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//! An [`ArrayInitializer<T>`] is an object that knows how to initialize the memory for a `[T]`.
//! It is useful for initializing `[T; N]` or (of more relevance for this crate) initializing
//! a `varlen::array::Array<T>`.
//!
//! # Examples
//!
//! Initializing a `[T; N]` from an initializer:
//!
//! ```
//! use varlen::prelude::*;
//!
//! let arr: [u16; 4] = new_array(FillWithDefault);
//! assert_eq!([0, 0, 0, 0], arr);
//!
//! let arr2: [u16; 4] = new_array(FillSequentially(|i| (i * 2) as u16));
//! assert_eq!([0, 2, 4, 6], arr2);
//! ```
//!
//! Initializing a `varlen::array::Array<T>` from an initializer:
//!
//! ```
//! use varlen::prelude::*;
//!
//! let arr: VBox<Array<u16>> = VBox::new(SizedInit(4, FillSequentially(|i| (i * 2) as u16)));
//! assert_eq!(&[0, 2, 4, 6], &arr[..]);
//! ```
use core::mem::MaybeUninit;
trait HasUninit: Sized {
const UNINIT: MaybeUninit<Self>;
}
impl<T> HasUninit for T {
const UNINIT: MaybeUninit<Self> = MaybeUninit::uninit();
}
/// Initializes an array from an initializer.
///
/// # Examples
///
/// ```
/// use varlen::prelude::*;
///
/// let arr: [u16; 4] = new_array(FillSequentially(|i| (i * 2) as u16));
/// assert_eq!([0, 2, 4, 6], arr);
/// ```
pub fn new_array<T, const N: usize>(init: impl ArrayInitializer<T>) -> [T; N] {
let mut data = [HasUninit::UNINIT; N];
init.initialize(&mut data);
unsafe {
// Safety:
// * guaranteed to be initialized by safety requirement on ArrayInitializer
// * size and alignment are guaranteed to match between [MaybeUninit<T>; N]
// and [T; N]
transmute_workaround(data)
}
}
/// An object that is able to initialize an array of `T` values.
///
/// # Examples
///
/// An initializer that fills an array from end to start:
///
/// ```
/// use varlen::prelude::*;
/// use std::mem::MaybeUninit;
///
/// struct WriteBackwardsPowersOf3;
/// unsafe impl ArrayInitializer<u64> for WriteBackwardsPowersOf3 {
/// fn initialize(self, dst: &mut [MaybeUninit<u64>]) {
/// let mut v = 1;
/// for slot in dst.iter_mut().rev() {
/// slot.write(v);
/// v *= 3;
/// }
/// }
/// }
///
/// assert_eq!([81, 27, 9, 3, 1], varlen::array_init::new_array(WriteBackwardsPowersOf3));
/// ```
pub unsafe trait ArrayInitializer<T> {
/// Fills the slice.
///
/// ```
/// use std::mem::MaybeUninit;
/// use varlen::prelude::*;
/// const UNINIT_U16: MaybeUninit<u16> = MaybeUninit::uninit();
/// let mut arr = [UNINIT_U16; 4];
/// FillWithDefault.initialize(&mut arr[..]);
/// for slot in arr {
/// assert_eq!(0, unsafe { slot.assume_init() });
/// }
/// ```
fn initialize(self, dst: &mut [MaybeUninit<T>]);
}
/// Fills an array from a prefix of an iterator.
///
/// # Example
///
/// ```
/// use varlen::prelude::*;
/// let iter = std::iter::successors(Some(3), |i| Some(i * 3));
/// let arr: [u16; 5] = new_array(FromIterPrefix(iter));
/// assert_eq!(arr, [3, 9, 27, 81, 243]);
/// ```
///
/// # Panics
///
/// Panics if the iterator yields fewer elements than the desired output size.
///
/// ```should_panic
/// use varlen::prelude::*;
/// let iter = std::iter::successors(Some(3), |i| Some(i * 3));
/// let arr: [u16; 5] = new_array(FromIterPrefix(iter.take(3))); // Panics
/// ```
pub struct FromIterPrefix<Iter>(pub Iter);
unsafe impl<T, Iter: Iterator<Item = T>> ArrayInitializer<T> for FromIterPrefix<Iter> {
fn initialize(mut self, dst: &mut [MaybeUninit<T>]) {
for slot in dst.iter_mut() {
slot.write(
self.0
.next()
.unwrap_or_else(|| panic!("Iterator had too few elements")),
);
}
}
}
/// Fills an array by calling the function for every element, in ascending order by index.
///
/// The function is given the `usize` element index as a parameter.
