1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145

//! Bit masks for extracting bits.

use crate::lib::mem;
use super::cast::as_cast;
use super::num::UnsignedInteger;

/// Generate a bitwise mask for the lower `n` bits.
///
/// # Examples
///
/// ```text
/// # use lexical::util::lower_n_mask;
/// # pub fn main() {
/// lower_n_mask(2u32) -> b11u32;
/// # }
/// ```
#[inline]
pub(crate) fn lower_n_mask<N>(n: N)
    -> N
    where N:UnsignedInteger
{
    let bits: N = as_cast(mem::size_of::<N>() * 8);
    debug_assert!(n <= bits, "lower_n_mask() overflow in shl.");

    match n == bits {
        true  => N::max_value(),
        false => (N::ONE << n) - N::ONE,
    }
}

/// Calculate the halfway point for the lower `n` bits.
///
/// # Examples
///
/// ```text
/// # use lexical::util::lower_n_halfway;
/// # pub fn main() {
/// lower_n_halfway(2u32) -> b10u32;
/// # }
/// ```
#[inline]
pub(crate) fn lower_n_halfway<N>(n: N)
    -> N
    where N:UnsignedInteger
{
    let bits: N = as_cast(mem::size_of::<N>() * 8);
    debug_assert!(n <= bits, "lower_n_halfway() overflow in shl.");

    match n.is_zero() {
        true  => N::ZERO,
        false => nth_bit(n - N::ONE),
    }
}

/// Calculate a scalar factor of 2 above the halfway point.
///
/// # Examples
///
/// ```text
/// # use lexical::util::nth_bit;
/// # pub fn main() {
/// nth_bit(2u32) -> b100u32;
/// # }
/// ```
#[inline]
pub(crate) fn nth_bit<N>(n: N)
    -> N
    where N:UnsignedInteger
{
    let bits: N = as_cast(N::BITS);
    debug_assert!(n < bits, "nth_bit() overflow in shl.");

    N::ONE << n
}

/// Calculate a bitwise mask with `n` 1 bits starting at the `bit` position.
///
/// # Examples
///
/// ```text
/// # use lexical::util::internal_n_mask;
/// # pub fn main() {
/// internal_n_mask(4u32, 2u32) -> b1100u32;
/// internal_n_mask(10u32, 2u32) -> 0b1100000000;
/// # }
/// ```
#[inline]
pub(crate) fn internal_n_mask<N>(bit: N, n: N)
    -> N
    where N:UnsignedInteger
{
    let bits: N = as_cast(mem::size_of::<N>() * 8);
    debug_assert!(bit <= bits, "internal_n_halfway() overflow in shl.");
    debug_assert!(n <= bits, "internal_n_halfway() overflow in shl.");
    debug_assert!(bit >= n, "internal_n_halfway() overflow in sub.");

    lower_n_mask(bit) ^ lower_n_mask(bit - n)
}


// TESTS
// -----

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

    #[test]
    fn lower_n_mask_test() {
        assert_eq!(lower_n_mask(0u32), 0b0);
        assert_eq!(lower_n_mask(1u32), 0b1);
        assert_eq!(lower_n_mask(2u32), 0b11);
        assert_eq!(lower_n_mask(10u32), 0b1111111111);
        assert_eq!(lower_n_mask(32u32), 0b11111111111111111111111111111111);
    }

    #[test]
    fn lower_n_halfway_test() {
        assert_eq!(lower_n_halfway(0u32), 0b0);
        assert_eq!(lower_n_halfway(1u32), 0b1);
        assert_eq!(lower_n_halfway(2u32), 0b10);
        assert_eq!(lower_n_halfway(10u32), 0b1000000000);
        assert_eq!(lower_n_halfway(32u32), 0b10000000000000000000000000000000);
    }

    #[test]
    fn nth_bit_test() {
        assert_eq!(nth_bit(0u32), 0b1);
        assert_eq!(nth_bit(1u32), 0b10);
        assert_eq!(nth_bit(2u32), 0b100);
        assert_eq!(nth_bit(10u32), 0b10000000000);
        assert_eq!(nth_bit(31u32), 0b10000000000000000000000000000000);
    }

    #[test]
    fn internal_n_mask_test() {
        assert_eq!(internal_n_mask(1u32, 0u32), 0b0);
        assert_eq!(internal_n_mask(1u32, 1u32), 0b1);
        assert_eq!(internal_n_mask(2u32, 1u32), 0b10);
        assert_eq!(internal_n_mask(4u32, 2u32), 0b1100);
        assert_eq!(internal_n_mask(10u32, 2u32), 0b1100000000);
        assert_eq!(internal_n_mask(10u32, 4u32), 0b1111000000);
        assert_eq!(internal_n_mask(32u32, 4u32), 0b11110000000000000000000000000000);
    }
}