cache.rs (8380B)
1 use core::ops::Index; 2 /// A cache of `N` `T`s. When the cache is filled up, 3 /// a new entry overwrites the oldest entry. `Cache<T, N>` 4 /// cannot be expanded or shrunk. 5 pub struct Cache<T, const N: usize> { 6 /// The cache of `T`s. 7 vals: [T; N], 8 /// The number of `T`s in our cache. Once 9 /// `N` number of `T`s have been cached, this will 10 /// always be `N`. 11 len: usize, 12 /// The index position of the next insert. 13 next_head: usize, 14 } 15 impl<T, const N: usize> Cache<T, N> { 16 /// Returns the number of items cached. 17 /// This will always return `N` once at least 18 /// `N` items have been cached. 19 #[inline] 20 pub const fn len(&self) -> usize { 21 self.len 22 } 23 /// Returns true iff no items have been cached. 24 #[inline] 25 pub const fn is_empty(&self) -> bool { 26 self.len == 0 27 } 28 } 29 /// Implements the functions `get`, `get_unsafe`, and `push` as well as `Index<usize, Output = T>` 30 /// for each of the passed usize literals. 31 /// Only powers of two are allowed to be passed due to how the above functions are implemented. 32 /// If any other value is passed, the implementation of those functions will be wrong. 33 macro_rules! cache { 34 ( $( $x:literal),* ) => { 35 $( 36 impl<T> Cache<T, $x> { 37 /// Returns `Some(&T)` iff there is an item located 38 /// at index `idx`. Indexing begins at the newest 39 /// entry (i.e., `self.get(0)` will return the most recent entry 40 /// iff at least one item has been cached). 41 #[inline] 42 pub fn get(&self, idx: usize) -> Option<&T> { 43 (idx < self.len).then(|| self.vals.index(self.next_head.wrapping_sub(1).wrapping_sub(idx) & ($x - 1))) 44 } 45 /// Returns a `&T` at index position `idx % N`. In the event 46 /// `(idx % N) >= self.len()`, then `&T` references the 47 /// default value of `T` that was inserted at creation but *not* 48 /// actually inserted via `push`. 49 /// 50 /// # Correctness 51 /// 52 /// `idx < self.len()`; otherwise a value that was not actually inserted will be returned. 53 #[inline] 54 pub fn get_unchecked(&self, idx: usize) -> &T { 55 self.vals.index(self.next_head.wrapping_sub(1).wrapping_sub(idx) & ($x - 1)) 56 } 57 /// Adds `val` to the cache. In the event `N` values have already been cached, 58 /// the oldest entry is overwritten. 59 #[expect(clippy::arithmetic_side_effects, reason = "must, and overflow is not possible")] 60 #[inline] 61 pub fn push(&mut self, val: T) { 62 if self.len < $x { 63 self.len += 1; 64 } 65 // next_head is always less than or equal to N since we only implement 66 // this function when N is a power of 2. 67 self.vals[self.next_head] = val; 68 self.next_head = (self.next_head + 1) & ($x - 1); 69 } 70 } 71 impl<T> Index<usize> for Cache<T, $x> { 72 type Output = T; 73 #[inline] 74 fn index(&self, index: usize) -> &Self::Output { 75 self.get(index).unwrap() 76 } 77 } 78 )* 79 }; 80 } 81 // MUST only pass powers of two! Anything else will lead to wrong code. 82 cache!( 83 0x1, 84 0x2, 85 0x4, 86 0x8, 87 0x10, 88 0x20, 89 0x40, 90 0x80, 91 0x100, 92 0x200, 93 0x400, 94 0x800, 95 0x1000, 96 0x2000, 97 0x4000, 98 0x8000, 99 0x0001_0000, 100 0x0002_0000, 101 0x0004_0000, 102 0x0008_0000, 103 0x0010_0000, 104 0x0020_0000, 105 0x0040_0000, 106 0x0080_0000, 107 0x0100_0000, 108 0x0200_0000, 109 0x0400_0000, 110 0x0800_0000, 111 0x1000_0000, 112 0x2000_0000, 113 0x4000_0000, 114 0x8000_0000, 115 0x0001_0000_0000, 116 0x0002_0000_0000, 117 0x0004_0000_0000, 118 0x0008_0000_0000, 119 0x0010_0000_0000, 120 0x0020_0000_0000, 121 0x0040_0000_0000, 122 0x0080_0000_0000, 123 0x0100_0000_0000, 124 0x0200_0000_0000, 125 0x0400_0000_0000, 126 0x0800_0000_0000, 127 0x1000_0000_0000, 128 0x2000_0000_0000, 129 0x4000_0000_0000, 130 0x8000_0000_0000, 131 0x0001_0000_0000_0000, 132 0x0002_0000_0000_0000, 133 0x0004_0000_0000_0000, 134 0x0008_0000_0000_0000, 135 0x0010_0000_0000_0000, 136 0x0020_0000_0000_0000, 137 0x0040_0000_0000_0000, 138 0x0080_0000_0000_0000, 139 0x0100_0000_0000_0000, 140 0x0200_0000_0000_0000, 141 0x0400_0000_0000_0000, 142 0x0800_0000_0000_0000, 143 0x1000_0000_0000_0000, 144 0x2000_0000_0000_0000, 145 0x4000_0000_0000_0000, 146 0x8000_0000_0000_0000 147 ); 148 impl<T, const N: usize> Cache<T, N> 149 where 150 [T; N]: Default, 151 { 152 /// Returns an instance of itself pre-loaded with 153 /// the `N` instances of the default value of `T`. 