Avatar of bkhl

bkhl's solution

to Simple Linked List in the Rust Track

Published at Jan 05 2019 · 0 comments
Instructions
Test suite
Solution

Note:

This exercise has changed since this solution was written.

Write a simple linked list implementation that uses Elements and a List.

The linked list is a fundamental data structure in computer science, often used in the implementation of other data structures. They're pervasive in functional programming languages, such as Clojure, Erlang, or Haskell, but far less common in imperative languages such as Ruby or Python.

The simplest kind of linked list is a singly linked list. Each element in the list contains data and a "next" field pointing to the next element in the list of elements.

This variant of linked lists is often used to represent sequences or push-down stacks (also called a LIFO stack; Last In, First Out).

As a first take, lets create a singly linked list to contain the range (1..10), and provide functions to reverse a linked list and convert to and from arrays.

When implementing this in a language with built-in linked lists, implement your own abstract data type.

Implementation Hints

Do not implement the struct SimpleLinkedList as a wrapper around a Vec. Instead, allocate nodes on the heap.
This might be implemented as:

pub struct SimpleLinkedList<T> {
    head: Option<Box<Node<T>>>,
}

The head field points to the first element (Node) of this linked list.
This implementation also requires a struct Node with the following fields:

struct Node<T> {
    data: T,
    next: Option<Box<Node<T>>>,
}

data contains the stored data, and next points to the following node (if available) or None.

Why Option<Box<Node<T>>> and not just Option<Node<T>>?

Try it on your own. You will get the following error.

| struct Node<T>
| ^^^^^^^^^^^^^^ recursive type has infinite size
...
|     next: Option<Node<T>>,
|     --------------------- recursive without indirection

The problem is that at compile time the size of next must be known. Since next is recursive ("a node has a node has a node..."), the compiler does not know how much memory is to be allocated. In contrast, Box is a heap pointer with a defined size.

Rust Installation

Refer to the exercism help page for Rust installation and learning resources.

Writing the Code

Execute the tests with:

$ cargo test

All but the first test have been ignored. After you get the first test to pass, open the tests source file wich is located in the tests directory and remove the #[ignore] flag from the next test and get the tests to pass again. Each separate test is a function with #[test] flag above it. Continue, until you pass every test.

If you wish to run all tests without editing the tests source file, use:

$ cargo test -- --ignored

To run a specific test, for example some_test, you can use:

$ cargo test some_test

If the specfic test is ignored use:

$ cargo test some_test -- --ignored

To learn more about Rust tests refer to the online test documentation

Make sure to read the Modules chapter if you haven't already, it will help you with organizing your files.

Feedback, Issues, Pull Requests

The exercism/rust repository on GitHub is the home for all of the Rust exercises. If you have feedback about an exercise, or want to help implement new exercises, head over there and create an issue. Members of the rust track team are happy to help!

If you want to know more about Exercism, take a look at the contribution guide.

Source

Inspired by 'Data Structures and Algorithms with Object-Oriented Design Patterns in Ruby', singly linked-lists. http://www.brpreiss.com/books/opus8/html/page96.html#SECTION004300000000000000000

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

simple-linked-list.rs

extern crate simple_linked_list;

use simple_linked_list::SimpleLinkedList;

#[test]
fn test_new_list_is_empty() {
    let list: SimpleLinkedList<u32> = SimpleLinkedList::new();
    assert_eq!(list.len(), 0, "list's length must be 0");
}

#[test]
#[ignore]
fn test_push_increments_length() {
    let mut list: SimpleLinkedList<u32> = SimpleLinkedList::new();
    list.push(1);
    assert_eq!(list.len(), 1, "list's length must be 1");
    list.push(2);
    assert_eq!(list.len(), 2, "list's length must be 2");
}

#[test]
#[ignore]
fn test_pop_decrements_length() {
    let mut list: SimpleLinkedList<u32> = SimpleLinkedList::new();
    list.push(1);
    list.push(2);
    list.pop();
    assert_eq!(list.len(), 1, "list's length must be 1");
    list.pop();
    assert_eq!(list.len(), 0, "list's length must be 0");
}

