hw1_final.sml

(* Programming Languages University of Washington programming assignment 1 *)

(* date = date in format (y, m, d) *)
(* day of year = day of year number *)

(* Returns true only if date1 precedes date2 *)
fun is_older (date1 : int * int * int, date2 : int * int * int) =
    if #1 date1 <> #1 date2
    then #1 date1 < #1 date2 
    else 
	if #2 date1 <> #2 date2
	then #2 date1 < #2 date2
	else 
	    if #3 date1 <> #3 date2
	    then #3 date1 < #3 date2
	    else false

(* Returns the number of instances that month occurs in a list of dates *)
fun number_in_month (dates_list : (int * int * int) list, month : int) = 
    if dates_list = []
    then 0
    else
	let fun count (dates_list : (int * int * int) list, month: int) = 
		let val counter = 0
		in
		    if month = #2 (hd dates_list)
		    then counter + 1
		    else counter
		end
	in
	    if null (tl dates_list)
	    then count(dates_list, month)
	    else count(dates_list, month) + number_in_month(tl dates_list, month)
	end

(* Returns the number of occurrences that the elements of a list of months occur in a list of dates *)
fun number_in_months (dates_list : (int * int * int) list, months_list : int list) = 
    if months_list = []
    then 0
    else
	let val new_counter = 0
	in
	    if null (tl months_list)
	    then new_counter + number_in_month(dates_list, hd months_list)
	    else 
		if number_in_month(dates_list, hd months_list) > 0
		then new_counter + number_in_month(dates_list, hd months_list) + number_in_months(dates_list, tl months_list)
		else new_counter
	end

(* Returns elements of a list of dates that fall in a given month *)
fun dates_in_month (dates_list : (int * int * int) list, month : int) = 
    if number_in_month(dates_list, month) = 0
    then []
    else
	if month = #2 (hd dates_list)
	then (hd dates_list) :: dates_in_month(tl dates_list, month)
	else dates_in_month(tl dates_list, month)

(* Returns elements of a list of dates that correspond to an element in a list of months *)
fun dates_in_months (dates_list : (int * int * int) list, months_list : int list) =
    if number_in_months(dates_list, months_list) = 0
    then []
    else
	if dates_in_month(dates_list, hd months_list) <> []
	then dates_in_month(dates_list, hd months_list) @ dates_in_months(dates_list, tl months_list)
	else dates_in_months(dates_list, tl months_list)

(* Returns the nth element in a string list *)
fun get_nth (string_list : string list, index : int) =
    if index = 1
    then hd string_list
    else get_nth(tl string_list, index - 1)

(* Converts a date from (y, m, d) to "M d, y" format *)
fun date_to_string (date : int * int * int) =
    let val months_as_strings = 
	    [
	    "January ", 
	    "February ", 
	    "March ", 
	    "April ", 
	    "May ", 
	    "June ", 
	    "July ", 
	    "August ", 
	    "September ", 
	    "October ", 
	    "November ", 
	    "December "
	    ]
    in
	get_nth(months_as_strings, #2 date) ^ Int.toString(#3 date) ^ ", " ^ Int.toString(#1 date)
    end

(* Returns the number of elements in a list that can be summed together starting from the front without exceeding a given value *)
fun number_before_reaching_sum (sum : int, num_list : int list) = 
    if sum - hd num_list <= 0
    then 0
    else 1 + number_before_reaching_sum(sum - hd num_list, tl num_list)

(* Returns the month for a given day *)
fun what_month (day_of_year : int) = 
    let val day_of_year_list = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    in number_before_reaching_sum(day_of_year, day_of_year_list) + 1
    end

(* Returns a list with length equal to the number of days between 2 days of year, enumerated by month *)
fun month_range (date1 : int, date2 : int) = 
    if date1 <= date2
    then what_month(date1) :: month_range(date1 + 1, date2)
    else []

(* Returns the oldest date from a list of dates *)
fun oldest (dates : (int * int * int) list) = 
    if null dates
    then NONE
    else
	let fun max_date (dates : (int * int * int) list) =
	    if null (tl dates)
	    then hd dates
	    else 
		let val oldest_val = max_date(tl dates)
		in
		    if is_older(hd dates, oldest_val)
		    then hd dates
		    else oldest_val
		end
	in SOME (max_date dates)
	end