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//! A singly linked list intended to be chained along stack frames.
#![cfg_attr(not(test), no_std)]
use core::ops::Range;
/// A singly-linked list intended to be chained along stack frames.
///
/// See [module documentation](mod@self) for more.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
pub enum Stack<'a, T> {
/// An empty stack
Bottom,
/// A successor to a previous stack
Top { head: T, tail: &'a Self },
}
impl<'a, T> Stack<'a, T> {
/// A utility terminator for any stack.
/// ```
/// # use stackstack::Stack;
/// let b = Stack::BOTTOM;
/// let _: &Stack::<&str> = b;
/// assert!(b.is_empty());
/// ```
pub const BOTTOM: &'a Self = &Self::Bottom;
/// Returns a stack of length 1, containing only the given item.
/// ```
/// # use stackstack::Stack;
/// let a = Stack::of("a");
/// assert_eq!(a.len(), 1);
///
/// ```
pub const fn of(head: T) -> Self {
Self::Top {
head,
tail: Self::BOTTOM,
}
}
/// Return a new stack with the given element appended.
/// ```
/// # use stackstack::Stack;
/// let a = Stack::of("a");
/// let ab = a.pushed("b");
/// assert_eq!(a.len(), 1);
/// assert_eq!(ab.len(), 2);
/// assert!(ab.iter().eq(&["a", "b"]))
/// ```
pub const fn pushed(&'a self, head: T) -> Self {
Self::Top { head, tail: self }
}
/// Edit all the non-empty items in the given iterator to follow from `self`,
/// returning the head of the stack.
///
/// This is useful for appending items with some scratch space.
///
/// ```
/// # use stackstack::Stack;
/// let mut scratch = ["b", "c", "d"].map(Stack::of);
/// let a = Stack::of("a");
/// let abcd = a.chained(&mut scratch);
/// assert!(abcd.iter().eq(&["a", "b", "c", "d"]))
/// ```
pub fn chained(&'a self, iter: impl IntoIterator<Item = &'a mut Self>) -> &'a Self {
let mut curr = self;
for it in iter {
match it {
Stack::Bottom => continue,
Stack::Top { head: _, tail } => {
*tail = curr;
}
}
curr = it;
}
curr
}
/// Extend the current stack with each item from the given iterator, calling
/// the given closure on the result.
///
/// This creates a stack frame for each item in the iterator.
///
/// ```
/// use stackstack::Stack;
/// let a = Stack::of("a");
/// a.on_chained(["b", "c", "d"], |abcd| {
/// assert!(abcd.iter().eq(&["a", "b", "c", "d"]))
/// })
/// ```
pub fn on_chained<R>(
&self,
chain: impl IntoIterator<Item = T>,
on: impl FnOnce(&Stack<'_, T>) -> R,
) -> R {
let mut chain = chain.into_iter();
match chain.next() {
Some(head) => self.pushed(head).on_chained(chain, on),
None => on(self),
}
}
/// Return the number of items in the stack.
pub fn len(&self) -> usize {
match self {
Stack::Bottom => 0,
Stack::Top { head: _, tail } => tail.len() + 1,
}
}
/// Returns `true` if there are no items in the stack.
pub fn is_empty(&self) -> bool {
matches!(self, Stack::Bottom)
}
/// Get an item by 0-based index, from the bottom of the stack.
///
/// ```
/// # use stackstack::{Stack, stack};
/// let mut storage;
/// let abcd = stack!(["a", "b", "c", "d"] in storage);
/// assert_eq!(abcd.get(0), Some(&"a"));
/// assert_eq!(abcd.get(3), Some(&"d"));
/// assert_eq!(abcd.get(4), None);
/// ```
pub fn get(&self, ix: usize) -> Option<&T> {
let mut current = self;
let depth = self.len().checked_sub(ix + 1)?;
for _ in 0..depth {
match current {
Stack::Bottom => return None,
Stack::Top { head: _, tail } => current = tail,
}
}
match current {
Stack::Bottom => None,
Stack::Top { head, tail: _ } => Some(head),
}
}
/// Return an [`Iterator`] of items in the stack, from the bottom to the top.
///
/// Note that the returned item implements [`DoubleEndedIterator`].
///
/// ```
/// # use stackstack::{Stack, stack};
/// let mut storage;
/// let abcd = stack!(["a", "b", "c", "d"] in storage);
/// assert!(abcd.iter().eq(&["a", "b", "c", "d"]));
/// assert!(abcd.iter().rev().eq(&["d", "c", "b", "a"]));
/// ```
pub fn iter(&'a self) -> Iter<'a, T> {
Iter {
head: self,
ixs: 0..self.len(),
}
}
}
impl<'a, T> Default for Stack<'a, T> {
fn default() -> Self {
Self::Bottom
}
}
pub struct Iter<'a, T> {
head: &'a Stack<'a, T>,
ixs: Range<usize>,
}
impl<'a, T> Iterator for Iter<'a, T> {
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
self.head.get(self.ixs.next()?)
}
}
impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
fn next_back(&mut self) -> Option<Self::Item> {
self.head.get(self.ixs.next_back()?)
}
}
/// Convenience macro for creating a [`Stack`] in a single stack frame.
///
/// ```
/// # use stackstack::stack;
/// let mut storage;
/// let abcd = stack![["a", "b", "c", "d"] in storage];
/// assert!(abcd.iter().eq(&["a", "b", "c", "d"]))
/// ```
#[macro_export]
macro_rules! stack {
([$($expr:expr),* $(,)?] in $ident:ident) => {{
$ident = [$($expr),*].map($crate::Stack::of);
$crate::Stack::Bottom.chained($ident.iter_mut())
}};
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn on_chained() {
Stack::of(String::from("mary")).on_chained(
["had".into(), "a".into(), "little".into(), "lamb".into()],
|it| {
assert_eq!(
*it,
Stack::Top {
head: String::from("lamb"),
tail: &Stack::Top {
head: String::from("little"),
tail: &Stack::Top {
head: String::from("a"),
tail: &Stack::Top {
head: String::from("had"),
tail: &Stack::Top {
head: String::from("mary"),
tail: &Stack::Bottom
}
}
}
}
}
)
},
);
}
#[test]
fn get() {
Stack::of(String::from("mary")).on_chained(
["had".into(), "a".into(), "little".into(), "lamb".into()],
|it| {
assert_eq!(it.get(0).unwrap(), "mary");
assert_eq!(it.get(1).unwrap(), "had");
assert_eq!(it.get(2).unwrap(), "a");
assert_eq!(it.get(3).unwrap(), "little");
assert_eq!(it.get(4).unwrap(), "lamb");
assert_eq!(it.get(5), None);
},
);
}
#[test]
fn iter() {
Stack::of(String::from("mary")).on_chained(
["had".into(), "a".into(), "little".into(), "lamb".into()],
|it| {
assert_eq!(
it.iter().collect::<Vec<_>>(),
["mary", "had", "a", "little", "lamb",]
)
},
);
}
#[test]
fn rev() {
Stack::of(String::from("mary")).on_chained(
["had".into(), "a".into(), "little".into(), "lamb".into()],
|it| {
assert_eq!(
it.iter().rev().collect::<Vec<_>>(),
["lamb", "little", "a", "had", "mary",]
)
},
);
}
}