Add meetup exercise (#75)

Author: keiravillekode

config.json

@@ -513,6 +513,14 @@
         "prerequisites": [],
         "difficulty": 8
       },
+      {
+        "slug": "meetup",
+        "name": "Meetup",
+        "uuid": "6dcd24ed-902b-4fa4-9816-e28b87c157f0",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 8
+      },
       {
         "slug": "wordy",
         "name": "Wordy",

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 six 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": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "meetup.fut"
+    ],
+    "test": [
+      "test.fut"
+    ],
+    "example": [
+      ".meta/example.fut"
+    ]
+  },
+  "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.fut

@@ -0,0 +1,88 @@
+
+def equal [m] [n] (first: [m]u8) (second: [n]u8): bool =
+  if m != n then false else
+  let same = loop same = true for i < n do
+    if first[i] == second[i] then same else
+    false
+  in
+    same
+
+def write (start: i64) (len: i64) (value: i32) (buffer: *[]u8): *[]u8 =
+  let (buffer, _) = loop (buffer, value) = (buffer, value) for i < len do
+    let digit = '0' + u8.i32 (value % 10)
+    in
+      (buffer with [start + len - 1 - i] = digit, value / 10)
+  in
+    buffer
+
+def is_leap (year: i32): bool =
+  (year % 4 == 0) && ((year % 100 != 0) || (year % 400 == 0))
+
+def days_in_month (year: i32) (month: i32): i32 =
+  match month
+    case 1 -> 31
+    case 2 -> if is_leap year then 29 else 28
+    case 3 -> 31
+    case 4 -> 30
+    case 5 -> 31
+    case 6 -> 30
+    case 7 -> 31
+    case 8 -> 31
+    case 9 -> 30
+    case 10 -> 31
+    case 11 -> 30
+    case 12 -> 31
+    case _ -> assert false 0
+
+def week_concludes (year: i32) (month: i32) (week: []u8): i32 =
+  if equal week "first" then 7 else
+  if equal week "second" then 14 else
+  if equal week "third" then 21 else
+  if equal week "fourth" then 28 else
+  if equal week "teenth" then 19 else
+  if equal week "last" then days_in_month year month else
+  assert false 0
+
+def required_day (dayofweek: []u8): i32 =
+  if equal dayofweek "Monday" then 0 else
+  if equal dayofweek "Tuesday" then 1 else
+  if equal dayofweek "Wednesday" then 2 else
+  if equal dayofweek "Thursday" then 3 else
+  if equal dayofweek "Friday" then 4 else
+  if equal dayofweek "Saturday" then 5 else
+  if equal dayofweek "Sunday" then 6 else
+  assert false 0
+
+def month_offset (month: i32): i32 =
+  match month
+    case 1 -> 307  -- offset from the end of February of previous year
+    case 2 -> 338
+    case 3 -> 1
+    case 4 -> 32
+    case 5 -> 62
+    case 6 -> 93
+    case 7 -> 123
+    case 8 -> 154
+    case 9 -> 185
+    case 10 -> 215
+    case 11 -> 246
+    case 12 -> 276
+    case _ -> assert false 0
+
+def concluding_day (year: i32) (month: i32) (dayofmonth: i32): i32 =
+  let year = if month < 3 then year - 1 else year
+  in
+    (year + (year / 4) - (year / 100) + (year / 400) + (month_offset month) + dayofmonth) % 7
+
+def meetup_dayofmonth (year: i32) (month: i32) (week: []u8) (dayofweek: []u8): i32 =
+  let day = week_concludes year month week
+  let concluding = concluding_day year month day
+  let required = required_day dayofweek
+  let adjustment = if concluding < required then 7 else 0
+  in
+    day + required - (concluding + adjustment)
+
+def meetup (year: i32) (month: i32) (week: []u8) (dayofweek: []u8): []u8 =
+  let dayofmonth = meetup_dayofmonth year month week dayofweek
+  in
+    (write 0 4 year (write 5 2 month (write 8 2 dayofmonth (replicate 10 '-'))))