hibitset 0.6.4

Hierarchical bit set structure
Documentation 
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/// Type used for indexing.
pub type Index = u32;

/// Base two log of the number of bits in a usize.
#[cfg(target_pointer_width = "64")]
pub const BITS: usize = 6;
#[cfg(target_pointer_width = "32")]
pub const BITS: usize = 5;
/// Amount of layers in the hierarchical bitset.
pub const LAYERS: usize = 4;
pub const MAX: usize = BITS * LAYERS;
/// Maximum amount of bits per bitset.
pub const MAX_EID: usize = 2 << MAX - 1;

/// Layer0 shift (bottom layer, true bitset).
pub const SHIFT0: usize = 0;
/// Layer1 shift (third layer).
pub const SHIFT1: usize = SHIFT0 + BITS;
/// Layer2 shift (second layer).
pub const SHIFT2: usize = SHIFT1 + BITS;
/// Top layer shift.
pub const SHIFT3: usize = SHIFT2 + BITS;

pub trait Row: Sized + Copy {
    /// Location of the bit in the row.
    fn row(self, shift: usize) -> usize;

    /// Index of the row that the bit is in.
    fn offset(self, shift: usize) -> usize;

    /// Bitmask of the row the bit is in.
    #[inline(always)]
    fn mask(self, shift: usize) -> usize {
        1usize << self.row(shift)
    }
}

impl Row for Index {
    #[inline(always)]
    fn row(self, shift: usize) -> usize {
        ((self >> shift) as usize) & ((1 << BITS) - 1)
    }

    #[inline(always)]
    fn offset(self, shift: usize) -> usize {
        self as usize / (1 << shift)
    }
}

/// Helper method for getting parent offsets of 3 layers at once.
///
/// Returns them in (Layer0, Layer1, Layer2) order.
#[inline]
pub fn offsets(bit: Index) -> (usize, usize, usize) {
    (bit.offset(SHIFT1), bit.offset(SHIFT2), bit.offset(SHIFT3))
}

/// Finds the highest bit that splits set bits of the `usize`
/// to half (rounding up).
///
/// Returns `None` if the `usize` has only one or zero set bits.
///
/// # Examples
/// ````rust,ignore
/// use hibitset::util::average_ones;
///
/// assert_eq!(Some(4), average_ones(0b10110));
/// assert_eq!(Some(5), average_ones(0b100010));
/// assert_eq!(None, average_ones(0));
/// assert_eq!(None, average_ones(1));
/// ````
// TODO: Can 64/32 bit variants be merged to one implementation?
// Seems that this would need integer generics to do.
#[cfg(feature = "parallel")]
pub fn average_ones(n: usize) -> Option<usize> {
    #[cfg(target_pointer_width = "64")]
    let average = average_ones_u64(n as u64).map(|n| n as usize);

    #[cfg(target_pointer_width = "32")]
    let average = average_ones_u32(n as u32).map(|n| n as usize);

    average
}

#[cfg(all(any(test, target_pointer_width = "32"), feature = "parallel"))]
fn average_ones_u32(n: u32) -> Option<u32> {
    // !0 / ((1 << (1 << n)) | 1)
    const PAR: [u32; 5] = [!0 / 0x3, !0 / 0x5, !0 / 0x11, !0 / 0x101, !0 / 0x10001];

    // Counting set bits in parallel
    let a = n - ((n >> 1) & PAR[0]);
    let b = (a & PAR[1]) + ((a >> 2) & PAR[1]);
    let c = (b + (b >> 4)) & PAR[2];
    let d = (c + (c >> 8)) & PAR[3];
    let mut cur = d >> 16;
    let count = (d + cur) & PAR[4];
    if count <= 1 {
        return None;
    }

    // Amount of set bits that are wanted for both sides
    let mut target = count / 2;

