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Learning Exercise
Elixir's standard library offers 4 different modules for working with dates and time, each with its own struct.
The Date module. A Date struct can be created with the ~D sigil.
~D[2021-01-01]
The Time module. A Time struct can be created with the ~T sigil.
~T[12:00:00]
The NaiveDateTime module for datetimes without a timezone. A NaiveDateTime struct can be created with the ~N sigil.
~N[2021-01-01 12:00:00]
The DateTime module for datetimes with a timezone. Using this module for timezones other than UTC requires an external dependency, a timezone database.
To compare dates or times to one another, look for a compare or diff function in the corresponding module. Comparison operators such as ==, >, and < seem to work, but they don't do a correct semantic comparison for those structs.
Your librarian friend has asked you to extend her library software to automatically calculate late fees. Her current system stores the exact date and time of a book checkout as an ISO8601 datetime string. She runs a local library in a small town in Ghana, which uses the GMT timezone (UTC +0), doesn't use daylight saving time, and doesn't need to worry about other timezones.
Implement the LibraryFees.datetime_from_string/1 function. It should take an ISO8601 datetime string as an argument, and return a NaiveDateTime struct.
LibraryFees.datetime_from_string("2021-01-01T13:30:45Z")
# => ~N[2021-01-01 13:30:45]
If a book was checked out before noon, the reader has 28 days to return it. If it was checked out at or after noon, it's 29 days.
Implement the LibraryFees.before_noon?/1 function. It should take a NaiveDateTime struct and return a boolean.
LibraryFees.before_noon?(~N[2021-01-12 08:23:03])
# => true
Based on the checkout datetime, calculate the return date.
Implement the LibraryFees.return_date/1 function. It should take a NaiveDateTime struct and return a Date struct, either 28 or 29 days later.
LibraryFees.return_date(~N[2020-11-28 15:55:33])
# => ~D[2020-12-27]
The library has a flat rate for late returns. To be able to calculate the fee, we need to know how many days after the return date the book was actually returned.
Implement the LibraryFees.days_late/2 function. It should take a Date struct - the planned return date, and a NaiveDateTime struct - the actual return datetime.
If the actual return date is on an earlier or the same day as the planned return datetime, the function should return 0. Otherwise, the function should return the difference between those two dates in days.
The library tracks both the date and time of the actual return of the book for statistical purposes, but doesn't use the time when calculating late fees.
LibraryFees.days_late(~D[2020-12-27], ~N[2021-01-03 09:23:36])
# => 7
The library has a special offer for returning books on Mondays.
Implement the LibraryFees.monday?/1 function. It should take a NaiveDateTime struct and return a boolean.
LibraryFees.monday?(~N[2021-01-03 13:30:45Z])
# => false
Implement the LibraryFees.calculate_late_fee/3 function. It should take three arguments - two ISO8601 datetime strings, checkout datetime and actual return datetime, and the late fee for one day. It should return the total late fee according to how late the actual return of the book was.
Include the special Monday offer. If you return the book on Monday, your late fee is 50% off, rounded down.
# Sunday, 7 days late
LibraryFees.calculate_late_fee("2020-11-28T15:55:33Z", "2021-01-03T13:30:45Z", 100)
# => 700
# one day later, Monday, 8 days late
LibraryFees.calculate_late_fee("2020-11-28T15:55:33Z", "2021-01-04T09:02:11Z", 100)
# => 400
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