mod.rs

mod chunks_exact;
mod merge;

pub use chunks_exact::BitChunksExact;
pub(crate) use merge::merge_reversed;

use crate::trusted_len::TrustedLen;
pub use crate::types::BitChunk;
use crate::types::BitChunkIter;

/// Trait representing an exact iterator over bytes in [`BitChunk`].
pub trait BitChunkIterExact<B: BitChunk>: TrustedLen<Item = B> {
    /// The remainder of the iterator.
    fn remainder(&self) -> B;

    /// The number of items in the remainder
    fn remainder_len(&self) -> usize;

    /// An iterator over individual items of the remainder
    #[inline]
    fn remainder_iter(&self) -> BitChunkIter<B> {
        BitChunkIter::new(self.remainder(), self.remainder_len())
    }
}

/// This struct is used to efficiently iterate over bit masks by loading bytes on
/// the stack with alignments of `uX`. This allows efficient iteration over bitmaps.
#[derive(Debug)]
pub struct BitChunks<'a, T: BitChunk> {
    chunk_iterator: std::slice::ChunksExact<'a, u8>,
    current: T,
    remainder_bytes: &'a [u8],
    last_chunk: T,
    remaining: usize,
    /// offset inside a byte
    bit_offset: usize,
    len: usize,
    phantom: std::marker::PhantomData<T>,
}

/// writes `bytes` into `dst`.
#[inline]
fn copy_with_merge<T: BitChunk>(dst: &mut T::Bytes, bytes: &[u8], bit_offset: usize) {
    bytes
        .windows(2)
        .chain(std::iter::once([bytes[bytes.len() - 1], 0].as_ref()))
        .take(size_of::<T>())
        .enumerate()
        .for_each(|(i, w)| {
            let val = merge_reversed(w[0], w[1], bit_offset);
            dst[i] = val;
        });
}

impl<'a, T: BitChunk> BitChunks<'a, T> {
    /// Creates a [`BitChunks`].
    pub fn new(slice: &'a [u8], offset: usize, len: usize) -> Self {
        assert!(offset + len <= slice.len() * 8);

        let slice = &slice[offset / 8..];
        let bit_offset = offset % 8;
        let size_of = size_of::<T>();

        let bytes_len = len / 8;
        let bytes_upper_len = (len + bit_offset).div_ceil(8);
        let mut chunks = slice[..bytes_len].chunks_exact(size_of);

        let remainder = &slice[bytes_len - chunks.remainder().len()..bytes_upper_len];

        let remainder_bytes = if chunks.len() == 0 { slice } else { remainder };

        let last_chunk = remainder_bytes
            .first()
            .map(|first| {
                let mut last = T::zero().to_ne_bytes();
                last[0] = *first;
                T::from_ne_bytes(last)
            })
            .unwrap_or_else(T::zero);

        let remaining = chunks.size_hint().0;

        let current = chunks
            .next()
            .map(|x| match x.try_into() {
                Ok(a) => T::from_ne_bytes(a),
                Err(_) => unreachable!(),
            })
            .unwrap_or_else(T::zero);

        Self {
            chunk_iterator: chunks,
            len,
            current,
            remaining,
            remainder_bytes,
            last_chunk,
            bit_offset,
            phantom: std::marker::PhantomData,
        }
    }

    #[inline]
    fn load_next(&mut self) {
        self.current = match self.chunk_iterator.next().unwrap().try_into() {
            Ok(a) => T::from_ne_bytes(a),
            Err(_) => unreachable!(),
        };
    }

    /// Returns the remainder [`BitChunk`].
    pub fn remainder(&self) -> T {
        // remaining bytes may not fit in `size_of::<T>()`. We complement
        // them to fit by allocating T and writing to it byte by byte
        let mut remainder = T::zero().to_ne_bytes();

        let remainder = match (self.remainder_bytes.is_empty(), self.bit_offset == 0) {
            (true, _) => remainder,
            (false, true) => {
                // all remaining bytes
                self.remainder_bytes
                    .iter()
                    .take(size_of::<T>())
                    .enumerate()
                    .for_each(|(i, val)| remainder[i] = *val);

                remainder
            },
            (false, false) => {
                // all remaining bytes
                copy_with_merge::<T>(&mut remainder, self.remainder_bytes, self.bit_offset);
                remainder
            },
        };
        let mut remainder = T::from_ne_bytes(remainder);
        let mask = (T::one() << self.remainder_len()) - T::one();
        remainder &= mask;
        remainder
    }

    /// Returns the remainder bits in [`BitChunks::remainder`].
    pub fn remainder_len(&self) -> usize {
        self.len - (size_of::<T>() * ((self.len / 8) / size_of::<T>()) * 8)
    }
}

impl<T: BitChunk> Iterator for BitChunks<'_, T> {
    type Item = T;

    #[inline]
    fn next(&mut self) -> Option<T> {
        if self.remaining == 0 {
            return None;
        }

        let current = self.current;
        let combined = if self.bit_offset == 0 {
            // fast case where there is no offset. In this case, there is bit-alignment
            // at byte boundary and thus the bytes correspond exactly.
            if self.remaining >= 2 {
                self.load_next();
            }
            current
        } else {
            let next = if self.remaining >= 2 {
                // case where `next` is complete and thus we can take it all
                self.load_next();
                self.current
            } else {
                // case where the `next` is incomplete and thus we take the remaining
                self.last_chunk
            };
            merge_reversed(current, next, self.bit_offset)
        };

        self.remaining -= 1;
        Some(combined)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        // it contains always one more than the chunk_iterator, which is the last
        // one where the remainder is merged into current.
        (self.remaining, Some(self.remaining))
    }
}

impl<T: BitChunk> BitChunkIterExact<T> for BitChunks<'_, T> {
    #[inline]
    fn remainder(&self) -> T {
        self.remainder()
    }

    #[inline]
    fn remainder_len(&self) -> usize {
        self.remainder_len()
    }
}

impl<T: BitChunk> ExactSizeIterator for BitChunks<'_, T> {
    #[inline]
    fn len(&self) -> usize {
        self.chunk_iterator.len()
    }
}

unsafe impl<T: BitChunk> TrustedLen for BitChunks<'_, T> {}