154 /// Note these default instances are treated as though 155 /// they don't exist (i.e., `self.is_empty()` will return true). 156 #[inline] 157 #[must_use] 158 pub fn new() -> Self { 159 Self::default() 160 } 161 } 162 impl<T, const N: usize> Default for Cache<T, N> 163 where 164 [T; N]: Default, 165 { 166 #[inline] 167 fn default() -> Self { 168 Self { 169 vals: Default::default(), 170 len: 0, 171 next_head: 0, 172 } 173 } 174 } 175 #[cfg(test)] 176 mod tests { 177 use super::Cache; 178 #[test] 179 fn test_len() { 180 let mut c = Cache::<bool, 1>::new(); 181 assert_eq!(0, c.len()); 182 c.push(false); 183 assert_eq!(1, c.len()); 184 c.push(false); 185 assert_eq!(1, c.len()); 186 } 187 #[test] 188 fn test_is_empty() { 189 let mut c = Cache::<bool, 1>::new(); 190 assert!(c.is_empty()); 191 c.push(false); 192 assert!(!c.is_empty()); 193 } 194 #[test] 195 fn test_get() { 196 let mut c = Cache::<bool, 4>::new(); 197 assert!(c.get(0).is_none()); 198 assert!(c.get(1).is_none()); 199 assert!(c.get(2).is_none()); 200 assert!(c.get(3).is_none()); 201 assert!(c.get(4).is_none()); 202 assert!(c.get(5).is_none()); 203 assert!(c.get(usize::MAX).is_none()); 204 c.push(true); 205 assert!(c.get(0).unwrap()); 206 assert!(c.get(1).is_none()); 207 assert!(c.get(2).is_none()); 208 assert!(c.get(3).is_none()); 209 assert!(c.get(4).is_none()); 210 assert!(c.get(5).is_none()); 211 assert!(c.get(usize::MAX).is_none()); 212 c.push(false); 213 assert!(!c.get(0).unwrap()); 214 assert!(c.get(1).unwrap()); 215 assert!(c.get(2).is_none()); 216 assert!(c.get(3).is_none()); 217 assert!(c.get(4).is_none()); 218 assert!(c.get(5).is_none()); 219 assert!(c.get(usize::MAX).is_none()); 220 c.push(false); 221 assert!(!c.get(0).unwrap()); 222 assert!(!c.get(1).unwrap()); 223 assert!(c.get(2).unwrap()); 224 assert!(c.get(3).is_none()); 225 assert!(c.get(4).is_none()); 226 assert!(c.get(5).is_none()); 227 assert!(c.get(usize::MAX).is_none()); 228 c.push(true); 229 assert!(c.get(0).unwrap()); 230 assert!(!c.get(1).unwrap()); 231 assert!(!c.get(2).unwrap()); 232 assert!(c.get(3).unwrap()); 233 assert!(c.get(4).is_none()); 234 assert!(c.get(5).is_none()); 235 assert!(c.get(usize::MAX).is_none()); 236 c.push(true); 237 assert!(c.get(0).unwrap()); 238 assert!(c.get(1).unwrap()); 239 assert!(!c.get(2).unwrap()); 240 assert!(!c.get(3).unwrap()); 241 assert!(c.get(4).is_none()); 242 assert!(c.get(5).is_none()); 243 assert!(c.get(usize::MAX).is_none()); 244 } 245 #[test] 246 fn test_get_unsafe() { 247 let mut c = Cache::<bool, 4>::new(); 248 assert!(!c.get_unchecked(0)); 249 assert!(!c.get_unchecked(1)); 250 assert!(!c.get_unchecked(2)); 251 assert!(!c.get_unchecked(3)); 252 assert!(!c.get_unchecked(4)); 253 c.push(true); 254 assert!(c.get_unchecked(0)); 255 assert!(!c.get_unchecked(1)); 256 assert!(!c.get_unchecked(2)); 257 assert!(!c.get_unchecked(3)); 258 assert!(c.get_unchecked(4)); 259 } 260 #[test] 261 fn test_index() { 262 let mut c = Cache::<bool, 4>::new(); 263 c.push(true); 264 assert!(c[0]); 265 } 266 #[test] 267 #[should_panic] 268 fn test_index_panic() { 269 let c = Cache::<bool, 4>::new(); 270 assert!(c[0]); 271 } 272 #[test] 273 fn test_push() { 274 let mut c = Cache::<bool, 4>::new(); 275 c.push(true); 276 assert!(c[0]); 277 c.push(true); 278 assert!(c[0]); 279 c.push(false); 280 assert!(!c[0]); 281 c.push(true); 282 assert!(c[0]); 283 c.push(false); 284 assert!(!c[0]); 285 c.push(false); 286 assert!(!c[0]); 287 } 288 #[test] 289 fn test_new() { 290 _ = Cache::<bool, 0>::new(); 291 _ = Cache::<bool, 32>::new(); 292 _ = Cache::<bool, 31>::new(); 293 } 294 }