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    //! Simple stack-allocated vector.

    #![cfg(not(feature = "alloc"))]
    #![doc(hidden)]

    use crate::bigint;
    use core::{cmp, mem, ops, ptr, slice};

    /// Simple stack vector implementation.
    #[derive(Clone)]
    pub struct StackVec {
        /// The raw buffer for the elements.
        data: [mem::MaybeUninit<bigint::Limb>; bigint::BIGINT_LIMBS],
        /// The number of elements in the array (we never need more than u16::MAX).
        length: u16,
    }

    #[allow(clippy::new_without_default)]
    impl StackVec {
        /// Construct an empty vector.
        #[inline]
        pub const fn new() -> Self {
            Self {
                length: 0,
                data: [mem::MaybeUninit::uninit(); bigint::BIGINT_LIMBS],
            }
        }

        /// Construct a vector from an existing slice.
        #[inline]
        pub fn try_from(x: &[bigint::Limb]) -> Option<Self> {
            let mut vec = Self::new();
            vec.try_extend(x)?;
            Some(vec)
        }

        /// Sets the length of a vector.
        ///
        /// This will explicitly set the size of the vector, without actually
        /// modifying its buffers, so it is up to the caller to ensure that the
        /// vector is actually the specified size.
        ///
        /// # Safety
        ///
        /// Safe as long as `len` is less than `BIGINT_LIMBS`.
        #[inline]
        pub unsafe fn set_len(&mut self, len: usize) {
            // Constant is `u16::MAX` for older Rustc versions.
            debug_assert!(len <= 0xffff);
            debug_assert!(len <= bigint::BIGINT_LIMBS);
            self.length = len as u16;
        }

        /// The number of elements stored in the vector.
        #[inline]
        pub const fn len(&self) -> usize {
            self.length as usize
        }

        /// If the vector is empty.
        #[inline]
        pub const fn is_empty(&self) -> bool {
            self.len() == 0
        }

        /// The number of items the vector can hold.
        #[inline]
        pub const fn capacity(&self) -> usize {
            bigint::BIGINT_LIMBS as usize
        }

        /// Append an item to the vector, without bounds checking.
        ///
        /// # Safety
        ///
        /// Safe if `self.len() < self.capacity()`.
        #[inline]
        pub unsafe fn push_unchecked(&mut self, value: bigint::Limb) {
            debug_assert!(self.len() < self.capacity());
            // SAFETY: safe, capacity is less than the current size.
            unsafe {
                ptr::write(self.as_mut_ptr().add(self.len()), value);
                self.length += 1;
            }
        }

        /// Append an item to the vector.
        #[inline]
        pub fn try_push(&mut self, value: bigint::Limb) -> Option<()> {
            if self.len() < self.capacity() {
                // SAFETY: safe, capacity is less than the current size.
                unsafe { self.push_unchecked(value) };
                Some(())
            } else {
                None
            }
        }

        /// Remove an item from the end of a vector, without bounds checking.
        ///
        /// # Safety
        ///
        /// Safe if `self.len() > 0`.
        #[inline]
        pub unsafe fn pop_unchecked(&mut self) -> bigint::Limb {
            debug_assert!(!self.is_empty());
            // SAFETY: safe if `self.length > 0`.
            // We have a trivial drop and copy, so this is safe.
            self.length -= 1;
            unsafe { ptr::read(self.as_mut_ptr().add(self.len())) }
        }

        /// Remove an item from the end of the vector and return it, or None if empty.
        #[inline]
        pub fn pop(&mut self) -> Option<bigint::Limb> {
            if self.is_empty() {
                None
            } else {
                // SAFETY: safe, since `self.len() > 0`.
                unsafe { Some(self.pop_unchecked()) }
            }
        }

        /// Add items from a slice to the vector, without bounds checking.
        ///
        /// # Safety
        ///
        /// Safe if `self.len() + slc.len() <= self.capacity()`.
        #[inline]
        pub unsafe fn extend_unchecked(&mut self, slc: &[bigint::Limb]) {
            let index = self.len();
            let new_len = index + slc.len();
            debug_assert!(self.len() + slc.len() <= self.capacity());
            let src = slc.as_ptr();
            // SAFETY: safe if `self.len() + slc.len() <= self.capacity()`.
            unsafe {
                let dst = self.as_mut_ptr().add(index);
                ptr::copy_nonoverlapping(src, dst, slc.len());
                self.set_len(new_len);
            }
        }

        /// Copy elements from a slice and append them to the vector.
        #[inline]
        pub fn try_extend(&mut self, slc: &[bigint::Limb]) -> Option<()> {
            if self.len() + slc.len() <= self.capacity() {
                // SAFETY: safe, since `self.len() + slc.len() <= self.capacity()`.
                unsafe { self.extend_unchecked(slc) };
                Some(())
            } else {
                None
            }
        }

