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/*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at https://mozilla.org/MPL/2.0/.
*/
//! Breadth-first exhaustive `zip` for repeatable iterators.
//! Behavior matches the following pseudocode specification:
//! - Initialize a counter `i` at zero.
//! - When propmted, pull the first element from each iterator.
//! - If any iterator is empty, return `None`.
//! - When prompted again, advance only the last iterator.
//! - Continue to do so until the last iterator terminates or reaches its `i`th element.
//! - When it does so, reset it and pull the next element from the second-to-last iterator.
//! - Repeat this process until we exhaust the first iterator.
//! - When you've done that, increase `i` and repeat.
//! - Once `i` exceeds the longest iterator's length, we're done: return `None`.
#![cfg_attr(not(test), no_std)]
#![deny(warnings)]
#![warn(
clippy::all,
clippy::missing_docs_in_private_items,
clippy::nursery,
clippy::pedantic,
clippy::restriction,
clippy::cargo,
missing_docs,
rustdoc::all
)]
#![allow(
clippy::blanket_clippy_restriction_lints,
clippy::cognitive_complexity,
clippy::expect_used,
clippy::implicit_return,
clippy::inline_always,
clippy::needless_borrowed_reference,
clippy::panic,
clippy::question_mark_used,
clippy::separated_literal_suffix,
clippy::string_add,
clippy::unwrap_used
)]
#[cfg(test)]
mod test;
/// End of a recursive implementation of a breadth-first exhaustive `zip`.
#[derive(Clone)]
pub struct BaseCase(bool);
/// Index and value.
#[derive(Clone, Debug)]
struct Indexed<Type> {
/// Index.
index: usize,
/// Value.
value: Type,
}
/// Recursive implementation of a breadth-first exhaustive `zip`.
#[derive(Clone)]
pub struct BreadthFirstZipped<Head: Iterator + Clone, Tail: BreadthFirst>
where
Head::Item: Clone,
{
/// Enumerated iterator for this current "index" in the recursive scheme.
head: ::core::iter::Enumerate<Head>,
/// Copy of the original iterator to allow rewinding later.
orig_head: ::core::iter::Enumerate<Head>,
/// Implementations for the rest of the list.
tail: Tail,
/// Current value: we don't always advance every call.
current: Indexed<Head::Item>,
}
impl<Head: Iterator + Clone, Tail: BreadthFirst> BreadthFirstZipped<Head, Tail>
where
Head::Item: Clone,
{
/// Initialize a new recursive node of a breadth-first zip implementation.
/// # Errors
/// If any iterator is empty.
#[inline(always)]
pub fn new(head: Head, tail: Tail) -> Result<Self, &'static str> {
#![allow(clippy::shadow_unrelated)]
let orig_head = head.enumerate();
let mut head = orig_head.clone();
let current = head
.next()
.ok_or("Tried to breadth-first zip an empty iterator")
.map(|(index, value)| Indexed { index, value })?;
Ok(Self {
head,
orig_head,
tail,
current,
})
}
}
/// Flatten a nested tuple like `(A, (B, (C, ())))` to a flat one like `(A, B, C)`
pub trait Flatten {
/// Flat tuple, e.g. `(A, B, C)`, not `(A, (B, (C, ())))`.
type Flattened;
/// Flatten e.g. `(A, (B, (C, ())))` into `(A, B, C)`.
#[must_use]
fn flatten(self) -> Self::Flattened;
}
impl Flatten for () {
type Flattened = Self;
#[inline(always)]
#[must_use]
fn flatten(self) -> Self::Flattened {}
}
/// Sealed traits.
mod sealed {
/// Either `BaseCase` or `BreadthFirst<Whatever, ...>` eventually ending in `BaseCase` on the right-hand side.
pub trait BreadthFirst {}
impl BreadthFirst for super::BaseCase {}
impl<Head: Iterator + Clone, Tail: super::BreadthFirst> BreadthFirst
for super::BreadthFirstZipped<Head, Tail>
where
Head::Item: Clone,
{
}
}
/// Helper trait returning a nested list that will be turned into a flat list for a huge but finite range of tuple sizes.
pub trait BreadthFirst: Clone + sealed::BreadthFirst {
/// Depth of recursion.
const DEPTH: usize;
/// Output of `advance` if successful.
type Advance: Flatten;
/// Fallibly choose the next output.
