// Copyright 2017 Matt Brubeck. See the COPYRIGHT file at the top-level
// directory of this distribution and at http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! [`SmallBitVec`] is a bit vector, a vector of single-bit values stored compactly in memory.
//!
//! SmallBitVec grows dynamically, like the standard `Vec<T>` type.  It can hold up to about one
//! word of bits inline (without a separate heap allocation).  If the number of bits exceeds this
//! inline capacity, it will allocate a buffer on the heap.
//!
//! [`SmallBitVec`]: struct.SmallBitVec.html
//!
//! # Example
//!
//! ```
//! use smallbitvec::SmallBitVec;
//!
//! let mut v = SmallBitVec::new();
//! v.push(true);
//! v.push(false);
//!
//! assert_eq!(v[0], true);
//! assert_eq!(v[1], false);
//! ```

#![no_std]

extern crate alloc;

use alloc::{vec, vec::Vec, boxed::Box};

use core::cmp::max;
use core::fmt;
use core::hash;
use core::iter::{DoubleEndedIterator, ExactSizeIterator, FromIterator};
use core::mem::{forget, replace, size_of};
use core::ops::{Index, Range};
use core::slice;

/// Creates a [`SmallBitVec`] containing the arguments.
///
/// `sbvec!` allows `SmallBitVec`s to be defined with the same syntax as array expressions.
/// There are two forms of this macro:
///
/// - Create a [`SmallBitVec`] containing a given list of elements:
///
/// ```
/// # #[macro_use] extern crate smallbitvec;
/// # use smallbitvec::SmallBitVec;
/// # fn main() {
/// let v = sbvec![true, false, true];
/// assert_eq!(v[0], true);
/// assert_eq!(v[1], false);
/// assert_eq!(v[2], true);
/// # }
/// ```
///
/// - Create a [`SmallBitVec`] from a given element and size:
///
/// ```
/// # #[macro_use] extern crate smallbitvec;
/// # use smallbitvec::SmallBitVec;
/// # fn main() {
/// let v = sbvec![true; 3];
/// assert!(v.into_iter().eq(vec![true, true, true].into_iter()));
/// # }
/// ```

#[macro_export]
macro_rules! sbvec {
    ($elem:expr; $n:expr) => (
        $crate::SmallBitVec::from_elem($n, $elem)
    );
    ($($x:expr),*) => (
        [$($x),*].iter().cloned().collect::<$crate::SmallBitVec>()
    );
    ($($x:expr,)*) => (
        sbvec![$($x),*]
    );
}


// FIXME: replace this with `debug_assert!` when it’s usable in `const`:
// * https://github.com/rust-lang/rust/issues/49146
// * https://github.com/rust-lang/rust/issues/51999
macro_rules! const_debug_assert_le {
    ($left: ident <= $right: expr) =>  {
        #[cfg(debug_assertions)]
        // Causes an `index out of bounds` panic if `$left` is too large
        [(); $right + 1][$left];
    }
}

#[cfg(test)]
mod tests;

/// A resizable bit vector, optimized for size and inline storage.
///
/// `SmallBitVec` is exactly one word wide. Depending on the required capacity, this word
/// either stores the bits inline, or it stores a pointer to a separate buffer on the heap.
pub struct SmallBitVec {
    data: usize,
}

/// Total number of bits per word.
#[inline(always)]
const fn inline_bits() -> usize {
    size_of::<usize>() * 8
}

/// For an inline vector, all bits except two can be used as storage capacity:
///
/// - The rightmost bit is set to zero to signal an inline vector.
/// - The position of the rightmost nonzero bit encodes the length.
#[inline(always)]
const fn inline_capacity() -> usize {
    inline_bits() - 2
}

/// Left shift amount to access the nth bit
#[inline(always)]
const fn inline_shift(n: usize) -> usize {
    const_debug_assert_le!(n <= inline_capacity());
    // The storage starts at the leftmost bit.
    inline_bits() - 1 - n
}

/// An inline vector with the nth bit set.
#[inline(always)]
const fn inline_index(n: usize) -> usize {
    1 << inline_shift(n)
}

/// An inline vector with the leftmost `n` bits set.
#[inline(always)]
fn inline_ones(n: usize) -> usize {
    if n == 0 {
        0
    } else {
        !0 << (inline_bits() - n)
    }
}

