On the subject of inverting hashes, you can invert a hash via
reverse like so:
my %hash = ( a => 1, b => 2, c => 3 );
my %inverted_hash = reverse %hash;
To see the results, we can use Data::Dumper:
use Data::Dumper;
print Dumper(\(%hash, %inverted_hash));
# Output:
# $VAR1 = {
# 'c' => 3,
# 'a' => 1,
# 'b' => 2
# };
# $VAR2 = {
# '1' => 'a',
# '3' => 'c',
# '2' => 'b'
# };
As you note, however, not all hashes have one-to-one mappings to their
inverted forms. In such a case the inverted form will have fewer
key-value pairs than the original hash:
%hash = ( a => 1, b => 2, c => 2 );
%inverted_hash = reverse %hash;
print Dumper(\(%hash, %inverted_hash));
# Output:
# $VAR1 = {
# 'c' => 2,
# 'a' => 1,
# 'b' => 2
# };
# $VAR2 = {
# '1' => 'a',
# '2' => 'c'
# };
You can use this property to test whether you have lost information
during inversion:
print "keys were lost during inversion\n"
if keys %inverted_hash < keys %hash;
Cheers,
Tom