1// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2// file at the top-level directory of this distribution and at
3// http://rust-lang.org/COPYRIGHT.
4//
5// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8// option. This file may not be copied, modified, or distributed
9// except according to those terms.
10
11//! Union-find implementation. The main type is `UnificationTable`.
12//!
13//! You can define your own type for the *keys* in the table, but you
14//! must implement `UnifyKey` for that type. The assumption is that
15//! keys will be newtyped integers, hence we require that they
16//! implement `Copy`.
17//!
18//! Keys can have values associated with them. The assumption is that
19//! these values are cheaply cloneable (ideally, `Copy`), and some of
20//! the interfaces are oriented around that assumption. If you just
21//! want the classical "union-find" algorithm where you group things
22//! into sets, use the `Value` type of `()`.
23//!
24//! When you have keys with non-trivial values, you must also define
25//! how those values can be merged. As part of doing this, you can
26//! define the "error" type to return on error; if errors are not
27//! possible, use `NoError` (an uninstantiable struct). Using this
28//! type also unlocks various more ergonomic methods (e.g., `union()`
29//! in place of `unify_var_var()`).
30//!
31//! The best way to see how it is used is to read the `tests.rs` file;
32//! search for e.g. `UnitKey`.
33
34use std::fmt::Debug;
35use std::marker;
36use std::ops::Range;
37
38use snapshot_vec::{self as sv, UndoLog};
39use undo_log::{UndoLogs, VecLog};
40
41mod backing_vec;
42pub use self::backing_vec::{
43 Delegate, InPlace, UnificationStore, UnificationStoreBase, UnificationStoreMut,
44};
45
46#[cfg(feature = "persistent")]
47pub use self::backing_vec::Persistent;
48
49#[cfg(test)]
50mod tests;
51
52/// This trait is implemented by any type that can serve as a type
53/// variable. We call such variables *unification keys*. For example,
54/// this trait is implemented by `IntVid`, which represents integral
55/// variables.
56///
57/// Each key type has an associated value type `V`. For example, for
58/// `IntVid`, this is `Option<IntVarValue>`, representing some
59/// (possibly not yet known) sort of integer.
60///
61/// Clients are expected to provide implementations of this trait; you
62/// can see some examples in the `test` module.
63pub trait UnifyKey: Copy + Clone + Debug + PartialEq {
64 type Value: UnifyValue;
65
66 fn index(&self) -> u32;
67
68 fn from_index(u: u32) -> Self;
69
70 fn tag() -> &'static str;
71
72 /// You should return first the key that should be used as root,
73 /// then the other key (that will then point to the new root).
74 ///
75 /// NB. The only reason to implement this method is if you want to
76 /// control what value is returned from `find()`. In general, it
77 /// is better to let the unification table determine the root,
78 /// since overriding the rank can cause execution time to increase
79 /// dramatically.
80 #[allow(unused_variables)]
81 fn order_roots(
82 a: Self,
83 a_value: &Self::Value,
84 b: Self,
85 b_value: &Self::Value,
86 ) -> Option<(Self, Self)> {
87 None
88 }
89}
90
91/// Trait implemented for **values** associated with a unification
92/// key. This trait defines how to merge the values from two keys that
93/// are unioned together. This merging can be fallible. If you attempt
94/// to union two keys whose values cannot be merged, then the error is
95/// propagated up and the two keys are not unioned.
96///
97/// This crate provides implementations of `UnifyValue` for `()`
98/// (which is infallible) and `Option<T>` (where `T: UnifyValue`). The
99/// option implementation merges two sum-values using the `UnifyValue`
100/// implementation of `T`.
101///
102/// See also `EqUnifyValue`, which is a convenience trait for cases
103/// where the "merge" operation succeeds only if the two values are
104/// equal.
105pub trait UnifyValue: Clone + Debug {
106 /// Defines the type to return when merging of two values fails.
107 /// If merging is infallible, use the special struct `NoError`
108 /// found in this crate, which unlocks various more convenient
109 /// methods on the unification table.
110 type Error;
111
112 /// Given two values, produce a new value that combines them.
113 /// If that is not possible, produce an error.
114 fn unify_values(value1: &Self, value2: &Self) -> Result<Self, Self::Error>;
115}
116
117/// A convenient helper for unification values which must be equal or
118/// else an error occurs. For example, if you are unifying types in a
119/// simple functional language, this may be appropriate, since (e.g.)
120/// you can't unify a type variable bound to `int` with one bound to
121/// `float` (but you can unify two type variables both bound to
122/// `int`).