    // Binary search
    let mut result = 32;
    {
        let mut descend = |child, child_stride, child_mask| {
            if cur < target {
                result -= 2 * child_stride;
                target -= cur;
            }
            // Descend to upper half or lower half
            // depending on are we over or under
            cur = (child >> (result - child_stride)) & child_mask;
        };
        //(!PAR[n] & (PAR[n] + 1)) - 1
        descend(c, 8, 16 - 1); // PAR[3]
        descend(b, 4, 8 - 1); // PAR[2]
        descend(a, 2, 4 - 1); // PAR[1]
        descend(n, 1, 2 - 1); // PAR[0]
    }
    if cur < target {
        result -= 1;
    }

    Some(result - 1)
}

#[cfg(all(any(test, target_pointer_width = "64"), feature = "parallel"))]
fn average_ones_u64(n: u64) -> Option<u64> {
    // !0 / ((1 << (1 << n)) | 1)
    const PAR: [u64; 6] = [
        !0 / 0x3,
        !0 / 0x5,
        !0 / 0x11,
        !0 / 0x101,
        !0 / 0x10001,
        !0 / 0x100000001,
    ];

    // Counting set bits in parallel
    let a = n - ((n >> 1) & PAR[0]);
    let b = (a & PAR[1]) + ((a >> 2) & PAR[1]);
    let c = (b + (b >> 4)) & PAR[2];
    let d = (c + (c >> 8)) & PAR[3];
    let e = (d + (d >> 16)) & PAR[4];
    let mut cur = e >> 32;
    let count = (e + cur) & PAR[5];
    if count <= 1 {
        return None;
    }

    // Amount of set bits that are wanted for both sides
    let mut target = count / 2;

    // Binary search
    let mut result = 64;
    {
        let mut descend = |child, child_stride, child_mask| {
            if cur < target {
                result -= 2 * child_stride;
                target -= cur;
            }
            // Descend to upper half or lower half
            // depending on are we over or under
            cur = (child >> (result - child_stride)) & child_mask;
        };
        //(!PAR[n] & (PAR[n] + 1)) - 1
        descend(d, 16, 256 - 1); // PAR[4]
        descend(c, 8, 16 - 1); // PAR[3]
        descend(b, 4, 8 - 1); // PAR[2]
        descend(a, 2, 4 - 1); // PAR[1]
        descend(n, 1, 2 - 1); // PAR[0]
    }
    if cur < target {
        result -= 1;
    }

    Some(result - 1)
}

#[cfg(all(test, feature = "parallel"))]
mod test_average_ones {
    use super::*;
    #[test]
    fn parity_0_average_ones_u32() {
        struct EvenParity(u32);

        impl Iterator for EvenParity {
            type Item = u32;
            fn next(&mut self) -> Option<Self::Item> {
                if self.0 == u32::max_value() {
                    return None;
                }
                self.0 += 1;
                while self.0.count_ones() & 1 != 0 {
                    if self.0 == u32::max_value() {
                        return None;
                    }
                    self.0 += 1;
                }
                Some(self.0)
            }
        }

        let steps = 1000;
        for i in 0..steps {
            let pos = i * (u32::max_value() / steps);
            for i in EvenParity(pos).take(steps as usize) {
                let mask = (1 << average_ones_u32(i).unwrap_or(31)) - 1;
                assert_eq!((i & mask).count_ones(), (i & !mask).count_ones(), "{:x}", i);
            }
        }
    }

    #[test]
    fn parity_1_average_ones_u32() {
        struct OddParity(u32);

        impl Iterator for OddParity {
            type Item = u32;
            fn next(&mut self) -> Option<Self::Item> {
                if self.0 == u32::max_value() {
                    return None;
                }
                self.0 += 1;
                while self.0.count_ones() & 1 == 0 {
                    if self.0 == u32::max_value() {
                        return None;
                    }
                    self.0 += 1;
                }
                Some(self.0)
            }
        }

        let steps = 1000;
        for i in 0..steps {
            let pos = i * (u32::max_value() / steps);
            for i in OddParity(pos).take(steps as usize) {
                let mask = (1 << average_ones_u32(i).unwrap_or(31)) - 1;
                let a = (i & mask).count_ones();
                let b = (i & !mask).count_ones();
                if a < b {
                    assert_eq!(a + 1, b, "{:x}", i);
                } else if b < a {
                    assert_eq!(a, b + 1, "{:x}", i);
                } else {
                    panic!("Odd parity shouldn't split in exactly half");
                }
            }
        }
    }