/// If the rightmost bit of `data` is set, then the remaining bits of `data`
/// are a pointer to a heap allocation.
const HEAP_FLAG: usize = 1;

/// The allocation will contain a `Header` followed by a [Storage] buffer.
type Storage = usize;

/// The number of bits in one `Storage`.
#[inline(always)]
fn bits_per_storage() -> usize {
    size_of::<Storage>() * 8
}

/// Data stored at the start of the heap allocation.
///
/// `Header` must have the same alignment as `Storage`.
struct Header {
    /// The number of bits in this bit vector.
    len: Storage,

    /// The number of elements in the [usize] buffer that follows this header.
    buffer_len: Storage,
}

impl Header {
    /// Create a heap allocation with enough space for a header,
    /// plus a buffer of at least `cap` bits, each initialized to `val`.
    fn new(cap: usize, len: usize, val: bool) -> *mut Header {
        let alloc_len = header_len() + buffer_len(cap);
        let init = if val { !0 } else { 0 };

        let v: Vec<Storage> = vec![init; alloc_len];

        let buffer_len = v.capacity() - header_len();
        let header_ptr = v.as_ptr() as *mut Header;

        forget(v);

        unsafe {
            (*header_ptr).len = len;
            (*header_ptr).buffer_len = buffer_len;
        }
        header_ptr
    }
}

/// The number of `Storage` elements to allocate to hold a header.
#[inline(always)]
fn header_len() -> usize {
    size_of::<Header>() / size_of::<Storage>()
}

/// The minimum number of `Storage` elements to hold at least `cap` bits.
#[inline(always)]
fn buffer_len(cap: usize) -> usize {
    (cap + bits_per_storage() - 1) / bits_per_storage()
}

/// A typed representation of a `SmallBitVec`'s internal storage.
///
/// The layout of the data inside both enum variants is a private implementation detail.
pub enum InternalStorage {
    /// The internal representation of a `SmallBitVec` that has not spilled to a
    /// heap allocation.
    Inline(usize),

    /// The contents of the heap allocation of a spilled `SmallBitVec`.
    Spilled(Box<[usize]>),
}

impl SmallBitVec {
    /// Create an empty vector.
    #[inline]
    pub const fn new() -> SmallBitVec {
        SmallBitVec {
            data: inline_index(0),
        }
    }

    /// Create a vector containing `len` bits, each set to `val`.
    #[inline]
    pub fn from_elem(len: usize, val: bool) -> SmallBitVec {
        if len <= inline_capacity() {
            return SmallBitVec {
                data: if val {
                    inline_ones(len + 1)
                } else {
                    inline_index(len)
                },
            };
        }
        let header_ptr = Header::new(len, len, val);
        SmallBitVec {
            data: (header_ptr as usize) | HEAP_FLAG,
        }
    }

    /// Create an empty vector with enough storage pre-allocated to store at least `cap` bits
    /// without resizing.
    #[inline]
    pub fn with_capacity(cap: usize) -> SmallBitVec {
        // Use inline storage if possible.
        if cap <= inline_capacity() {
            return SmallBitVec::new();
        }

        // Otherwise, allocate on the heap.
        let header_ptr = Header::new(cap, 0, false);
        SmallBitVec {
            data: (header_ptr as usize) | HEAP_FLAG,
        }
    }

    /// The number of bits stored in this bit vector.
    #[inline]
    pub fn len(&self) -> usize {
        if self.is_inline() {
            // The rightmost nonzero bit is a sentinel.  All bits to the left of
            // the sentinel bit are the elements of the bit vector.
            inline_bits() - self.data.trailing_zeros() as usize - 1
        } else {
            self.header().len
        }
    }

    /// Returns `true` if this vector contains no bits.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    /// The number of bits that can be stored in this bit vector without re-allocating.
    #[inline]
    pub fn capacity(&self) -> usize {
        if self.is_inline() {
            inline_capacity()
        } else {
            self.header().buffer_len * bits_per_storage()
        }
    }

    /// Get the nth bit in this bit vector.
    #[inline]
    pub fn get(&self, n: usize) -> Option<bool> {
        if n < self.len() {
            Some(unsafe { self.get_unchecked(n) })
        } else {
            None
        }
    }

