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- Applications often need to
create objects from Strings,

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and for example, in our application,

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we have a String representing
year, month, day,

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and we need to convert it
into a date object, NaiveDate.

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So in util.rs, gonna see these functions

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to achieve that effect and prompt_for_date

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and date_from_str.

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Okay, I've written these functions

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to help me with date String conversions.

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Okay then, so in util.rs, so first of all,

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let's give you a quick reminder,

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prompt_for_string, and the
purpose of the function,

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you call the function.

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You pass in a message, a prompt message

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to tell the user what you want,

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and it should return a String.

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So it displays the
message that we passed in

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asking the user to enter something.

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I create an empty String object here,

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and then in the std::io module,
there's a std::in function.

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Basically it represents
the standard input device.

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It's like cin in C++.

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It's like Console in C#.

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Anyway, from the input
device, I read a line, okay?

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And I store it in the String.

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So it's a bit like scanf
in C, if you remember that.

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It returns a result, so I
unwrap the result, okay?

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And I return the String
that's been entered.

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The String here, which is empty initially,

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will be populated by
the read_line function,

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and then I return that
String back to the user.

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So that was easy enough.

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I've got a similar kinda
thing, prompt_for_date.

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It takes a message asking
the user, you know,

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please enter a date, and it's
meant to return a NaiveDate.

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So it calls the prompt_for_string method

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that we just looked at
to display the message,

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and that gives me back a String.

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Okay, so that's a bit
of reuse going on there.

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I've reused my prompt_for_string
function already,

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gives me back a String
that the user has entered.

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I get rid of any white space

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in case the user had spaces
at the beginning and end,

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and then I pass that function
into date_from_str down here.

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Okay, so it turns out
that this date_from_str

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is a generally useful function
that I might use elsewhere

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where you've got a
String or a String slice,

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and it parses it to give you
back a date object, NaiveDate.

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So this is the interesting
function really, I guess.

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Here's a String. Parse
that String into NaiveDate.

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So it turns out it's
just a simple wrapper.

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The NaiveDate structure
has a parse_from_str

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associated function or static function.

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You give it a String that
contains a stringified date,

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and then you tell it the format to expect.

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So obviously you can have
a look at the documentation

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in the NaiveDate structure to see

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what the different options are here.

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%Y, four-digit year.

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%m, month. %d, day.

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So it needs to know the
format of the incoming String

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to see where's the year
part, where's the month part,

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where's the date part
in the incoming String,

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and again, I unwrap that result

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and I return it back to the code.

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All right, so you could argue
that this function here,

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I could've just called that
function directly up here,

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and I could've done that,

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but sometimes it's nice to just wrap up

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some low-level API
behind your own function

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because that function
could in the future evolve

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to do something a little
bit more complete.

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Okay, so I was quite happy
with those functions.

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So the next thing we're going
to look at is how to actually,

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in the context of my application scenario,

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which is all about visits to Europe

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in a 180-day period in Schengen,

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how do I actually implement that logic,

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the actual scenario, in my application?

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And it turns out I had to do quite a lot

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of date manipulation.

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So we're gonna look at that now,

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and this will be the
end of our discussions.

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This is the only bit
that we haven't yet seen.

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In menu.rs, we're going to see the options

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for basically displaying the dates

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within a certain period of interest,

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which is the whole point of the demo.

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So there's a couple of
functions we're gonna look at.

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Get_days_prior_to is a
function I've written

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and
display_days_within_period_of_interest.

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I'm trying to go for the world record

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for the longest function name ever.

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So let's have a look at those then.

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So in menu.rs, just a quick reminder

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just to kind of set the scene,

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the menu function, or do_menu,

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it displays a list of options to the user,

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and we've covered all of
these apart from option 2.

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How do I calculate the number of days

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when I've been away from a specified date?

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Right, so if the user chooses option 2,

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it calls the
display_days_within_period_of_interest,

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and it gives you all the visits,
it takes in all the visits,

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and it calculates how many
days you've been away.

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So let's have a look at this function now,

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display_days_within_period_of_interest.

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So here we are.

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I'm gonna go through this
function in a couple of steps.

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I'm just gonna give us a
little bit more space as well.

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I'm gonna run the application
actually before I do this

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just to remind you of
what that option does.

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Let's open up a new terminal.

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I run the application, cargo run.

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Remember, when the application starts,

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it immediately loads in
the data from data.txt,

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and it builds the collection of visits.

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So it's already done all that,

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and then it displays the menu,

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and its option 2 is what
we're interested in,

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calculate visit days
within period of interest.

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So I'll choose option 2, okay?

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And you see the first thing
it does, I'm in here now.

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What reference date do you
want to use? Year, month, day.

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All right, so I'll enter, let's say,

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2023-12-31,

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New Year's Eve 2023.

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When I press Enter, this is the function

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which is currently going to be processing.

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So I pass that message into the function.

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This function is currently running,

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and it's waiting for me
to press Enter down here.

