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
// Original implementation taken from rust-memchr.
// Copyright 2015 Andrew Gallant, bluss and Nicolas Koch

use crate::cmp;
use crate::mem;

const LO_USIZE: usize = usize::repeat_u8(0x01);
const HI_USIZE: usize = usize::repeat_u8(0x80);
const USIZE_BYTES: usize = mem::size_of::<usize>();

/// Returns `true` if `x` contains any zero byte.
///
/// From *Matters Computational*, J. Arndt:
///
/// "The idea is to subtract one from each of the bytes and then look for
/// bytes where the borrow propagated all the way to the most significant
/// bit."
#[inline]
const fn contains_zero_byte(x: usize) -> bool {
    x.wrapping_sub(LO_USIZE) & !x & HI_USIZE != 0
}

#[cfg(target_pointer_width = "16")]
#[inline]
const fn repeat_byte(b: u8) -> usize {
    (b as usize) << 8 | b as usize
}

#[cfg(not(target_pointer_width = "16"))]
#[inline]
const fn repeat_byte(b: u8) -> usize {
    (b as usize) * (usize::MAX / 255)
}

/// Returns the first index matching the byte `x` in `text`.
#[must_use]
#[inline]
pub const fn memchr(x: u8, text: &[u8]) -> Option<usize> {
    // Fast path for small slices.
    if text.len() < 2 * USIZE_BYTES {
        return memchr_naive(x, text);
    }

    memchr_aligned(x, text)
}

#[inline]
const fn memchr_naive(x: u8, text: &[u8]) -> Option<usize> {
    let mut i = 0;

    // FIXME(const-hack): Replace with `text.iter().pos(|c| *c == x)`.
    while i < text.len() {
        if text[i] == x {
            return Some(i);
        }

        i += 1;
    }

    None
}

const fn memchr_aligned(x: u8, text: &[u8]) -> Option<usize> {
    // Scan for a single byte value by reading two `usize` words at a time.
    //
    // Split `text` in three parts
    // - unaligned initial part, before the first word aligned address in text
    // - body, scan by 2 words at a time
    // - the last remaining part, < 2 word size

    // search up to an aligned boundary
    let len = text.len();
    let ptr = text.as_ptr();
    let mut offset = ptr.align_offset(USIZE_BYTES);

    if offset > 0 {
        offset = cmp::min(offset, len);
        if let Some(index) = memchr_naive(x, &text[..offset]) {
            return Some(index);
        }
    }

    // search the body of the text
    let repeated_x = repeat_byte(x);
    while offset <= len - 2 * USIZE_BYTES {
        // SAFETY: the while's predicate guarantees a distance of at least 2 * usize_bytes
        // between the offset and the end of the slice.
        unsafe {
            let u = *(ptr.add(offset) as *const usize);
            let v = *(ptr.add(offset + USIZE_BYTES) as *const usize);

            // break if there is a matching byte
            let zu = contains_zero_byte(u ^ repeated_x);
            let zv = contains_zero_byte(v ^ repeated_x);
            if zu || zv {
                break;
            }
        }
        offset += USIZE_BYTES * 2;
    }

    // Find the byte after the point the body loop stopped.
    // FIXME(const-hack): Use `?` instead.
    if let Some(i) = memchr_naive(x, &text[offset..]) { Some(offset + i) } else { None }
}

/// Returns the last index matching the byte `x` in `text`.
#[must_use]
pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
    // Scan for a single byte value by reading two `usize` words at a time.
    //
    // Split `text` in three parts:
    // - unaligned tail, after the last word aligned address in text,
    // - body, scanned by 2 words at a time,
    // - the first remaining bytes, < 2 word size.
    let len = text.len();
    let ptr = text.as_ptr();
    type Chunk = usize;

    let (min_aligned_offset, max_aligned_offset) = {
        // We call this just to obtain the length of the prefix and suffix.
        // In the middle we always process two chunks at once.
        // SAFETY: transmuting `[u8]` to `[usize]` is safe except for size differences
        // which are handled by `align_to`.
        let (prefix, _, suffix) = unsafe { text.align_to::<(Chunk, Chunk)>() };
        (prefix.len(), len - suffix.len())
    };

    let mut offset = max_aligned_offset;
    if let Some(index) = text[offset..].iter().rposition(|elt| *elt == x) {
        return Some(offset + index);
    }

    // Search the body of the text, make sure we don't cross min_aligned_offset.
    // offset is always aligned, so just testing `>` is sufficient and avoids possible
    // overflow.
    let repeated_x = repeat_byte(x);
    let chunk_bytes = mem::size_of::<Chunk>();

    while offset > min_aligned_offset {
        // SAFETY: offset starts at len - suffix.len(), as long as it is greater than
        // min_aligned_offset (prefix.len()) the remaining distance is at least 2 * chunk_bytes.
        unsafe {
            let u = *(ptr.add(offset - 2 * chunk_bytes) as *const Chunk);
            let v = *(ptr.add(offset - chunk_bytes) as *const Chunk);

            // Break if there is a matching byte.
            let zu = contains_zero_byte(u ^ repeated_x);
            let zv = contains_zero_byte(v ^ repeated_x);
            if zu || zv {
                break;
            }
        }
        offset -= 2 * chunk_bytes;
    }

    // Find the byte before the point the body loop stopped.
    text[..offset].iter().rposition(|elt| *elt == x)
}