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// Copyright 2012-2023 The Rust Project Developers. 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.

// FIXME(Gankro): BitVec and BitSet are very tightly coupled. Ideally (for
// maintenance), they should be in separate files/modules, with BitSet only
// using BitVec's public API. This will be hard for performance though, because
// `BitVec` will not want to leak its internal representation while its internal
// representation as `u32`s must be assumed for best performance.

// (1) Be careful, most things can overflow here because the amount of bits in
// memory can overflow `usize`.
// (2) Make sure that the underlying vector has no excess length:
// E. g. `nbits == 16`, `storage.len() == 2` would be excess length,
// because the last word isn't used at all. This is important because some
// methods rely on it (for *CORRECTNESS*).
// (3) Make sure that the unused bits in the last word are zeroed out, again
// other methods rely on it for *CORRECTNESS*.
// (4) `BitSet` is tightly coupled with `BitVec`, so any changes you make in
// `BitVec` will need to be reflected in `BitSet`.

//! # Description
//!
//! Dynamic collections implemented with compact bit vectors.
//!
//! # Examples
//!
//! This is a simple example of the [Sieve of Eratosthenes][sieve]
//! which calculates prime numbers up to a given limit.
//!
//! [sieve]: http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
//!
//! ```
//! use bit_vec::BitVec;
//!
//! let max_prime = 10000;
//!
//! // Store the primes as a BitVec
//! let primes = {
//! // Assume all numbers are prime to begin, and then we
//! // cross off non-primes progressively
//! let mut bv = BitVec::from_elem(max_prime, true);
//!
//! // Neither 0 nor 1 are prime
//! bv.set(0, false);
//! bv.set(1, false);
//!
//! for i in 2.. 1 + (max_prime as f64).sqrt() as usize {
//! // if i is a prime
//! if bv[i] {
//! // Mark all multiples of i as non-prime (any multiples below i * i
//! // will have been marked as non-prime previously)
//! for j in i.. {
//! if i * j >= max_prime {
//! break;
//! }
//! bv.set(i * j, false)
//! }
//! }
//! }
//! bv
//! };
//!
//! // Simple primality tests below our max bound
//! let print_primes = 20;
//! print!("The primes below {} are: ", print_primes);
//! for x in 0..print_primes {
//! if primes.get(x).unwrap_or(false) {
//! print!("{} ", x);
//! }
//! }
//! println!();
//!
//! let num_primes = primes.iter().filter(|x| *x).count();
//! println!("There are {} primes below {}", num_primes, max_prime);
//! assert_eq!(num_primes, 1_229);
//! ```

#![doc(html_root_url = "https://docs.rs/bit-vec/0.8.0")]
#![no_std]

#[cfg(any(test, feature = "std"))]
#[macro_use]
extern crate std;
#[cfg(feature = "std")]
use std::rc::Rc;
#[cfg(feature = "std")]
use std::string::String;
#[cfg(feature = "std")]
use std::vec::Vec;

#[cfg(feature = "serde")]
extern crate serde;
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};
#[cfg(feature = "borsh")]
extern crate borsh;
#[cfg(feature = "miniserde")]
extern crate miniserde;
#[cfg(feature = "nanoserde")]
extern crate nanoserde;
#[cfg(feature = "nanoserde")]
use nanoserde::{DeBin, DeJson, DeRon, SerBin, SerJson, SerRon};

#[cfg(not(feature = "std"))]
#[macro_use]
extern crate alloc;
#[cfg(not(feature = "std"))]
use alloc::rc::Rc;
#[cfg(not(feature = "std"))]
use alloc::string::String;
#[cfg(not(feature = "std"))]
use alloc::vec::Vec;

use core::cell::RefCell;
use core::cmp;
use core::cmp::Ordering;
use core::fmt::{self, Write};
use core::hash;
use core::iter::repeat;
use core::iter::FromIterator;
use core::mem;
use core::ops::*;
use core::slice;

type MutBlocks<'a, B> = slice::IterMut<'a, B>;

/// Abstracts over a pile of bits (basically unsigned primitives)
pub trait BitBlock:
 Copy
 + Add<Self, Output = Self>
 + Sub<Self, Output = Self>
 + Shl<usize, Output = Self>
 + Shr<usize, Output = Self>
 + Not<Output = Self>
 + BitAnd<Self, Output = Self>
 + BitOr<Self, Output = Self>
 + BitXor<Self, Output = Self>
 + Rem<Self, Output = Self>
 + Eq
 + Ord
 + hash::Hash
{
 /// How many bits it has
 fn bits() -> usize;
 /// How many bytes it has
 #[inline]
 fn bytes() -> usize {
 Self::bits() / 8
 }
 /// Convert a byte into this type (lowest-order bits set)
 fn from_byte(byte: u8) -> Self;
 /// Count the number of 1's in the bitwise repr
 fn count_ones(self) -> usize;
 /// Count the number of 0's in the bitwise repr
 fn count_zeros(self) -> usize {
 Self::bits() - self.count_ones()
 }
 /// Get `0`
 fn zero() -> Self;
 /// Get `1`
 fn one() -> Self;
}

macro_rules! bit_block_impl {
 ($(($t: ident, $size: expr)),*) => ($(
 impl BitBlock for $t {
 #[inline]
 fn bits() -> usize { $size }
 #[inline]
 fn from_byte(byte: u8) -> Self { $t::from(byte) }
 #[inline]
 fn count_ones(self) -> usize { self.count_ones() as usize }
 #[inline]
 fn count_zeros(self) -> usize { self.count_zeros() as usize }
 #[inline]
 fn one() -> Self { 1 }
 #[inline]
 fn zero() -> Self { 0 }
 }
 )*)
}

bit_block_impl! {
 (u8, 8),
 (u16, 16),
 (u32, 32),
 (u64, 64),
 (usize, core::mem::size_of::<usize>() * 8)
}

fn reverse_bits(byte: u8) -> u8 {
 let mut result = 0;
 for i in 0..u8::bits() {
 result |= ((byte >> i) & 1) << (u8::bits() - 1 - i);
 }
 result
}

static TRUE: bool = true;
static FALSE: bool = false;

/// The bitvector type.
///
/// # Examples
///
/// ```
/// use bit_vec::BitVec;
///
/// let mut bv = BitVec::from_elem(10, false);
///
/// // insert all primes less than 10
/// bv.set(2, true);
/// bv.set(3, true);
/// bv.set(5, true);
/// bv.set(7, true);
/// println!("{:?}", bv);
/// println!("total bits set to true: {}", bv.iter().filter(|x| *x).count());
///
/// // flip all values in bitvector, producing non-primes less than 10
/// bv.negate();
/// println!("{:?}", bv);
/// println!("total bits set to true: {}", bv.iter().filter(|x| *x).count());
///
/// // reset bitvector to empty
/// bv.clear();
/// println!("{:?}", bv);
/// println!("total bits set to true: {}", bv.iter().filter(|x| *x).count());
/// ```
#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
#[cfg_attr(
 feature = "borsh",
 derive(borsh::BorshDeserialize, borsh::BorshSerialize)
)]
#[cfg_attr(
 feature = "miniserde",
 derive(miniserde::Deserialize, miniserde::Serialize)
)]
#[cfg_attr(
 feature = "nanoserde",
 derive(DeBin, DeJson, DeRon, SerBin, SerJson, SerRon)
)]
pub struct BitVec {
 /// Internal representation of the bit vector
 storage: Vec,
 /// The number of valid bits in the internal representation
 nbits: usize,
}

