poptrie 0.1.1

A pure-rust implementation of poptrie
Documentation 
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
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162

use core::ops::{Shl, Shr};

/// A trait for types that can be used as keys in the [poptrie](crate::Poptrie).
///
/// This is currently not sealed to allow for custom `Key` types.
/// Future versions may change this, which would be a breaking change.
///
/// - Needs to inform its bit width and a `to_u8` method that returns its 8 least significant bits.
/// - Needs to implement `rotate_right` method that rotates the key by `n` bits to the right.
///
/// # Examples
///
/// Implementing `Key` for a custom newtype:
/// ```
/// use poptrie::Key;
/// use core::ops::{Shl, Shr};
///
/// #[derive(Clone, Copy)]
/// struct MyAddr(u32);
///
/// impl Shl<u8> for MyAddr {
///     type Output = Self;
///     fn shl(self, n: u8) -> Self { MyAddr(self.0 << n) }
/// }
///
/// impl Shr<u8> for MyAddr {
///     type Output = Self;
///     fn shr(self, n: u8) -> Self { MyAddr(self.0 >> n) }
/// }
///
/// impl Key for MyAddr {
///     const BITS: u8 = 32;
///     fn to_u8(self) -> u8 { self.0 as u8 }
///     fn rotate_right(self, n: u32) -> Self { MyAddr(self.0.rotate_right(n)) }
/// }
///
/// assert_eq!(MyAddr::BITS, 32);
/// assert_eq!(MyAddr(0xdeadbeef).to_u8(), 0xef);
/// ```
pub trait Key:
    Copy + Shl<u8, Output = Self> + Shr<u8, Output = Self> + Sized
{
    /// The number of bits in the key.
    const BITS: u8;
    /// Converts the least significant bits of the key to a u8.
    fn to_u8(self) -> u8;
    /// Rotates the key to the right by `n` bits.
    fn rotate_right(self, n: u32) -> Self;
}

impl Key for u32 {
    const BITS: u8 = 32;
    #[inline(always)]
    fn to_u8(self) -> u8 {
        self as u8
    }

    #[inline(always)]
    fn rotate_right(self, n: u32) -> Self {
        self.rotate_right(n)
    }
}

impl Key for u128 {
    const BITS: u8 = 128;
    #[inline(always)]
    fn to_u8(self) -> u8 {
        self as u8
    }

    #[inline(always)]
    fn rotate_right(self, n: u32) -> Self {
        self.rotate_right(n)
    }
}

/// Extract `len` bits from `key`, starting from `offset` bits from the most
/// significant bit. Bit position is exact (msb aligned) and zero-padded to the right.
/// - `len` must be at most 8 since where returning a `u8`.
///``` text
/// MSB |------------------------key-------------------------| LSB
///     |------ offset ------|----- len ----|---remaining----|
///                           ^^^^^^^^^^^^^^
///                           extracted bits
///```
/// If `len` + `offset` > K::BITS, the extraction will be zero-padded from the right:
///``` text
/// MSB |------------------------key-------------------------| LSB
///     |---------------- offset -----------------|----- len ----|
///                                               |^^^^^^^^^^0000|
///                                                extracted bits
///```
#[inline(always)]
pub(crate) fn extract_bits<K>(key: K, offset: u8, len: u8) -> u8
where
    K: Key,
{
    (key.rotate_right((K::BITS - offset).wrapping_sub(len) as u32)).to_u8()
        & ((1u16 << len) - 1) as u8
}

/// Extracts `len` bits from `key`, starting from `offset` bits from the most
/// significant bit. Significant bits are lsb-aligned and zero-padded to the left.
/// - `len` must be at most 8 since where returning a `u8`.
///
/// ``` text
/// MSB |------------------------key-------------------------| LSB
///     |------ offset ------|----- len ----|---remaining----|
///                           ^^^^^^^^^^^^^^
///                           extracted bits
///```
/// If `len` + `offset` > `K::BITS`, the extraction will be saturated:
///
/// ``` text
/// MSB |------------------------key-------------------------| LSB
///     |---------------- offset -----------------|----- len ----|
///                                           |0000^^^^^^^^^^|
///                                            extracted bits
///```
#[inline(always)]
pub(crate) fn extract_bits_saturated<K>(key: K, offset: u8, len: u8) -> u8
where
    K: Key,
{
    // TODO: Check if mask version is faster
    let remaining = u8::saturating_sub(K::BITS - offset, len);
    if remaining + offset == K::BITS {
        return 0;
    }
    (key << offset >> (remaining + offset)).to_u8()
}

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

    #[test]
    fn test_extraction() {
        let key: u32 = 0b11111111_01010101_11110000_11001100;
        assert_eq!(extract_bits(key, 0, 8), 0b11111111);
        assert_eq!(extract_bits(key, 8, 8), 0b01010101);
        assert_eq!(extract_bits(key, 16, 8), 0b11110000);
        assert_eq!(extract_bits(key, 24, 8), 0b11001100);
    }

    #[test]
    fn test_saturated_extraction() {
        let key: u32 = 0b000000_000000_000000_000000_000000_01;
        assert_eq!(extract_bits_saturated(key, 30, 6), 1);
        let key: u32 = 0b000001_000001_000001_000001_000001_01;
        assert_eq!(extract_bits_saturated(key, 30, 6), 1);
    }

    #[test]
    fn test_msb_alignement() {
        let key: u32 = 0b000000_000000_000000_000000_000000_01;
        assert_eq!(extract_bits(key, 30, 6), 0b0001_0000);
        let key: u32 = 0b000001_000001_000001_000001_000001_01;
        assert_eq!(extract_bits(key, 30, 6), 0b0001_0000);
        assert_eq!(extract_bits(key, 0, 6), 0b0000_0001);
    }
}