        /// Truncate vector to new length, dropping any items after `len`.
        ///
        /// # Safety
        ///
        /// Safe as long as `len <= self.capacity()`.
        unsafe fn truncate_unchecked(&mut self, len: usize) {
            debug_assert!(len <= self.capacity());
            self.length = len as u16;
        }

        /// Resize the buffer, without bounds checking.
        ///
        /// # Safety
        ///
        /// Safe as long as `len <= self.capacity()`.
        #[inline]
        pub unsafe fn resize_unchecked(&mut self, len: usize, value: bigint::Limb) {
            debug_assert!(len <= self.capacity());
            let old_len = self.len();
            if len > old_len {
                // We have a trivial drop, so there's no worry here.
                // Just, don't set the length until all values have been written,
                // so we don't accidentally read uninitialized memory.

                // SAFETY: safe if `len < self.capacity()`.
                let count = len - old_len;
                for index in 0..count {
                    unsafe {
                        let dst = self.as_mut_ptr().add(old_len + index);
                        ptr::write(dst, value);
                    }
                }
                self.length = len as u16;
            } else {
                // SAFETY: safe since `len < self.len()`.
                unsafe { self.truncate_unchecked(len) };
            }
        }

        /// Try to resize the buffer.
        ///
        /// If the new length is smaller than the current length, truncate
        /// the input. If it's larger, then append elements to the buffer.
        #[inline]
        pub fn try_resize(&mut self, len: usize, value: bigint::Limb) -> Option<()> {
            if len > self.capacity() {
                None
            } else {
                // SAFETY: safe, since `len <= self.capacity()`.
                unsafe { self.resize_unchecked(len, value) };
                Some(())
            }
        }

        // HI

        /// Get the high 64 bits from the vector.
        #[inline(always)]
        pub fn hi64(&self) -> (u64, bool) {
            bigint::hi64(self)
        }

        // FROM

        /// Create StackVec from u64 value.
        #[inline(always)]
        pub fn from_u64(x: u64) -> Self {
            bigint::from_u64(x)
        }

        // MATH

        /// Normalize the integer, so any leading zero values are removed.
        #[inline]
        pub fn normalize(&mut self) {
            bigint::normalize(self)
        }

        /// Get if the big integer is normalized.
        #[inline]
        pub fn is_normalized(&self) -> bool {
            bigint::is_normalized(self)
        }

        /// AddAssign small integer.
        #[inline]
        pub fn add_small(&mut self, y: bigint::Limb) -> Option<()> {
            bigint::small_add(self, y)
        }

        /// MulAssign small integer.
        #[inline]
        pub fn mul_small(&mut self, y: bigint::Limb) -> Option<()> {
            bigint::small_mul(self, y)
        }
    }

    impl PartialEq for StackVec {
        #[inline]
        #[allow(clippy::op_ref)]
        fn eq(&self, other: &Self) -> bool {
            use core::ops::Deref;
            self.len() == other.len() && self.deref() == other.deref()
        }
    }

    impl Eq for StackVec {
    }

    impl cmp::PartialOrd for StackVec {
        #[inline]
        fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
            Some(bigint::compare(self, other))
        }
    }

    impl cmp::Ord for StackVec {
        #[inline]
        fn cmp(&self, other: &Self) -> cmp::Ordering {
            bigint::compare(self, other)
        }
    }

    impl ops::Deref for StackVec {
        type Target = [bigint::Limb];
        #[inline]
        fn deref(&self) -> &[bigint::Limb] {
            // SAFETY: safe since `self.data[..self.len()]` must be initialized
            // and `self.len() <= self.capacity()`.
            unsafe {
                let ptr = self.data.as_ptr() as *const bigint::Limb;
                slice::from_raw_parts(ptr, self.len())
            }
        }
    }

    impl ops::DerefMut for StackVec {
        #[inline]
        fn deref_mut(&mut self) -> &mut [bigint::Limb] {
            // SAFETY: safe since `self.data[..self.len()]` must be initialized
            // and `self.len() <= self.capacity()`.
            unsafe {
                let ptr = self.data.as_mut_ptr() as *mut bigint::Limb;
                slice::from_raw_parts_mut(ptr, self.len())
            }
        }
    }

    impl ops::MulAssign<&[bigint::Limb]> for StackVec {
        #[inline]
        fn mul_assign(&mut self, rhs: &[bigint::Limb]) {
            bigint::large_mul(self, rhs).unwrap();
        }
    }