    #[test]
    fn empty_average_ones_u32() {
        assert_eq!(None, average_ones_u32(0));
    }

    #[test]
    fn singleton_average_ones_u32() {
        for i in 0..32 {
            assert_eq!(None, average_ones_u32(1 << i), "{:x}", i);
        }
    }

    #[test]
    fn parity_0_average_ones_u64() {
        struct EvenParity(u64);

        impl Iterator for EvenParity {
            type Item = u64;
            fn next(&mut self) -> Option<Self::Item> {
                if self.0 == u64::max_value() {
                    return None;
                }
                self.0 += 1;
                while self.0.count_ones() & 1 != 0 {
                    if self.0 == u64::max_value() {
                        return None;
                    }
                    self.0 += 1;
                }
                Some(self.0)
            }
        }

        let steps = 1000;
        for i in 0..steps {
            let pos = i * (u64::max_value() / steps);
            for i in EvenParity(pos).take(steps as usize) {
                let mask = (1 << average_ones_u64(i).unwrap_or(63)) - 1;
                assert_eq!((i & mask).count_ones(), (i & !mask).count_ones(), "{:x}", i);
            }
        }
    }

    #[test]
    fn parity_1_average_ones_u64() {
        struct OddParity(u64);

        impl Iterator for OddParity {
            type Item = u64;
            fn next(&mut self) -> Option<Self::Item> {
                if self.0 == u64::max_value() {
                    return None;
                }
                self.0 += 1;
                while self.0.count_ones() & 1 == 0 {
                    if self.0 == u64::max_value() {
                        return None;
                    }
                    self.0 += 1;
                }
                Some(self.0)
            }
        }

        let steps = 1000;
        for i in 0..steps {
            let pos = i * (u64::max_value() / steps);
            for i in OddParity(pos).take(steps as usize) {
                let mask = (1 << average_ones_u64(i).unwrap_or(63)) - 1;
                let a = (i & mask).count_ones();
                let b = (i & !mask).count_ones();
                if a < b {
                    assert_eq!(a + 1, b, "{:x}", i);
                } else if b < a {
                    assert_eq!(a, b + 1, "{:x}", i);
                } else {
                    panic!("Odd parity shouldn't split in exactly half");
                }
            }
        }
    }

    #[test]
    fn empty_average_ones_u64() {
        assert_eq!(None, average_ones_u64(0));
    }

    #[test]
    fn singleton_average_ones_u64() {
        for i in 0..64 {
            assert_eq!(None, average_ones_u64(1 << i), "{:x}", i);
        }
    }

    #[test]
    fn average_ones_agree_u32_u64() {
        let steps = 1000;
        for i in 0..steps {
            let pos = i * (u32::max_value() / steps);
            for i in pos..steps {
                assert_eq!(
                    average_ones_u32(i),
                    average_ones_u64(i as u64).map(|n| n as u32),
                    "{:x}",
                    i
                );
            }
        }
    }

    #[test]
    fn specific_values() {
        assert_eq!(Some(4), average_ones_u32(0b10110));
        assert_eq!(Some(5), average_ones_u32(0b100010));
        assert_eq!(None, average_ones_u32(0));
        assert_eq!(None, average_ones_u32(1));

        assert_eq!(Some(4), average_ones_u64(0b10110));
        assert_eq!(Some(5), average_ones_u64(0b100010));
        assert_eq!(None, average_ones_u64(0));
        assert_eq!(None, average_ones_u64(1));
    }
}