hybrid-array: implement concat and split methods

Convenient methods for concatenation and splitting. The size of the
result is calculated in the compile time.

This implementation was inspired by GenericArray implementation of
Concat and Split traits.

Author: Ruslan Piasetskyi

hybrid-array/src/lib.rs

@@ -32,10 +32,12 @@ use core::{
     cmp::Ordering,
     fmt::{self, Debug},
     hash::{Hash, Hasher},
-    ops::{Deref, DerefMut, Index, IndexMut, Range},
+    mem::{ManuallyDrop, MaybeUninit},
+    ops::{Add, Deref, DerefMut, Index, IndexMut, Range, Sub},
+    ptr,
     slice::{Iter, IterMut},
 };
-use typenum::Unsigned;
+use typenum::{Diff, Sum, Unsigned};
 
 /// Hybrid typenum-based and const generic array type.
 ///
@@ -44,6 +46,10 @@ use typenum::Unsigned;
 #[repr(transparent)]
 pub struct Array<T, U: ArraySize>(pub U::ArrayType<T>);
 
+type SplitResult<T, U, N> = (Array<T, N>, Array<T, Diff<U, N>>);
+type SplitRefResult<'a, T, U, N> = (&'a Array<T, N>, &'a Array<T, Diff<U, N>>);
+type SplitRefMutResult<'a, T, U, N> = (&'a mut Array<T, N>, &'a mut Array<T, Diff<U, N>>);
+
 impl<T, U> Array<T, U>
 where
     U: ArraySize,
@@ -126,6 +132,74 @@ where
     {
         Self::ref_from_slice(slice).clone()
     }
+
+    /// Concatenates `self` with `other`.
+    #[inline]
+    pub fn concat<N>(self, other: Array<T, N>) -> Array<T, Sum<U, N>>
+    where
+        N: ArraySize,
+        U: Add<N>,
+        Sum<U, N>: ArraySize,
+    {
+        let mut result = MaybeUninit::uninit();
+        let result_ptr = result.as_mut_ptr() as *mut Self;
+
+        unsafe {
+            ptr::write(result_ptr, self);
+            ptr::write(result_ptr.add(1) as *mut _, other);
+            result.assume_init()
+        }
+    }
+
+    /// Splits `self` at index `N` in two arrays.
+    ///
+    /// New arrays hold the original memory from `self`.
+    #[inline]
+    pub fn split<N>(self) -> SplitResult<T, U, N>
+    where
+        U: Sub<N>,
+        N: ArraySize,
+        Diff<U, N>: ArraySize,
+    {
+        unsafe {
+            let array = ManuallyDrop::new(self);
+            let head = ptr::read(array.as_ptr() as *const _);
+            let tail = ptr::read(array.as_ptr().add(N::USIZE) as *const _);
+            (head, tail)
+        }
+    }
+
+    /// Splits `&self` at index `N` in two array references.
+    #[inline]
+    pub fn split_ref<N>(&self) -> SplitRefResult<'_, T, U, N>
+    where
+        U: Sub<N>,
+        N: ArraySize,
+        Diff<U, N>: ArraySize,
+    {
+        unsafe {
+            let array_ptr = self.as_ptr();
+            let head = &*(array_ptr as *const _);
+            let tail = &*(array_ptr.add(N::USIZE) as *const _);
+            (head, tail)
+        }
+    }
+
+    /// Splits `&mut self` at index `N` in two mutable array references.
+    #[inline]
+    pub fn split_ref_mut<N>(&mut self) -> SplitRefMutResult<'_, T, U, N>
+    where
+        U: Sub<N>,
+        N: ArraySize,
+        Diff<U, N>: ArraySize,
+    {
+        unsafe {
+            let array_ptr = self.as_mut_ptr();
+            let head = &mut *(array_ptr as *mut _);
+            let tail = &mut *(array_ptr.add(N::USIZE) as *mut _);
+            (head, tail)
+        }
+    }
 }
 
 impl<T, U, const N: usize> AsRef<[T; N]> for Array<T, U>
@@ -698,7 +772,7 @@ impl_array_size! {
 mod tests {
     use super::ByteArray;
     use crate::Array;
-    use typenum::{U0, U3, U6, U7};
+    use typenum::{U0, U2, U3, U4, U6, U7};
 
     const EXAMPLE_SLICE: &[u8] = &[1, 2, 3, 4, 5, 6];
 
@@ -729,4 +803,43 @@ mod tests {
 
         assert!(<&ByteArray::<U7>>::try_from(EXAMPLE_SLICE).is_err());
     }
+
+    #[test]
+    fn concat() {
+        let prefix = ByteArray::<U2>::clone_from_slice(&EXAMPLE_SLICE[..2]);
+        let suffix = ByteArray::<U4>::clone_from_slice(&EXAMPLE_SLICE[2..]);
+
+        let array = prefix.concat(suffix);
+        assert_eq!(array.as_slice(), EXAMPLE_SLICE);
+    }
+
+    #[test]
+    fn split() {
+        let array = ByteArray::<U6>::clone_from_slice(EXAMPLE_SLICE);
+
+        let (prefix, suffix) = array.split::<U2>();
+
+        assert_eq!(prefix.as_slice(), &EXAMPLE_SLICE[..2]);
+        assert_eq!(suffix.as_slice(), &EXAMPLE_SLICE[2..]);
+    }
+
+    #[test]
+    fn split_ref() {
+        let array = ByteArray::<U6>::clone_from_slice(EXAMPLE_SLICE);
+
+        let (prefix, suffix) = array.split_ref::<U3>();
+
+        assert_eq!(prefix.as_slice(), &EXAMPLE_SLICE[..3]);
+        assert_eq!(suffix.as_slice(), &EXAMPLE_SLICE[3..]);
+    }
+
+    #[test]
+    fn split_ref_mut() {
+        let array = &mut ByteArray::<U6>::clone_from_slice(EXAMPLE_SLICE);
+
+        let (prefix, suffix) = array.split_ref_mut::<U4>();
+
+        assert_eq!(prefix.as_slice(), &EXAMPLE_SLICE[..4]);
+        assert_eq!(suffix.as_slice(), &EXAMPLE_SLICE[4..]);
+    }
 }