vortex_buffer/bit/

buf_mut.rs

// SPDX-License-Identifier: Apache-2.0
// SPDX-FileCopyrightText: Copyright the Vortex contributors

use bitvec::view::BitView;

use crate::bit::{get_bit_unchecked, set_bit_unchecked, unset_bit_unchecked};
use crate::{BitBuffer, BufferMut, ByteBufferMut, buffer_mut};

/// A mutable bitset buffer that allows random access to individual bits for set and get.
///
///
/// # Example
/// ```
/// use vortex_buffer::BitBufferMut;
///
/// let mut bools = BitBufferMut::new_unset(10);
/// bools.set_to(9, true);
/// for i in 0..9 {
///    assert!(!bools.value(i));
/// }
/// assert!(bools.value(9));
///
/// // Freeze into a new bools vector.
/// let bools = bools.freeze();
/// ```
///
/// See also: [`BitBuffer`].
pub struct BitBufferMut {
    buffer: ByteBufferMut,
    offset: usize,
    len: usize,
}

impl BitBufferMut {
    /// Create new bit buffer from given byte buffer and logical bit length
    pub fn from_buffer(buffer: ByteBufferMut, offset: usize, len: usize) -> Self {
        assert!(
            len <= buffer.len() * 8,
            "Buffer len {} is too short for the given length {len}",
            buffer.len()
        );
        Self {
            buffer,
            offset,
            len,
        }
    }

    /// Create a new empty mutable bit buffer with requested capacity (in bits).
    pub fn with_capacity(capacity: usize) -> Self {
        Self {
            buffer: BufferMut::with_capacity(capacity.div_ceil(8)),
            offset: 0,
            len: 0,
        }
    }

    /// Create a new mutable buffer with requested `len` and all bits set to `true`.
    pub fn new_set(len: usize) -> Self {
        Self {
            buffer: buffer_mut![0xFF; len.div_ceil(8)],
            offset: 0,
            len,
        }
    }

    /// Create a new mutable buffer with requested `len` and all bits set to `false`.
    pub fn new_unset(len: usize) -> Self {
        Self {
            buffer: BufferMut::zeroed(len.div_ceil(8)),
            offset: 0,
            len,
        }
    }

    /// Create a new empty `BitBufferMut`.
    #[inline(always)]
    pub fn empty() -> Self {
        Self::with_capacity(0)
    }

    /// Create a new mutable buffer with requested `len` and all bits set to `value`.
    pub fn full(value: bool, len: usize) -> Self {
        if value {
            Self::new_set(len)
        } else {
            Self::new_unset(len)
        }
    }

    /// Get the current populated length of the buffer.
    #[inline(always)]
    pub fn len(&self) -> usize {
        self.len
    }

    /// True if the buffer has length 0.
    #[inline(always)]
    pub fn is_empty(&self) -> bool {
        self.len == 0
    }

    /// Get the value at the requested index.
    #[inline(always)]
    pub fn value(&self, index: usize) -> bool {
        assert!(index < self.len);
        // SAFETY: checked by assertion
        unsafe { self.value_unchecked(index) }
    }

    /// Get the value at the requested index without bounds checking.
    ///
    /// # Safety
    ///
    /// The caller must ensure that `index` is less than the length of the buffer.
    #[inline(always)]
    pub unsafe fn value_unchecked(&self, index: usize) -> bool {
        unsafe { get_bit_unchecked(self.buffer.as_ptr(), self.offset + index) }
    }

    /// Get the bit capacity of the buffer.
    #[inline(always)]
    pub fn capacity(&self) -> usize {
        (self.buffer.capacity() * 8) - self.offset
    }

    /// Reserve additional bit capacity for the buffer.
    pub fn reserve(&mut self, additional: usize) {
        let required_capacity = (self.offset + self.len + additional).div_ceil(8);
        let buffer_capacity = self.buffer.capacity();
        if required_capacity > self.buffer.capacity() {
            let additional = required_capacity - buffer_capacity;
            self.buffer.reserve(additional);
        }
    }

    /// Set the bit at `index` to the given boolean value.
    ///
    /// This operation is checked so if `index` exceeds the buffer length, this will panic.
    pub fn set_to(&mut self, index: usize, value: bool) {
        if value {
            self.set(index);
        } else {
            self.unset(index);
        }
    }