// FIXME(Gankro): NopeNopeNopeNopeNope (wait for IndexGet to be a thing)
impl<B: BitBlock> Index<usize> for BitVec {
 type Output = bool;

 #[inline]
 fn index(&self, i: usize) -> &bool {
 if self.get(i).expect("index out of bounds") {
 &TRUE
 } else {
 &FALSE
 }
 }
}

/// Computes how many blocks are needed to store that many bits
fn blocks_for_bits<B: BitBlock>(bits: usize) -> usize {
 // If we want 17 bits, dividing by 32 will produce 0. So we add 1 to make sure we
 // reserve enough. But if we want exactly a multiple of 32, this will actually allocate
 // one too many. So we need to check if that's the case. We can do that by computing if
 // bitwise AND by `32 - 1` is 0. But LLVM should be able to optimize the semantically
 // superior modulo operator on a power of two to this.
 //
 // Note that we can technically avoid this branch with the expression
 // `(nbits + U32_BITS - 1) / 32::BITS`, but if nbits is almost usize::MAX this will overflow.
 if bits % B::bits() == 0 {
 bits / B::bits()
 } else {
 bits / B::bits() + 1
 }
}

/// Computes the bitmask for the final word of the vector
fn mask_for_bits<B: BitBlock>(bits: usize) -> B {
 // Note especially that a perfect multiple of U32_BITS should mask all 1s.
 (!B::zero()) >> ((B::bits() - bits % B::bits()) % B::bits())
}

type B = u32;

impl BitVec<u32> {
 /// Creates an empty `BitVec`.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 /// let mut bv = BitVec::new();
 /// ```
 #[inline]
 pub fn new() -> Self {
 Default::default()
 }

 /// Creates a `BitVec` that holds `nbits` elements, setting each element
 /// to `bit`.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(10, false);
 /// assert_eq!(bv.len(), 10);
 /// for x in bv.iter() {
 /// assert_eq!(x, false);
 /// }
 /// ```
 #[inline]
 pub fn from_elem(nbits: usize, bit: bool) -> Self {
 let nblocks = blocks_for_bits::(nbits);
 let mut bit_vec = BitVec {
 storage: vec![if bit { !B::zero() } else { B::zero() }; nblocks],
 nbits,
 };
 bit_vec.fix_last_block();
 bit_vec
 }

 /// Constructs a new, empty `BitVec` with the specified capacity.
 ///
 /// The bitvector will be able to hold at least `capacity` bits without
 /// reallocating. If `capacity` is 0, it will not allocate.
 ///
 /// It is important to note that this function does not specify the
 /// *length* of the returned bitvector, but only the *capacity*.
 #[inline]
 pub fn with_capacity(nbits: usize) -> Self {
 BitVec {
 storage: Vec::with_capacity(blocks_for_bits::(nbits)),
 nbits: 0,
 }
 }

 /// Transforms a byte-vector into a `BitVec`. Each byte becomes eight bits,
 /// with the most significant bits of each byte coming first. Each
 /// bit becomes `true` if equal to 1 or `false` if equal to 0.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let bv = BitVec::from_bytes(&[0b10100000, 0b00010010]);
 /// assert!(bv.eq_vec(&[true, false, true, false,
 /// false, false, false, false,
 /// false, false, false, true,
 /// false, false, true, false]));
 /// ```
 pub fn from_bytes(bytes: &[u8]) -> Self {
 let len = bytes
 .len()
 .checked_mul(u8::bits())
 .expect("capacity overflow");
 let mut bit_vec = BitVec::with_capacity(len);
 let complete_words = bytes.len() / B::bytes();
 let extra_bytes = bytes.len() % B::bytes();

 bit_vec.nbits = len;

 for i in 0..complete_words {
 let mut accumulator = B::zero();
 for idx in 0..B::bytes() {
 accumulator |= B::from_byte(reverse_bits(bytes[i * B::bytes() + idx])) << (idx * 8)
 }
 bit_vec.storage.push(accumulator);
 }

 if extra_bytes > 0 {
 let mut last_word = B::zero();
 for (i, &byte) in bytes[complete_words * B::bytes()..].iter().enumerate() {
 last_word |= B::from_byte(reverse_bits(byte)) << (i * 8);
 }
 bit_vec.storage.push(last_word);
 }

 bit_vec
 }

 /// Creates a `BitVec` of the specified length where the value at each index
 /// is `f(index)`.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let bv = BitVec::from_fn(5, |i| { i % 2 == 0 });
 /// assert!(bv.eq_vec(&[true, false, true, false, true]));
 /// ```
 #[inline]
 pub fn from_fn<F>(len: usize, mut f: F) -> Self
 where
 F: FnMut(usize) -> bool,
 {
 let mut bit_vec = BitVec::from_elem(len, false);
 for i in 0..len {
 bit_vec.set(i, f(i));
 }
 bit_vec
 }
}

impl<B: BitBlock> BitVec {
 /// Applies the given operation to the blocks of self and other, and sets
 /// self to be the result. This relies on the caller not to corrupt the
 /// last word.
 #[inline]
 fn process<F>(&mut self, other: &BitVec, mut op: F) -> bool
 where
 F: FnMut(B, B) -> B,
 {
 assert_eq!(self.len(), other.len());
 debug_assert_eq!(self.storage.len(), other.storage.len());
 let mut changed_bits = B::zero();
 for (a, b) in self.blocks_mut().zip(other.blocks()) {
 let w = op(*a, b);
 changed_bits = changed_bits | (*a ^ w);
 *a = w;
 }
 changed_bits != B::zero()
 }

 /// Iterator over mutable refs to the underlying blocks of data.
 #[inline]
 fn blocks_mut(&mut self) -> MutBlocks {
 // (2)
 self.storage.iter_mut()
 }

 /// Iterator over the underlying blocks of data
 #[inline]
 pub fn blocks(&self) -> Blocks {
 // (2)
 Blocks {
 iter: self.storage.iter(),
 }
 }

 /// Exposes the raw block storage of this `BitVec`.
 ///
 /// Only really intended for `BitSet`.
 #[inline]
 pub fn storage(&self) -> &[B] {
 &self.storage
 }

 /// Exposes the raw block storage of this `BitVec`.
 ///
 /// # Safety
 ///
 /// Can probably cause unsafety. Only really intended for `BitSet`.
 #[inline]
 pub unsafe fn storage_mut(&mut self) -> &mut Vec {
 &mut self.storage
 }

 /// Helper for procedures involving spare space in the last block.
 #[inline]
 fn last_block_with_mask(&self) -> Option<(B, B)> {
 let extra_bits = self.len() % B::bits();
 if extra_bits > 0 {
 let mask = (B::one() << extra_bits) - B::one();
 let storage_len = self.storage.len();
 Some((self.storage[storage_len - 1], mask))
 } else {
 None
 }
 }

 /// Helper for procedures involving spare space in the last block.
 #[inline]
 fn last_block_mut_with_mask(&mut self) -> Option<(&mut B, B)> {
 let extra_bits = self.len() % B::bits();
 if extra_bits > 0 {
 let mask = (B::one() << extra_bits) - B::one();
 let storage_len = self.storage.len();
 Some((&mut self.storage[storage_len - 1], mask))
 } else {
 None
 }
 }

