/// Iterates over a slice of `u64` in *big-endian* order, from arkworks.
#[derive(Debug)]
pub struct BitIteratorBE<Slice: AsRef<[u64]>> {
    s: Slice,
    n: usize,
}

impl<Slice: AsRef<[u64]>> BitIteratorBE<Slice> {
    pub fn new(s: Slice) -> Self {
        let n = s.as_ref().len() * 64;
        BitIteratorBE { s, n }
    }

    /// Construct an iterator that automatically skips any leading zeros.
    /// That is, it skips all zeros before the most-significant one.
    pub fn without_leading_zeros(s: Slice) -> impl Iterator<Item = bool> {
        Self::new(s).skip_while(|b| !b)
    }
}

impl<Slice: AsRef<[u64]>> Iterator for BitIteratorBE<Slice> {
    type Item = bool;

    fn next(&mut self) -> Option<bool> {
        if self.n == 0 {
            None
        } else {
            self.n -= 1;
            let part = self.n / 64;
            let bit = self.n - (64 * part);
            Some(self.s.as_ref()[part] & (1 << bit) > 0)
        }
    }
}

/// Iterates over a slice of `u64` in *little-endian* order, from arkworks.
#[derive(Debug)]
pub struct BitIteratorLE<Slice: AsRef<[u64]>> {
    s: Slice,
    n: usize,
    max_len: usize,
}

impl<Slice: AsRef<[u64]>> BitIteratorLE<Slice> {
    pub fn new(s: Slice) -> Self {
        let n = 0;
        let max_len = s.as_ref().len() * 64;
        BitIteratorLE { s, n, max_len }
    }

    /// Construct an iterator that automatically skips any trailing zeros.
    /// That is, it skips all zeros after the most-significant one.
    pub fn without_trailing_zeros(s: Slice) -> impl Iterator<Item = bool> {
        let mut first_trailing_zero = 0;
        for (i, limb) in s.as_ref().iter().enumerate().rev() {
            first_trailing_zero = i * 64 + (64 - limb.leading_zeros()) as usize;
            if *limb != 0 {
                break;
            }
        }
        let mut iter = Self::new(s);
        iter.max_len = first_trailing_zero;
        iter
    }
}

impl<Slice: AsRef<[u64]>> Iterator for BitIteratorLE<Slice> {
    type Item = bool;

    fn next(&mut self) -> Option<bool> {
        if self.n == self.max_len {
            None
        } else {
            let part = self.n / 64;
            let bit = self.n - (64 * part);
            self.n += 1;

            Some(self.s.as_ref()[part] & (1 << bit) > 0)
        }
    }
}
