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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647
//! Cursor-like helper which allows removing elements from vector without //! moving the tail every time. //! //! ## `Vec::remove` comparison //! //! If you'll use common `Vec::remove` to remove only some elements starting //! from the head, you'll have bad times because it will copy a lot of elements: //! ```text //! vec's memory: [0, 1, 2, 3, 4] //! //! > vec.remove(0); //! vec's memory: [-, 1, 2, 3, 4] //! vec's memory: [1, 2, 3, 4, -] // copy of 4 elements (the whole tail) //! //! > vec.remove(1); //! vec's memory: [1, -, 3, 4, -] //! vec's memory: [1, 3, 4, -, -] // copy of 2 elements //! //! > vec.remove(2); //! vec's memory: [1, 3, -, -, -] //! ``` //! //! Whereas [`Removing`] uses `swap`s: //! //! ```text //! vec's memory: [0, 1, 2, 3, 4] //! rem's ptr: ^ //! //! > let rem = vec.removing(); //! > rem.next().unwrap().remove(); //! vec's memory: [-, 1, 2, 3, 4] //! rem's ptr: ^ //! //! > rem.next().unwrap(); //! vec's memory: [1, -, 2, 3, 4] // one copy of 1 //! rem's ptr: ^ //! //! > rem.next().unwrap().remove(); //! vec's memory: [1, -, -, 3, 4] //! rem's ptr: ^ //! //! > rem.next().unwrap(); //! vec's memory: [1, 3, -, -, 4] // one copy of 3 //! rem's ptr: ^ //! //! > rem.next().unwrap().remove(); //! vec's memory: [1, 3, -, -, -] //! ``` //! //! ## no_std support //! //! This crate supports `#![no_std]` but requires `alloc` (we are working with //! vec after all) #![deny(missing_docs)] #![cfg_attr(not(test), no_std)] extern crate alloc; use alloc::vec::Vec; use core::{mem::ManuallyDrop, ptr}; // FIXME(waffle): add support for backwards iteration /// A cursor-like structure made for cheap iterative removing of elements from /// vector. /// /// Essentially overhead of the process is one `ptr::copy` for each ignored /// element + one `ptr::copy` for the not yielded tail. /// /// For the comparison with manual [`Vec::remove`] see [self#] /// /// Also, an analog to this may be [`VecDeque`] (if you need to only pop /// elements from either of ends) or (unstable as of writing this) /// [`Vec::drain_filter`] (if you need to remove all matching elements). /// /// However note 2 differences between [`Vec::drain_filter`] and [`Removing`]: /// - [`Removing`] is not iterator (this makes it more universal but less /// convenient to use) /// - [`Removing`] does **not** remove elements from the vec when you drop it /// /// The main method of this struct is [`next`] which returns [`Entry`] which can /// be used to mutate the vec. /// /// [`VecDeque`]: alloc::collections::VecDeque /// [`next`]: Removing::next /// /// ## Examples /// /// ``` /// use vecrem::VecExt; /// /// let mut vec: Vec<_> = (0..17).collect(); /// let mut out = Vec::new(); /// { /// let mut rem = vec.removing(); /// /// while let Some(entry) = rem.next() { /// let value = *entry.value(); /// if value >= 10 { /// break; /// } /// /// if value % 2 == 0 { /// out.push(entry.remove()); /// } /// } /// } /// /// // All ignored and not yielded elements are in the original vec /// assert_eq!(vec, [1, 3, 5, 7, 9, 10, 11, 12, 13, 14, 15, 16]); /// /// assert_eq!