Add meetup (#482)

Author: keiravillekode

config.json

@@ -953,6 +953,14 @@
         "prerequisites": [],
         "difficulty": 5
       },
+      {
+        "slug": "meetup",
+        "name": "Meetup",
+        "uuid": "a625f21c-b468-478a-b23b-1b3fd02fa2a7",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 5
+      },
       {
         "slug": "pythagorean-triplet",
         "name": "Pythagorean Triplet",

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.zig"
+    ],
+    "test": [
+      "test_meetup.zig"
+    ],
+    "example": [
+      ".meta/example.zig"
+    ]
+  },
+  "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.zig

@@ -0,0 +1,105 @@
+const std = @import("std");
+
+pub const Month = enum {
+    january,
+    february,
+    march,
+    april,
+    may,
+    june,
+    july,
+    august,
+    september,
+    october,
+    november,
+    december,
+};
+
+pub const Week = enum {
+    first,
+    second,
+    third,
+    fourth,
+    teenth,
+    last,
+};
+
+pub const DayOfWeek = enum {
+    monday,
+    tuesday,
+    wednesday,
+    thursday,
+    friday,
+    saturday,
+    sunday,
+};
+
+fn isLeapYear(year: u12) bool {
+    return (year % 4 == 0) and ((year % 100 != 0) or (year % 400 == 0));
+}
+
+fn daysInMonth(year: u12, month: Month) u6 {
+    return switch (month) {
+        .january => 31,
+        .february => if (isLeapYear(year)) 29 else 28,
+        .march => 31,
+        .april => 30,
+        .may => 31,
+        .june => 30,
+        .july => 31,
+        .august => 31,
+        .september => 30,
+        .october => 31,
+        .november => 30,
+        .december => 31,
+    };
+}
+
+fn weekConcludes(year: u12, month: Month, week: Week) u6 {
+    return switch (week) {
+        .first => 7,
+        .second => 14,
+        .third => 21,
+        .fourth => 28,
+        .teenth => 19,
+        .last => daysInMonth(year, month),
+    };
+}
+
+fn monthOffset(month: Month) u12 {
+    return switch (month) {
+        .january => 307, // offset from the end of February of previous year
+        .february => 338,
+        .march => 1,
+        .april => 32,
+        .may => 62,
+        .june => 93,
+        .july => 123,
+        .august => 154,
+        .september => 185,
+        .october => 215,
+        .november => 246,
+        .december => 276,
+    };
+}
+
+fn concludingDay(year: u12, month: Month, day_of_month: u6) u12 {
+    const y = if (month == .january or month == .february) (year - 1) else year;
+    return (y + (y / 4) - (y / 100) + (y / 400) + monthOffset(month) + day_of_month) % 7;
+}
+
+pub fn meetupDayOfMonth(year: u12, month: Month, week: Week, day_of_week: DayOfWeek) u12 {
+    const day = weekConcludes(year, month, week);
+    const concluding = concludingDay(year, month, day);
+    const required = @intFromEnum(day_of_week);
+    const adjustment: u12 = if (concluding < required) 7 else 0;
+    return day + required - (concluding + adjustment);
+}
+
+pub fn meetup(year: u12, month: Month, week: Week, day_of_week: DayOfWeek) [10]u8 {
+    const day_of_month = meetupDayOfMonth(year, month, week, day_of_week);
+
+    var buffer: [10]u8 = undefined;
+    _ = std.fmt.bufPrint(&buffer, "{d:0>4}-{d:0>2}-{d:0>2}", .{ year, 1 + @intFromEnum(month), day_of_month }) catch unreachable;
+    return buffer;
+}