a->[0] cmp $b->[0] }  # $a and $b are always defined
    map { [ $_ ] }
    @_;
  return $ans[0];
}

package Diffpackage;

# this is the same as previous mycmp
sub mycmp {
  my ($first, $second) = @_;
  return 1 if $first ne ( mysort($first, $second) );
  return -1 if $first eq ( mysort($second, $first) );
  return 0;
}

# this is the same as previous mysort
sub mysort {
  my @ans =
    map { $_->[0] }
    sort { $a->[0] cmp $b->[0] }   # this is where $a and $b can be undefined
    map { [ $_ ] }
    @_;
  return $ans[0];
}

##</code><code>##

Individual calls to mycmp()...
Same package:      a b => a b
Different package: a b => a b
Same package:      b a => a b
Different package: b a => a b
Same package:      c b => b c
Different package: c b => b c
Same package:      b c => b c
Different package: b c => b c

Multiple calls to mycmp() in sort...
Same package:      a b c => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: a b c => a b c

Same package:      b a c => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: b a c => a b c  <== first two inputs sort correctly

Same package:      c b a => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: c b a => b c a  <== first two inputs sort correctly

Same package:      b c a => a b c
Use of uninitialized value in string comparison (cmp) at ./testsort3.pl line 77.
(Snip repeated error)
Different package: b c a => b c a