///
/// # Examples
///
/// ```
/// use varlen::prelude::*;
/// let arr: [u16; 5] = new_array(FillSequentially(|i| (i as u16) * 2));
/// assert_eq!(arr, [0, 2, 4, 6, 8]);
/// ```
///
/// Stateful functions are also allowed:
///
/// ```
/// use varlen::prelude::*;
/// let mut state = 1;
/// let arr: [u16; 5] = new_array(FillSequentially(|_| {
/// state = state * 3;
/// state
/// }));
/// assert_eq!(arr, [3, 9, 27, 81, 243]);
/// ```
pub struct FillSequentially<Lambda>(pub Lambda);
unsafe impl<T, Lambda: FnMut(usize) -> T> ArrayInitializer<T> for FillSequentially<Lambda> {
fn initialize(mut self, dst: &mut [MaybeUninit<T>]) {
for (i, slot) in dst.iter_mut().enumerate() {
slot.write(self.0(i));
}
}
}
/// Fills an array with [`Default::default()`].
///
/// # Examples
///
/// ```
/// use varlen::prelude::*;
/// let arr: [u16; 5] = new_array(FillWithDefault);
/// assert_eq!(arr, [0, 0, 0, 0, 0]);
/// ```
pub struct FillWithDefault;
unsafe impl<T: Default> ArrayInitializer<T> for FillWithDefault {
fn initialize(self, dst: &mut [MaybeUninit<T>]) {
FillSequentially(|_i| Default::default()).initialize(dst)
}
}
/// Fills an array by copying from a source array. Source and destination must have identical length.
///
/// # Examples
///
/// ```
/// use varlen::prelude::*;
/// let arr: [u16; 5] = new_array(CopyFrom(&[3, 1, 4, 1, 5]));
/// assert_eq!(arr, [3, 1, 4, 1, 5]);
/// ```
///
/// # Panics
///
/// Panics if the source and destination arrays have different length.
///
/// ```should_panic
/// use varlen::prelude::*;
/// let arr: [u16; 6] = new_array(CopyFrom(&[3, 1, 4, 1, 5])); // Panic
/// ```
///
/// ```should_panic
/// use varlen::prelude::*;
/// let arr: [u16; 4] = new_array(CopyFrom(&[3, 1, 4, 1, 5])); // Panic
/// ```
pub struct CopyFrom<'a, T>(pub &'a [T]);
unsafe impl<'a, T: Copy> ArrayInitializer<T> for CopyFrom<'a, T> {
fn initialize(self, dst: &mut [MaybeUninit<T>]) {
// TODO(reinerp): Use MaybeUninit::write_slice once it stabilizes.
// Safety: MaybeUninit<T> and T have the same layout.
let src: &[MaybeUninit<T>] = unsafe { core::mem::transmute(self.0) };
dst.copy_from_slice(src)
}
}
/// Fills an array by cloning from a source array. Source and destination must have identical length.
///
/// # Examples
///
/// ```
/// use varlen::prelude::*;
///
/// let src = ["hello".to_string(), "world".to_string()];
/// let arr: [String; 2] = new_array(CloneFrom(&src));
/// assert_eq!(src, arr);
/// ```
///
/// # Panics
///
/// Panics if the source and destination have different lengths.
///
/// ```should_panic
/// # use varlen::prelude::*;
/// #
/// # let src = ["hello".to_string(), "world".to_string()];
/// let arr: [String; 1] = new_array(CloneFrom(&src)); // Panics
/// ```
pub struct CloneFrom<'a, T>(pub &'a [T]);
unsafe impl<'a, T: Clone> ArrayInitializer<T> for CloneFrom<'a, T> {
fn initialize(self, dst: &mut [MaybeUninit<T>]) {
assert_eq!(self.0.len(), dst.len());
for (src, dst) in self.0.iter().zip(dst.iter_mut()) {
dst.write(src.clone());
}
}
}
/// Fills an array by moving from a source array. Source and destination must have identical length.
///
/// # Examples
///
/// ```
/// use varlen::prelude::*;
///
/// let src = ["hello".to_string(), "world".to_string()];
/// let arr: [String; 2] = new_array(MoveFrom(src));
/// assert_eq!(arr, ["hello".to_string(), "world".to_string()]);
/// ```
///
/// # Panics
///
/// Panics if the source and destination have different lengths.
///
/// ```should_panic
/// # use varlen::prelude::*;
/// #
/// # let src = ["hello".to_string(), "world".to_string()];
/// let arr: [String; 1] = new_array(MoveFrom(src)); // Panics
/// ```
pub struct MoveFrom<T, const N: usize>(pub [T; N]);
unsafe impl<T, const N: usize> ArrayInitializer<T> for MoveFrom<T, N> {
fn initialize(self, dst: &mut [MaybeUninit<T>]) {
assert_eq!(dst.len(), N);
for (src, dst) in self.0.into_iter().zip(dst.iter_mut()) {
dst.write(src);
}
}
}
/// Same as `transmute`, but supports types whose sizes are not known until monomorphization.
unsafe fn transmute_workaround<T, U>(e: T) -> U {
assert!(core::alloc::Layout::new::<T>() == core::alloc::Layout::new::<U>());
let u = core::mem::transmute_copy(&e);
core::mem::forget(e);
u
}