; Roman numeral decoder

; First, some useful substring utilities
to starts_with? :string :prefix
  if empty? :prefix [output "true]
  if empty? :string [output "false]
  if not equal? first :string first :prefix [output "false]
  output starts_with? butfirst :string butfirst :prefix
end

to remove_prefix :string :prefix
  if or empty? :prefix not starts_with? :string :prefix [output :string]
  output remove_prefix butfirst :string butfirst :prefix
end

; Our list of Roman numeral values
make "values [[M 1000] [CM 900] [D  500] [CD 400] [C  100] [XC 90] [L 50]
              [XL 40]  [X  10]  [IX 9]   [V  5]   [IV 4]   [I  1]]

; Function to do the work
to from_roman :str
 local "n make "n 0
 foreach :values [
   local "s make "s first ?
   local "v make "v last ?
   while [starts_with? :str :s] [
     make "n sum n :v
     make "str remove_prefix :str :s
   ]
 ]
 output :n
end

foreach [MCMXC MDCLXVI MMVIII] [print (sentence (word ? "|: |) from_roman ?)]
bye