    /// Set the bit at `index` to the given boolean value without checking bounds.
    ///
    /// # Safety
    ///
    /// The caller must ensure that `index` does not exceed the largest bit index in the backing buffer.
    pub unsafe fn set_to_unchecked(&mut self, index: usize, value: bool) {
        if value {
            // SAFETY: checked by caller
            unsafe { self.set_unchecked(index) }
        } else {
            // SAFETY: checked by caller
            unsafe { self.unset_unchecked(index) }
        }
    }

    /// Set a position to `true`.
    ///
    /// This operation is checked so if `index` exceeds the buffer length, this will panic.
    pub fn set(&mut self, index: usize) {
        assert!(index < self.len, "index {index} exceeds len {}", self.len);

        // SAFETY: checked by assertion
        unsafe { self.set_unchecked(index) };
    }

    /// Set a position to `false`.
    ///
    /// This operation is checked so if `index` exceeds the buffer length, this will panic.
    pub fn unset(&mut self, index: usize) {
        assert!(index < self.len, "index {index} exceeds len {}", self.len);

        // SAFETY: checked by assertion
        unsafe { self.unset_unchecked(index) };
    }

    /// Set the bit at `index` to `true` without checking bounds.
    ///
    /// # Safety
    ///
    /// The caller must ensure that `index` does not exceed the largest bit index in the backing buffer.
    pub unsafe fn set_unchecked(&mut self, index: usize) {
        // SAFETY: checked by caller
        unsafe { set_bit_unchecked(self.buffer.as_mut_ptr(), self.offset + index) }
    }

    /// Unset the bit at `index` without checking bounds.
    ///
    /// # Safety
    ///
    /// The caller must ensure that `index` does not exceed the largest bit index in the backing buffer.
    pub unsafe fn unset_unchecked(&mut self, index: usize) {
        // SAFETY: checked by caller
        unsafe { unset_bit_unchecked(self.buffer.as_mut_ptr(), self.offset + index) }
    }

    /// Truncate the buffer to the given length.
    pub fn truncate(&mut self, len: usize) {
        if len > self.len {
            return;
        }

        let new_len_bytes = (self.offset + len).div_ceil(8);
        self.buffer.truncate(new_len_bytes);
        self.len = len;
    }

    /// Append a new boolean into the bit buffer, incrementing the length.
    pub fn append(&mut self, value: bool) {
        if value {
            self.append_true()
        } else {
            self.append_false()
        }
    }

    /// Append a new true value to the buffer.
    pub fn append_true(&mut self) {
        let bit_pos = self.offset + self.len;
        let byte_pos = bit_pos / 8;
        let bit_in_byte = bit_pos % 8;

        // Ensure buffer has enough bytes
        if byte_pos >= self.buffer.len() {
            self.buffer.push(0u8);
        }

        // Set the bit
        self.buffer.as_mut_slice()[byte_pos] |= 1 << bit_in_byte;
        self.len += 1;
    }

    /// Append a new false value to the buffer.
    pub fn append_false(&mut self) {
        let bit_pos = self.offset + self.len;
        let byte_pos = bit_pos / 8;
        let bit_in_byte = bit_pos % 8;

        // Ensure buffer has enough bytes
        if byte_pos >= self.buffer.len() {
            self.buffer.push(0u8);
        }

        // Bit is already 0 if we just pushed a new byte, otherwise ensure it's unset
        if bit_in_byte != 0 {
            self.buffer.as_mut_slice()[byte_pos] &= !(1 << bit_in_byte);
        }

        self.len += 1;
    }

    /// Append several boolean values into the bit buffer. After this operation,
    /// the length will be incremented by `n`.
    ///
    /// Panics if the buffer does not have `n` slots left.
    pub fn append_n(&mut self, value: bool, n: usize) {
        if n == 0 {
            return;
        }

        let start_bit_pos = self.offset + self.len;
        let end_bit_pos = start_bit_pos + n;
        let required_bytes = end_bit_pos.div_ceil(8);

        // Ensure buffer has enough bytes
        if required_bytes > self.buffer.len() {
            self.buffer.push_n(0x00, required_bytes - self.buffer.len());
        }

        let fill_byte = if value { 0xFF } else { 0x00 };

