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``````//! This crate makes it possible to initialise arrays from iterators.
//!
//! # Examples:
//!
//! ```rust
//! use from_iter::FromIterator;
//!
//! let iter = (0..).map(|i| i * 2);
//! let array = <[i32; 8]>::from_iter(iter);
//! assert_eq!(array, [0, 2, 4, 6, 8, 10, 12, 14]);
//! ```
//!
//! ```rust
//! use from_iter::FromIterator;
//!
//! let first = vec![1, 1, 2, 3, 5, 8, 13, 21, 34].into_iter();
//! let even_fibonaccis = first.filter(|n| n % 2 == 0);
//! let array = <[i32; 3]>::from_iter(even_fibonaccis);
//! ```
//!
//! ```rust
//! use from_iter::FromIterator;
//!
//! let short_iterator = vec![1, 2, 3].into_iter();
//! let long_array = match <[i32; 1000]>::try_from_iter(short_iterator) {
//!     Ok(long_array) => long_array,
//!     Err(e) => {
//!         eprintln!("{}", e);
//!         return;
//!     }
//! };
//! ```
//!
//! Note that the [from_iter](crate::FromIterator::from_iter) method will panic if the iterator
//! does not provide enough elements to fill the entire array. To avoid this, consider using the
//!
//! Both methods will ignore any extra elements in the iterator.
//!

use std::{error, fmt, mem};

/// This trait contains the [from_iter](crate::FromIterator::from_iter) and
/// [try_from_iter](crate::FromIterator::try_from_iter) methods.
///
/// Example:
/// ```rust
/// use from_iter::FromIterator;
///
/// let iter = (0..).map(|i| i * 2);
/// let array = <[i32; 8]>::from_iter(iter);
/// assert_eq!(array, [0, 2, 4, 6, 8, 10, 12, 14]);
/// ```
pub trait FromIterator<A>: Sized {
/// This method fills an array using the given iterator. Note that it
/// will panic if the iterator doesn't contain enough items.
fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self;

/// This method fills an array using the given iterator. If there aren't
/// enough items available, it will return a [FromIteratorError].
fn try_from_iter<T: IntoIterator<Item = A>>(iter: T) -> Result<Self, FromIteratorError>;

/// This method fills an array using the given iterator. Note that it
/// will panic if the iterator doesn't contain enough items, or there are more elements than
/// expected.
fn from_iter_exact<T: IntoIterator<Item = A>>(iter: T) -> Self;

/// This method fills an array using the given iterator.
///
/// - If there aren't enough items available,it will return [FromIteratorExactError::NotEnoughElement].
/// - If there are more elements than expected, it will return [FromIteratorExactError::TooManyElements].
///   This variant contains the last value returned from the given iterator,
///   since we consume the excessive element from the iterator (if any)
///   in order to check if the length matches.
fn try_from_iter_exact<T: IntoIterator<Item = A>>(
iter: T,
) -> Result<Self, FromIteratorExactError<A>>;
}

/// This represents the error when there aren't enough items available to fill the
/// entire array.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct FromIteratorError;

impl fmt::Display for FromIteratorError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "iterator exhausted unexpectedly")
}
}

/// This represents the error when there aren't enough items available to fill the
/// entire array, or there are more elements than expected.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum FromIteratorExactError<T> {
NotEnoughElement,
TooManyElements(T),
}

impl<T> fmt::Display for FromIteratorExactError<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match self {
FromIteratorExactError::NotEnoughElement => {
write!(f, "iterator exhausted unexpectedly")
}
FromIteratorExactError::TooManyElements(_) => {
write!(f, "iterator has more elements than expected")
}
}
}
}

impl error::Error for FromIteratorError {}

impl<A, const N: usize> FromIterator<A> for [A; N] {
fn from_iter<T: IntoIterator<Item = A>>(iter: T) -> Self {
Self::try_from_iter(iter).expect("iterator exhausted unexpectedly")
}

fn try_from_iter<T: IntoIterator<Item = A>>(iter: T) -> Result<Self, FromIteratorError> {
try_from_iter_impl(iter, false).map_err(|_| FromIteratorError)
}

fn from_iter_exact<T: IntoIterator<Item = A>>(iter: T) -> Self {
use FromIteratorExactError::*;
match Self::try_from_iter_exact(iter) {
Ok(ret) => ret,
Err(NotEnoughElement) => panic!("iterator exhausted unexpectedly"),
Err(TooManyElements(_)) => panic!("iterator has more elements than expected"),
}
}

fn try_from_iter_exact<T: IntoIterator<Item = A>>(
iter: T,
) -> Result<Self, FromIteratorExactError<A>> {
try_from_iter_impl(iter, true)
}
}

fn try_from_iter_impl<A, T: IntoIterator<Item = A>, const N: usize>(
iter: T,
check_next: bool,
) -> Result<[A; N], FromIteratorExactError<A>> {
use FromIteratorExactError::*;

let mut iterator = iter.into_iter();

// use [MaybeUninit::uninit_array] when that method stabilizes
let mut array: [mem::MaybeUninit<A>; N] = unsafe {
// This `assume_init` call is safe because we are initialising
// a bunch of `MaybeUninit`s, which do not require initialisation.
mem::MaybeUninit::uninit().assume_init()
};

for elem in &mut array[..] {
// now fill the array using the iterator
*elem = mem::MaybeUninit::new(iterator.next().ok_or(NotEnoughElement)?);
}

if check_next {
if let Some(next) = iterator.next() {
return Err(TooManyElements(next));
}
}

let array_ptr = &array as *const _ as *const [A; N];

// use [MaybeUninit::array_assume_init] when that method stabilizes
Ok(unsafe {
// This requires the pointer to be valid, properly aligned, and correctly
// initialised. It would be better to use [std::mem::transmute] here,
// but that is not possible because the types depend on the const
// parameter `N`.
})
}

#[cfg(test)]
mod tests {
use super::{FromIterator, FromIteratorExactError::*};

#[test]
fn it_works() {
let iter = (0..).map(|i| i * 2);
let arr = <[i32; 8]>::from_iter(iter);
assert_eq!(arr, [0, 2, 4, 6, 8, 10, 12, 14]);
}

#[test]
#[allow(unused_imports)]
fn no_conflict() {
use std::iter::FromIterator;
let _arr = <[i32; 8]>::from_iter(0..);
}

#[test]
fn try_from_iter() {
let huge_array = <[i32; 1000]>::try_from_iter(0..).unwrap();
assert_eq!(huge_array[0], 0);
assert_eq!(huge_array[1], 1);
assert_eq!(huge_array[2], 2);
assert_eq!(huge_array[999], 999);
}

#[test]
fn try_from_iter_error() {
<[i32; 1000]>::try_from_iter(std::iter::once(5)).unwrap_err();
}

#[test]
fn test_from_iter_exact() {
assert_eq!(
<[i32; 6]>::from_iter_exact((0..6).map(|i| i * 2)),
[0, 2, 4, 6, 8, 10]
);
}

#[test]
fn test_try_from_iter_exact() {
assert_eq!(
<[i32; 6]>::try_from_iter_exact((0..6).map(|i| i * 2)),
Ok([0, 2, 4, 6, 8, 10])
);
assert_eq!(
<[i32; 6]>::try_from_iter_exact((0..5).map(|i| i * 2)),
Err(NotEnoughElement)
);
let mut even_numbers = (0..).map(|i| i * 2);
assert_eq!(
<[i32; 6]>::try_from_iter_exact(&mut even_numbers),
Err(TooManyElements(12))
);
assert_eq!(even_numbers.next(), Some(14));
}
}
``````