 /// An operation might screw up the unused bits in the last block of the
 /// `BitVec`. As per (3), it's assumed to be all 0s. This method fixes it up.
 fn fix_last_block(&mut self) {
 if let Some((last_block, used_bits)) = self.last_block_mut_with_mask() {
 *last_block = *last_block & used_bits;
 }
 }

 /// Operations such as change detection for xnor, nor and nand are easiest
 /// to implement when unused bits are all set to 1s.
 fn fix_last_block_with_ones(&mut self) {
 if let Some((last_block, used_bits)) = self.last_block_mut_with_mask() {
 *last_block = *last_block | !used_bits;
 }
 }

 /// Check whether last block's invariant is fine.
 fn is_last_block_fixed(&self) -> bool {
 if let Some((last_block, used_bits)) = self.last_block_with_mask() {
 last_block & !used_bits == B::zero()
 } else {
 true
 }
 }

 /// Ensure the invariant for the last block.
 ///
 /// An operation might screw up the unused bits in the last block of the
 /// `BitVec`.
 ///
 /// This method fails in case the last block is not fixed. The check
 /// is skipped outside testing.
 #[inline]
 fn ensure_invariant(&self) {
 if cfg!(test) {
 debug_assert!(self.is_last_block_fixed());
 }
 }

 /// Retrieves the value at index `i`, or `None` if the index is out of bounds.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let bv = BitVec::from_bytes(&[0b01100000]);
 /// assert_eq!(bv.get(0), Some(false));
 /// assert_eq!(bv.get(1), Some(true));
 /// assert_eq!(bv.get(100), None);
 ///
 /// // Can also use array indexing
 /// assert_eq!(bv[1], true);
 /// ```
 #[inline]
 pub fn get(&self, i: usize) -> Option<bool> {
 self.ensure_invariant();
 if i >= self.nbits {
 return None;
 }
 let w = i / B::bits();
 let b = i % B::bits();
 self.storage
 .get(w)
 .map(|&block| (block & (B::one() << b)) != B::zero())
 }

 /// Retrieves the value at index `i`, without doing bounds checking.
 ///
 /// For a safe alternative, see `get`.
 ///
 /// # Safety
 ///
 /// Calling this method with an out-of-bounds index is undefined behavior
 /// even if the resulting reference is not used.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let bv = BitVec::from_bytes(&[0b01100000]);
 /// unsafe {
 /// assert_eq!(bv.get_unchecked(0), false);
 /// assert_eq!(bv.get_unchecked(1), true);
 /// }
 /// ```
 #[inline]
 pub unsafe fn get_unchecked(&self, i: usize) -> bool {
 self.ensure_invariant();
 let w = i / B::bits();
 let b = i % B::bits();
 let block = *self.storage.get_unchecked(w);
 block & (B::one() << b) != B::zero()
 }

 /// Retrieves a smart pointer to the value at index `i`, or `None` if the index is out of bounds.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_bytes(&[0b01100000]);
 /// *bv.get_mut(0).unwrap() = true;
 /// *bv.get_mut(1).unwrap() = false;
 /// assert!(bv.get_mut(100).is_none());
 /// assert_eq!(bv, BitVec::from_bytes(&[0b10100000]));
 /// ```
 #[inline]
 pub fn get_mut(&mut self, index: usize) -> Option<MutBorrowedBit> {
 self.get(index).map(move |value| MutBorrowedBit {
 vec: Rc::new(RefCell::new(self)),
 index,
 #[cfg(debug_assertions)]
 old_value: value,
 new_value: value,
 })
 }

 /// Retrieves a smart pointer to the value at index `i`, without doing bounds checking.
 ///
 /// # Safety
 ///
 /// Calling this method with out-of-bounds `index` may cause undefined behavior even when
 /// the result is not used.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_bytes(&[0b01100000]);
 /// unsafe {
 /// *bv.get_unchecked_mut(0) = true;
 /// *bv.get_unchecked_mut(1) = false;
 /// }
 /// assert_eq!(bv, BitVec::from_bytes(&[0b10100000]));
 /// ```
 #[inline]
 pub unsafe fn get_unchecked_mut(&mut self, index: usize) -> MutBorrowedBit {
 let value = self.get_unchecked(index);
 MutBorrowedBit {
 #[cfg(debug_assertions)]
 old_value: value,
 new_value: value,
 vec: Rc::new(RefCell::new(self)),
 index,
 }
 }

 /// Sets the value of a bit at an index `i`.
 ///
 /// # Panics
 ///
 /// Panics if `i` is out of bounds.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(5, false);
 /// bv.set(3, true);
 /// assert_eq!(bv[3], true);
 /// ```
 #[inline]
 pub fn set(&mut self, i: usize, x: bool) {
 self.ensure_invariant();
 assert!(
 i < self.nbits,
 "index out of bounds: {:?} >= {:?}",
 i,
 self.nbits
 );
 let w = i / B::bits();
 let b = i % B::bits();
 let flag = B::one() << b;
 let val = if x {
 self.storage[w] | flag
 } else {
 self.storage[w] & !flag
 };
 self.storage[w] = val;
 }

 /// Sets all bits to 1.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let before = 0b01100000;
 /// let after = 0b11111111;
 ///
 /// let mut bv = BitVec::from_bytes(&[before]);
 /// bv.set_all();
 /// assert_eq!(bv, BitVec::from_bytes(&[after]));
 /// ```
 #[inline]
 pub fn set_all(&mut self) {
 self.ensure_invariant();
 for w in &mut self.storage {
 *w = !B::zero();
 }
 self.fix_last_block();
 }

 /// Flips all bits.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let before = 0b01100000;
 /// let after = 0b10011111;
 ///
 /// let mut bv = BitVec::from_bytes(&[before]);
 /// bv.negate();
 /// assert_eq!(bv, BitVec::from_bytes(&[after]));
 /// ```
 #[inline]
 pub fn negate(&mut self) {
 self.ensure_invariant();
 for w in &mut self.storage {
 *w = !*w;
 }
 self.fix_last_block();
 }

 /// Calculates the union of two bitvectors. This acts like the bitwise `or`
 /// function.
 ///
 /// Sets `self` to the union of `self` and `other`. Both bitvectors must be
 /// the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different lengths.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100100;
 /// let b = 0b01011010;
 /// let res = 0b01111110;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.union(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[deprecated(since = "0.7.0", note = "Please use the 'or' function instead")]
 #[inline]
 pub fn union(&mut self, other: &Self) -> bool {
 self.or(other)
 }

 /// Calculates the intersection of two bitvectors. This acts like the
 /// bitwise `and` function.
 ///
 /// Sets `self` to the intersection of `self` and `other`. Both bitvectors
 /// must be the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different lengths.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100100;
 /// let b = 0b01011010;
 /// let res = 0b01000000;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.intersect(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[deprecated(since = "0.7.0", note = "Please use the 'and' function instead")]
 #[inline]
 pub fn intersect(&mut self, other: &Self) -> bool {
 self.and(other)
 }

 /// Calculates the bitwise `or` of two bitvectors.
 ///
 /// Sets `self` to the union of `self` and `other`. Both bitvectors must be
 /// the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different lengths.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100100;
 /// let b = 0b01011010;
 /// let res = 0b01111110;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.or(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[inline]
 pub fn or(&mut self, other: &Self) -> bool {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 self.process(other, |w1, w2| (w1 | w2))
 }

