Meetup

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:

    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.

Working with Dates in Prolog

Prolog's built-in support for dates is fairly rudimentary. A much nicer way to work with dates is via the [date_time package][date_time-package].

Installing the date_time package

If your installed Swipl version is fairly up-to-date, you can install the package by running:

swipl pack install date_time

If that doesn't work, you can run:

swipl -g 'pack_install(date_time)'

Using the date_time package

Ad the following code to the top of your solution file to use the date_time package:

:- use_module(library(date_time)).