added from(S) functionality and tests

roman.erl

@@ -1,24 +1,41 @@
 -module(roman).
 -include_lib("eunit/include/eunit.hrl").
 -export([to/1]).
+-export([from/1]).
 
 to(N) -> to_roman(N,"").
+from(S) -> from_roman(S,0).
 
-to_roman(N,S) when N >= 1000 -> to_roman(N-1000,string:concat(S,"M"));
-to_roman(N,S) when N >= 900 -> to_roman(N-900,string:concat(S,"CM"));
-to_roman(N,S) when N >= 500 -> to_roman(N-500,string:concat(S,"D"));
-to_roman(N,S) when N >= 400 -> to_roman(N-400,string:concat(S,"CD"));
-to_roman(N,S) when N >= 100 -> to_roman(N-100,string:concat(S,"C"));
-to_roman(N,S) when N >= 90 -> to_roman(N-90,string:concat(S,"XC"));
-to_roman(N,S) when N >= 50 -> to_roman(N-50,string:concat(S,"L"));
-to_roman(N,S) when N >= 40 -> to_roman(N-40,string:concat(S,"XL"));
-to_roman(N,S) when N >= 10 -> to_roman(N-10,string:concat(S,"X"));
-to_roman(N,S) when N >= 9 -> to_roman(N-9,string:concat(S,"IX"));
-to_roman(N,S) when N >= 5 -> to_roman(N-5,string:concat(S,"V"));
-to_roman(N,S) when N >= 4 -> to_roman(N-4,string:concat(S,"IV"));
-to_roman(N,S) when N >= 1 -> to_roman(N-1,string:concat(S,"I"));
+to_roman(N,S) when N >= 1000 -> to_roman(N-1000,S ++ "M");
+to_roman(N,S) when N >= 900 -> to_roman(N-900,S ++ "CM");
+to_roman(N,S) when N >= 500 -> to_roman(N-500,S ++ "D");
+to_roman(N,S) when N >= 400 -> to_roman(N-400,S ++ "CD");
+to_roman(N,S) when N >= 100 -> to_roman(N-100,S ++ "C");
+to_roman(N,S) when N >= 90 -> to_roman(N-90,S ++ "XC");
+to_roman(N,S) when N >= 50 -> to_roman(N-50,S ++ "L");
+to_roman(N,S) when N >= 40 -> to_roman(N-40,S ++ "XL");
+to_roman(N,S) when N >= 10 -> to_roman(N-10,S ++ "X");
+to_roman(N,S) when N >= 9 -> to_roman(N-9,S ++ "IX");
+to_roman(N,S) when N >= 5 -> to_roman(N-5,S ++ "V");
+to_roman(N,S) when N >= 4 -> to_roman(N-4,S ++ "IV");
+to_roman(N,S) when N >= 1 -> to_roman(N-1,S ++ "I");
 to_roman(0,S) -> S.
 
+from_roman("M" ++ S,N) -> from_roman(S,N+1000);
+from_roman("CM" ++ S,N) -> from_roman(S,N+900);
+from_roman("D" ++ S,N) -> from_roman(S,N+500);
+from_roman("CD" ++ S,N) -> from_roman(S,N+400);
+from_roman("C" ++ S,N) -> from_roman(S,N+100);
+from_roman("XC" ++ S,N) -> from_roman(S,N+90);
+from_roman("L" ++ S,N) -> from_roman(S,N+50);
+from_roman("XL" ++ S,N) -> from_roman(S,N+40);
+from_roman("X" ++ S,N) -> from_roman(S,N+10);
+from_roman("IX" ++ S,N) -> from_roman(S,N+9);
+from_roman("V" ++ S,N) -> from_roman(S,N+5);
+from_roman("IV" ++ S,N) -> from_roman(S,N+4);
+from_roman("I" ++ S,N) -> from_roman(S,N+1);
+from_roman("",N) -> N.
+
 roman_one_test() -> "I" = to(1).
 roman_two_test() -> "II" = to(2).
 roman_three_test() -> "III" = to(3).
@@ -39,3 +56,13 @@ roman_four_hundred_test() -> "CD" = to(400).
 roman_thousand_test() -> "M" = to(1000).
 roman_nine_hundred_test() -> "CM" = to(900).
 roman_nineteen_sixty_eight_test() -> "MCMLXVIII" = to(1968).
+
+roman_from_one_test() -> 1 = from("I").
+roman_from_two_test() -> 2 = from("II").
+roman_from_three_test() -> 3 = from("III").
+roman_from_four_test() -> 4 = from("IV").
+roman_from_five_test() -> 5 = from("V").
+roman_from_nine_test() -> 9 = from("IX").
+roman_from_ten_test() -> 10 = from("X").
+roman_from_eighteen_test() ->
+    18 = from("XVIII").