#[must_use]
fn advance(&mut self, index_sum: usize) -> Option<Self::Advance>;
/// Rewind the iterator back to its starting point
fn rewind(&mut self);
}
impl BreadthFirst for BaseCase {
const DEPTH: usize = 0;
type Advance = ();
#[inline(always)]
#[must_use]
fn advance(&mut self, index_sum: usize) -> Option<Self::Advance> {
(index_sum == 0 && self.0).then(|| {
self.0 = false;
})
}
#[inline(always)]
fn rewind(&mut self) {
self.0 = true;
}
}
impl<Head: Iterator + Clone, Tail: BreadthFirst> BreadthFirst for BreadthFirstZipped<Head, Tail>
where
Head::Item: Clone,
(Head::Item, Tail::Advance): Flatten,
{
const DEPTH: usize = Tail::DEPTH + 1;
type Advance = (Head::Item, Tail::Advance);
#[inline(always)]
#[must_use]
fn advance(&mut self, index_sum: usize) -> Option<Self::Advance> {
loop {
if let Some(tail) = self
.tail
.advance(index_sum.checked_sub(self.current.index)?)
{
return Some((self.current.value.clone(), tail));
}
self.current = (self.current.index < index_sum).then(|| {
self.head
.next()
.map(|(index, value)| Indexed { index, value })
})??;
self.tail.rewind();
}
}
#[inline(always)]
fn rewind(&mut self) {
self.head = self.orig_head.clone();
self.tail.rewind();
self.current = self
.head
.next()
.map(|(index, value)| Indexed { index, value })
.unwrap();
}
}
/// Unflatten a tuple like `(A, B, C)` to `BreadthFirstZipped<A, BreadthFirstZipped<B, BreadthFirstZipped<C, BaseCase>>>`.
trait Unflatten {
/// E.g. `BreadthFirstZipped<A, BreadthFirstZipped<B, BreadthFirstZipped<C, BaseCase>>>`, not `(A, B, C)`.
type Unflattened: BreadthFirst;
/// Unflatten a tuple like `(A, B, C)` to `BreadthFirstZipped<A, BreadthFirstZipped<B, BreadthFirstZipped<C, BaseCase>>>`.
fn unflatten(self) -> Result<Self::Unflattened, &'static str>;
}
impl Unflatten for () {
type Unflattened = BaseCase;
#[inline(always)]
fn unflatten(self) -> Result<Self::Unflattened, &'static str> {
Ok(BaseCase(true))
}
}
/// Helper struct for a breadth-first zip: a counter controlling the maximum index sum of the internal recursive implementation.
pub struct BreadthFirstManager<Tail: BreadthFirst> {
/// Recursive implementation.
tail: Tail,
/// "Global" counter to allow the maximum possible sum of indices.
index_sum: usize,
}
impl<Tail: BreadthFirst> BreadthFirstManager<Tail> {
/// Initialize a new breadth-first algorithm.
#[inline(always)]
#[must_use]
pub const fn new(tail: Tail) -> Self {
Self { tail, index_sum: 0 }
}
}
/// Zip a tuple into a lazy breadth-first traversal of each possible combination with a monotonically increasing sum of indices.
pub trait BreadthFirstZip {
/// Rearrangement of input into a nested tuple.
type Nested: BreadthFirst;
/// Lazy breadth-first exhaustive `zip` that guarantees a monotonically increasing sum of indices.
/// # Errors
/// If any iterator is empty.
fn breadth_first_zip(self) -> Result<BreadthFirstManager<Self::Nested>, &'static str>;
}
impl BreadthFirstZip for () {
type Nested = BaseCase;
#[inline(always)]
fn breadth_first_zip(self) -> Result<BreadthFirstManager<Self::Nested>, &'static str> {
self.unflatten().map(BreadthFirstManager::new)
}
}
#[allow(clippy::missing_trait_methods)]
impl<Tail: BreadthFirst> Iterator for BreadthFirstManager<Tail> {
type Item = <Tail::Advance as Flatten>::Flattened;
#[inline(always)]
#[must_use]
fn next(&mut self) -> Option<Self::Item> {
self.tail
.advance(self.index_sum)
.map_or_else(
|| {
self.index_sum = self.index_sum.checked_add(1)?;
self.tail.rewind();
self.tail.advance(self.index_sum)
},
Some,
)
.map(Flatten::flatten)
}
}
breadth_first_zip_macros::implement!(); // Implement traits for (A,), (A, B), (A, B, C), (A, B, C, D), ...