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//! Dynamic array initialisation is very dangerous currently. //! The safest way is to initialize one with a default value //! //! ``` //! let mut array = [0; 32]; //! for i in 0..32 { //! array[i] = i; //! } //! ``` //! //! This is not possible in general though. For any type `[T; N]`, //! T either needs to be [`Copy`], or there needs to be a `const t: T`. //! This is definitely not always the case. //! //! The second problem is efficiency. In the example above, we are //! filling an array with zeros, just to replace them. While the //! compiler can sometimes optimise this away, it's nice to have the guarantee. //! //! So, what's the alternative? How about [`MaybeUninit`]! Although, it's not that simple. //! Take the following example, which uses completely safe Rust! Can you spot the error? //! //! ```should_panic //! # #![feature(maybe_uninit_uninit_array)] //! # #![feature(maybe_uninit_extra)] //! # use std::mem::MaybeUninit; //! let mut uninit: [MaybeUninit<String>; 8] = MaybeUninit::uninit_array(); //! uninit[0].write("foo".to_string()); //! uninit[1].write("bar".to_string()); //! uninit[2].write("baz".to_string()); //! panic!("oops"); //! ``` //! //! Did you spot it? Right there is a memory leak. The key here is that //! [`MaybeUninit`] **does not** implement [`Drop`]. This makes sense //! since the value could be uninitialized, and calling [`Drop`] on an //! uninitialized value is undefined behaviour. The result of this is that //! the 3 [`String`] values we did initialize never got dropped! //! Now, this is safe according to Rust. Leaking memory is not undefined //! behaviour. But it's still not something we should promote. //! //! What other options do we have? The only solution is to provide a new //! `struct` that wraps the array, and properly implements [`Drop`]. That //! way, if `drop` is called, we can make sure any initialized values get //! dropped properly. This is exactly what [`ArrayBuilder`] provides. //! //! ```should_panic //! use array_builder::ArrayBuilder; //! let mut uninit: ArrayBuilder<String, 8> = ArrayBuilder::new(); //! uninit.push("foo".to_string()); //! uninit.push("bar".to_string()); //! uninit.push("baz".to_string()); //! panic!("oops"); // ArrayBuilder drops the 3 values above for you //! ``` //! //! ``` //! use array_builder::ArrayBuilder; //! let mut uninit: ArrayBuilder<String, 3> = ArrayBuilder::new(); //! uninit.push("foo".to_string()); //! uninit.push("bar".to_string()); //! uninit.push("baz".to_string()); //! let array: [String; 3] = uninit.build().unwrap(); //! ``` //! //! You can also take a peek at what the current set of initialised values are //! //! ``` //! use array_builder::ArrayBuilder; //! let mut uninit: ArrayBuilder<usize, 4> = ArrayBuilder::new(); //! uninit.push(1); //! uninit.push(2); //! uninit.push(3); //! //! // we can't build just yet //! let mut uninit = uninit.build().unwrap_err(); //! let slice: &[usize] = &uninit; //! assert_eq!(&[1, 2, 3], slice); //! //! uninit.push(4); //! assert_eq!([1, 2, 3, 4], uninit.build().unwrap()); //! ``` use core::{ cmp, fmt, mem::{self, ManuallyDrop, MaybeUninit}, ops::{Deref, DerefMut}, ptr, slice, }; /// ArrayBuilder makes it easy to dynamically build arrays safely /// and efficiently. /// /// ``` /// use array_builder::ArrayBuilder; /// /// struct ArrayIterator<I: Iterator, const N: usize> { /// builder: ArrayBuilder<I::Item, N>, /// iter: I, /// } /// /// impl<I: Iterator, const N: usize> Iterator for ArrayIterator<I, N> { /// type Item = [I::Item; N]; /// /// fn next(&mut self) -> Option<Self::Item> { /// for _ in self.builder.len()..N { /// self.builder.push(self.iter.next()?); /// } /// self.builder.take().build().ok() /// } /// } /// /// impl<I: Iterator, const N: usize> ArrayIterator<I, N> { /// pub fn new(i: impl IntoIterator<IntoIter=I>) -> Self { /// Self { /// builder: ArrayBuilder::new(), /// iter: i.into_iter(), /// } /// } /// /// pub fn remaining(&self) -> &[I::Item] { /// &self.builder /// } /// } /// /// let mut i = ArrayIterator::new(0..10); /// assert_eq!(Some([0, 1, 2, 3]), i.next()); /// assert_eq!(Some([4, 5, 6, 7]), i.next()); /// assert_eq!(None, i.next()); /// assert_eq!