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00 Seasons.md

Seasons

Well, it's Hurricane Season here in the Gulf South. But that's not the kind of seasons I'm talking about now.

Your job is to take a month (keyword) day (number) and a hemisphere (:north or :south) and determine which season it is (return a keyword), according to this handy table.

Start       End         North  South
March 1     May 31      Spring Autumn
June 1      August 31   Summer Winter
September 1 November 30 Autumn Spring
December 1  February 29 Winter Summer

Example:

(which-season :north :march 5) ;=> :spring
(which-season :south :december 25) ;=> :summer

Thanks to this site for the challenge idea where it is considered Hard level in JavaScript.

Please submit your solutions as comments to this gist. Discussion is welcome.

ugly but works

(def northern
  {:january :winter,
   :february :winter,
   :march :spring,
   :april :spring,
   :may :spring,
   :june :summer,
   :july :summer,
   :august :summer
   :september :fall,
   :october :fall,
   :november :fall,
   :december :winter})

(def reverse-season
 {:winter :summer
  :summer :winter
  :spring :autumn
  :autumn :spring})

(defn which-season [hemi month _date]
  (cond-> (northern month)
    (#{:south} hemi) reverse-season))

This solution parses the text and also takes care of the days at the beginning of the year, which are tricky:

(ns season.core
  (:require [clojure.string :refer [lower-case]]
            [clojure.edn :as edn]))

(def input
  "Start       End         North  South
   March 1     May 31      Spring Autumn
   June 1      August 31   Summer Winter
   September 1 November 30 Autumn Spring
   December 1  February 29 Winter Summer")

(def months
  (apply array-map
         [:january 31 :february 29 :march 31 :april 30 :may 31 :june 30
          :july 31 :august 31 :september 30 :october 31 :november 30 :december 31]))

(defn day-of-year [month day-of-month]
  (let [days  (->> (take-while #(not= month %) (keys months))
                   (map months))]
    (reduce + day-of-month days)))

(def keywordize
  (comp keyword lower-case))

(defn normalize [start end]
  (let [end (if (> start end) (+ end 365) end)]
    [start end]))

(defn make-range [[month1 day1 month2 day2 season1 season2]]
  (let [start   (day-of-year (keywordize month1) (edn/read-string day1))
        end     (day-of-year (keywordize month2) (edn/read-string day2))
        seasons {:north (keywordize season1) :south (keywordize season2)}]
    [(normalize start end) seasons]))

(def make-ranges-table
  (delay (->> (re-seq #"\w+" input)
              (drop 4)
              (partition 6)
              (map make-range))))

(defn which-season* [hemi month day]
  (let [doy (day-of-year month day)
        row (first
             (filter (fn [[[lo hi] _]] (<= lo doy hi))
                     @make-ranges-table))
        [_ seasons] row]
    (get seasons hemi)))

(defn which-season [hemi month day]
  (or (which-season* hemi month day)
      (which-season* hemi month (+ 365 day))))

(comment
  (which-season :north :march 5) ;=> :spring
  (which-season :south :december 25) ;=> :summer
  (which-season :south :january 2) ;=> :summer
  )

A simpler F# version:

    type Hemisphere = North | South
    
    type Month = January | February | March | April | May | June | July | August | September | October | November | December
    
    type Season = Spring | Summer | Autumn | Winter

    let whichSeason hemisphere month day =
        let invertSeason season =
            match season with
            | Spring -> Autumn
            | Summer -> Winter
            | Autumn -> Spring
            | Winter -> Summer

        let northernSeason month =
            match month with
            | March | April | May -> Spring
            | June | July | August -> Summer
            | September | October | November -> Autumn
            | December | January | February -> Winter
        
        match hemisphere with
            | North -> northernSeason month
            | South -> invertSeason (northernSeason month)

I first started with a nested map with month and then hemisphere for keys.
Then I wanted to extend the solution to also cover astronomical seasons, while keeping the (get-in seasons [date hemisphere]) pattern.

Here it goes

(def month->int (zipmap [:january :february :march :april :may :june :july :august :september :october :november :december]
                        (range)))

(defn date<= [& dates]
  (->> dates
       (map (fn [[month day]] [(month->int month) day]))
       (partition 2 1)
       (map #(apply compare %))
       (every? #(<= % 0))))

(def seasons-astronomical
  (reify clojure.lang.ILookup
    (valAt [this date]
      (cond
        (date<= [:march     21] date [:june      20]) {:north :spring :south :autumn}
        (date<= [:june      21] date [:september 22]) {:north :summer :south :winter}
        (date<= [:september 23] date [:december  20]) {:north :autumn :south :spring}
        (or (date<= [:december 21] date [:december 31])
            (date<= [:january   1] date [:march    20])) {:north :winter :south :summer}))))


(def seasons-meteorological
  (reify clojure.lang.ILookup
    (valAt [this [month _]]
      (let [by-season {#{:march     :april   :may}      {:north :spring :south :autumn}
                       #{:june      :july    :august}   {:north :summer :south :winter}
                       #{:september :october :november} {:north :autumn :south :spring}
                       #{:december  :january :february} {:north :winter :south :summer}}
            by-month (into {}
                           (mapcat
                            (fn [[months by-hemisphere]]
                              (for [month months]
                                [month by-hemisphere]))
                            by-season))]
        (by-month month)))))

(defn which-season
  ([hemisphere month day]
   (get-in seasons-meteorological [[month day] hemisphere]))
  ([seasons hemisphere month day]
   (get-in seasons [[month day] hemisphere])))


(which-season :north :march 5) ;=> :spring
(which-season :south :december 25) ;=> :summer

(which-season seasons-astronomical :north :march 5) ;=> :winter
(which-season seasons-astronomical :south :december 25) ;=> :summer

;; Vector of seasons in order containing the months in each season.
;; Starts with spring in northern hemisphere (autumn in southern hemisphere).
(def seasons [#{:march :april :may}
              #{:june :july :august}
              #{:september :october :november}
              #{:december :january :february}])

;; Can be left out if we don't validate days
(defn max-days-in-month [month]
  (condp contains? month
    #{:february}                         29
    #{:april :june :september :november} 30
    31))

(def season-names {:north [:spring :summer :autumn :winter]
                   :south [:autumn :winter :spring :summer]})

(defn season-index [month]
  (first (keep-indexed #(when (%2 month) %1) seasons)))

(defn which-season
  [hemisphere month day]
  {:pre [(contains? season-names hemisphere)
         (season-index month)
         (<= 1 day (max-days-in-month month))]}
  (let [seasons (hemisphere season-names)]
    (seasons (season-index month))))