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

//! Allocation-free hex formatting.
//!
//! Given that most, if not all, of the types that we hex-format are of constant byte size (Hash,
//! Address, Identity), this hex implementation lets you format to hex without needing to allocate
//! a `String` on the heap.

use core::{fmt, ops, str};

#[derive(Copy, Clone)]
pub struct HexString<const N: usize> {
    s: [HexByte; N],
}

pub fn encode<const N: usize>(bytes: &[u8; N]) -> HexString<N> {
    let s = bytes.map(HexByte::from_byte);
    HexString { s }
}

impl<const N: usize> HexString<N> {
    #[inline(always)]
    pub fn as_str(&self) -> &str {
        self
    }

    #[inline(always)]
    pub fn as_bytes(&self) -> &[u8] {
        str::as_bytes(self)
    }
}

impl<const N: usize> ops::Deref for HexString<N> {
    type Target = str;
    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        HexByte::as_str(&self.s)
    }
}

impl<const N: usize> AsRef<str> for HexString<N> {
    fn as_ref(&self) -> &str {
        self
    }
}

impl<const N: usize> AsRef<[u8]> for HexString<N> {
    fn as_ref(&self) -> &[u8] {
        self.as_bytes()
    }
}

impl<const N: usize> fmt::Display for HexString<N> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad(self)
    }
}

impl<const N: usize> fmt::Debug for HexString<N> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.pad(self)
    }
}

#[derive(Copy, Clone)]
#[repr(u8)]
enum HexNybble {
    _0 = b'0',
    _1 = b'1',
    _2 = b'2',
    _3 = b'3',
    _4 = b'4',
    _5 = b'5',
    _6 = b'6',
    _7 = b'7',
    _8 = b'8',
    _9 = b'9',
    _A = b'a',
    _B = b'b',
    _C = b'c',
    _D = b'd',
    _E = b'e',
    _F = b'f',
}

#[rustfmt::skip]
const NYBBLE_LOOKUP: [HexNybble; 16] = [
    HexNybble::_0, HexNybble::_1, HexNybble::_2, HexNybble::_3, HexNybble::_4, HexNybble::_5, HexNybble::_6, HexNybble::_7,
    HexNybble::_8, HexNybble::_9, HexNybble::_A, HexNybble::_B, HexNybble::_C, HexNybble::_D, HexNybble::_E, HexNybble::_F,
];

#[derive(Copy, Clone)]
#[repr(transparent)]
struct HexByte([HexNybble; 2]);

static BYTE_LOOKUP: [HexByte; 256] = {
    let mut arr = [HexByte::ZERO; 256];
    let mut i = 0;
    while i < arr.len() {
        let (hi, lo) = (i >> 4, i & 0x0F);
        let byte = HexByte([NYBBLE_LOOKUP[hi], NYBBLE_LOOKUP[lo]]);
        arr[i] = byte;
        i += 1;
    }
    arr
};

impl HexByte {
    const ZERO: HexByte = HexByte([HexNybble::_0, HexNybble::_0]);

    #[inline(always)]
    fn from_byte(b: u8) -> Self {
        BYTE_LOOKUP[b as usize]
    }

    #[inline(always)]
    fn as_nybbles(this: &[Self]) -> &[HexNybble] {
        // SAFETY: HexByte is repr(transparent) over [HexNybble; 2]
        let arrays = unsafe { &*(this as *const [HexByte] as *const [[HexNybble; 2]]) };
        // SAFETY: this is equivalent to the unstable [[T; N]].flatten() -> &[T] method
        // TODO: switch to <[[T; N]]>::flatten() once stabilized
        unsafe { arrays.align_to::<HexNybble>().1 }
    }
    #[inline(always)]
    fn as_str(this: &[Self]) -> &str {
        let nybbles = HexByte::as_nybbles(this);
        // SAFETY: a HexNybble can only be valid ascii, which is always valid utf8
        unsafe { str::from_utf8_unchecked(HexNybble::as_bytes(nybbles)) }
    }
}

impl HexNybble {
    #[inline(always)]
    fn as_bytes(this: &[Self]) -> &[u8] {
        // SAFETY: HexNybble is repr(u8)
        unsafe { &*(this as *const [HexNybble] as *const [u8]) }
    }
}