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When I press Enter, it'll do
the work we just looked at,

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and it'll give me back a date,

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and that date object will represent

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the 31st of December, 2023.

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So I press Enter.

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What it's then gonna do, obviously,

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is to display all the results.

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So I wanna remind you a couple of things.

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First of all, it displays two
batches of results, right?

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Go back up to the top of the output.

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So there are lots of visits here.

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The first set of results is chronological.

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Okay, so from the date that I specified,

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the 31st of December, 2023, it
displays the number of days.

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It takes the 31st of December,

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and it displays the number
of days I've been away

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for 180 days prior to that date, okay?

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So each of these things being
displayed here is a Record.

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Remember the Record structure?

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Let me give you a quick reminder of that.

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The Record structure, it says
how many days were you away,

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and what's the date

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that we're talking about, okay?

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So if it says if the date

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was the 31st of December,

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it'll tell me how many days were you away

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180 days prior to that, okay?

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So from this date here,
the 31st of December,

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I was away 67 days

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in the 180-day period prior to that,

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but it doesn't just display
the number of days I was away

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on the 31st of December.

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It also displays the number of
days on the 30th of December

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and the 29th of December.

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The whole point of the application

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was not to just give me
a single snapshot in time

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but to show me a historical
record of all dates,

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so not only for the date that I've entered

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but all other dates, and it
basically does all dates.

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It'll say, if I scroll
down here, for example,

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let's just pick a random
date, the 5th of November.

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That's Bonfire Night, Guy
Fawkes Night in the UK.

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It says, "In your current
calendar on the 11th,

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on the 5th of November,
you will have been away,

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in the 180 days before
the 5th of November,

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you will have been away
for 88 days," all right?

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So that's what's going
on in my application.

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It's gonna build collection of Records.

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Each one of these is a Record with a date

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and the number of days I've been away

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in the 180-day period before that.

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So it takes, this date here
is the date that I entered,

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the 31st of December,
2023, and it basically,

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it'll iterate over all dates
from now, from that date,

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up to and including the current
date going backwards, okay?

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Each time around the loop,

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it'll subtract one day from the date.

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First time around the loop,

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the date is the 31st of December.

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31st of December. That's the current date.

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Is the date greater than or
equal to the current date?

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I mean today. Today here
is the, check phone.

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00:10:18,720 --> 00:10:21,213
It's the 26th of May I'm here.

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So it's a bit tricky,

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00:10:23,760 --> 00:10:27,030
but there's an API to
get the current date.

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It's the 26th of May.

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So this loop is gonna go
from the 31st of December

200
00:10:31,350 --> 00:10:35,130
all the way down to the 26th of May,

201
00:10:35,130 --> 00:10:37,290
decrementing as we go.

202
00:10:37,290 --> 00:10:38,643
So if I scroll down here,

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00:10:39,570 --> 00:10:41,470
it's gonna have to scroll quite a lot.

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How far is it gonna go?

205
00:10:43,860 --> 00:10:46,620
There we go, down to
today, the 26th of May,

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00:10:46,620 --> 00:10:48,600
as my phone just told me.

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00:10:48,600 --> 00:10:52,170
Each time round, and I'll
show you how it does this,

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00:10:52,170 --> 00:10:55,350
it'll say if we're talking
about a particular date,

209
00:10:55,350 --> 00:10:57,630
so let's again pick a date at random,

210
00:10:57,630 --> 00:10:59,460
the, oh, I'll tell you what we'll do.

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00:10:59,460 --> 00:11:02,310
We'll choose the 11th of August.

212
00:11:02,310 --> 00:11:06,240
That was our wedding date back
in 1990, the 11th of August.

213
00:11:06,240 --> 00:11:07,620
So let's say that's the date.

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00:11:07,620 --> 00:11:09,180
It's gonna iterate through all the dates,

215
00:11:09,180 --> 00:11:11,700
but let's say that's the
current date it's looking at.

216
00:11:11,700 --> 00:11:14,940
All right, so it'll
say prior to that date,

217
00:11:14,940 --> 00:11:17,850
look at all the visits that
you'd been away altogether

218
00:11:17,850 --> 00:11:21,930
and tell me in the 180
days prior to that date,

219
00:11:21,930 --> 00:11:26,430
okay, so 180 days prior

220
00:11:26,430 --> 00:11:30,423
to the 11th of August, I was away 74 days.

221
00:11:34,080 --> 00:11:36,960
So it'll tell me how many
days I was away, 74 days,

222
00:11:36,960 --> 00:11:38,700
and then it creates a new Record.

223
00:11:38,700 --> 00:11:41,130
It'll create a Record with the date,

224
00:11:41,130 --> 00:11:44,137
like the 11th of August, and it'll say,

225
00:11:44,137 --> 00:11:46,977
"On that date, you were away 74 days."

226
00:11:47,940 --> 00:11:52,290
Okay, so I add that to
my vector of all Records,

227
00:11:52,290 --> 00:11:54,780
and then I decrement my date counter

228
00:11:54,780 --> 00:11:56,550
to go back to the previous day.

