signature PARSE_FILE =
sig
  val parseLine: string -> AppType.triangle option
end

structure ParseFile :> PARSE_FILE =
struct
  datatype triangle_token =
    X
  | Y
  | R
  | G
  | B
  | COORD of Real32.real
  | UNKNOWN of string

  val zero: Real32.real = 0.0

  fun extractTriangle lst =
    case lst of
      [ X
      , COORD x1
      , Y
      , COORD y1

      , X
      , COORD x2
      , Y
      , COORD y2

      , X
      , COORD x3
      , Y
      , COORD y3
      ] =>
        (* file format not specifying any colours *)
        SOME
          { x1 = x1
          , y1 = y1
          , x2 = x2
          , y2 = y2
          , x3 = x3
          , y3 = y3
          , r = zero
          , g = zero
          , b = zero
          }
    | [ X
      , COORD x1
      , Y
      , COORD y1

      , X
      , COORD x2
      , Y
      , COORD y2

      , X
      , COORD x3
      , Y
      , COORD y3

      , R
      , COORD r
      , G
      , COORD g
      , B
      , COORD b
      ] =>
        (* file format specifying rgb *)
        SOME
          { x1 = x1
          , y1 = y1
          , x2 = x2
          , y2 = y2
          , x3 = x3
          , y3 = y3
          , r = r
          , g = g
          , b = b
          }
    | _ => NONE

  fun tokeniseString str =
    if str = "x" then
      X
    else if str = "y" then
      Y
    else if str = "r" then
      R
    else if str = "g" then
      G
    else if str = "b" then
      B
    else
      case Real32.fromString str of
        SOME num => COORD num
      | NONE => UNKNOWN str

  fun helpParseLine (line, pos, acc, wordStartPos) =
    if pos = String.size line then
      List.rev acc
    else
      let
        val chr = String.sub (line, pos)
      in
        if Char.isSpace chr then
          if pos > 0 andalso Char.isSpace (String.sub (line, pos - 1)) then
            (* if previous character is space, just proceed to next character *)
            helpParseLine (line, pos + 1, acc, wordStartPos)
          else
            let
              (* current character is space, but previous character is not, 
               * which means we have some text to substring and tokenise 
               * before proceeding to next character *)
              val strToken =
                String.substring (line, wordStartPos, pos - wordStartPos)
              val token = tokeniseString strToken
            in
              helpParseLine (line, pos + 1, token :: acc, pos)
            end
        else if pos > 0 andalso Char.isSpace (String.sub (line, pos - 1)) then
          (* previous character was space but current character is not,
           * meaning that we have hit the start of a new word *)
          helpParseLine (line, pos + 1, acc, pos)
        else
          (* just proceed to next character *)
          helpParseLine (line, pos + 1, acc, wordStartPos)
      end

  fun parseLine line =
    let val lst = helpParseLine (line, 0, [], 0)
    in extractTriangle lst
    end
end