 /// Calculates the bitwise `and` of two bitvectors.
 ///
 /// Sets `self` to the intersection of `self` and `other`. Both bitvectors
 /// must be the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different lengths.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100100;
 /// let b = 0b01011010;
 /// let res = 0b01000000;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.and(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[inline]
 pub fn and(&mut self, other: &Self) -> bool {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 self.process(other, |w1, w2| (w1 & w2))
 }

 /// Calculates the difference between two bitvectors.
 ///
 /// Sets each element of `self` to the value of that element minus the
 /// element of `other` at the same index. Both bitvectors must be the same
 /// length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different length.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100100;
 /// let b = 0b01011010;
 /// let a_b = 0b00100100; // a - b
 /// let b_a = 0b00011010; // b - a
 ///
 /// let mut bva = BitVec::from_bytes(&[a]);
 /// let bvb = BitVec::from_bytes(&[b]);
 ///
 /// assert!(bva.difference(&bvb));
 /// assert_eq!(bva, BitVec::from_bytes(&[a_b]));
 ///
 /// let bva = BitVec::from_bytes(&[a]);
 /// let mut bvb = BitVec::from_bytes(&[b]);
 ///
 /// assert!(bvb.difference(&bva));
 /// assert_eq!(bvb, BitVec::from_bytes(&[b_a]));
 /// ```
 #[inline]
 pub fn difference(&mut self, other: &Self) -> bool {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 self.process(other, |w1, w2| (w1 & !w2))
 }

 /// Calculates the xor of two bitvectors.
 ///
 /// Sets `self` to the xor of `self` and `other`. Both bitvectors must be
 /// the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different length.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100110;
 /// let b = 0b01010100;
 /// let res = 0b00110010;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.xor(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[inline]
 pub fn xor(&mut self, other: &Self) -> bool {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 self.process(other, |w1, w2| (w1 ^ w2))
 }

 /// Calculates the nand of two bitvectors.
 ///
 /// Sets `self` to the nand of `self` and `other`. Both bitvectors must be
 /// the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different length.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100110;
 /// let b = 0b01010100;
 /// let res = 0b10111011;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.nand(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[inline]
 pub fn nand(&mut self, other: &Self) -> bool {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 self.fix_last_block_with_ones();
 let result = self.process(other, |w1, w2| !(w1 & w2));
 self.fix_last_block();
 result
 }

 /// Calculates the nor of two bitvectors.
 ///
 /// Sets `self` to the nor of `self` and `other`. Both bitvectors must be
 /// the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different length.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100110;
 /// let b = 0b01010100;
 /// let res = 0b10001001;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.nor(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[inline]
 pub fn nor(&mut self, other: &Self) -> bool {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 self.fix_last_block_with_ones();
 let result = self.process(other, |w1, w2| !(w1 | w2));
 self.fix_last_block();
 result
 }

 /// Calculates the xnor of two bitvectors.
 ///
 /// Sets `self` to the xnor of `self` and `other`. Both bitvectors must be
 /// the same length. Returns `true` if `self` changed.
 ///
 /// # Panics
 ///
 /// Panics if the bitvectors are of different length.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let a = 0b01100110;
 /// let b = 0b01010100;
 /// let res = 0b11001101;
 ///
 /// let mut a = BitVec::from_bytes(&[a]);
 /// let b = BitVec::from_bytes(&[b]);
 ///
 /// assert!(a.xnor(&b));
 /// assert_eq!(a, BitVec::from_bytes(&[res]));
 /// ```
 #[inline]
 pub fn xnor(&mut self, other: &Self) -> bool {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 self.fix_last_block_with_ones();
 let result = self.process(other, |w1, w2| !(w1 ^ w2));
 self.fix_last_block();
 result
 }

 /// Returns `true` if all bits are 1.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(5, true);
 /// assert_eq!(bv.all(), true);
 ///
 /// bv.set(1, false);
 /// assert_eq!(bv.all(), false);
 /// ```
 #[inline]
 pub fn all(&self) -> bool {
 self.ensure_invariant();
 let mut last_word = !B::zero();
 // Check that every block but the last is all-ones...
 self.blocks().all(|elem| {
 let tmp = last_word;
 last_word = elem;
 tmp == !B::zero()
 // and then check the last one has enough ones
 }) && (last_word == mask_for_bits(self.nbits))
 }

 /// Returns the number of ones in the binary representation.
 ///
 /// Also known as the
 /// [Hamming weight](https://en.wikipedia.org/wiki/Hamming_weight).
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(100, true);
 /// assert_eq!(bv.count_ones(), 100);
 ///
 /// bv.set(50, false);
 /// assert_eq!(bv.count_ones(), 99);
 /// ```
 #[inline]
 pub fn count_ones(&self) -> u64 {
 self.ensure_invariant();
 // Add the number of ones of each block.
 self.blocks().map(|elem| elem.count_ones() as u64).sum()
 }

 /// Returns the number of zeros in the binary representation.
 ///
 /// Also known as the opposite of
 /// [Hamming weight](https://en.wikipedia.org/wiki/Hamming_weight).
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(100, false);
 /// assert_eq!(bv.count_zeros(), 100);
 ///
 /// bv.set(50, true);
 /// assert_eq!(bv.count_zeros(), 99);
 /// ```
 #[inline]
 pub fn count_zeros(&self) -> u64 {
 self.ensure_invariant();
 // Add the number of zeros of each block.
 let extra_zeros = (B::bits() - (self.len() % B::bits())) % B::bits();
 self.blocks()
 .map(|elem| elem.count_zeros() as u64)
 .sum::<u64>()
 - extra_zeros as u64
 }

 /// Returns an iterator over the elements of the vector in order.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let bv = BitVec::from_bytes(&[0b01110100, 0b10010010]);
 /// assert_eq!(bv.iter().filter(|x| *x).count(), 7);
 /// ```
 #[inline]
 pub fn iter(&self) -> Iter {
 self.ensure_invariant();
 Iter {
 bit_vec: self,
 range: 0..self.nbits,
 }
 }

 /// Returns an iterator over mutable smart pointers to the elements of the vector in order.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut a = BitVec::from_elem(8, false);
 /// a.iter_mut().enumerate().for_each(|(index, mut bit)| {
 /// *bit = if index % 2 == 1 { true } else { false };
 /// });
 /// assert!(a.eq_vec(&[
 /// false, true, false, true, false, true, false, true
 /// ]));
 /// ```
 #[inline]
 pub fn iter_mut(&mut self) -> IterMut {
 self.ensure_invariant();
 let nbits = self.nbits;
 IterMut {
 vec: Rc::new(RefCell::new(self)),
 range: 0..nbits,
 }
 }