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00:11:56,550 --> 00:12:00,870
Okay, so the previous day
was the 10th of August,

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and it'll say, "Okay then,
for the 10th of August,

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00:12:04,290 --> 00:12:05,760
with all the visits that you had,

232
00:12:05,760 --> 00:12:07,450
how many days were you away

233
00:12:08,545 --> 00:12:11,340
in the 180-day period before then?"

234
00:12:11,340 --> 00:12:15,847
So 180 days before the 10th
of August, it'll tell me,

235
00:12:15,847 --> 00:12:18,150
"Oh, it was 74 days away there."

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00:12:18,150 --> 00:12:22,353
That days, I push it
into a new Record again.

237
00:12:23,580 --> 00:12:26,190
Obviously I need to explain
how this function works.

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00:12:26,190 --> 00:12:28,503
I'll get on to that in a moment.

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00:12:29,820 --> 00:12:31,260
By the time it gets to here,

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00:12:31,260 --> 00:12:34,710
it has gathered a collection
of all the Record objects,

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00:12:34,710 --> 00:12:37,440
and I then display them
in chronological order.

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00:12:37,440 --> 00:12:40,680
Well, actually, it's
reverse chronological order.

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00:12:40,680 --> 00:12:43,740
For each Record, I
iterate over my Records,

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00:12:43,740 --> 00:12:45,780
and for each Record I display it.

245
00:12:45,780 --> 00:12:46,613
Remember,

246
00:12:48,390 --> 00:12:51,240
my Record type implements
the Display trait,

247
00:12:51,240 --> 00:12:54,360
so it can output via the format function.

248
00:12:54,360 --> 00:12:59,360
It can output a Record
object as the number of days

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00:12:59,460 --> 00:13:01,440
and the date that we're talking about,

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00:13:01,440 --> 00:13:04,410
the number of days and the
date that we're talking about.

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00:13:04,410 --> 00:13:07,350
So that's coming together
quite nicely now.

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00:13:07,350 --> 00:13:09,450
So it displays all the information

253
00:13:09,450 --> 00:13:11,100
in reverse chronological order.

254
00:13:11,100 --> 00:13:12,180
So that was useful for me.

255
00:13:12,180 --> 00:13:14,850
So I can see where the hotspots are.

256
00:13:14,850 --> 00:13:18,393
It looks quite busy around July.

257
00:13:20,310 --> 00:13:22,980
Right, so if I go back,
it'll keep on looping

258
00:13:22,980 --> 00:13:26,670
till the bottom of the
current date basically,

259
00:13:26,670 --> 00:13:30,900
but then it also displays
the same information

260
00:13:30,900 --> 00:13:32,940
in reverse number of days.

261
00:13:32,940 --> 00:13:35,310
Ultimately, I want to be able to see

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00:13:35,310 --> 00:13:39,240
what's the top number of days
I've been away in that period,

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00:13:39,240 --> 00:13:42,090
so I sort my records.

264
00:13:42,090 --> 00:13:46,447
When you sort the records, it
has to compare records to say,

265
00:13:46,447 --> 00:13:49,080
"Is your number of days
greater than or less than?"

266
00:13:49,080 --> 00:13:52,770
So sorting, in order to
sort a structure type

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00:13:52,770 --> 00:13:54,510
or a vector of structures,

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00:13:54,510 --> 00:13:58,110
you have to implement
comparability, and I did.

269
00:13:58,110 --> 00:14:03,060
So my Record type
implemented the order trait

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00:14:03,060 --> 00:14:05,757
and the partial order trait, okay?

271
00:14:05,757 --> 00:14:07,680
And that gives me sortability.

272
00:14:07,680 --> 00:14:10,950
It can then compare based on days.

273
00:14:10,950 --> 00:14:13,200
It compares one record against another,

274
00:14:13,200 --> 00:14:17,010
and it's the days that will
influence the ordering.

275
00:14:17,010 --> 00:14:19,530
So then, having sorted the records,

276
00:14:19,530 --> 00:14:23,583
these records, this vector
is now in time order.

277
00:14:26,059 --> 00:14:28,380
So not in time order, in
the number of days order.

278
00:14:28,380 --> 00:14:32,040
So I can then display the
records in the number of days,

279
00:14:32,040 --> 00:14:34,290
okay, so the highest
number of days appearing

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00:14:34,290 --> 00:14:35,190
at the top of the list.

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00:14:35,190 --> 00:14:36,960
It says, "Watch out.

282
00:14:36,960 --> 00:14:41,520
On the 4th of November, in
the 180 days prior to that,

283
00:14:41,520 --> 00:14:43,530
you'll have been away 89 days."

284
00:14:43,530 --> 00:14:47,793
That's very close to the 90-day
margin for the Brexit rule.

285
00:14:48,960 --> 00:14:51,410
I still can't believe
Brexit, to be quite honest.