(&[8, 9], i.remaining()); /// ``` pub struct ArrayBuilder<T, const N: usize> { buf: [MaybeUninit<T>; N], len: usize, } impl<T: Clone, const N: usize> Clone for ArrayBuilder<T, N> { fn clone(&self) -> Self { let mut new = Self::new(); new.len = self.len(); new.deref_mut().clone_from_slice(self.deref()); new } } impl<T: fmt::Debug, const N: usize> fmt::Debug for ArrayBuilder<T, N> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { f.debug_struct("ArrayBuilder") .field("capacity", &N) .field("length", &self.len) .field("values", &self.deref()) .finish() } } impl<T, U, const N: usize> cmp::PartialEq<ArrayBuilder<U, N>> for ArrayBuilder<T, N> where T: cmp::PartialEq<U>, { fn eq(&self, other: &ArrayBuilder<U, N>) -> bool { self.deref() == other.deref() } } impl<T: cmp::Eq, const N: usize> cmp::Eq for ArrayBuilder<T, N> {} impl<T: cmp::PartialOrd, const N: usize> cmp::PartialOrd for ArrayBuilder<T, N> { fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> { self.deref().partial_cmp(other.deref()) } } impl<T: cmp::Ord, const N: usize> cmp::Ord for ArrayBuilder<T, N> { fn cmp(&self, other: &Self) -> cmp::Ordering { self.deref().cmp(other.deref()) } } impl<T, const N: usize> Drop for ArrayBuilder<T, N> { fn drop(&mut self) { self.clear() } } impl<T, const N: usize> Deref for ArrayBuilder<T, N> { type Target = [T]; fn deref(&self) -> &[T] { unsafe { slice::from_raw_parts(self.as_ptr(), self.len) } } } impl<T, const N: usize> DerefMut for ArrayBuilder<T, N> { fn deref_mut(&mut self) -> &mut [T] { unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) } } } impl<T, const N: usize> From<[T; N]> for ArrayBuilder<T, N> { fn from(array: [T; N]) -> Self { Self { buf: unsafe { ptr::read(array.as_ptr() as *const [MaybeUninit<T>; N]) }, len: N, } } } impl<T, const N: usize> ArrayBuilder<T, N> { const UNINIT: MaybeUninit<T> = MaybeUninit::uninit(); /// Create a new ArrayBuilder, backed by an uninitialised [T; N] pub fn new() -> Self { Self { buf: [Self::UNINIT; N], len: 0, } } /// Get the number of initialized values in the ArrayBuilder pub fn len(&self) -> usize { self.len } /// Get whether the ArrayBuilder is full pub fn is_full(&self) -> bool { self.len == N } /// Get whether the ArrayBuilder is empty pub fn is_empty(&self) -> bool { self.len == 0 } /// Empties the ArrayBuilder pub fn clear(&mut self) { let s: &mut [T] = self; unsafe { ptr::drop_in_place(s); } self.len = 0; } fn as_ptr(&self) -> *const T { self.buf.as_ptr() as _ } fn as_mut_ptr(&mut self) -> *mut T { self.buf.as_mut_ptr() as _ } /// Pushes the value onto the ArrayBuilder /// /// Panics: /// If the ArrayBuilder is full pub fn push(&mut self, t: T) { assert!(self.len < N); unsafe { self.push_unchecked(t); } } /// Pushes the value onto the ArrayBuilder if there is space /// Otherwise, returns Err(t) pub fn try_push(&mut self, t: T) -> Result<(), T> { if self.len < N { unsafe { self.push_unchecked(t); } Ok(()) } else { Err(t) } } /// Pushes the value onto the ArrayBuilder /// /// Safety: /// The ArrayBuilder must not be full pub unsafe fn push_unchecked(&mut self, t: T) { ptr::write(self.as_mut_ptr().add(self.len), t); self.len += 1; } /// Pops the last value on the ArrayBuilder /// Returns None if the ArrayBuilder is empty /// /// ``` /// use array_builder::ArrayBuilder; /// let mut builder: ArrayBuilder<usize, 4> = [1, 2, 3, 4].into(); /// let t = builder.pop().unwrap(); /// builder.push(t * t); /// assert_eq!(Ok([1, 2, 3, 16]), builder.build()); /// ``` pub fn pop(&mut self) -> Option<T> { if self.len > 0 { unsafe { Some(self.pop_unchecked()) } } else { None } } /// Pops the last value on the ArrayBuilder /// /// Safety: /// The ArrayBuilder must not be empty pub unsafe fn pop_unchecked(&mut self) -> T { self.len -= 1; ptr::read(self.as_ptr().add(self.len)) } /// Converts the ArrayBuilder into a [T; N]. /// If the ArrayBuilder is not full, returns Err(self) pub fn build(self) -> Result<[T; N], Self> { if self.len == N { unsafe { Ok(self.build_unchecked()) } } else { Err(self) } } /// Converts the ArrayBuilder into a [T; N]. /// /// Safety: /// The ArrayBuilder must be full pub unsafe fn build_unchecked(self) -> [T; N] { let self_ = ManuallyDrop::new(self); ptr::read(self_.as_ptr() as *const [T; N]) } /// Takes the value out of the ArrayBuilder /// Leaving an empty ArrayBuilder in it's place. /// /// ``` /// use array_builder::ArrayBuilder; /// let mut builder1: ArrayBuilder<usize, 4> = [1, 2, 3, 4].into(); /// assert!(builder1.is_full()); /// let builder2 = builder1.take(); /// assert!(builder1.is_empty()); /// assert!(builder2.is_full()); /// ``` pub fn take(&mut self) -> Self { mem::replace(self, Self::new()) } }