 /// Moves all bits from `other` into `Self`, leaving `other` empty.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut a = BitVec::from_bytes(&[0b10000000]);
 /// let mut b = BitVec::from_bytes(&[0b01100001]);
 ///
 /// a.append(&mut b);
 ///
 /// assert_eq!(a.len(), 16);
 /// assert_eq!(b.len(), 0);
 /// assert!(a.eq_vec(&[true, false, false, false, false, false, false, false,
 /// false, true, true, false, false, false, false, true]));
 /// ```
 pub fn append(&mut self, other: &mut Self) {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());

 let b = self.len() % B::bits();
 let o = other.len() % B::bits();
 let will_overflow = (b + o > B::bits()) || (o == 0 && b != 0);

 self.nbits += other.len();
 other.nbits = 0;

 if b == 0 {
 self.storage.append(&mut other.storage);
 } else {
 self.storage.reserve(other.storage.len());

 for block in other.storage.drain(..) {
 {
 let last = self.storage.last_mut().unwrap();
 *last = *last | (block << b);
 }
 self.storage.push(block >> (B::bits() - b));
 }

 // Remove additional block if the last shift did not overflow
 if !will_overflow {
 self.storage.pop();
 }
 }
 }

 /// Splits the `BitVec` into two at the given bit,
 /// retaining the first half in-place and returning the second one.
 ///
 /// # Panics
 ///
 /// Panics if `at` is out of bounds.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 /// let mut a = BitVec::new();
 /// a.push(true);
 /// a.push(false);
 /// a.push(false);
 /// a.push(true);
 ///
 /// let b = a.split_off(2);
 ///
 /// assert_eq!(a.len(), 2);
 /// assert_eq!(b.len(), 2);
 /// assert!(a.eq_vec(&[true, false]));
 /// assert!(b.eq_vec(&[false, true]));
 /// ```
 pub fn split_off(&mut self, at: usize) -> Self {
 self.ensure_invariant();
 assert!(at <= self.len(), "`at` out of bounds");

 let mut other = BitVec::::default();

 if at == 0 {
 mem::swap(self, &mut other);
 return other;
 } else if at == self.len() {
 return other;
 }

 let w = at / B::bits();
 let b = at % B::bits();
 other.nbits = self.nbits - at;
 self.nbits = at;
 if b == 0 {
 // Split at block boundary
 other.storage = self.storage.split_off(w);
 } else {
 other.storage.reserve(self.storage.len() - w);

 {
 let mut iter = self.storage[w..].iter();
 let mut last = *iter.next().unwrap();
 for &cur in iter {
 other.storage.push((last >> b) | (cur << (B::bits() - b)));
 last = cur;
 }
 other.storage.push(last >> b);
 }

 self.storage.truncate(w + 1);
 self.fix_last_block();
 }

 other
 }

 /// Returns `true` if all bits are 0.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(10, false);
 /// assert_eq!(bv.none(), true);
 ///
 /// bv.set(3, true);
 /// assert_eq!(bv.none(), false);
 /// ```
 #[inline]
 pub fn none(&self) -> bool {
 self.blocks().all(|w| w == B::zero())
 }

 /// Returns `true` if any bit is 1.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(10, false);
 /// assert_eq!(bv.any(), false);
 ///
 /// bv.set(3, true);
 /// assert_eq!(bv.any(), true);
 /// ```
 #[inline]
 pub fn any(&self) -> bool {
 !self.none()
 }

 /// Organises the bits into bytes, such that the first bit in the
 /// `BitVec` becomes the high-order bit of the first byte. If the
 /// size of the `BitVec` is not a multiple of eight then trailing bits
 /// will be filled-in with `false`.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(3, true);
 /// bv.set(1, false);
 ///
 /// assert_eq!(bv.to_bytes(), [0b10100000]);
 ///
 /// let mut bv = BitVec::from_elem(9, false);
 /// bv.set(2, true);
 /// bv.set(8, true);
 ///
 /// assert_eq!(bv.to_bytes(), [0b00100000, 0b10000000]);
 /// ```
 pub fn to_bytes(&self) -> Vec<u8> {
 self.ensure_invariant();
 // Oh lord, we're mapping this to bytes bit-by-bit!
 fn bit<B: BitBlock>(bit_vec: &BitVec, byte: usize, bit: usize) -> u8 {
 let offset = byte * 8 + bit;
 if offset >= bit_vec.nbits {
 0
 } else {
 (bit_vec[offset] as u8) << (7 - bit)
 }
 }

 let len = self.nbits / 8 + if self.nbits % 8 == 0 { 0 } else { 1 };
 (0..len)
 .map(|i| {
 bit(self, i, 0)
 | bit(self, i, 1)
 | bit(self, i, 2)
 | bit(self, i, 3)
 | bit(self, i, 4)
 | bit(self, i, 5)
 | bit(self, i, 6)
 | bit(self, i, 7)
 })
 .collect()
 }

 /// Compares a `BitVec` to a slice of `bool`s.
 /// Both the `BitVec` and slice must have the same length.
 ///
 /// # Panics
 ///
 /// Panics if the `BitVec` and slice are of different length.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let bv = BitVec::from_bytes(&[0b10100000]);
 ///
 /// assert!(bv.eq_vec(&[true, false, true, false,
 /// false, false, false, false]));
 /// ```
 #[inline]
 pub fn eq_vec(&self, v: &[bool]) -> bool {
 assert_eq!(self.nbits, v.len());
 self.iter().zip(v.iter().cloned()).all(|(b1, b2)| b1 == b2)
 }

 /// Shortens a `BitVec`, dropping excess elements.
 ///
 /// If `len` is greater than the vector's current length, this has no
 /// effect.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_bytes(&[0b01001011]);
 /// bv.truncate(2);
 /// assert!(bv.eq_vec(&[false, true]));
 /// ```
 #[inline]
 pub fn truncate(&mut self, len: usize) {
 self.ensure_invariant();
 if len < self.len() {
 self.nbits = len;
 // This fixes (2).
 self.storage.truncate(blocks_for_bits::(len));
 self.fix_last_block();
 }
 }

 /// Reserves capacity for at least `additional` more bits to be inserted in the given
 /// `BitVec`. The collection may reserve more space to avoid frequent reallocations.
 ///
 /// # Panics
 ///
 /// Panics if the new capacity overflows `usize`.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(3, false);
 /// bv.reserve(10);
 /// assert_eq!(bv.len(), 3);
 /// assert!(bv.capacity() >= 13);
 /// ```
 #[inline]
 pub fn reserve(&mut self, additional: usize) {
 let desired_cap = self
 .len()
 .checked_add(additional)
 .expect("capacity overflow");
 let storage_len = self.storage.len();
 if desired_cap > self.capacity() {
 self.storage
 .reserve(blocks_for_bits::(desired_cap) - storage_len);
 }
 }

 /// Reserves the minimum capacity for exactly `additional` more bits to be inserted in the
 /// given `BitVec`. Does nothing if the capacity is already sufficient.
 ///
 /// Note that the allocator may give the collection more space than it requests. Therefore
 /// capacity can not be relied upon to be precisely minimal. Prefer `reserve` if future
 /// insertions are expected.
 ///
 /// # Panics
 ///
 /// Panics if the new capacity overflows `usize`.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_elem(3, false);
 /// bv.reserve(10);
 /// assert_eq!(bv.len(), 3);
 /// assert!(bv.capacity() >= 13);
 /// ```
 #[inline]
 pub fn reserve_exact(&mut self, additional: usize) {
 let desired_cap = self
 .len()
 .checked_add(additional)
 .expect("capacity overflow");
 let storage_len = self.storage.len();
 if desired_cap > self.capacity() {
 self.storage
 .reserve_exact(blocks_for_bits::(desired_cap) - storage_len);
 }
 }

