bit-string 0.1.0

A compact owned bit string type with editing, matching, and bitwise operations.
Documentation 
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use alloc::vec::Vec;
use int_interval::UsizeCO;

use super::*;

#[inline]
pub(crate) fn last_word_mask(len: usize) -> u64 {
    let rem = len % WORD_BITS;
    if rem == 0 {
        u64::MAX
    } else {
        (1u64 << rem) - 1
    }
}

#[inline]
pub(crate) fn mask_unused_bits(bits: &mut [u64], len: usize) {
    if let Some(last) = bits.last_mut() {
        *last &= last_word_mask(len);
    }
}

#[inline]
pub(crate) fn word_len(bit_len: usize) -> usize {
    bit_len / WORD_BITS + usize::from(bit_len % WORD_BITS != 0)
}

#[inline]
pub(crate) fn zero_words(words: usize) -> Box<[u64]> {
    let mut bits = Vec::with_capacity(words);
    bits.resize(words, 0);
    bits.into_boxed_slice()
}

#[inline]
pub(crate) fn shrink_words(bits: &[u64], words: usize) -> Box<[u64]> {
    let mut out = Vec::with_capacity(words);
    out.extend_from_slice(&bits[..words]);
    out.into_boxed_slice()
}

#[inline]
pub(crate) fn bit_at(bits: &[u64], index: usize) -> bool {
    bits[index / WORD_BITS] & (1u64 << (index % WORD_BITS)) != 0
}

#[inline]
pub(crate) fn set_bit(bits: &mut [u64], index: usize, value: bool) {
    let word = index / WORD_BITS;
    let mask = 1u64 << (index % WORD_BITS);

    if value {
        bits[word] |= mask;
    } else {
        bits[word] &= !mask;
    }
}

#[inline]
pub(crate) fn low_mask(bits: usize) -> u64 {
    if bits == WORD_BITS {
        u64::MAX
    } else {
        (1u64 << bits) - 1
    }
}

#[inline]
pub(crate) fn read_chunk(src: &[u64], bit_start: usize) -> u64 {
    let word = bit_start / WORD_BITS;
    let shift = bit_start % WORD_BITS;

    let lo = src.get(word).copied().unwrap_or(0) >> shift;

    if shift == 0 {
        lo
    } else {
        let hi = src.get(word + 1).copied().unwrap_or(0);
        lo | (hi << (WORD_BITS - shift))
    }
}

#[inline]
pub(crate) fn write_chunk(dst: &mut [u64], bit_start: usize, value: u64, len: usize) {
    let value = value & low_mask(len);
    let word = bit_start / WORD_BITS;
    let shift = bit_start % WORD_BITS;

    dst[word] |= value << shift;

    if shift != 0 && word + 1 < dst.len() {
        dst[word + 1] |= value >> (WORD_BITS - shift);
    }
}

#[inline]
pub(crate) fn copy_bits(
    src: &[u64],
    src_start: usize,
    dst: &mut [u64],
    dst_start: usize,
    len: usize,
) {
    let mut done = 0usize;

    while done < len {
        let take = (len - done).min(WORD_BITS);
        let chunk = read_chunk(src, src_start + done);
        write_chunk(dst, dst_start + done, chunk, take);
        done += take;
    }
}

#[inline]
pub(crate) fn assert_interval_in_bounds(interval: UsizeCO, len: usize) {
    assert!(
        interval.end_excl() <= len,
        "bit string interval out of bounds: {}..{}, len={}",
        interval.start(),
        interval.end_excl(),
        len
    );
}

#[cfg(test)]
mod tests_for_assert_interval_in_bounds;
#[cfg(test)]
mod tests_for_bit_at;
#[cfg(test)]
mod tests_for_copy_bits;
#[cfg(test)]
mod tests_for_last_word_mask;
#[cfg(test)]
mod tests_for_low_mask;
#[cfg(test)]
mod tests_for_mask_unused_bits;
#[cfg(test)]
mod tests_for_read_chunk;
#[cfg(test)]
mod tests_for_set_bit;
#[cfg(test)]
mod tests_for_shrink_words;
#[cfg(test)]
mod tests_for_word_len;
#[cfg(test)]
mod tests_for_write_chunk;
#[cfg(test)]
mod tests_for_zero_words;