Add meetup exercise

config.json

@@ -738,6 +738,14 @@
         "practices": [],
         "prerequisites": [],
         "difficulty": 3
+      },
+      {
+        "slug": "meetup",
+        "name": "Meetup",
+        "uuid": "398c1590-2e8f-445e-83cb-2b762be63b2b",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
       }
     ]
   },

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

@@ -0,0 +1,34 @@
+# Instructions
+
+Your task is to find the exact date of a meetup, given a month, year, weekday and week.
+
+There are five week values to consider: `first`, `second`, `third`, `fourth`, `last`, `teenth`.
+
+For example, you might be asked to find the date for the meetup on the first Monday in January 2018 (January 1, 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 teenth Saturday of August 1953 (August 15, 1953)
+
+## Teenth
+
+The teenth week refers to the seven days in a month that end in '-teenth' (13th, 14th, 15th, 16th, 17th, 18th and 19th).
+
+If asked to find the teenth Saturday of August, 1953, we check its calendar:
+
+```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
+```
+
+From this we find that the teenth Saturday is August 15, 1953.

exercises/practice/meetup/.docs/introduction.md

@@ -0,0 +1,29 @@
+# Introduction
+
+Every month, your partner meets up with their best friend.
+Both of them have very busy schedules, making it challenging to find a suitable date!
+Given your own busy schedule, your partner always double-checks potential meetup dates with you:
+
+- "Can I meet up on the first Friday of next month?"
+- "What about the third Wednesday?"
+- "Maybe the last Sunday?"
+
+In this month's call, your partner asked you this question:
+
+- "I'd like to meet up on the teenth Thursday; is that okay?"
+
+Confused, you ask what a "teenth" day is.
+Your partner explains that a teenth day, a concept they made up, refers to the days in a month that end in '-teenth':
+
+- 13th (thirteenth)
+- 14th (fourteenth)
+- 15th (fifteenth)
+- 16th (sixteenth)
+- 17th (seventeenth)
+- 18th (eighteenth)
+- 19th (nineteenth)
+
+As there are also seven weekdays, it is guaranteed that each day of the week has _exactly one_ teenth day each month.
+
+Now that you understand the concept of a teenth day, you check your calendar.
+You don't have anything planned on the teenth Thursday, so you happily confirm the date with your partner.

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

@@ -0,0 +1,18 @@
+{
+  "authors": [
+    "glennj"
+  ],
+  "files": {
+    "solution": [
+      "meetup.coffee"
+    ],
+    "test": [
+      "meetup.spec.coffee"
+    ],
+    "example": [
+      ".meta/example.coffee"
+    ]
+  },
+  "blurb": "Calculate the date of meetups.",
+  "source": "Jeremy Hinegardner mentioned a Boulder meetup that happens on the Wednesteenth of every month"
+}

exercises/practice/meetup/.meta/example.coffee

@@ -0,0 +1,26 @@
+weekdays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
+meetup = ({year, month, week, dayofweek}) ->
+  # javascript uses 0-based months
+  # initialize the date's time to 12:00:00 to avoid daylight saving problems
+  date = new Date year, month - 1, 1, 12
+
+  days = Array.from({length: 7}, -> [])
+
+  while date.getMonth() == month - 1
+    days[date.getDay()].push new Date date
+    date.setDate (date.getDate() + 1)
+
+  dayIdx = weekdays.indexOf dayofweek
+  targetDays = days[dayIdx]
+
+  wantedDay = switch week
+    when 'first'  then targetDays.at 0
+    when 'second' then targetDays.at 1
+    when 'third'  then targetDays.at 2
+    when 'fourth' then targetDays.at 3
+    when 'last'   then targetDays.at -1
+    when 'teenth' then targetDays.find (d) -> d.getDate() >= 13
+
+  wantedDay.toISOString().replace /T.+/, ''
+
+module.exports = meetup