        // Calculate byte positions
        let start_byte = start_bit_pos / 8;
        let start_bit = start_bit_pos % 8;
        let end_byte = end_bit_pos / 8;
        let end_bit = end_bit_pos % 8;

        let slice = self.buffer.as_mut_slice();

        if start_byte == end_byte {
            // All bits are in the same byte
            let mask = ((1u8 << (end_bit - start_bit)) - 1) << start_bit;
            if value {
                slice[start_byte] |= mask;
            } else {
                slice[start_byte] &= !mask;
            }
        } else {
            // Fill the first partial byte
            if start_bit != 0 {
                let mask = !((1u8 << start_bit) - 1);
                if value {
                    slice[start_byte] |= mask;
                } else {
                    slice[start_byte] &= !mask;
                }
            }

            // Fill the complete middle bytes
            let fill_start = if start_bit != 0 {
                start_byte + 1
            } else {
                start_byte
            };
            let fill_end = end_byte;
            if fill_start < fill_end {
                slice[fill_start..fill_end].fill(fill_byte);
            }

            // Fill the last partial byte
            if end_bit != 0 {
                let mask = (1u8 << end_bit) - 1;
                if value {
                    slice[end_byte] |= mask;
                } else {
                    slice[end_byte] &= !mask;
                }
            }
        }

        self.len += n;
    }

    /// Append a [`BitBuffer`] to this [`BitBufferMut`]
    ///
    /// This efficiently copies all bits from the source buffer to the end of this buffer.
    pub fn append_buffer(&mut self, buffer: &BitBuffer) {
        let bit_len = buffer.len();
        if bit_len == 0 {
            return;
        }

        let start_bit_pos = self.offset + self.len;
        let end_bit_pos = start_bit_pos + bit_len;
        let required_bytes = end_bit_pos.div_ceil(8);

        // Ensure buffer has enough bytes
        if required_bytes > self.buffer.len() {
            self.buffer.push_n(0x00, required_bytes - self.buffer.len());
        }

        // Use bitvec for efficient bit copying
        let self_slice = self
            .buffer
            .as_mut_slice()
            .view_bits_mut::<bitvec::prelude::Lsb0>();
        let other_slice = buffer
            .inner()
            .as_slice()
            .view_bits::<bitvec::prelude::Lsb0>();

        // Copy from source buffer (accounting for its offset) to destination (accounting for our offset + len)
        let source_range = buffer.offset()..buffer.offset() + bit_len;
        self_slice[start_bit_pos..end_bit_pos].copy_from_bitslice(&other_slice[source_range]);

        self.len += bit_len;
    }

    /// Freeze the buffer in its current state into an immutable `BoolBuffer`.
    pub fn freeze(self) -> BitBuffer {
        BitBuffer::new_with_offset(self.buffer.freeze(), self.len, self.offset)
    }

    /// Get the underlying bytes as a slice
    pub fn as_slice(&self) -> &[u8] {
        self.buffer.as_slice()
    }

    /// Get the underlying bytes as a mutable slice
    pub fn as_mut_slice(&mut self) -> &mut [u8] {
        self.buffer.as_mut_slice()
    }
}

impl Default for BitBufferMut {
    fn default() -> Self {
        Self::with_capacity(0)
    }
}

impl From<&[bool]> for BitBufferMut {
    fn from(value: &[bool]) -> Self {
        let mut buf = BitBufferMut::new_unset(value.len());
        for (i, &v) in value.iter().enumerate() {
            if v {
                // SAFETY: i is in bounds
                unsafe { buf.set_unchecked(i) }
            }
        }
        buf
    }
}

impl From<Vec<bool>> for BitBufferMut {
    fn from(value: Vec<bool>) -> Self {
        value.as_slice().into()
    }
}

impl FromIterator<bool> for BitBufferMut {
    fn from_iter<T: IntoIterator<Item = bool>>(iter: T) -> Self {
        let iter = iter.into_iter();
        let (low, high) = iter.size_hint();
        if let Some(len) = high {
            let mut buf = BitBufferMut::new_unset(len);
            for (i, v) in iter.enumerate() {
                if v {
                    // SAFETY: i is in bounds
                    unsafe { buf.set_unchecked(i) }
                }
            }
            buf
        } else {
            let mut buf = BitBufferMut::with_capacity(low);
            for v in iter {
                buf.append(v);
            }
            buf
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::bit::buf_mut::BitBufferMut;
    use crate::{BufferMut, bitbuffer, bitbuffer_mut, buffer_mut};

