1/*!
2Contains architecture independent routines.
3
4These routines are often used as a "fallback" implementation when the more
5specialized architecture dependent routines are unavailable.
6*/
7
8pub mod memchr;
9pub mod packedpair;
10pub mod rabinkarp;
11#[cfg(feature = "alloc")]
12pub mod shiftor;
13pub mod twoway;
14
15/// Returns true if and only if `needle` is a prefix of `haystack`.
16///
17/// This uses a latency optimized variant of `memcmp` internally which *might*
18/// make this faster for very short strings.
19///
20/// # Inlining
21///
22/// This routine is marked `inline(always)`. If you want to call this function
23/// in a way that is not always inlined, you'll need to wrap a call to it in
24/// another function that is marked as `inline(never)` or just `inline`.
25#[inline(always)]
26pub fn is_prefix(haystack: &[u8], needle: &[u8]) -> bool {
27 needle.len() <= haystack.len()
28 && is_equal(&haystack[..needle.len()], y:needle)
29}
30
31/// Returns true if and only if `needle` is a suffix of `haystack`.
32///
33/// This uses a latency optimized variant of `memcmp` internally which *might*
34/// make this faster for very short strings.
35///
36/// # Inlining
37///
38/// This routine is marked `inline(always)`. If you want to call this function
39/// in a way that is not always inlined, you'll need to wrap a call to it in
40/// another function that is marked as `inline(never)` or just `inline`.
41#[inline(always)]
42pub fn is_suffix(haystack: &[u8], needle: &[u8]) -> bool {
43 needle.len() <= haystack.len()
44 && is_equal(&haystack[haystack.len() - needle.len()..], y:needle)
45}
46
47/// Compare corresponding bytes in `x` and `y` for equality.
48///
49/// That is, this returns true if and only if `x.len() == y.len()` and
50/// `x[i] == y[i]` for all `0 <= i < x.len()`.
51///
52/// # Inlining
53///
54/// This routine is marked `inline(always)`. If you want to call this function
55/// in a way that is not always inlined, you'll need to wrap a call to it in
56/// another function that is marked as `inline(never)` or just `inline`.
57///
58/// # Motivation
59///
60/// Why not use slice equality instead? Well, slice equality usually results in
61/// a call out to the current platform's `libc` which might not be inlineable
62/// or have other overhead. This routine isn't guaranteed to be a win, but it
63/// might be in some cases.
64#[inline(always)]
65pub fn is_equal(x: &[u8], y: &[u8]) -> bool {
66 if x.len() != y.len() {
67 return false;
68 }
69 // SAFETY: Our pointers are derived directly from borrowed slices which
70 // uphold all of our safety guarantees except for length. We account for
71 // length with the check above.
72 unsafe { is_equal_raw(x.as_ptr(), y.as_ptr(), n:x.len()) }
73}
74
75/// Compare `n` bytes at the given pointers for equality.
76///
77/// This returns true if and only if `*x.add(i) == *y.add(i)` for all
78/// `0 <= i < n`.
79///
80/// # Inlining
81///
82/// This routine is marked `inline(always)`. If you want to call this function
83/// in a way that is not always inlined, you'll need to wrap a call to it in
84/// another function that is marked as `inline(never)` or just `inline`.
85///
86/// # Motivation
87///
88/// Why not use slice equality instead? Well, slice equality usually results in
89/// a call out to the current platform's `libc` which might not be inlineable
90/// or have other overhead. This routine isn't guaranteed to be a win, but it
91/// might be in some cases.
92///
93/// # Safety
94///
95/// * Both `x` and `y` must be valid for reads of up to `n` bytes.
96/// * Both `x` and `y` must point to an initialized value.
97/// * Both `x` and `y` must each point to an allocated object and
98/// must either be in bounds or at most one byte past the end of the
99/// allocated object. `x` and `y` do not need to point to the same allocated
100/// object, but they may.
101/// * Both `x` and `y` must be _derived from_ a pointer to their respective
102/// allocated objects.
103/// * The distance between `x` and `x+n` must not overflow `isize`. Similarly
104/// for `y` and `y+n`.
105/// * The distance being in bounds must not rely on "wrapping around" the
106/// address space.
107#[inline(always)]
108pub unsafe fn is_equal_raw(
109 mut x: *const u8,
110 mut y: *const u8,
111 mut n: usize,
112) -> bool {
113 // When we have 4 or more bytes to compare, then proceed in chunks of 4 at
114 // a time using unaligned loads.
115 //
116 // Also, why do 4 byte loads instead of, say, 8 byte loads? The reason is
117 // that this particular version of memcmp is likely to be called with tiny
118 // needles. That means that if we do 8 byte loads, then a higher proportion
119 // of memcmp calls will use the slower variant above. With that said, this
120 // is a hypothesis and is only loosely supported by benchmarks. There's
121 // likely some improvement that could be made here. The main thing here
122 // though is to optimize for latency, not throughput.
