``````package Math::Calc::Units::Convert;
use base 'Exporter';
use strict;
use vars qw(@EXPORT_OK);
BEGIN { @EXPORT_OK = qw(convert reduce canonical find_top construct); };

use Math::Calc::Units::Convert::Multi qw(to_canonical);

# convert : value x unit -> value
#
# The lower-level conversion routines really only know how to convert
# things to canonical units. But this routine may be called with eg
# 120 minutes -> hours. So we convert both the current and target to
# canonical units, and divide the first by the second. (Doesn't work
# for adding units that aren't multiples of each other, but that's not
# what this tool is for anyway.)
sub convert {
my (\$from, \$unit) = @_;

my \$to = [ 1, \$unit ];

my \$canon_from = canonical(\$from);
my \$canon_to = canonical(\$to);

die "conversion between incompatible units"
if not same_units(\$canon_from->[1], \$canon_to->[1]);

return [ \$canon_from->[0] / \$canon_to->[0], \$unit ];
}

# Are the (canonical) units compatible? (They must have exactly the
# same base units, and each must be raised to exactly the same power.)
sub same_units {
my (\$u1, \$u2) = @_;
return if keys %\$u1 != keys %\$u2;
while (my (\$bu1, \$bp1) = each %\$u1) {
return if ! exists \$u2->{\$bu1};
return if \$bp1 != \$u2->{\$bu1};
}
return 1;
}

sub canonical {
my (\$v) = @_;
my \$c = to_canonical(\$v->[1]);
my \$w = [ \$v->[0] * \$c->[0], \$c->[1] ];
return \$w;
}

sub reduce {
my (\$v) = @_;