Add meetup exercise (#133)

config.json

@@ -466,6 +466,14 @@
         "prerequisites": [],
         "difficulty": 7
       },
+      {
+        "slug": "meetup",
+        "name": "Meetup",
+        "uuid": "dd0716b7-09d4-4fb5-95c4-af79983f41f1",
+        "practices": [],
+        "prerequisites": [],
+        "difficulty": 8
+      },
       {
         "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 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": [
+    "keiravillekode"
+  ],
+  "files": {
+    "solution": [
+      "meetup.s"
+    ],
+    "test": [
+      "meetup_test.c"
+    ],
+    "example": [
+      ".meta/example.s"
+    ]
+  },
+  "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.s

@@ -0,0 +1,189 @@
+.equ FIRST, 1
+.equ SECOND, 2
+.equ THIRD, 3
+.equ FOURTH, 4
+.equ TEENTH, 5
+.equ LAST, 6
+
+.equ MONDAY, 1
+.equ TUESDAY, 2
+.equ WEDNESDAY, 3
+.equ THURSDAY, 4
+.equ FRIDAY, 5
+.equ SATURDAY, 6
+.equ SUNDAY, 7
+
+.data
+
+/* offset from end February for the start of the specified month */
+offset_array:
+        .hword -1
+        .hword 307
+        .hword 338
+        .hword 1 /* March */
+        .hword 32
+        .hword 62
+        .hword 93
+        .hword 123
+        .hword 154
+        .hword 185
+        .hword 215
+        .hword 246
+        .hword 276
+        .hword 307
+        .hword 338 /* February */
+
+/* day in month for last day in specified week */
+week_array:
+        .byte -1
+        .byte 7 /* first */
+        .byte 14
+        .byte 21
+        .byte 28
+        .byte 19 /* teenth */
+
+.text
+.globl meetup
+
+/* void print_date(char *buffer, int year, int month, int day); */
+print_date:
+        mov    w9, #'-'
+        mov    w10, 10                  /* divisor */
+
+        udiv    w11, w1, w10            /* quotient */
+        msub    w13, w11, w10, w1       /* remainder (year) */
+        add     w13, w13, #'0'
+        strb    w13, [x0, #3]
+
+        udiv    w12, w11, w10           /* quotient */
+        msub    w13, w12, w10, w11      /* remainder (decade) */
+        add     w13, w13, #'0'
+        strb    w13, [x0, #2]
+
+        udiv    w11, w12, w10           /* quotient */
+        msub    w13, w11, w10, w12      /* remainder (century) */
+        add     w13, w13, #'0'
+        strb    w13, [x0, #1]
+        add     w11, w11, #'0'
+        strb    w11, [x0]
+
+        strb    w9, [x0, #4]
+
+        udiv    w11, w2, w10            /* quotient */
+        msub    w13, w11, w10, w2       /* remainder (month) */
+        add     w13, w13, #'0'
+        strb    w13, [x0, #6]
+        add     w11, w11, #'0'
+        strb    w11, [x0, #5]
+
+        strb    w9, [x0, #7]
+
+        udiv    w11, w3, w10            /* quotient */
+        msub    w13, w11, w10, w3       /* remainder (day) */
+        add     w13, w13, #'0'
+        strb    w13, [x0, #9]
+        add     w11, w11, #'0'
+        strb    w11, [x0, #8]
+
+        strb    wzr, [x0, #10]
+        ret
+
+/* int leap_year(void *unused, int year); */
+leap_year:
+        mov     w10, #100               /* divisor */
+        udiv    w12, w1, w10            /* quotient, i.e. century */
+        msub    w13, w12, w10, w1       /* remainder, i.e. year within century */
+        tst     w13, w13                /* check if 0 */
+        csel    w0, w12, w13, eq        /* either century, or year within century */
+        tst     w0, #3                  /* check if multiple of 4 */
+        cset    w0, eq
+        ret
+
+/* int days_in_month(void *unused, int year, int month); */
+days_in_month:
+        cmp     w2, #2
+        beq     .february
+
+        adrp    x9, offset_array
+        add     x9, x9, :lo12:offset_array
+
+        lsl     w10, w2, #1             /* month * sizeof(hword) */
+        ldrh    w11, [x9, x10]          /* days offset for required month */
+
+        add     w10, w10, #2            /* (month + 1) * sizeof(hword) */
+        ldrh    w0, [x9, x10]           /* days offset for following month */
+
+        sub     w0, w0, w11
+        ret
+
+.february:
+        mov     x11, lr
+        bl      leap_year
+        add     w0, w0, #28
+        ret     x11
+
+/* int week_concludes(void *unused, int year, int month, week_t week); */
+week_concludes:
+        cmp     w3, LAST
+        beq     days_in_month           /* tail call */
+
+        adrp    x9, week_array
+        add     x9, x9, :lo12:week_array
+        ldrb    w0, [x9, x3]            /* day in month at end of requested week */
+        ret
+
+/* dayofweek_t day_of_week(void *unused, int year, int month, int day); */
+day_of_week:
+        mov     w12, w2                 /* month */
+        mov     w11, w1                 /* year */
+
+        add     w10, w12, #12           /* Consider January and February to be the 13th and 14th months of */
+        sub     w9, w11, #1             /* the previous year */
+        cmp     w2, #2
+        csel    w12, w12, w10, hi       /* Conditionally adjust month */
+        csel    w11, w11, w9, hi        /* Conditionally adjust year */
+
+        lsl     w12, w12, #1            /* (adjusted month) * sizeof(hword) */
+        adrp    x9, offset_array
+        add     x9, x9, :lo12:offset_array
+        ldrh    w13, [x9, x12]          /* days offset for required month */
+
+        mov     w0, w11                 /* adjusted year */
+
+        lsr     w11, w11, #2            /* adjusted year / 4 */
+        add     w0, w0, w11
+
+        mov     w9, #25
+        udiv    w11, w11, w9            /* adjusted year / 100 */
+        sub     w0, w0, w11
+
+        lsr     w11, w11, #2            /* adjusted year / 400 */
+        add     w0, w0, w11
+
+        add     w0, w0, w13             /* add offset */
+        add     w0, w0, w3              /* add day in month */
+
+        mov     w9, #7
+        udiv    w10, w0, w9             /* quotient */
+        msub    w0, w10, w9, w0         /* remainder */
+        add     w0, w0, #1              /* day of week, in range 1..7 */
+        ret
+
+/* extern void meetup(char *buffer, int year, int month, week_t week, dayofweek_t dayofweek); */
+meetup:
+        mov     x14, x0                 /* buffer */
+        mov     x15, lr                 /* return address */
+        bl      week_concludes
+
+        mov     w3, w0                  /* day of month when week concludes */
+        bl      day_of_week
+
+        sub     w0, w4, w0              /* delta: requested day of week minus current day of week */
+        sub     w9, w0, #7
+        cmp     w0, wzr
+        csel    w0, w0, w9, le          /* if delta was positive, subtract 7 */
+        add     w3, w3, w0              /* required day of month */
+        mov     x0, x14                 /* buffer */
+        bl      print_date
+
+        ret     x15