(* Cat Huston - s0346906 *)
(* Functional Programming and Specification - Practical 1, Exercise 1 *)




fun inString letter s = (* takes a character and a string as an argument and returns whether the character is present in the string *)
	let fun inString' letter[] = false (* set to false so if the character isn't present in the string the function retuns false *)
		| inString' letter (y::ys) = (* the function checks the first character in the current string the calls itself recursively on the rest of the string *)
			if letter = y then true (* the character is present in the string *)
				else inString' letter ys
	in
		inString' letter (explode s) (* splits the string into a list of characters *)
end;
  



fun getCode letter =  (* takes a character as an argument and returns a code  *)
	if inString (Char.toLower letter) "aehiouwy"  then #"0" (* the character will be removed *)
		else if inString (Char.toLower letter) "bfpv" then #"1"
		else if inString (Char.toLower letter) "cgjkqsxz" then #"2"
		else if inString (Char.toLower letter) "dt" then #"3"
		else if (Char.toLower letter) = #"l" then #"4"
		else if inString (Char.toLower letter) "mn" then #"5"
		else if (Char.toLower letter) = #"r" then #"6"
			else #"0"; (* the character is not a letter and will be removed *)




fun dropReps[] = [] (* drops the repeated characters from the string *)
	| dropReps [x] = [x]
	| dropReps (x::y::st) = (* the function checks the first two characters in the string then calls itself on the second character and the rest of the string *)
		if getCode x = getCode y then dropReps(x::st) (* drops y and resplits the string *)
			else x:: dropReps(y::st) (* retains the y and resplits the string with the y *)




fun dropChars[] = [] (* drops the characters with code zero from the string *)
	| dropChars (x::xs) = (* the function checks the first character in the string then calls itself on the rest of the string *)
			if getCode x = #"0" then dropChars xs (* drops the character and splits the string *)
				else x:: dropChars xs (* retains the character and splits the string *)




fun codeSt[] = [] (* takes a list of characters and returns a list of the codes *)
	| codeSt (x::st) = 
		getCode x :: codeSt st




fun take 0 _ = [] (* takes a list of characters and returns a list of length 4 *)
	| take n [] = #"0" :: take (n-1) []
	| take n (x::xs) = x :: take (n-1) xs




 fun soundex st = (* main function *)
	let val (c::cs) = dropReps (explode st)
	in
		implode (c :: take 3 (codeSt (dropChars cs)))
	end;