    /// Get the last bit in this bit vector.
    #[inline]
    pub fn last(&self) -> Option<bool> {
        self.len().checked_sub(1).map(|n| unsafe { self.get_unchecked(n) })
    }

    /// Get the nth bit in this bit vector, without bounds checks.
    #[inline]
    pub unsafe fn get_unchecked(&self, n: usize) -> bool {
        if self.is_inline() {
            self.data & inline_index(n) != 0
        } else {
            let buffer = self.buffer();
            let i = n / bits_per_storage();
            let offset = n % bits_per_storage();
            *buffer.get_unchecked(i) & (1 << offset) != 0
        }
    }

    /// Set the nth bit in this bit vector to `val`.  Panics if the index is out of bounds.
    #[inline]
    pub fn set(&mut self, n: usize, val: bool) {
        assert!(n < self.len(), "Index {} out of bounds", n);
        unsafe {
            self.set_unchecked(n, val);
        }
    }

    /// Set the nth bit in this bit vector to `val`, without bounds checks.
    #[inline]
    pub unsafe fn set_unchecked(&mut self, n: usize, val: bool) {
        if self.is_inline() {
            if val {
                self.data |= inline_index(n);
            } else {
                self.data &= !inline_index(n);
            }
        } else {
            let buffer = self.buffer_mut();
            let i = n / bits_per_storage();
            let offset = n % bits_per_storage();
            if val {
                *buffer.get_unchecked_mut(i) |= 1 << offset;
            } else {
                *buffer.get_unchecked_mut(i) &= !(1 << offset);
            }
        }
    }

    /// Append a bit to the end of the vector.
    ///
    /// ```
    /// use smallbitvec::SmallBitVec;
    /// let mut v = SmallBitVec::new();
    /// v.push(true);
    ///
    /// assert_eq!(v.len(), 1);
    /// assert_eq!(v.get(0), Some(true));
    /// ```
    #[inline]
    pub fn push(&mut self, val: bool) {
        let idx = self.len();
        if idx == self.capacity() {
            self.reserve(1);
        }
        unsafe {
            self.set_len(idx + 1);
            self.set_unchecked(idx, val);
        }
    }

    /// Remove the last bit from the vector and return it, if there is one.
    ///
    /// ```
    /// use smallbitvec::SmallBitVec;
    /// let mut v = SmallBitVec::new();
    /// v.push(false);
    ///
    /// assert_eq!(v.pop(), Some(false));
    /// assert_eq!(v.len(), 0);
    /// assert_eq!(v.pop(), None);
    /// ```
    #[inline]
    pub fn pop(&mut self) -> Option<bool> {
        self.len().checked_sub(1).map(|last| unsafe {
            let val = self.get_unchecked(last);
            self.set_len(last);
            val
        })
    }

    /// Remove and return the bit at index `idx`, shifting all later bits toward the front.
    ///
    /// Panics if the index is out of bounds.
    #[inline]
    pub fn remove(&mut self, idx: usize) -> bool {
        let len = self.len();
        let val = self[idx];

        if self.is_inline() {
            // Shift later bits, including the length bit, toward the front.
            let mask = !inline_ones(idx);
            let new_vals = (self.data & mask) << 1;
            self.data = (self.data & !mask) | (new_vals & mask);
        } else {
            let first = idx / bits_per_storage();
            let offset = idx % bits_per_storage();
            let count = buffer_len(len);
            {
                // Shift bits within the first storage block.
                let buf = self.buffer_mut();
                let mask = !0 << offset;
                let new_vals = (buf[first] & mask) >> 1;
                buf[first] = (buf[first] & !mask) | (new_vals & mask);
            }
            // Shift bits in subsequent storage blocks.
            for i in (first + 1)..count {
                // Move the first bit into the previous block.
                let bit_idx = i * bits_per_storage();
                unsafe {
                    let first_bit = self.get_unchecked(bit_idx);
                    self.set_unchecked(bit_idx - 1, first_bit);
                }
                // Shift the remaining bits.
                self.buffer_mut()[i] >>= 1;
            }
            // Decrement the length.
            unsafe {
                self.set_len(len - 1);
            }
        }
        val
    }