 /// Returns the capacity in bits for this bit vector. Inserting any
 /// element less than this amount will not trigger a resizing.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::new();
 /// bv.reserve(10);
 /// assert!(bv.capacity() >= 10);
 /// ```
 #[inline]
 pub fn capacity(&self) -> usize {
 self.storage.capacity().saturating_mul(B::bits())
 }

 /// Grows the `BitVec` in-place, adding `n` copies of `value` to the `BitVec`.
 ///
 /// # Panics
 ///
 /// Panics if the new len overflows a `usize`.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_bytes(&[0b01001011]);
 /// bv.grow(2, true);
 /// assert_eq!(bv.len(), 10);
 /// assert_eq!(bv.to_bytes(), [0b01001011, 0b11000000]);
 /// ```
 pub fn grow(&mut self, n: usize, value: bool) {
 self.ensure_invariant();

 // Note: we just bulk set all the bits in the last word in this fn in multiple places
 // which is technically wrong if not all of these bits are to be used. However, at the end
 // of this fn we call `fix_last_block` at the end of this fn, which should fix this.

 let new_nbits = self.nbits.checked_add(n).expect("capacity overflow");
 let new_nblocks = blocks_for_bits::(new_nbits);
 let full_value = if value { !B::zero() } else { B::zero() };

 // Correct the old tail word, setting or clearing formerly unused bits
 let num_cur_blocks = blocks_for_bits::(self.nbits);
 if self.nbits % B::bits() > 0 {
 let mask = mask_for_bits::(self.nbits);
 if value {
 let block = &mut self.storage[num_cur_blocks - 1];
 *block = *block | !mask;
 } else {
 // Extra bits are already zero by invariant.
 }
 }

 // Fill in words after the old tail word
 let stop_idx = cmp::min(self.storage.len(), new_nblocks);
 for idx in num_cur_blocks..stop_idx {
 self.storage[idx] = full_value;
 }

 // Allocate new words, if needed
 if new_nblocks > self.storage.len() {
 let to_add = new_nblocks - self.storage.len();
 self.storage.extend(repeat(full_value).take(to_add));
 }

 // Adjust internal bit count
 self.nbits = new_nbits;

 self.fix_last_block();
 }

 /// Removes the last bit from the `BitVec`, and returns it. Returns `None` if the `BitVec` is empty.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::from_bytes(&[0b01001001]);
 /// assert_eq!(bv.pop(), Some(true));
 /// assert_eq!(bv.pop(), Some(false));
 /// assert_eq!(bv.len(), 6);
 /// ```
 #[inline]
 pub fn pop(&mut self) -> Option<bool> {
 self.ensure_invariant();

 if self.is_empty() {
 None
 } else {
 let i = self.nbits - 1;
 let ret = self[i];
 // (3)
 self.set(i, false);
 self.nbits = i;
 if self.nbits % B::bits() == 0 {
 // (2)
 self.storage.pop();
 }
 Some(ret)
 }
 }

 /// Pushes a `bool` onto the end.
 ///
 /// # Examples
 ///
 /// ```
 /// use bit_vec::BitVec;
 ///
 /// let mut bv = BitVec::new();
 /// bv.push(true);
 /// bv.push(false);
 /// assert!(bv.eq_vec(&[true, false]));
 /// ```
 #[inline]
 pub fn push(&mut self, elem: bool) {
 if self.nbits % B::bits() == 0 {
 self.storage.push(B::zero());
 }
 let insert_pos = self.nbits;
 self.nbits = self.nbits.checked_add(1).expect("Capacity overflow");
 self.set(insert_pos, elem);
 }

 /// Returns the total number of bits in this vector
 #[inline]
 pub fn len(&self) -> usize {
 self.nbits
 }

 /// Sets the number of bits that this `BitVec` considers initialized.
 ///
 /// # Safety
 ///
 /// Almost certainly can cause bad stuff. Only really intended for `BitSet`.
 #[inline]
 pub unsafe fn set_len(&mut self, len: usize) {
 self.nbits = len;
 }

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

 /// Clears all bits in this vector.
 #[inline]
 pub fn clear(&mut self) {
 self.ensure_invariant();
 for w in &mut self.storage {
 *w = B::zero();
 }
 }

 /// Shrinks the capacity of the underlying storage as much as
 /// possible.
 ///
 /// It will drop down as close as possible to the length but the
 /// allocator may still inform the underlying storage that there
 /// is space for a few more elements/bits.
 pub fn shrink_to_fit(&mut self) {
 self.storage.shrink_to_fit();
 }

 /// Inserts a given bit at index `at`, shifting all bits after by one
 ///
 /// # Panics
 /// Panics if `at` is out of bounds for `BitVec`'s length (that is, if `at > BitVec::len()`)
 ///
 /// # Examples
 ///```
 /// use bit_vec::BitVec;
 ///
 /// let mut b = BitVec::new();
 ///
 /// b.push(true);
 /// b.push(true);
 /// b.insert(1, false);
 ///
 /// assert!(b.eq_vec(&[true, false, true]));
 ///```
 ///
 /// # Time complexity
 /// Takes O([`len`]) time. All items after the insertion index must be
 /// shifted to the right. In the worst case, all elements are shifted when
 /// the insertion index is 0.
 ///
 /// [`len`]: Self::len
 pub fn insert(&mut self, at: usize, bit: bool) {
 assert!(
 at <= self.nbits,
 "insertion index (is {at}) should be <= nbits (is {nbits})",
 nbits = self.nbits
 );

 let last_block_bits = self.nbits % B::bits();
 let block_at = at / B::bits(); // needed block
 let bit_at = at % B::bits(); // index within the block

 if last_block_bits == 0 {
 self.storage.push(B::zero());
 }

 self.nbits += 1;

 let mut carry = self.storage[block_at] >> (B::bits() - 1);
 let lsbits_mask = (B::one() << bit_at) - B::one();
 let set_bit = if bit { B::one() } else { B::zero() } << bit_at;
 self.storage[block_at] = (self.storage[block_at] & lsbits_mask)
 | ((self.storage[block_at] & !lsbits_mask) << 1)
 | set_bit;

 for block_ref in &mut self.storage[block_at + 1..] {
 let curr_carry = *block_ref >> (B::bits() - 1);
 *block_ref = *block_ref << 1 | carry;
 carry = curr_carry;
 }
 }
}

impl<B: BitBlock> Default for BitVec {
 #[inline]
 fn default() -> Self {
 BitVec {
 storage: Vec::new(),
 nbits: 0,
 }
 }
}

impl<B: BitBlock> FromIterator<bool> for BitVec {
 #[inline]
 fn from_iter<I: IntoIterator<Item = bool>>(iter: I) -> Self {
 let mut ret: Self = Default::default();
 ret.extend(iter);
 ret
 }
}

impl<B: BitBlock> Extend<bool> for BitVec {
 #[inline]
 fn extend<I: IntoIterator<Item = bool>>(&mut self, iterable: I) {
 self.ensure_invariant();
 let iterator = iterable.into_iter();
 let (min, _) = iterator.size_hint();
 self.reserve(min);
 for element in iterator {
 self.push(element)
 }
 }
}

impl<B: BitBlock> Clone for BitVec {
 #[inline]
 fn clone(&self) -> Self {
 self.ensure_invariant();
 BitVec {
 storage: self.storage.clone(),
 nbits: self.nbits,
 }
 }