    #[test]
    fn test_bits_mut() {
        let mut bools = bitbuffer_mut![false; 10];
        bools.set_to(0, true);
        bools.set_to(9, true);

        let bools = bools.freeze();
        assert!(bools.value(0));
        for i in 1..=8 {
            assert!(!bools.value(i));
        }
        assert!(bools.value(9));
    }

    #[test]
    fn test_append_n() {
        let mut bools = BitBufferMut::with_capacity(10);
        assert_eq!(bools.len(), 0);
        assert!(bools.is_empty());

        bools.append(true);
        bools.append_n(false, 8);
        bools.append_n(true, 1);

        let bools = bools.freeze();

        assert_eq!(bools.true_count(), 2);
        assert!(bools.value(0));
        assert!(bools.value(9));
    }

    #[test]
    fn test_with_offset_zero() {
        // Test basic operations when offset is 0
        let buf = BufferMut::zeroed(2);
        let mut bit_buf = BitBufferMut::from_buffer(buf, 0, 16);

        // Set some bits
        bit_buf.set(0);
        bit_buf.set(7);
        bit_buf.set(8);
        bit_buf.set(15);

        // Verify values
        assert!(bit_buf.value(0));
        assert!(bit_buf.value(7));
        assert!(bit_buf.value(8));
        assert!(bit_buf.value(15));
        assert!(!bit_buf.value(1));
        assert!(!bit_buf.value(9));

        // Verify underlying bytes
        assert_eq!(bit_buf.as_slice()[0], 0b10000001);
        assert_eq!(bit_buf.as_slice()[1], 0b10000001);
    }

    #[test]
    fn test_with_offset_within_byte() {
        // Test operations with offset=3 (within first byte)
        let buf = buffer_mut![0b11111111, 0b00000000, 0b00000000];
        let mut bit_buf = BitBufferMut::from_buffer(buf, 3, 10);

        // Initially, bits 3-7 from first byte are set (5 bits)
        // and bits 0-4 from second byte are unset (5 bits more)
        assert!(bit_buf.value(0)); // bit 3 of byte 0
        assert!(bit_buf.value(4)); // bit 7 of byte 0
        assert!(!bit_buf.value(5)); // bit 0 of byte 1

        // Set a bit in the second byte's range
        bit_buf.set(7);
        assert!(bit_buf.value(7));

        // Unset a bit in the first byte's range
        bit_buf.unset(0);
        assert!(!bit_buf.value(0));
    }

    #[test]
    fn test_with_offset_byte_boundary() {
        // Test operations with offset=8 (exactly one byte)
        let buf = buffer_mut![0xFF, 0x00, 0xFF];
        let mut bit_buf = BitBufferMut::from_buffer(buf, 8, 16);

        // Buffer starts at byte 1, so all bits should be unset initially
        for i in 0..8 {
            assert!(!bit_buf.value(i));
        }
        // Next byte has all bits set
        for i in 8..16 {
            assert!(bit_buf.value(i));
        }

        // Set some bits
        bit_buf.set(0);
        bit_buf.set(3);
        assert!(bit_buf.value(0));
        assert!(bit_buf.value(3));
    }

    #[test]
    fn test_with_large_offset() {
        // Test with offset=13 (one byte + 5 bits)
        let buf = buffer_mut![0xFF, 0xFF, 0xFF, 0xFF];
        let mut bit_buf = BitBufferMut::from_buffer(buf, 13, 10);

        // All bits should initially be set
        for i in 0..10 {
            assert!(bit_buf.value(i));
        }

        // Unset some bits
        bit_buf.unset(0);
        bit_buf.unset(5);
        bit_buf.unset(9);

        assert!(!bit_buf.value(0));
        assert!(bit_buf.value(1));
        assert!(!bit_buf.value(5));
        assert!(!bit_buf.value(9));
    }

    #[test]
    fn test_append_with_offset() {
        // Create buffer with offset
        let buf = buffer_mut![0b11100000]; // First 3 bits unset, last 5 set
        let mut bit_buf = BitBufferMut::from_buffer(buf, 3, 0); // Start at bit 3, len=0

