(*
 * Konzepte höherer Programmiersprachen (SS 2006)
 *
 * Übungsblatt 10 / Aufgabe 2
 *)

type date = {day:int; month:int; year:int};;


(* Aufgabe 1a *)
let leap_year y = (y mod 4 = 0) && ((y mod 100) <> 0 || (y mod 400 = 0));;

let days_per_month m y =
  if m mod 2 = 0
  then (if leap_year y
	then 28
	else 29)
  else (if List.mem m [2;4;6;9;11]
	then 30
	else 31);;

let next_day d =
  if d.day < (days_per_month d.month d.year)
  then {day = d.day + 1; month = d.month; year = d.year}
  else (if d.month < 12
	then {day = 1; month = d.month + 1; year = d.year}
	else {day = 1; month = 1; year = d.year + 1});;


(* Aufgabe 1b *)

let before d1 d2 = (d1.year < d2.year)
  || (d1.year = d2.year && (d1.month < d2.month
			    || (d1.month = d2.month
				&& d1.day < d2.day)));;


(* Aufgabe 1c *)

let rec difference_from_to d1 d2 =
  if before d1 d2
  then 1 + difference_from_to (next_day d1) d2
  else 0;;


let difference d1 d2 =
  if before d1 d2
  then difference_from_to d1 d2
  else 0 - (difference_from_to d2 d1);;


(* Aufgabe 1d *)

let day_of_the_week_text i =
  List.nth ["so"; "mo"; "di"; "mi"; "do"; "fr"; "sa"] i;;

let day_of_the_week d =
  let sunday = {day = 21; month = 5; year = 2006} in
    day_of_the_week_text ((difference sunday d) mod 7);;