(out, [0, 2, 4, 6, 8]) /// ``` /// /// ## forget behavior /// /// In the same way as [`Vec::drain_filter`], for efficient work, [`Removing`] /// needs to temporarily break vec's invariants leaving it in an inconsistent /// state. The state is made normal in `Drop`. /// /// However in rust running destructors is not guaranteed (see [`mem::forget`], /// [`ManuallyDrop`]). As such, on construction [`Removing`] sets vec's len to /// `0` (and restores it in `Drop`), this means that if [`Removing`] gets leaked /// or forgotten - the elements of the vector are forgotten too. /// /// ``` /// use vecrem::VecExt; /// /// let mut vec = vec![0, 1, 2, 3, 4]; /// core::mem::forget(vec.removing()); /// assert_eq!(vec, []); /// ``` /// /// [`mem::forget`]: core::mem::forget /// [`ManuallyDrop`]: core::mem::ManuallyDrop pub struct Removing<'a, T> { // Type invariants: // - vec.capacity() >= len // - vec[..slot] is initialized (note: due to safety guarantees vec.len is set to 0, so this is // not 'real' indexing) // - vec[curr..len] is initialized // - vec[..len] is the same allocation vec: &'a mut Vec<T>, slot: usize, curr: usize, len: usize, } /* Example of how the lib works, step-by-step: * * Imagine vec with items A-F: * * [A, B, C, D, E, F] * * 1. `Removing` is created * * [A, B, C, D, E, F] * \ * curr, slot * * 2. `.next()` is used, entry is not removed * * [A, B, C, D, E, F] * \ * curr, slot * * 3. `.next().remove()` * * slot * / * [A, _, C, D, E, F] * \ * curr * * 4. `.next().remove()` * * slot * / * [A, _, _, D, E, F] * \ * curr * * 5. `.next()` * * slot * / * [A, D, _, _, E, F] * \ * curr * * 5. Removing::drop moves the rest of elements & restores `vec`'s len (in * this case to 4): * * [A, D, E, F, _, _] */ impl<'a, T> Removing<'a, T> { /// Creates new [`Removing`] instance from given vec. /// /// See also: [`VecExt::removing`] #[inline] pub fn new(vec: &'a mut Vec<T>) -> Self { let len = vec.len(); // ## Safety // // - 0 <= vec.len() for any vec // - vec.len()..0 is always none elements => all needed elements are initialize unsafe { // This is needed to safely break vec invariants & not expose safety bags when vec.set_len(0); } Self { vec, slot: 0, curr: 0, len, } } /// Returns [`Entry`] for the next element. #[inline] #[allow(clippy::should_implement_trait)] pub fn next(&mut self) -> Option<Entry<'_, 'a, T>> { if self.is_empty() { None } else { // ## Safety // // Is not empty. Some(Entry { rem: self }) } } /// Returns number of remaining elements in this pseudo-iterator /// /// ## Examples /// /// ``` /// use vecrem::VecExt; /// /// let mut vec = vec![0, 1, 2, 3, 4]; /// let mut rem = vec.removing(); /// assert_eq!(rem.len(), 5); /// /// rem.next(); /// assert_eq!(rem.len(), 4); /// ``` pub fn len(&self) -> usize { self.len - self.curr } /// Return `true` if all elements of the underling vector were either /// ignored or [`remove`]d (i.e.: when [`next`] will return `None`) /// /// [`remove`]: Entry::remove /// [`next`]: Removing::next #[inline] pub fn is_empty(&self) -> bool { self.curr == self.len } // private /// ## Safety /// /// offset **must** be `<= vec.capacity()` unsafe fn ptr_mut(&mut self, offset: usize) -> *mut T { // Safety must be uphold be the caller self.vec.as_mut_ptr().add(offset) } fn slot_mut(&mut self) -> *mut T { // ## Safety // // `slot` always points to the content of the vector unsafe { self.ptr_mut(self.slot) } } fn curr_mut(&mut self) -> *mut T { // ## Safety // // `curr` always points to the content of the vector unsafe { self.ptr_mut(self.curr) } } } impl<T> Drop for