 #[inline]
 fn clone_from(&mut self, source: &Self) {
 debug_assert!(source.is_last_block_fixed());
 self.nbits = source.nbits;
 self.storage.clone_from(&source.storage);
 }
}

impl<B: BitBlock> PartialOrd for BitVec {
 #[inline]
 fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
 Some(self.cmp(other))
 }
}

impl<B: BitBlock> Ord for BitVec {
 #[inline]
 fn cmp(&self, other: &Self) -> Ordering {
 self.ensure_invariant();
 debug_assert!(other.is_last_block_fixed());
 let mut a = self.iter();
 let mut b = other.iter();
 loop {
 match (a.next(), b.next()) {
 (Some(x), Some(y)) => match x.cmp(&y) {
 Ordering::Equal => {}
 otherwise => return otherwise,
 },
 (None, None) => return Ordering::Equal,
 (None, _) => return Ordering::Less,
 (_, None) => return Ordering::Greater,
 }
 }
 }
}

impl<B: BitBlock> fmt::Display for BitVec {
 fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
 self.ensure_invariant();
 for bit in self {
 fmt.write_char(if bit { '1' } else { '0' })?;
 }
 Ok(())
 }
}

impl<B: BitBlock> fmt::Debug for BitVec {
 fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
 self.ensure_invariant();
 let mut storage = String::with_capacity(self.len() + self.len() / B::bits());
 for (i, bit) in self.iter().enumerate() {
 if i != 0 && i % B::bits() == 0 {
 storage.push(' ');
 }
 storage.push(if bit { '1' } else { '0' });
 }
 fmt.debug_struct("BitVec")
 .field("storage", &storage)
 .field("nbits", &self.nbits)
 .finish()
 }
}

impl<B: BitBlock> hash::Hash for BitVec {
 #[inline]
 fn hash<H: hash::Hasher>(&self, state: &mut H) {
 self.ensure_invariant();
 self.nbits.hash(state);
 for elem in self.blocks() {
 elem.hash(state);
 }
 }
}

impl<B: BitBlock> cmp::PartialEq for BitVec {
 #[inline]
 fn eq(&self, other: &Self) -> bool {
 if self.nbits != other.nbits {
 self.ensure_invariant();
 other.ensure_invariant();
 return false;
 }
 self.blocks().zip(other.blocks()).all(|(w1, w2)| w1 == w2)
 }
}

impl<B: BitBlock> cmp::Eq for BitVec {}

/// An iterator for `BitVec`.
#[derive(Clone)]
pub struct Iter<'a, B: 'a = u32> {
 bit_vec: &'a BitVec,
 range: Range<usize>,
}

#[derive(Debug)]
pub struct MutBorrowedBit<'a, B: 'a + BitBlock> {
 vec: Rc<RefCell<&'a mut BitVec>>,
 index: usize,
 #[cfg(debug_assertions)]
 old_value: bool,
 new_value: bool,
}

/// An iterator for mutable references to the bits in a `BitVec`.
pub struct IterMut<'a, B: 'a + BitBlock = u32> {
 vec: Rc<RefCell<&'a mut BitVec>>,
 range: Range<usize>,
}

impl<'a, B: 'a + BitBlock> IterMut<'a, B> {
 fn get(&mut self, index: Option<usize>) -> Option<MutBorrowedBit<'a, B>> {
 let index = index?;
 let value = (*self.vec).borrow().get(index)?;
 Some(MutBorrowedBit {
 vec: self.vec.clone(),
 index,
 #[cfg(debug_assertions)]
 old_value: value,
 new_value: value,
 })
 }
}

impl<'a, B: BitBlock> Deref for MutBorrowedBit<'a, B> {
 type Target = bool;

 fn deref(&self) -> &Self::Target {
 &self.new_value
 }
}

impl<'a, B: BitBlock> DerefMut for MutBorrowedBit<'a, B> {
 fn deref_mut(&mut self) -> &mut Self::Target {
 &mut self.new_value
 }
}

impl<'a, B: BitBlock> Drop for MutBorrowedBit<'a, B> {
 fn drop(&mut self) {
 let mut vec = (*self.vec).borrow_mut();
 #[cfg(debug_assertions)]
 debug_assert_eq!(
 Some(self.old_value),
 vec.get(self.index),
 "Mutably-borrowed bit was modified externally!"
 );
 vec.set(self.index, self.new_value);
 }
}

impl<'a, B: BitBlock> Iterator for Iter<'a, B> {
 type Item = bool;

 #[inline]
 fn next(&mut self) -> Option<bool> {
 // NB: indexing is slow for extern crates when it has to go through &TRUE or &FALSE
 // variables. get is more direct, and unwrap is fine since we're sure of the range.
 self.range.next().map(|i| self.bit_vec.get(i).unwrap())
 }

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

impl<'a, B: BitBlock> Iterator for IterMut<'a, B> {
 type Item = MutBorrowedBit<'a, B>;

 #[inline]
 fn next(&mut self) -> Option<Self::Item> {
 let index = self.range.next();
 self.get(index)
 }

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

impl<'a, B: BitBlock> DoubleEndedIterator for Iter<'a, B> {
 #[inline]
 fn next_back(&mut self) -> Option<bool> {
 self.range.next_back().map(|i| self.bit_vec.get(i).unwrap())
 }
}

impl<'a, B: BitBlock> DoubleEndedIterator for IterMut<'a, B> {
 #[inline]
 fn next_back(&mut self) -> Option<Self::Item> {
 let index = self.range.next_back();
 self.get(index)
 }
}

impl<'a, B: BitBlock> ExactSizeIterator for Iter<'a, B> {}

impl<'a, B: BitBlock> ExactSizeIterator for IterMut<'a, B> {}

impl<'a, B: BitBlock> IntoIterator for &'a BitVec {
 type Item = bool;
 type IntoIter = Iter<'a, B>;

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

pub struct IntoIter {
 bit_vec: BitVec,
 range: Range<usize>,
}

impl<B: BitBlock> Iterator for IntoIter {
 type Item = bool;

 #[inline]
 fn next(&mut self) -> Option<bool> {
 self.range.next().map(|i| self.bit_vec.get(i).unwrap())
 }
}

impl<B: BitBlock> DoubleEndedIterator for IntoIter {
 #[inline]
 fn next_back(&mut self) -> Option<bool> {
 self.range.next_back().map(|i| self.bit_vec.get(i).unwrap())
 }
}

impl<B: BitBlock> ExactSizeIterator for IntoIter {}

impl<B: BitBlock> IntoIterator for BitVec {
 type Item = bool;
 type IntoIter = IntoIter;

 #[inline]
 fn into_iter(self) -> IntoIter {
 let nbits = self.nbits;
 IntoIter {
 bit_vec: self,
 range: 0..nbits,
 }
 }
}

/// An iterator over the blocks of a `BitVec`.
#[derive(Clone)]
pub struct Blocks<'a, B: 'a> {
 iter: slice::Iter<'a, B>,
}

impl<'a, B: BitBlock> Iterator for Blocks<'a, B> {
 type Item = B;

 #[inline]
 fn next(&mut self) -> Option {
 self.iter.next().cloned()
 }

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

impl<'a, B: BitBlock> DoubleEndedIterator for Blocks<'a, B> {
 #[inline]
 fn next_back(&mut self) -> Option {
 self.iter.next_back().cloned()
 }
}

impl<'a, B: BitBlock> ExactSizeIterator for Blocks<'a, B> {}

#[cfg(test)]
mod tests {
 use super::{BitVec, Iter, Vec};