        // Append some bits
        bit_buf.append(false); // Should use bit 3
        bit_buf.append(true); // Should use bit 4
        bit_buf.append(true); // Should use bit 5

        assert_eq!(bit_buf.len(), 3);
        assert!(!bit_buf.value(0));
        assert!(bit_buf.value(1));
        assert!(bit_buf.value(2));
    }

    #[test]
    fn test_append_n_with_offset_crossing_boundary() {
        // Create buffer with offset that will cross byte boundary when appending
        let buf = BufferMut::zeroed(4);
        let mut bit_buf = BitBufferMut::from_buffer(buf, 5, 0);

        // Append enough bits to cross into next byte
        bit_buf.append_n(true, 10); // 5 bits left in first byte, then 5 in second

        assert_eq!(bit_buf.len(), 10);
        for i in 0..10 {
            assert!(bit_buf.value(i));
        }

        // Verify the underlying bytes
        // Bits 5-7 of byte 0 should be set (3 bits)
        // Bits 0-6 of byte 1 should be set (7 bits)
        assert_eq!(bit_buf.as_slice()[0], 0b11100000);
        assert_eq!(bit_buf.as_slice()[1], 0b01111111);
    }

    #[test]
    fn test_truncate_with_offset() {
        let buf = buffer_mut![0xFF, 0xFF];
        let mut bit_buf = BitBufferMut::from_buffer(buf, 4, 12);

        assert_eq!(bit_buf.len(), 12);

        // Truncate to 8 bits
        bit_buf.truncate(8);
        assert_eq!(bit_buf.len(), 8);

        // Truncate to 3 bits
        bit_buf.truncate(3);
        assert_eq!(bit_buf.len(), 3);

        // Truncating to larger length should be no-op
        bit_buf.truncate(10);
        assert_eq!(bit_buf.len(), 3);
    }

    #[test]
    fn test_capacity_with_offset() {
        // Use exact buffer size to test capacity calculation
        let buf = buffer_mut![0, 0, 0, 0, 0, 0, 0, 0, 0, 0]; // Exactly 10 bytes = 80 bits
        let bit_buf = BitBufferMut::from_buffer(buf, 5, 0);

        // Capacity should be at least buffer length minus offset
        // (may be more due to allocator rounding)
        assert!(bit_buf.capacity() >= 75);
        // And should account for offset
        assert_eq!(bit_buf.capacity() % 8, (80 - 5) % 8);
    }

    #[test]
    fn test_reserve_with_offset() {
        // Use exact buffer to test reserve
        let buf = buffer_mut![0, 0]; // Exactly 2 bytes = 16 bits
        let mut bit_buf = BitBufferMut::from_buffer(buf, 3, 0);

        // Current capacity should be at least 13 bits (16 - 3)
        let initial_capacity = bit_buf.capacity();
        assert!(initial_capacity >= 13);

        // Reserve 20 more bits (need total of offset 3 + len 0 + additional 20 = 23 bits)
        bit_buf.reserve(20);

        // Should now have at least 20 bits of capacity
        assert!(bit_buf.capacity() >= 20);
    }

    #[test]
    fn test_freeze_with_offset() {
        let buf = buffer_mut![0b11110000, 0b00001111];
        let mut bit_buf = BitBufferMut::from_buffer(buf, 4, 8);

        // Set some bits
        bit_buf.set(0);
        bit_buf.set(7);

        // Freeze and verify offset is preserved
        let frozen = bit_buf.freeze();
        assert_eq!(frozen.offset(), 4);
        assert_eq!(frozen.len(), 8);

        // Verify values through frozen buffer
        assert!(frozen.value(0));
        assert!(frozen.value(7));
    }

    #[cfg_attr(miri, ignore)] // bitvec crate uses a ptr cast that Miri doesn't support
    #[test]
    fn test_append_buffer_with_offsets() {
        // Create source buffer with offset
        let source = bitbuffer![false, false, true, true, false, true];

        // Create destination buffer with offset
        let buf = BufferMut::zeroed(4);
        let mut dest = BitBufferMut::from_buffer(buf, 3, 0);

        // Append 2 initial bits
        dest.append(true);
        dest.append(false);

        // Append the source buffer
        dest.append_buffer(&source);

        assert_eq!(dest.len(), 8);
        assert!(dest.value(0)); // Our first append
        assert!(!dest.value(1)); // Our second append
        assert!(!dest.value(2)); // From source[0]
        assert!(!dest.value(3)); // From source[1]
        assert!(dest.value(4)); // From source[2]
        assert!(dest.value(5)); // From source[3]
        assert!(!dest.value(6)); // From source[4]
        assert!(dest.value(7)); // From source[5]
    }

