public/subjects/sales/README.md

## sales

### Instructions

You are going to make a shopping system. It will have a store where the products will be saved, and a cart which will contain items from which a receipt will be generated.

**"Buy three, get one free".**

The store is having a promotion. The cheapest of three items will be free. But there is a problem with the printer interface, it cannot receive any zero values. We can create a workaround. We will reduce all of the values in the cart by a small amount to show the correct total price. You can see the example to see how it works.

You will have to implement for the `Cart` structure the following **functions**:

- `new`: that will initialize the cart.
- `insert_item`: will receive a reference to `Store` and a `String`. Just like the name says, it will insert the item to the cart.
- `generate_receipt`: returns a vector of sorted floats. This function must generate the receipt just like the example below, using the promotion. It should save the result in the `receipt` field.

### Expected Function

```rust
#[derive(Debug, Clone, PartialEq)]
pub struct Store {
    pub products: Vec<(String, f32)>,
}
impl Store {
    pub fn new(products: Vec<(String, f32)>) -> Store {
        Store { products }
    }
}

#[derive(Debug, Clone, PartialEq)]
pub struct Cart {
    // expected public fields
}
impl Cart {
    pub fn new() -> Cart {}
    pub fn insert_item(&mut self, s: &Store, ele: String) {}
    pub fn generate_receipt(&mut self) -> Vec<f32> {}
}
```

### Example

```
[1.23, 3.12, 23.1]` => `[1.17, 2.98, 22.06]
```

Because `1.17 + 2.98 + 22.06` == `0 + 3.12 + 23.1`

Floats are rounded with a precision of two decimals which can create small discrepancies as per the example above.

This is a percentage calculation, and it can be applied to a set of three items. If the client purchases 9 items, they will receive three for free, with the discount applied to all items.

```
[1.23, 23.1, 3.12, 9.75, 1.75, 23.75, 2.75, 1.64, 15.23] => [1.16, 1.55, 1.65, 2.6, 2.94, 9.2, 14.38, 21.8, 22.42]
```


```
[3.12, 9.75, 1.75, 23.75, 2.75, 1.64, 15.23] => [1.54, 1.65, 2.59, 2.94, 9.18, 14.34, 22.36]
```

> Hint: Closures are the way.


### Usage

Here is a program to test your function,

```rust
use sales::*;

fn main() {
    let store = Store::new(vec![
        (String::from("product A"), 1.23),
        (String::from("product B"), 23.1),
        (String::from("product C"), 3.12)]);

    println!("{:?}", store);

    let mut cart = Cart::new();
    cart.insert_item(&store, String::from("product A"));
    cart.insert_item(&store, String::from("product B"));
    cart.insert_item(&store, String::from("product C"));

    println!("{:?}", cart.generate_receipt());

    println!("{:?}", cart);
}
```

And its output:

```console
$ cargo run
Store { products: [("product A", 1.23), ("product B", 23.1), ("product C", 3.12)] }
[1.17, 2.98, 22.06]
Cart { items: [("product A", 1.23), ("product B", 23.1), ("product C", 3.12)], receipt: [1.17, 2.98, 22.06] }
$
```

### Notions

- [closures](https://doc.rust-lang.org/rust-by-example/fn/closures.html)