 // This is stupid, but I want to differentiate from a "random" 32
 const U32_BITS: usize = 32;

 #[test]
 fn test_display_output() {
 assert_eq!(format!("{}", BitVec::new()), "");
 assert_eq!(format!("{}", BitVec::from_elem(1, true)), "1");
 assert_eq!(format!("{}", BitVec::from_elem(8, false)), "00000000")
 }

 #[test]
 fn test_debug_output() {
 assert_eq!(
 format!("{:?}", BitVec::new()),
 "BitVec { storage: \"\", nbits: 0 }"
 );
 assert_eq!(
 format!("{:?}", BitVec::from_elem(1, true)),
 "BitVec { storage: \"1\", nbits: 1 }"
 );
 assert_eq!(
 format!("{:?}", BitVec::from_elem(8, false)),
 "BitVec { storage: \"00000000\", nbits: 8 }"
 );
 assert_eq!(
 format!("{:?}", BitVec::from_elem(33, true)),
 "BitVec { storage: \"11111111111111111111111111111111 1\", nbits: 33 }"
 );
 assert_eq!(
 format!(
 "{:?}",
 BitVec::from_bytes(&[0b111, 0b000, 0b1110, 0b0001, 0b11111111, 0b00000000])
 ),
 "BitVec { storage: \"00000111000000000000111000000001 1111111100000000\", nbits: 48 }"
 )
 }

 #[test]
 fn test_0_elements() {
 let act = BitVec::new();
 let exp = Vec::new();
 assert!(act.eq_vec(&exp));
 assert!(act.none() && act.all());
 }

 #[test]
 fn test_1_element() {
 let mut act = BitVec::from_elem(1, false);
 assert!(act.eq_vec(&[false]));
 assert!(act.none() && !act.all());
 act = BitVec::from_elem(1, true);
 assert!(act.eq_vec(&[true]));
 assert!(!act.none() && act.all());
 }

 #[test]
 fn test_2_elements() {
 let mut b = BitVec::from_elem(2, false);
 b.set(0, true);
 b.set(1, false);
 assert_eq!(format!("{}", b), "10");
 assert!(!b.none() && !b.all());
 }

 #[test]
 fn test_10_elements() {
 // all 0

 let mut act = BitVec::from_elem(10, false);
 assert!(
 (act.eq_vec(&[false, false, false, false, false, false, false, false, false, false]))
 );
 assert!(act.none() && !act.all());
 // all 1

 act = BitVec::from_elem(10, true);
 assert!((act.eq_vec(&[true, true, true, true, true, true, true, true, true, true])));
 assert!(!act.none() && act.all());
 // mixed

 act = BitVec::from_elem(10, false);
 act.set(0, true);
 act.set(1, true);
 act.set(2, true);
 act.set(3, true);
 act.set(4, true);
 assert!((act.eq_vec(&[true, true, true, true, true, false, false, false, false, false])));
 assert!(!act.none() && !act.all());
 // mixed

 act = BitVec::from_elem(10, false);
 act.set(5, true);
 act.set(6, true);
 act.set(7, true);
 act.set(8, true);
 act.set(9, true);
 assert!((act.eq_vec(&[false, false, false, false, false, true, true, true, true, true])));
 assert!(!act.none() && !act.all());
 // mixed

 act = BitVec::from_elem(10, false);
 act.set(0, true);
 act.set(3, true);
 act.set(6, true);
 act.set(9, true);
 assert!((act.eq_vec(&[true, false, false, true, false, false, true, false, false, true])));
 assert!(!act.none() && !act.all());
 }

 #[test]
 fn test_31_elements() {
 // all 0

 let mut act = BitVec::from_elem(31, false);
 assert!(act.eq_vec(&[
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, false, false, false
 ]));
 assert!(act.none() && !act.all());
 // all 1

 act = BitVec::from_elem(31, true);
 assert!(act.eq_vec(&[
 true, true, true, true, true, true, true, true, true, true, true, true, true, true,
 true, true, true, true, true, true, true, true, true, true, true, true, true, true,
 true, true, true
 ]));
 assert!(!act.none() && act.all());
 // mixed

 act = BitVec::from_elem(31, false);
 act.set(0, true);
 act.set(1, true);
 act.set(2, true);
 act.set(3, true);
 act.set(4, true);
 act.set(5, true);
 act.set(6, true);
 act.set(7, true);
 assert!(act.eq_vec(&[
 true, true, true, true, true, true, true, true, false, false, false, false, false,
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, false, false
 ]));
 assert!(!act.none() && !act.all());
 // mixed

 act = BitVec::from_elem(31, false);
 act.set(16, true);
 act.set(17, true);
 act.set(18, true);
 act.set(19, true);
 act.set(20, true);
 act.set(21, true);
 act.set(22, true);
 act.set(23, true);
 assert!(act.eq_vec(&[
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, true, true, true, true, true, true, true, true, false,
 false, false, false, false, false, false
 ]));
 assert!(!act.none() && !act.all());
 // mixed

 act = BitVec::from_elem(31, false);
 act.set(24, true);
 act.set(25, true);
 act.set(26, true);
 act.set(27, true);
 act.set(28, true);
 act.set(29, true);
 act.set(30, true);
 assert!(act.eq_vec(&[
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, false, false, false, false, false, false, false, false,
 true, true, true, true, true, true, true
 ]));
 assert!(!act.none() && !act.all());
 // mixed

 act = BitVec::from_elem(31, false);
 act.set(3, true);
 act.set(17, true);
 act.set(30, true);
 assert!(act.eq_vec(&[
 false, false, false, true, false, false, false, false, false, false, false, false,
 false, false, false, false, false, true, false, false, false, false, false, false,
 false, false, false, false, false, false, true
 ]));
 assert!(!act.none() && !act.all());
 }

 #[test]
 fn test_32_elements() {
 // all 0

 let mut act = BitVec::from_elem(32, false);
 assert!(act.eq_vec(&[
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, false, false, false, false
 ]));
 assert!(act.none() && !act.all());
 // all 1

 act = BitVec::from_elem(32, true);
 assert!(act.eq_vec(&[
 true, true, true, true, true, true, true, true, true, true, true, true, true, true,
 true, true, true, true, true, true, true, true, true, true, true, true, true, true,
 true, true, true, true
 ]));
 assert!(!act.none() && act.all());
 // mixed

 act = BitVec::from_elem(32, false);
 act.set(0, true);
 act.set(1, true);
 act.set(2, true);
 act.set(3, true);
 act.set(4, true);
 act.set(5, true);
 act.set(6, true);
 act.set(7, true);
 assert!(act.eq_vec(&[
 true, true, true, true, true, true, true, true, false, false, false, false, false,
 false, false, false, false, false, false, false, false, false, false, false, false,
 false, false, false, false, false, false, false
 ]));
 assert!(!act.none() && !act.all());
 // mixed

 act = BitVec::from_elem(32, false);
 act.set(16, true);
 act.set(17, true);
 act.set(18, true);
 act.set(19, true);
 act.set(20, true);
 act.set(21, true);
 act.set(22, true);
 act.set(23, true);
 assert!(act.eq_vec(&[
--> --------------------

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