Removing<'_, T> { fn drop(&mut self) { unsafe { let len = self.len(); if len != 0 { // Copy the tail // [A, B, -, -, -, C, D, E, F] => [A, B, C, D, E, F, D*, E*, F*] // * logically this memory is uninitialized i.e. it's a move ptr::copy(self.curr_mut(), self.slot_mut(), len); } // Slot points to the first uninitialized element self.vec.set_len(self.slot + len) } } } /// Vec entry. /// /// Entry allows you to [`remove`], [read] or [mutate] the element /// behind it. /// /// The only way to get this struct is to call [`Removing::next`] method. /// /// [`remove`]: Entry::remove /// [read]: Entry::value /// [mutate]: Entry::value_mut /// /// ## forget behavior /// /// When [`Entry`] destructor (drop) is not runned (this can be achieved via /// [`mem::forget`], [`ManuallyDrop`], etc) the entry is not skiped and the next /// call to [`Removing::next`] returns the same entry. /// /// ``` /// use core::mem; /// use vecrem::VecExt; /// /// let mut vec = vec![1, 2, 3]; /// { /// let mut rem = vec.removing(); /// /// let a = rem.next().unwrap(); /// assert_eq!(a.value(), &1); /// mem::forget(a); /// /// let b = rem.next().unwrap(); /// assert_eq!(b.value(), &1); /// mem::forget(b); /// } /// assert_eq!(vec, [1, 2, 3]); /// ``` /// /// [`mem::forget`]: core::mem::forget /// [`ManuallyDrop`]: core::mem::ManuallyDrop pub struct Entry<'a, 'rem, T> { // Type invariants: !self.rem.is_empty() rem: &'a mut Removing<'rem, T>, } impl<T> Entry<'_, '_, T> { /// Remove element behind this entry. #[inline] pub fn remove(self) -> T { // Prevents `self` from dropping (Self::drop would skip element) let mut this = ManuallyDrop::new(self); unsafe { let curr = this.curr_mut(); this.rem.curr += 1; // This read logically uninitializes mem behind `curr` ptr, but we've just moved // it, so it's ok. // // ## Safety // // Pointer is valid for reads by `Removing` invatiants. ptr::read(curr) } } /// Get access to the element behind this entry. #[inline] pub fn value(&self) -> &T { unsafe { // ## Safety // //`Entry` type invariants ensure that `curr` ptr is valid. &*self.curr_ptr() } } /// Get unique access to the element behind this entry. #[inline] pub fn value_mut(&mut self) -> &mut T { unsafe { // ## Safety // //`Entry` type invariants ensure that `curr` ptr is valid. &mut *self.curr_mut() } } /// Get unique access to the element after the element behind this entry. /// /// Returns `None` if this entry corresponds to the last item in the vec. #[inline] pub fn peek_next(&mut self) -> Option<&mut T> { let next = self.rem.curr + 1; if next >= self.rem.len { return None; } unsafe { // ## Safety // // We've just checked bounds Some(&mut *self.rem.ptr_mut(next)) } } // private fn curr_mut(&mut self) -> *mut T { self.rem.curr_mut() } fn curr_ptr(&self) -> *const T { // ## Safety // // `curr` always points to the content of the vector unsafe { self.rem.vec.as_ptr().add(self.rem.curr) } } fn slot_mut(&mut self) -> *mut T { self.rem.slot_mut() } } impl<T> Drop for Entry<'_, '_, T> { fn drop(&mut self) { // Skips element swaping it with the first empty slot. unsafe { // ## Safety // // `Entry` type invariants ensure that `self.rem.curr` points to a valid value, // and `self.rem.slot` is writable. ptr::copy(self.curr_mut(), self.slot_mut(), 1); self.rem.slot += 1; self.rem.curr += 1; } } } /// Extension for [`Vec`] which adds [`removing`] method /// /// [`Vec`]: alloc::vec::Vec /// [`removing`]: VecExt::removing pub trait VecExt<T> { /// Creates new [`Removing`] instance from given vec. fn removing(&mut self) -> Removing<T>; } impl<T> VecExt<T> for Vec<T> { #[inline] fn removing(&mut self) -> Removing<T> { Removing::new(self) } } #[cfg(test)] mod tests { use crate::VecExt; use core::{fmt::Debug, mem, ops::Rem}; /// Returns non-copy type that can help miri detect safety bugs fn f(i: i32) -> impl Clone + Debug + Eq + PartialOrd<i32> + Rem<i32, Output = i32> { #[derive(Clone, Debug, PartialEq, Eq)] struct NoCopy(i32); impl PartialEq<i32> for NoCopy { fn eq(&self, other: &i32) -> bool { self.0.eq(other) } } impl PartialOrd<i32> for NoCopy { fn partial_cmp(&self, other: &i32) -> Option<core::cmp::Ordering> { self.0.partial_cmp(other) } } impl Rem<i32> for NoCopy { type Output = i32; fn rem(self, rem: i32) -> i32 { self.0 % rem } } NoCopy(i) } fn zf(_i: i32) -> impl Debug + Eq { #[derive(Debug, PartialEq, Eq)] struct No; No } #[test] fn clear() { let mut vec: Vec<_> = (0..10).map(f).collect(); let mut out = Vec::with_capacity(10); let mut rem = vec.removing(); while let Some(entry) = rem.next() { out.push(entry.remove()); } assert_eq!(out, (0..10).map(f).collect::<Vec<_>>()) } #[test] fn skip() { let mut vec: Vec<_> = (0..10).map(f).collect(); let mut rem = vec.removing(); while let Some(_entry) = rem.next() {} } #[test] fn forget_entry() { let mut vec = vec![f(0)]; { let mut rem = vec.removing(); let mut timeout = 0..100; while let (Some(entry), Some(_)) = (rem.next(), timeout.next()) { mem::forget(entry) } assert_eq!(timeout.len(), 0); } assert_eq!(vec, [f(0)]); } #[test] fn zforget_entry() { let mut vec = vec![zf(0)]; { let mut rem = vec.removing(); let mut timeout = 0..100; while let (Some(entry), Some(_)) = (rem.next(), timeout.next()) { mem::forget(entry) } assert_eq!(timeout.len(), 0); } assert_eq!(vec, [zf(0)]); } #[test] // leaks mem #[cfg(not(miri))] fn forget() { let mut vec: Vec<_> = (0..10).map(f).collect(); let mut rem = vec.removing(); if let Some(entry) = rem.next() { entry.remove(); } if let Some(entry) = rem.next() { entry.remove(); } if let Some(entry) = rem.next() { entry.remove(); } mem::forget(rem); assert_eq!(vec, [0; 0]); } #[test] fn drop() { let mut vec: Vec<_> = (0..10).map(f).collect(); mem::drop(vec.removing()); assert_eq!(vec, (0..10).map(f).collect::<Vec<_>>()); } #[test] fn even() { let mut vec: Vec<_> = (0..10).map(f).collect(); let mut out = Vec::with_capacity(10); let mut rem = vec.removing(); while let Some(entry) = rem.next() { if entry.value().clone() % 2 == 0 { out.push(entry.remove()); } } mem::drop(rem); assert_eq!(out, [0, 2, 4, 6, 8]); assert_eq!(vec, [1, 3, 5, 7, 9]); } #[test] fn break_() { let mut vec: Vec<_> = (0..10).map(f).collect(); let mut rem = vec.removing(); while let Some(entry) = rem.next() { if *entry.value() >= 5 { break; } entry.remove(); } mem::drop(rem); assert_eq!(vec, [5, 6, 7, 8, 9]); } #[test] fn test() { let mut vec: Vec<_> = (0..3).map(f).collect(); { let mut rem = vec.removing(); rem.next().unwrap(); assert_eq!(rem.next().unwrap().remove(), 1); rem.next().unwrap(); } assert_eq!(vec, [0, 2]); } #[test] fn zst() { let mut vec: Vec<_> = (0..3).map(zf).collect(); { let mut rem = vec.removing(); rem.next().unwrap(); rem.next().unwrap().remove(); rem.next().unwrap(); } assert_eq!(vec.len(), 2); } }