Adjust subtractives to match standard form

README.md

@@ -24,6 +24,35 @@ Future enhancements will include expansions to printf/sprintf with a custom form
 
 While it handles both additive and subtractive Roman numerals, it doesn't check that they're properly formatted. For instance 'IC' should be a compile-time error but instead it'll generate 101 as if nothing of consequence happened.
 
+REFERENCE
+=========
+
+According to [Wikipedia](https://en.wikipedia.org/wiki/Roman_numerals), the standard form is:
+
++---+-----------+----------+------+-------+
+|   | Thousands | Hundreds | Tens | Units |
++---+-----------+----------+------+-------+
+| 1 | M         | C        | X    | I     |
++---+-----------+----------+------+-------+
+| 2 | MM        | CC       | XX   | II    |
++---+-----------+----------+------+-------+
+| 3 | MMM       | CCC      | XXX  | III   |
++---+-----------+----------+------+-------+
+| 4 |           | CD       | XL   | IV    |
++---+-----------+----------+------+-------+
+| 5 |           | D        | L    | V     |
++---+-----------+----------+------+-------+
+| 6 |           | DC       | LX   | VI    |
++---+-----------+----------+------+-------+
+| 7 |           | DCC      | LXX  | VII   |
++---+-----------+----------+------+-------+
+| 8 |           | DCCC     | LXXX | VIII  |
++---+-----------+----------+------+-------+
+| 9 |           | CM       | XC   | IX    |
++---+-----------+----------+------+-------+
+
+The numerals for 4 (IV) and 9 (IX) are written using subtractive notation,where the smaller symbol (I) is subtracted from the larger one (V, or X), thus avoiding the clumsier IIII and VIIII. Subtractive notation is also used for 40 (XL), 90 (XC), 400 (CD) and 900 (CM). These are the only subtractive forms in standard use.
+
 AUTHOR
 ======
 

lib/Slang/Roman.rakumod

@@ -64,25 +64,24 @@ my sub to-roman(Int:D $val) is export {
 my sub to-number(Str:D $value) is export {
 
     # Find subtractives and convert them to additives
-    #
-    # IV => IIII ( 5 - 1 == 4 )
-    # IX => VIIII ( 10 - 1 == 9 )
-    # XL => XXXX ( 50 - 10 == 40 )
-    # IL => XXXXVIIII ( 50 - 1 == 49 )
-    # XC => LXXXX ( 100 - 10 == 90 )
-    # CD => CCCC ( 500 - 100 == 400 )
-    # CM => DCCCC ( 1000 - 100 == 900 )
-
-    $value.subst(/
-      (<[ I Ⅰ X Ⅹ C Ⅽ M Ⅿ ↂ ]>)
-      (<[ V Ⅴ X Ⅹ L Ⅼ C Ⅽ D Ⅾ M Ⅿ ↁ ↂ ↇ ↈ ]>)
-    /, {
-        %char-map{$0} < %char-map{$1}
-          ?? to-roman(%char-map{$1} - %char-map{$0})
-          !! $0 ~ $1
-    }, :global).comb.map({
-        %char-map{$_} // die "Unexpected '$_' in Roman numeral"
-    }).sum
+
+    my %subtractives = (
+        IV =>  'IIII',  #( 5 - 1 == 4 )
+        IX => 'VIIII',  #( 10 - 1 == 9 )
+        XL =>  'XXXX',  #( 50 - 10 == 40 )
+        XC => 'LXXXX',  #( 100 - 10 == 90 )
+        CD =>  'CCCC',  #( 500 - 100 == 400 )
+        CM => 'DCCCC',  #( 1000 - 100 == 900 )
+    );
+
+    my Pair $p = (.keys => .values) given %subtractives;
+
+    $value
+        .trans( $p )
+        .comb.map({
+            %char-map{$_} // die "Unexpected '$_' in Roman numeral"
+        })
+        .sum
 }
 
 my role Grammar {

t/01-basic.rakutest

@@ -29,8 +29,8 @@ subtest sub {
     is 0rXIV, 14, q{Roman numeral 14 subtractive};
     is 0rXIX, 19, q{Roman numeral 19 subtractive};
     is 0rXLIV, 44, q{Roman numeral 44 subtractive};
-    is 0rIC, 99, q{Roman numeral 99 subtractive};
-    is 0rMIM, 1999, q{Roman numeral 1999 subtractive};
+    is 0rXC, 90, q{Roman numeral 90 subtractive};
+    is 0rMCM, 1900, q{Roman numeral 1900 subtractive};
   }, 'subtractive';
 
   is 0rMMXVI, 2016, q{Year of module release};