123
124 // SAFETY: The caller is responsible for ensuring the pointers we get are
125 // valid and readable for at least `n` bytes. We also do unaligned loads,
126 // so there's no need to ensure we're aligned. (This is justified by this
127 // routine being specifically for short strings.)
128 while n >= 4 {
129 let vx = x.cast::<u32>().read_unaligned();
130 let vy = y.cast::<u32>().read_unaligned();
131 if vx != vy {
132 return false;
133 }
134 x = x.add(4);
135 y = y.add(4);
136 n -= 4;
137 }
138 // If we don't have enough bytes to do 4-byte at a time loads, then
139 // do partial loads. Note that I used to have a byte-at-a-time
140 // loop here and that turned out to be quite a bit slower for the
141 // memmem/pathological/defeat-simple-vector-alphabet benchmark.
142 if n >= 2 {
143 let vx = x.cast::<u16>().read_unaligned();
144 let vy = y.cast::<u16>().read_unaligned();
145 if vx != vy {
146 return false;
147 }
148 x = x.add(2);
149 y = y.add(2);
150 n -= 2;
151 }
152 if n > 0 {
153 if x.read() != y.read() {
154 return false;
155 }
156 }
157 true
158}
159
160#[cfg(test)]
161mod tests {
162 use super::*;
163
164 #[test]
165 fn equals_different_lengths() {
166 assert!(!is_equal(b"", b"a"));
167 assert!(!is_equal(b"a", b""));
168 assert!(!is_equal(b"ab", b"a"));
169 assert!(!is_equal(b"a", b"ab"));
170 }
171
172 #[test]
173 fn equals_mismatch() {
174 let one_mismatch = [
175 (&b"a"[..], &b"x"[..]),
176 (&b"ab"[..], &b"ax"[..]),
177 (&b"abc"[..], &b"abx"[..]),
178 (&b"abcd"[..], &b"abcx"[..]),
179 (&b"abcde"[..], &b"abcdx"[..]),
180 (&b"abcdef"[..], &b"abcdex"[..]),
181 (&b"abcdefg"[..], &b"abcdefx"[..]),
182 (&b"abcdefgh"[..], &b"abcdefgx"[..]),
183 (&b"abcdefghi"[..], &b"abcdefghx"[..]),
184 (&b"abcdefghij"[..], &b"abcdefghix"[..]),
185 (&b"abcdefghijk"[..], &b"abcdefghijx"[..]),
186 (&b"abcdefghijkl"[..], &b"abcdefghijkx"[..]),
187 (&b"abcdefghijklm"[..], &b"abcdefghijklx"[..]),
188 (&b"abcdefghijklmn"[..], &b"abcdefghijklmx"[..]),
189 ];
190 for (x, y) in one_mismatch {
191 assert_eq!(x.len(), y.len(), "lengths should match");
192 assert!(!is_equal(x, y));
193 assert!(!is_equal(y, x));
194 }
195 }
196
197 #[test]
198 fn equals_yes() {
199 assert!(is_equal(b"", b""));
200 assert!(is_equal(b"a", b"a"));
201 assert!(is_equal(b"ab", b"ab"));
202 assert!(is_equal(b"abc", b"abc"));
203 assert!(is_equal(b"abcd", b"abcd"));
204 assert!(is_equal(b"abcde", b"abcde"));
205 assert!(is_equal(b"abcdef", b"abcdef"));
206 assert!(is_equal(b"abcdefg", b"abcdefg"));
207 assert!(is_equal(b"abcdefgh", b"abcdefgh"));
208 assert!(is_equal(b"abcdefghi", b"abcdefghi"));
209 }
210
211 #[test]
212 fn prefix() {
213 assert!(is_prefix(b"", b""));
214 assert!(is_prefix(b"a", b""));
215 assert!(is_prefix(b"ab", b""));
216 assert!(is_prefix(b"foo", b"foo"));
217 assert!(is_prefix(b"foobar", b"foo"));
218
219 assert!(!is_prefix(b"foo", b"fob"));
220 assert!(!is_prefix(b"foobar", b"fob"));
221 }
222
223 #[test]
224 fn suffix() {
225 assert!(is_suffix(b"", b""));
226 assert!(is_suffix(b"a", b""));
227 assert!(is_suffix(b"ab", b""));
228 assert!(is_suffix(b"foo", b"foo"));
229 assert!(is_suffix(b"foobar", b"bar"));
230
231 assert!(!is_suffix(b"foo", b"goo"));
232 assert!(!is_suffix(b"foobar", b"gar"));
233 }
234}
235