#[test]
#[ignore]
fn test_pop_returns_last_added_element() {
    let mut list: SimpleLinkedList<u32> = SimpleLinkedList::new();
    list.push(1);
    list.push(2);
    assert_eq!(list.pop(), Some(2), "Element must be 2");
    assert_eq!(list.pop(), Some(1), "Element must be 1");
    assert_eq!(list.pop(), None, "No element should be contained in list");
}

#[test]
#[ignore]
fn test_peek_returns_head_element() {
    let mut list: SimpleLinkedList<u32> = SimpleLinkedList::new();
    assert_eq!(list.peek(), None, "No element should be contained in list");
    list.push(2);
    assert_eq!(list.peek(), Some(&2), "Element must be 2");
    assert_eq!(list.peek(), Some(&2), "Element must be still 2");
}

#[test]
#[ignore]
fn test_from_slice() {
    let array = ["1", "2", "3", "4"];
    let mut list = SimpleLinkedList::from(array.as_ref());
    assert_eq!(list.pop(), Some("4"));
    assert_eq!(list.pop(), Some("3"));
    assert_eq!(list.pop(), Some("2"));
    assert_eq!(list.pop(), Some("1"));
}

#[test]
#[ignore]
fn test_reverse() {
    let mut list: SimpleLinkedList<u32> = SimpleLinkedList::new();
    list.push(1);
    list.push(2);
    list.push(3);
    let mut rev_list = list.rev();
    assert_eq!(rev_list.pop(), Some(1));
    assert_eq!(rev_list.pop(), Some(2));
    assert_eq!(rev_list.pop(), Some(3));
    assert_eq!(rev_list.pop(), None);
}

#[test]
#[ignore]
fn test_into_vector() {
    let mut v = Vec::new();
    let mut s = SimpleLinkedList::new();
    for i in 1..4 {
        v.push(i);
        s.push(i);
    }
    let s_as_vec: Vec<i32> = s.into();
    assert_eq!(v, s_as_vec);
}
pub struct SimpleLinkedList<T> {
    head: Option<Box<Node<T>>>,
}

struct Node<T> {
    value: T,
    tail: Option<Box<Node<T>>>,
}

impl<T> Node<T> {
    fn len(&self) -> usize {
        let mut c = 1;
        let mut tail = &self.tail;
        while let &Some(ref boxed_node) = tail {
            c += 1;
            tail = &boxed_node.tail;
        }
        c
    }

    fn peek(&self) -> &T {
        &self.value
    }
}

impl<T> SimpleLinkedList<T> {
    pub fn new() -> Self {
        SimpleLinkedList { head: None }
    }

    pub fn len(&self) -> usize {
        match &self.head {
            &None => 0,
            &Some(ref boxed_node) => boxed_node.len(),
        }
    }

    pub fn push(&mut self, element: T) {
        let new_head = Box::new(Node {
            value: element,
            tail: std::mem::replace(&mut self.head, None),
        });
        self.head = Some(new_head);
    }

    pub fn pop(&mut self) -> Option<T> {
        match std::mem::replace(&mut self.head, None) {
            Some(boxed_node) => {
                let node = *boxed_node;
                self.head = node.tail;
                Some(node.value)
            }
            None => None,
        }
    }

    pub fn peek(&self) -> Option<&T> {
        match self.head {
            Some(ref boxed_node) => Some(boxed_node.peek()),
            None => None,
        }
    }
}

impl<T: Clone> SimpleLinkedList<T> {
    pub fn rev(&self) -> SimpleLinkedList<T> {
        let mut list = Self::new();
        match self.head {
            Some(ref boxed_node) => {
                let mut node = boxed_node;
                loop {
                    list.push(node.value.clone());
                    match node.tail {
                        Some(ref boxed_node) => {
                            node = boxed_node;
                        }
                        None => break,
                    }
                }
            }
            None => {}
        }
        list
    }
}

impl<'a, T: Clone> From<&'a [T]> for SimpleLinkedList<T> {
    fn from(item: &[T]) -> Self {
        let mut list = Self::new();
        for element in item.iter() {
            list.push(element.clone());
        }
        list
    }
}

impl<T> Into<Vec<T>> for SimpleLinkedList<T> {
    fn into(mut self) -> Vec<T> {
        let mut vec = Vec::new();
        while let Some(value) = self.pop() {
            vec.insert(0, value);
        }
        vec
    }
}

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?