    #[test]
    fn test_set_unset_unchecked_with_offset() {
        let buf = BufferMut::zeroed(3);
        let mut bit_buf = BitBufferMut::from_buffer(buf, 7, 10);

        unsafe {
            bit_buf.set_unchecked(0);
            bit_buf.set_unchecked(5);
            bit_buf.set_unchecked(9);
        }

        assert!(bit_buf.value(0));
        assert!(bit_buf.value(5));
        assert!(bit_buf.value(9));

        unsafe {
            bit_buf.unset_unchecked(5);
        }

        assert!(!bit_buf.value(5));
    }

    #[test]
    fn test_value_unchecked_with_offset() {
        let buf = buffer_mut![0b11110000, 0b00001111];
        let bit_buf = BitBufferMut::from_buffer(buf, 4, 8);

        unsafe {
            // First 4 bits of logical buffer come from bits 4-7 of first byte (all 1s)
            assert!(bit_buf.value_unchecked(0));
            assert!(bit_buf.value_unchecked(3));

            // Next 4 bits come from bits 0-3 of second byte (all 1s)
            assert!(bit_buf.value_unchecked(4));
            assert!(bit_buf.value_unchecked(7));
        }
    }

    #[test]
    fn test_append_alternating_with_offset() {
        let buf = BufferMut::zeroed(4);
        let mut bit_buf = BitBufferMut::from_buffer(buf, 2, 0);

        // Append alternating pattern across byte boundaries
        for i in 0..20 {
            bit_buf.append(i % 2 == 0);
        }

        assert_eq!(bit_buf.len(), 20);
        for i in 0..20 {
            assert_eq!(bit_buf.value(i), i % 2 == 0);
        }
    }

    #[test]
    fn test_new_set_new_unset() {
        let set_buf = bitbuffer_mut![true; 10];
        let unset_buf = bitbuffer_mut![false; 10];

        for i in 0..10 {
            assert!(set_buf.value(i));
            assert!(!unset_buf.value(i));
        }

        assert_eq!(set_buf.len(), 10);
        assert_eq!(unset_buf.len(), 10);
    }

    #[test]
    fn test_append_n_false_with_offset() {
        let buf = BufferMut::zeroed(4);
        let mut bit_buf = BitBufferMut::from_buffer(buf, 5, 0);

        bit_buf.append_n(false, 15);

        assert_eq!(bit_buf.len(), 15);
        for i in 0..15 {
            assert!(!bit_buf.value(i));
        }
    }

    #[test]
    fn test_append_n_true_with_offset() {
        let buf = BufferMut::zeroed(4);
        let mut bit_buf = BitBufferMut::from_buffer(buf, 5, 0);

        bit_buf.append_n(true, 15);

        assert_eq!(bit_buf.len(), 15);
        for i in 0..15 {
            assert!(bit_buf.value(i));
        }
    }

    #[test]
    fn test_mixed_operations_with_offset() {
        // Complex test combining multiple operations with offset
        let buf = BufferMut::zeroed(5);
        let mut bit_buf = BitBufferMut::from_buffer(buf, 3, 0);

        // Append some bits
        bit_buf.append_n(true, 5);
        bit_buf.append_n(false, 3);
        bit_buf.append(true);

        assert_eq!(bit_buf.len(), 9);

        // Set and unset
        bit_buf.set(6); // Was false, now true
        bit_buf.unset(2); // Was true, now false

        // Verify
        assert!(bit_buf.value(0));
        assert!(bit_buf.value(1));
        assert!(!bit_buf.value(2)); // Unset
        assert!(bit_buf.value(3));
        assert!(bit_buf.value(4));
        assert!(!bit_buf.value(5));
        assert!(bit_buf.value(6)); // Set
        assert!(!bit_buf.value(7));
        assert!(bit_buf.value(8));

        // Truncate
        bit_buf.truncate(6);
        assert_eq!(bit_buf.len(), 6);

        // Freeze and verify offset preserved
        let frozen = bit_buf.freeze();
        assert_eq!(frozen.offset(), 3);
        assert_eq!(frozen.len(), 6);
    }
}