    /// Remove all elements from the vector, without deallocating its buffer.
    #[inline]
    pub fn clear(&mut self) {
        unsafe {
            self.set_len(0);
        }
    }

    /// Reserve capacity for at least `additional` more elements to be inserted.
    ///
    /// May reserve more space than requested, to avoid frequent reallocations.
    ///
    /// Panics if the new capacity overflows `usize`.
    ///
    /// Re-allocates only if `self.capacity() < self.len() + additional`.
    #[inline]
    pub fn reserve(&mut self, additional: usize) {
        let old_cap = self.capacity();
        let new_cap = self.len()
            .checked_add(additional)
            .expect("capacity overflow");
        if new_cap <= old_cap {
            return;
        }
        // Ensure the new capacity is at least double, to guarantee exponential growth.
        let double_cap = old_cap.saturating_mul(2);
        self.reallocate(max(new_cap, double_cap));
    }

    /// Set the length of the vector. The length must not exceed the capacity.
    ///
    /// If this makes the vector longer, then the values of its new elements
    /// are not specified.
    #[inline]
    unsafe fn set_len(&mut self, len: usize) {
        debug_assert!(len <= self.capacity());
        if self.is_inline() {
            let sentinel = inline_index(len);
            let mask = !(sentinel - 1);
            self.data |= sentinel;
            self.data &= mask;
        } else {
            self.header_mut().len = len;
        }
    }

    /// Returns an iterator that yields the bits of the vector in order, as `bool` values.
    #[inline]
    pub fn iter(&self) -> Iter {
        Iter {
            vec: self,
            range: 0..self.len(),
        }
    }

    /// Returns an immutable view of a range of bits from this vec.
    /// ```
    /// #[macro_use] extern crate smallbitvec;
    /// let v = sbvec![true, false, true];
    /// let r = v.range(1..3);
    /// assert_eq!(r[1], true);
    /// ```
    #[inline]
    pub fn range(&self, range: Range<usize>) -> VecRange {
        assert!(range.end <= self.len(), "range out of bounds");
        VecRange { vec: &self, range }
    }

    /// Returns true if all the bits in the vec are set to zero/false.
    ///
    /// On an empty vector, returns true.
    #[inline]
    pub fn all_false(&self) -> bool {
        let mut len = self.len();
        if len == 0 {
            return true;
        }

        if self.is_inline() {
            let mask = inline_ones(len);
            self.data & mask == 0
        } else {
            for &storage in self.buffer() {
                if len >= bits_per_storage() {
                    if storage != 0 {
                        return false;
                    }
                    len -= bits_per_storage();
                } else {
                    let mask = (1 << len) - 1;
                    if storage & mask != 0 {
                        return false;
                    }
                    break;
                }
            }
            true
        }
    }

    /// Returns true if all the bits in the vec are set to one/true.
    ///
    /// On an empty vector, returns true.
    #[inline]
    pub fn all_true(&self) -> bool {
        let mut len = self.len();
        if len == 0 {
            return true;
        }

        if self.is_inline() {
            let mask = inline_ones(len);
            self.data & mask == mask
        } else {
            for &storage in self.buffer() {
                if len >= bits_per_storage() {
                    if storage != !0 {
                        return false;
                    }
                    len -= bits_per_storage();
                } else {
                    let mask = (1 << len) - 1;
                    if storage & mask != mask {
                        return false;
                    }
                    break;
                }
            }
            true
        }
    }

    /// Shorten the vector, keeping the first `len` elements and dropping the rest.
    ///
    /// If `len` is greater than or equal to the vector's current length, this has no
    /// effect.
    ///
    /// This does not re-allocate.
    pub fn truncate(&mut self, len: usize) {
        unsafe {
            if len < self.len() {
                self.set_len(len);
            }
        }
    }

    /// Resizes the vector so that its length is equal to `len`.
    ///
    /// If `len` is less than the current length, the vector simply truncated.
    ///
    /// If `len` is greater than the current length, `value` is appended to the
    /// vector until its length equals `len`.
    pub fn resize(&mut self, len: usize, value: bool) {
        let old_len = self.len();

        if len > old_len {
            unsafe {
                self.reallocate(len);
                self.set_len(len);
                for i in old_len..len {
                    self.set(i, value);
                }
            }
        } else {
            self.truncate(len);
        }
    }

