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// Copyright (C) 2019-2021 Aleo Systems Inc.
// This file is part of the snarkVM library.

// The snarkVM library is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// The snarkVM library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with the snarkVM library. If not, see <https://www.gnu.org/licenses/>.

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

#[allow(clippy::len_without_is_empty)]
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 new_without_leading_zeros(s: Slice) -> impl Iterator<Item = bool> {
        Self::new(s).skip_while(|b| !b)
    }

    pub fn len(&self) -> usize {
        self.n
    }
}

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.
#[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 new_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)
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_bititerator_be() {
        let mut five = BitIteratorBE::new(&[5]);

        for _ in 0..61 {
            assert_eq!(Some(false), five.next());
        }
        assert_eq!(Some(true), five.next());
        assert_eq!(Some(false), five.next());
        assert_eq!(Some(true), five.next());
        assert_eq!(None, five.next());
    }

    #[test]
    fn test_bititerator_be_without_leading_zeros() {
        let mut five = BitIteratorBE::new_without_leading_zeros(&[5]);

        assert_eq!(Some(true), five.next());
        assert_eq!(Some(false), five.next());
        assert_eq!(Some(true), five.next());
        assert_eq!(None, five.next());
    }

    #[test]
    fn test_bititerator_le() {
        let mut five = BitIteratorLE::new(&[5]);

        assert_eq!(Some(true), five.next());
        assert_eq!(Some(false), five.next());
        assert_eq!(Some(true), five.next());
        for _ in 0..61 {
            assert_eq!(Some(false), five.next());
        }
        assert_eq!(None, five.next());
    }

    #[test]
    fn test_bititerator_le_without_trailing_zeros() {
        let mut five = BitIteratorLE::new_without_trailing_zeros(&[5]);

        assert_eq!(Some(true), five.next());
        assert_eq!(Some(false), five.next());
        assert_eq!(Some(true), five.next());
        assert_eq!(None, five.next());
    }
}