Add meetup exercise (#131)

config.json

@@ -315,6 +315,16 @@
         "difficulty": 5,
         "topics": null
       },
+      {
+        "slug": "meetup",
+        "name": "Meetup",
+        "uuid": "ba096a09-dda5-45ae-bb90-51fe79b48c70",
+        "practices": [],
+        "prerequisites": [
+          "numbers"
+        ],
+        "difficulty": 4
+      },
       {
         "slug": "kindergarten-garden",
         "name": "Kindergarten Garden",

exercises/practice/meetup/.docs/instructions.md

@@ -0,0 +1,51 @@
+# Instructions
+
+Recurring monthly meetups are generally scheduled on the given weekday of a given week each month.
+In this exercise you will be given the recurring schedule, along with a month and year, and then asked to find the exact date of the meetup.
+
+For example a meetup might be scheduled on the _first Monday_ of every month.
+You might then be asked to find the date that this meetup will happen in January 2018.
+In other words, you need to determine the date of the first Monday of January 2018.
+
+Similarly, you might be asked to find:
+
+- the third Tuesday of August 2019 (August 20, 2019)
+- the teenth Wednesday of May 2020 (May 13, 2020)
+- the fourth Sunday of July 2021 (July 25, 2021)
+- the last Thursday of November 2022 (November 24, 2022)
+
+The descriptors you are expected to process are: `first`, `second`, `third`, `fourth`, `last`, `teenth`.
+
+Note that descriptor `teenth` is a made-up word.
+
+It refers to the seven numbers that end in '-teen' in English: 13, 14, 15, 16, 17, 18, and 19.
+But general descriptions of dates use ordinal numbers, e.g. the _first_ Monday, the _third_ Tuesday.
+
+For the numbers ending in '-teen', that becomes:
+
+- 13th (thirteenth)
+- 14th (fourteenth)
+- 15th (fifteenth)
+- 16th (sixteenth)
+- 17th (seventeenth)
+- 18th (eighteenth)
+- 19th (nineteenth)
+
+So there are seven numbers ending in '-teen'.
+And there are also seven weekdays (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday).
+Therefore, it is guaranteed that each day of the week (Monday, Tuesday, ...) will have exactly one numbered day ending with "teen" each month.
+
+If asked to find the teenth Saturday of August, 1953 (or, alternately the "Saturteenth" of August, 1953), we need to look at the calendar for August 1953:
+
+```plaintext
+    August 1953
+Su Mo Tu We Th Fr Sa
+                   1
+ 2  3  4  5  6  7  8
+ 9 10 11 12 13 14 15
+16 17 18 19 20 21 22
+23 24 25 26 27 28 29
+30 31
+```
+
+The Saturday that has a number ending in '-teen' is August 15, 1953.

exercises/practice/meetup/.meta/config.json

@@ -0,0 +1,19 @@
+{
+  "authors": [
+    "ErikSchierboom"
+  ],
+  "files": {
+    "solution": [
+      "src/meetup.cljs"
+    ],
+    "test": [
+      "test/meetup_test.cljs"
+    ],
+    "example": [
+      ".meta/src/example.cljs"
+    ]
+  },
+  "blurb": "Calculate the date of meetups.",
+  "source": "Jeremy Hinegardner mentioned a Boulder meetup that happens on the Wednesteenth of every month",
+  "source_url": "https://twitter.com/copiousfreetime"
+}

exercises/practice/meetup/.meta/src/example.cljs

@@ -0,0 +1,66 @@
+(ns meetup)
+
+(def day-structure
+  {1 :sunday 2 :monday 3 :tuesday 4 :wednesday
+   5 :thursday 6 :friday 7 :saturday})
+
+(defn leap-year? [year]
+  (cond (zero? (mod year 400)) true
+        (zero? (mod year 100)) false
+        :else  (zero? (mod year 4))))
+
+(defn zellers-congruence [input_year input_month input_day]
+  (let [month (+ (mod (+ input_month 9) 12) 3)
+        year (- input_year (quot (- month input_month) 12))
+        century (quot year 100)
+        century-year (mod year 100)]
+    (mod (+ input_day
+            (quot (* 26 (inc month)) 10)
+            century-year
+            (quot century-year 4)
+            (quot century 4)
+            (* 5 century)) 7)))
+
+(defn get-day-counts [year]
+  {1 31, 2 (if (leap-year? year) 29 28), 3 31, 4 30
+   5 31, 6 30, 7 31, 8 31, 9 30, 10 31, 11 30, 12 31})
+
+(defn get-days
+  ([year month]
+   (get-days year month
+             (zellers-congruence year month 1)
+             (get-in (get-day-counts year) [month])))
+  ([year month start-day limit]
+   (loop [count 2
+          day (inc start-day)
+          day-arrangement {1 (get-in day-structure [start-day])}]
+     (if (not= count (inc limit))
+       (recur  (inc count)
+               (if (= (inc day) 8) 1 (inc day))
+               (assoc day-arrangement count
+                      (get-in day-structure [day])))
+       day-arrangement))))
+
+(defn filter-by-day [year month day]
+  (let [days (get-days year month)]
+    (apply hash-map (flatten (filter #(-> % val (= day)) days)))))
+
+(defn filter-keys [year month day style]
+  (let [days (filter-by-day year month day)
+        dates (sort (keys days))]
+    (cond
+      (= style :first)
+      (nth dates 0)
+      (= style :second)
+      (nth dates 1)
+      (= style :third)
+      (nth dates 2)
+      (= style :fourth)
+      (nth dates 3)
+      (= style :last)
+      (nth dates (dec (count dates)))
+      (= style :teenth)
+      (first (filter #(and (> % 12) (< % 20)) (vec dates))))))
+
+(defn meetup [month year day style]
+  [year month (filter-keys year month day style)])