    /// Resize the vector to have capacity for at least `cap` bits.
    ///
    /// `cap` must be at least as large as the length of the vector.
    fn reallocate(&mut self, cap: usize) {
        let old_cap = self.capacity();
        if cap <= old_cap {
            return;
        }
        assert!(self.len() <= cap);

        if self.is_heap() {
            let old_buffer_len = self.header().buffer_len;
            let new_buffer_len = buffer_len(cap);

            let old_alloc_len = header_len() + old_buffer_len;
            let new_alloc_len = header_len() + new_buffer_len;

            let old_ptr = self.header_raw() as *mut Storage;
            let mut v = unsafe { Vec::from_raw_parts(old_ptr, old_alloc_len, old_alloc_len) };
            v.resize(new_alloc_len, 0);
            v.shrink_to_fit();
            self.data = v.as_ptr() as usize | HEAP_FLAG;
            forget(v);

            self.header_mut().buffer_len = new_buffer_len;
        } else {
            let old_self = replace(self, SmallBitVec::with_capacity(cap));
            unsafe {
                self.set_len(old_self.len());
                for i in 0..old_self.len() {
                    self.set_unchecked(i, old_self.get_unchecked(i));
                }
            }
        }
    }

    /// If the vector owns a heap allocation, returns a pointer to the start of the allocation.
    ///
    /// The layout of the data at this allocation is a private implementation detail.
    #[inline]
    pub fn heap_ptr(&self) -> Option<*const usize> {
        if self.is_heap() {
            Some((self.data & !HEAP_FLAG) as *const Storage)
        } else {
            None
        }
    }

    /// Converts this `SmallBitVec` into its internal representation.
    ///
    /// The layout of the data inside both enum variants is a private implementation detail.
    #[inline]
    pub fn into_storage(self) -> InternalStorage {
        if self.is_heap() {
            let alloc_len = header_len() + self.header().buffer_len;
            let ptr = self.header_raw() as *mut Storage;
            let slice = unsafe { Box::from_raw(slice::from_raw_parts_mut(ptr, alloc_len)) };
            forget(self);
            InternalStorage::Spilled(slice)
        } else {
            InternalStorage::Inline(self.data)
        }
    }

    /// Creates a `SmallBitVec` directly from the internal storage of another
    /// `SmallBitVec`.
    ///
    /// # Safety
    ///
    /// This is highly unsafe.  `storage` needs to have been previously generated
    /// via `SmallBitVec::into_storage` (at least, it's highly likely to be
    /// incorrect if it wasn't.)  Violating this may cause problems like corrupting the
    /// allocator's internal data structures.
    ///
    /// # Examples
    ///
    /// ```
    /// # use smallbitvec::{InternalStorage, SmallBitVec};
    ///
    /// fn main() {
    ///     let v = SmallBitVec::from_elem(200, false);
    ///
    ///     // Get the internal representation of the SmallBitVec.
    ///     // unless we transfer its ownership somewhere else.
    ///     let storage = v.into_storage();
    ///
    ///     /// Make a copy of the SmallBitVec's data.
    ///     let cloned_storage = match storage {
    ///         InternalStorage::Spilled(vs) => InternalStorage::Spilled(vs.clone()),
    ///         inline => inline,
    ///     };
    ///
    ///     /// Create a new SmallBitVec from the coped storage.
    ///     let v = unsafe { SmallBitVec::from_storage(cloned_storage) };
    /// }
    /// ```
    pub unsafe fn from_storage(storage: InternalStorage) -> SmallBitVec {
        match storage {
            InternalStorage::Inline(data) => SmallBitVec { data },
            InternalStorage::Spilled(vs) => {
                let ptr = Box::into_raw(vs);
                SmallBitVec {
                    data: (ptr as *mut usize as usize) | HEAP_FLAG,
                }
            }
        }
    }

    /// If the rightmost bit is set, then we treat it as inline storage.
    #[inline]
    fn is_inline(&self) -> bool {
        self.data & HEAP_FLAG == 0
    }

    /// Otherwise, `data` is a pointer to a heap allocation.
    #[inline]
    fn is_heap(&self) -> bool {
        !self.is_inline()
    }

