# Lucian's Luscious Lasagna
Welcome to Lucian's Luscious Lasagna on Exercism's Rust Track.
If you need help running the tests or submitting your code, check out `HELP.md`.
If you get stuck on the exercise, check out `HINTS.md`, but try and solve it without using those first :)
## Introduction
In Rust, assigning a value to a name is referred to as a _binding_. Bindings are immutable unless declared with the `mut` keyword. As Rust is a statically-typed language, each binding has a type known at compile-time.
Bindings are most commonly defined using the `let` keyword. Specifying a binding's type is optional for most bindings, as Rust's _type inference_ can usually infer the type based on their value. A binding looks like this:
```rust
// Automatically inferred type
let fingers = 10;
```
Functions are _items_. Where bindings typically refer to a particular value, items refer to a unit of code organization, typically a function or a module, which is available throughout the lifetime of the program. A function automatically returns the result of its last expression. A function may have 0 or more parameters, which are bindings with a lifetime of the function call.
Type inference is theoretically possible for functions, but is disabled as an intentional language design choice. While this means that you need to spend a little more time when writing code to specify precisely what a function's input and output types are, you save the time when you're reading the code, because all the input and output types are explicitly defined.
```rust
fn add(x: i32, y: i32) -> i32 {
x + y
}
```
Invoking a function is done by specifying its name followed by parentheses. If the function requires parameters, an argument must be specified for each within the parentheses.
```rust
let five = add(2, 3);
```
If a binding's type cannot be inferred, the compiler will report an error. To fix this, add an explicit type annotation to the binding.
```rust
// Explicit type annotation
let fingers: i32 = 10;
```
Items in Rust can be used before or after they are defined, because they have a static lifetime. Bindings, on the other hand, can only be used _after_ they have been defined. Using a binding before it has been defined results in a compile error.
```rust
fn main() {
// `fn add` hasn't yet been defined, but that's perfectly ok
dbg!(add(3, 4));
}
fn add(x: i32, y: i32) -> i32 {
x + y
}
```
```rust
// this won't compile; `a` is used before its binding is defined
let b = a;
let a = x + y
```
Rust uses curly braces (`{}`) to define a scope. A binding defined within a scope can't escape from it. This means that scope is defined by indenting the code with spaces, relative to the line declaring the binding.
```rust
let a = 1;
dbg!(a); // 1
{
// Here, we re-bind `a` to a new value, which is still immutable.
// This technique is called _shadowing_. The new binding is constrained to
// this anonymous scope. Outside this scope, the previous binding still
// applies.
let a = 2;
let b = 3;
dbg!(a, b); // 2, 3
}
// can't use `b` anymore because it is out of scope
// dbg!(b);
// The shadowed `a` in the inner scope above has fallen out of scope,
// leaving us with our original binding.
dbg!(a); // 1
```
Rust items are often organized in modules. Each crate is implicitly a module, but it can define inner sub-modules of arbitrary depth. A module groups related functionality and is defined using the `mod` keyword.
```rust
mod calc_i32 {
fn add(a: i32, b: i32) -> i32 { a + b }
fn sub(a: i32, b: i32) -> i32 { a - b }
fn mul(a: i32, b: i32) -> i32 { a * b }
fn div(a: i32, b: i32) -> i32 { a / b }
}
```
Rust supports two types of comments. The keyword `//` indicates a single-line comment; everything following the keyword until the end of the line is ignored. The keywords `/*` and `*/` indicate a multi-line comment; everything within those two keywords is ignored. It is idiomatic and good practice to prefer single-line comments.
Rust also supports doc-comments, which show up in the generated documentation produced by `cargo doc`. Outer doc comments are formed with the keyword `///`, which acts identically to the `//` keyword. They apply to the item which follows them, such as a function:
```rust
/// The `add` function produces the sum of its arguments.
fn add(x: i32, y: i32) -> i32 { x + y }
```
Inner doc comments are formed with the keyword `//!`, which acts identically to the `//` keyword. They apply to the item enclosing them, such as a module:
```rust
mod my_cool_module {
//! This module is the bee's knees.
}
```
Doc comments can be of arbitrary length and contain markdown, which is rendered into the generated documentation.
## Instructions
In this exercise you're going to write some code to help you cook a brilliant lasagna from your favorite cooking book.
You have four tasks, all related to the time spent cooking the lasagna.
## 1. Define the expected oven time in minutes
Define the `expected_minutes_in_oven` binding to check how many minutes the lasagna should be in the oven. According to the cooking book, the expected oven time in minutes is 40:
```rust
expected_minutes_in_oven()
// Returns: 40
```
## 2. Calculate the remaining oven time in minutes
Define the `remaining_minutes_in_oven` function that takes the actual minutes the lasagna has been in the oven as a parameter and returns how many minutes the lasagna still has to remain in the oven, based on the expected time oven time in minutes from the previous task.
```rust
remaining_minutes_in_oven(30)
// Returns: 10
```
## 3. Calculate the preparation time in minutes
Define the `preparation_time_in_minutes` function that takes the number of layers you added to the lasagna as a parameter and returns how many minutes you spent preparing the lasagna, assuming each layer takes you 2 minutes to prepare.
```rust
preparation_time_in_minutes(2)
// Returns: 4
```
## 4. Calculate the elapsed time in minutes
Define the `elapsed_time_in_minutes` function that takes two parameters: the first parameter is the number of layers you added to the lasagna, and the second parameter is the number of minutes the lasagna has been in the oven. The function should return how many minutes you've worked on cooking the lasagna, which is the sum of the preparation time in minutes, and the time in minutes the lasagna has spent in the oven at the moment.
```rust
elapsed_time_in_minutes(3, 20)
// Returns: 26
```
## Source
### Created by
- @coriolinus
- @ErikSchierboom