assignment1.txt

(* A date is (year,month,day) int*int*int *)

(* evaluates to true if the first argument is a date that comes before 
   the second argument *)
fun is_older (date1 : int*int*int, date2 : int*int*int) =
    let val d1 = (#1 date1) * 365 + (#2 date1) * 31 + (#3 date1)
        val d2 = (#1 date2) * 365 + (#2 date2) * 31 + (#3 date2)
    in
        if d1 < d2
        then true
        else false
    end

(* returns count of dates for which the month of the date matches m *)
fun number_in_month (ds : (int*int*int) list, m : int) =
    if null ds
    then 0
    else if (#2 (hd ds)) = m
         then 1 + number_in_month(tl ds, m)
         else 0 + number_in_month(tl ds, m)

(* returns count of dates (ds) that fall within the list of months (ms) *)
fun number_in_months (ds : (int*int*int) list, ms : int list) =
    if null ms
    then 0
    else number_in_month(ds, hd ms) + number_in_months(ds, tl ms)

(* filters list of dates (ds) to return a list of dates that fall within
   month (m) *)
fun dates_in_month (ds : (int*int*int) list, m : int) =
    if null ds
    then []
    else if (#2 (hd ds)) = m
         then hd ds :: dates_in_month(tl ds, m)
         else dates_in_month(tl ds, m)

(* filters list of dates (ds) to return a list of dates that fall within
   any of the months (ms) listed *)
fun dates_in_months (ds : (int*int*int) list, ms : int list) =
    if null ms
    then []
    else dates_in_month(ds, hd ms) @ dates_in_months(ds, tl ms)

(* returns the nth string in a list of strings *)
fun get_nth (ss : string list, n : int) =
    let 
	fun nth (ss : string list, c : int) =
	    if c = n
            then hd ss
            else nth (tl ss, c+1)
    in 
	nth (ss, 1)
    end

(* formats a date (d) into a string, like "February 5, 2012" *)
fun date_to_string (d : (int*int*int)) =
    let 
        val mnths = ["January","February","March","April",
                     "May","June","July","August","September",
                     "October","November","December"]
    in
        get_nth(mnths, #2 d) ^ " " ^ Int.toString(#3 d) ^ ", " ^ 
        Int.toString(#1 d)
    end

(* returns the number of the elements in list (ns) that sum up to,
   but do not exceed, the target sum (sum) *)
fun number_before_reaching_sum (sum : int, ns : int list) =
    let
	fun sum_count (current_sum : int, idx : int, rns : int list) =
	    if current_sum >= sum
            then idx-1
            else sum_count(current_sum + (hd rns), idx+1, tl rns)
    in
	sum_count (0, 0, ns)
    end

(* Returns the number of the month (1..12) in which the given day of the 
   year (doy - (1..365)) falls, not considering leap years *)
fun what_month (doy : int) =
    let
	val mnthds = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    in
        number_before_reaching_sum (doy, mnthds)+1
    end

(* returns a lists of month number (int list, [m1,m2,..,mn]) where m1 is
   the month of day1, m2 is the month of day1+1, and mn is the month of
   day day2. [] if day1 > day2, otherwise lenght of day2-day1+1 *)
fun month_range (day1 : int, day2 : int) =
    let
	fun count_days (start : int) =
            if start > day2
            then []
            else what_month (start) :: count_days(start+1)
    in
        count_days(day1)
    end

(* returns the oldest date in the list of dates, but uses option: NONE
   if the list has no dates and SOME d if d is the oldest *)
fun oldest (ds : (int*int*int) list) =
    if null ds
    then NONE
    else
         let val tl_ans = oldest(tl ds)
         in if isSome tl_ans andalso is_older(valOf(tl_ans), hd ds)
            then tl_ans
            else SOME (hd ds)
         end