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+#!env raku
+#
+# Perl Weekly Challenge 47
+# see <https://perlweeklychallenge.org/blog/perl-weekly-challenge-047/>
+#
+# Task 1
+# Roman Calculator
+# Write a script that accepts two roman numbers and operation.
+# It should then perform the operation on the give
+# roman numbers and print the result.
+# For example,
+# perl ch-1.pl V + VI
+# It should print
+# XI
+#
+#
+# Example of output
+# % raku ch-1.p6  V + IX
+# XIV
+
+
+my %roman-to-arabic = :I(1), :V(5), :X(10), :L(50), :C(100), :D(500), :M(1000);
+
+
+# Function to convert a roman number into an arabic one.
+sub convert-roman-to-arabic( Str:D $roman  ) {
+    # convert to uppercase, reverse the string
+    # and translates into numbers. For example,
+    # IX => [ 10 1 ]
+    # then map so that the current value is multiplied by -1 if the
+    # previous one is higher
+    my @arabic-digits = $roman.uc.comb.reverse.map: {
+        state $last = 0;
+        my $value = %roman-to-arabic{ $_ };
+        $value *= -1 if $value < $last;
+        $last = $value;
+        $value;
+    };
+
+    return [+] @arabic-digits;
+}
+
+
+# Function to convert an arabic number into a roman one.
+sub convert-arabic-to-roman( Int $arabic  ) {
+
+    # special cases: what to subtract from every value
+    my %subtractors = 1_000 => 100,
+    500 => 100,
+    100 => 10,
+    50 =>  10,
+    10 =>  1,
+    5 =>  1,
+    1 => 0;
+
+    # reverse the map of the values so that each arabic value
+    # corresponds to a letter
+    my %translator = %roman-to-arabic.map( { $_.value => $_.key } );
+
+
+    return '' if ! $arabic;
+
+    for %subtractors.pairs.sort: { $^b.key.Int <=> $^a.key.Int } -> $pair {
+        my $subtractor = $pair.key.Int;
+        my $removing   = $pair.value.Int;
+
+        return %translator{ $subtractor } ~ convert-arabic-to-roman( $arabic - $subtractor.Int ) if $arabic >= $subtractor;
+        return %translator{ $removing } ~ convert-arabic-to-roman( $arabic + $removing.Int ) if $arabic >= $subtractor - $removing;
+
+    }
+
+}
+
+
+
+########################################
+# Main program
+########################################
+
+die "Usage: $*PROGRAM <operand> <operator> <operand>" if @*ARGS.elems != 3;
+
+my $operand-a = convert-roman-to-arabic( @*ARGS[0] );
+my $operand-b = convert-roman-to-arabic( @*ARGS[2] );
+
+my $result = do given @*ARGS[1].trim {
+    when '+' { $operand-a + $operand-b; }
+    when '-' { $operand-a - $operand-b; }
+    when '/' { $operand-a / $operand-b; }
+    when '*' { $operand-a * $operand-b; }
+};
+
+say convert-arabic-to-roman( $result );