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use polars_utils::slice::load_padded_le_u64;

use super::get_bit_unchecked;
use crate::trusted_len::TrustedLen;

/// An iterator over bits according to the [LSB](https://en.wikipedia.org/wiki/Bit_numbering#Least_significant_bit),
/// i.e. the bytes `[4u8, 128u8]` correspond to `[false, false, true, false, ..., true]`.
#[derive(Debug, Clone)]
pub struct BitmapIter<'a> {
    bytes: &'a [u8],
    word: u64,
    word_len: usize,
    rest_len: usize,
}

impl<'a> BitmapIter<'a> {
    /// Creates a new [`BitmapIter`].
    pub fn new(bytes: &'a [u8], offset: usize, len: usize) -> Self {
        if len == 0 {
            return Self {
                bytes,
                word: 0,
                word_len: 0,
                rest_len: 0,
            };
        }

        assert!(bytes.len() * 8 >= offset + len);
        let first_byte_idx = offset / 8;
        let bytes = &bytes[first_byte_idx..];
        let offset = offset % 8;

        // Make sure during our hot loop all our loads are full 8-byte loads
        // by loading the remainder now if it exists.
        let word = load_padded_le_u64(bytes) >> offset;
        let mod8 = bytes.len() % 8;
        let first_word_bytes = if mod8 > 0 { mod8 } else { 8 };
        let bytes = &bytes[first_word_bytes..];

        let word_len = (first_word_bytes * 8 - offset).min(len);
        let rest_len = len - word_len;
        Self {
            bytes,
            word,
            word_len,
            rest_len,
        }
    }
}

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

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        if self.word_len != 0 {
            let ret = self.word & 1 != 0;
            self.word >>= 1;
            self.word_len -= 1;
            return Some(ret);
        }

        if self.rest_len != 0 {
            self.word_len = self.rest_len.min(64);
            self.rest_len -= self.word_len;
            unsafe {
                let chunk = self.bytes.get_unchecked(..8).try_into().unwrap();
                self.word = u64::from_le_bytes(chunk);
                self.bytes = self.bytes.get_unchecked(8..);
            }

            let ret = self.word & 1 != 0;
            self.word >>= 1;
            self.word_len -= 1;
            return Some(ret);
        }

        None
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let exact = self.word_len + self.rest_len;
        (exact, Some(exact))
    }
}

impl<'a> DoubleEndedIterator for BitmapIter<'a> {
    #[inline]
    fn next_back(&mut self) -> Option<bool> {
        if self.rest_len > 0 {
            self.rest_len -= 1;
            Some(unsafe { get_bit_unchecked(self.bytes, self.rest_len) })
        } else if self.word_len > 0 {
            self.word_len -= 1;
            Some(self.word & (1 << self.word_len) != 0)
        } else {
            None
        }
    }
}

unsafe impl TrustedLen for BitmapIter<'_> {}
impl ExactSizeIterator for BitmapIter<'_> {}