    /// Get the header of a heap-allocated vector.
    #[inline]
    fn header_raw(&self) -> *mut Header {
        assert!(self.is_heap());
        (self.data & !HEAP_FLAG) as *mut Header
    }

    #[inline]
    fn header_mut(&mut self) -> &mut Header {
        unsafe { &mut *self.header_raw() }
    }

    #[inline]
    fn header(&self) -> &Header {
        unsafe { &*self.header_raw() }
    }

    /// Get the buffer of a heap-allocated vector.
    #[inline]
    fn buffer_raw(&self) -> *mut [Storage] {
        unsafe {
            let header_ptr = self.header_raw();
            let buffer_len = (*header_ptr).buffer_len;
            let buffer_ptr = (header_ptr as *mut Storage)
                .offset((size_of::<Header>() / size_of::<Storage>()) as isize);
            slice::from_raw_parts_mut(buffer_ptr, buffer_len)
        }
    }

    #[inline]
    fn buffer_mut(&mut self) -> &mut [Storage] {
        unsafe { &mut *self.buffer_raw() }
    }

    #[inline]
    fn buffer(&self) -> &[Storage] {
        unsafe { &*self.buffer_raw() }
    }
}

// Trait implementations:

impl fmt::Debug for SmallBitVec {
    #[inline]
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        fmt.debug_list()
            .entries(self.iter().map(|b| b as u8))
            .finish()
    }
}

impl Default for SmallBitVec {
    #[inline]
    fn default() -> Self {
        Self::new()
    }
}

impl PartialEq for SmallBitVec {
    fn eq(&self, other: &Self) -> bool {
        // Compare by inline representation
        if self.is_inline() && other.is_inline() {
            return self.data == other.data;
        }

        let len = self.len();
        if len != other.len() {
            return false;
        }

        // Compare by heap representation
        if self.is_heap() && other.is_heap() {
            let buf0 = self.buffer();
            let buf1 = other.buffer();

            let full_blocks = len / bits_per_storage();
            let remainder = len % bits_per_storage();

            if buf0[..full_blocks] != buf1[..full_blocks] {
                return false;
            }

            if remainder != 0 {
                let mask = (1 << remainder) - 1;
                if buf0[full_blocks] & mask != buf1[full_blocks] & mask {
                    return false;
                }
            }
            return true;
        }

        // Representations differ; fall back to bit-by-bit comparison
        Iterator::eq(self.iter(), other.iter())
    }
}

impl Eq for SmallBitVec {}

impl Drop for SmallBitVec {
    fn drop(&mut self) {
        if self.is_heap() {
            unsafe {
                let header_ptr = self.header_raw();
                let alloc_ptr = header_ptr as *mut Storage;
                let alloc_len = header_len() + (*header_ptr).buffer_len;
                Vec::from_raw_parts(alloc_ptr, alloc_len, alloc_len);
            }
        }
    }
}

impl Clone for SmallBitVec {
    fn clone(&self) -> Self {
        if self.is_inline() {
            return SmallBitVec { data: self.data };
        }

        let buffer_len = self.header().buffer_len;
        let alloc_len = header_len() + buffer_len;
        let ptr = self.header_raw() as *mut Storage;
        let raw_allocation = unsafe { slice::from_raw_parts(ptr, alloc_len) };

        let v = raw_allocation.to_vec();
        let header_ptr = v.as_ptr() as *mut Header;
        forget(v);
        SmallBitVec {
            data: (header_ptr as usize) | HEAP_FLAG,
        }
    }
}

impl Index<usize> for SmallBitVec {
    type Output = bool;

    #[inline(always)]
    fn index(&self, i: usize) -> &bool {
        assert!(i < self.len(), "index out of range");
        if self.get(i).unwrap() {
            &true
        } else {
            &false
        }
    }
}

impl hash::Hash for SmallBitVec {
    #[inline]
    fn hash<H: hash::Hasher>(&self, state: &mut H) {
        let len = self.len();
        len.hash(state);
        if self.is_inline() {
            reverse_bits(self.data & inline_ones(len)).hash(state);
        } else {
            let full_blocks = len / bits_per_storage();
            let remainder = len % bits_per_storage();
            let buffer = self.buffer();
            if full_blocks != 0 {
                buffer[..full_blocks].hash(state);
            }
            if remainder != 0 {
                let mask = (1 << remainder) - 1;
                (buffer[full_blocks] & mask).hash(state);
            }
        }
    }
}

impl Extend<bool> for SmallBitVec {
    #[inline]
    fn extend<I: IntoIterator<Item = bool>>(&mut self, iter: I) {
        let iter = iter.into_iter();

        let (min, _) = iter.size_hint();
        assert!(min <= usize::max_value(), "capacity overflow");
        self.reserve(min);

        for element in iter {
            self.push(element)
        }
    }
}

impl FromIterator<bool> for SmallBitVec {
    #[inline]
    fn from_iter<I: IntoIterator<Item = bool>>(iter: I) -> Self {
        let mut v = SmallBitVec::new();
        v.extend(iter);
        v
    }
}

impl IntoIterator for SmallBitVec {
    type Item = bool;
    type IntoIter = IntoIter;

    #[inline]
    fn into_iter(self) -> IntoIter {
        IntoIter {
            range: 0..self.len(),
            vec: self,
        }
    }
}

impl<'a> IntoIterator for &'a SmallBitVec {
    type Item = bool;
    type IntoIter = Iter<'a>;

    #[inline]
    fn into_iter(self) -> Iter<'a> {
        self.iter()
    }
}

/// An iterator that owns a SmallBitVec and yields its bits as `bool` values.
///
/// Returned from [`SmallBitVec::into_iter`][1].
///
/// [1]: struct.SmallBitVec.html#method.into_iter
pub struct IntoIter {
    vec: SmallBitVec,
    range: Range<usize>,
}

impl Iterator for IntoIter {
    type Item = bool;

    #[inline]
    fn next(&mut self) -> Option<bool> {
        self.range
            .next()
            .map(|i| unsafe { self.vec.get_unchecked(i) })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.range.size_hint()
    }
}

impl DoubleEndedIterator for IntoIter {
    #[inline]
    fn next_back(&mut self) -> Option<bool> {
        self.range
            .next_back()
            .map(|i| unsafe { self.vec.get_unchecked(i) })
    }
}

impl ExactSizeIterator for IntoIter {}

/// An iterator that borrows a SmallBitVec and yields its bits as `bool` values.
///
/// Returned from [`SmallBitVec::iter`][1].
///
/// [1]: struct.SmallBitVec.html#method.iter
pub struct Iter<'a> {
    vec: &'a SmallBitVec,
    range: Range<usize>,
}

impl<'a> Default for Iter<'a> {
    #[inline]
    fn default() -> Self {
        const EMPTY: &'static SmallBitVec = &SmallBitVec::new();
        Self {
            vec: EMPTY,
            range: 0..0,
        }
    }
}

impl<'a> Iterator for Iter<'a> {
    type Item = bool;

    #[inline]
    fn next(&mut self) -> Option<bool> {
        self.range
            .next()
            .map(|i| unsafe { self.vec.get_unchecked(i) })
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.range.size_hint()
    }
}

impl<'a> DoubleEndedIterator for Iter<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<bool> {
        self.range
            .next_back()
            .map(|i| unsafe { self.vec.get_unchecked(i) })
    }
}

impl<'a> ExactSizeIterator for Iter<'a> {}

/// An immutable view of a range of bits from a borrowed SmallBitVec.
///
/// Returned from [`SmallBitVec::range`][1].
///
/// [1]: struct.SmallBitVec.html#method.range
#[derive(Debug, Clone)]
pub struct VecRange<'a> {
    vec: &'a SmallBitVec,
    range: Range<usize>,
}

impl<'a> VecRange<'a> {
    #[inline]
    pub fn iter(&self) -> Iter<'a> {
        Iter {
            vec: self.vec,
            range: self.range.clone(),
        }
    }
}

impl<'a> Index<usize> for VecRange<'a> {
    type Output = bool;

    #[inline]
    fn index(&self, i: usize) -> &bool {
        let vec_i = i + self.range.start;
        assert!(vec_i < self.range.end, "index out of range");
        &self.vec[vec_i]
    }
}

fn reverse_bits(mut value: usize) -> usize {
    let mut result = 0;
    for _ in 0..inline_bits() {
        result <<= 1;
        result |= value & 1;
        value >>= 1;
    }
    result
}