123///
124/// Any type which implements `EqUnifyValue` automatially implements
125/// `UnifyValue`; if the two values are equal, merging is permitted.
126/// Otherwise, the error `(v1, v2)` is returned, where `v1` and `v2`
127/// are the two unequal values.
128pub trait EqUnifyValue: Eq + Clone + Debug {}
129
130impl<T: EqUnifyValue> UnifyValue for T {
131 type Error = (T, T);
132
133 fn unify_values(value1: &Self, value2: &Self) -> Result<Self, Self::Error> {
134 if value1 == value2 {
135 Ok(value1.clone())
136 } else {
137 Err((value1.clone(), value2.clone()))
138 }
139 }
140}
141
142/// A struct which can never be instantiated. Used
143/// for the error type for infallible cases.
144#[derive(Debug)]
145pub struct NoError {
146 _dummy: (),
147}
148
149/// Value of a unification key. We implement Tarjan's union-find
150/// algorithm: when two keys are unified, one of them is converted
151/// into a "redirect" pointing at the other. These redirects form a
152/// DAG: the roots of the DAG (nodes that are not redirected) are each
153/// associated with a value of type `V` and a rank. The rank is used
154/// to keep the DAG relatively balanced, which helps keep the running
155/// time of the algorithm under control. For more information, see
156/// <http://en.wikipedia.org/wiki/Disjoint-set_data_structure>.
157#[derive(PartialEq, Clone, Debug)]
158pub struct VarValue<K: UnifyKey> {
159 parent: K, // if equal to self, this is a root
160 value: K::Value, // value assigned (only relevant to root)
161 rank: u32, // max depth (only relevant to root)
162}
163
164/// Table of unification keys and their values. You must define a key type K
165/// that implements the `UnifyKey` trait. Unification tables can be used in two-modes:
166///
167/// - in-place (`UnificationTable<InPlace<K>>` or `InPlaceUnificationTable<K>`):
168/// - This is the standard mutable mode, where the array is modified
169/// in place.
170/// - To do backtracking, you can employ the `snapshot` and `rollback_to`
171/// methods.
172/// - persistent (`UnificationTable<Persistent<K>>` or `PersistentUnificationTable<K>`):
173/// - In this mode, we use a persistent vector to store the data, so that
174/// cloning the table is an O(1) operation.
175/// - This implies that ordinary operations are quite a bit slower though.
176/// - Requires the `persistent` feature be selected in your Cargo.toml file.
177#[derive(Clone, Debug, Default)]
178pub struct UnificationTable<S: UnificationStoreBase> {
179 /// Indicates the current value of each key.
180 values: S,
181}
182
183pub type UnificationStorage<K> = Vec<VarValue<K>>;
184pub type UnificationTableStorage<K> = UnificationTable<InPlace<K, UnificationStorage<K>, ()>>;
185
186/// A unification table that uses an "in-place" vector.
187#[allow(type_alias_bounds)]
188pub type InPlaceUnificationTable<
189 K: UnifyKey,
190 V: sv::VecLike<Delegate<K>> = Vec<VarValue<K>>,
191 L = VecLog<UndoLog<Delegate<K>>>,
192> = UnificationTable<InPlace<K, V, L>>;
193
194/// A unification table that uses a "persistent" vector.
195#[cfg(feature = "persistent")]
196#[allow(type_alias_bounds)]
197pub type PersistentUnificationTable<K: UnifyKey> = UnificationTable<Persistent<K>>;
198
199/// At any time, users may snapshot a unification table. The changes
200/// made during the snapshot may either be *committed* or *rolled back*.
201pub struct Snapshot<S: UnificationStore> {
202 // Link snapshot to the unification store `S` of the table.
203 marker: marker::PhantomData<S>,
204 snapshot: S::Snapshot,
205}
206
207impl<K: UnifyKey> VarValue<K> {
208 fn new_var(key: K, value: K::Value) -> VarValue<K> {
209 VarValue::new(parent:key, value, rank:0)
210 }
211
212 fn new(parent: K, value: K::Value, rank: u32) -> VarValue<K> {
213 VarValue {
214 parent: parent, // this is a root
215 value: value,
216 rank: rank,
217 }
218 }
219
220 fn redirect(&mut self, to: K) {
221 self.parent = to;
222 }
223
224 fn root(&mut self, rank: u32, value: K::Value) {
225 self.rank = rank;
226 self.value = value;
227 }
228}
229
230impl<K> UnificationTableStorage<K>
231where
232 K: UnifyKey,
233{
234 /// Creates a `UnificationTable` using an external `undo_log`, allowing mutating methods to be
235 /// called if `L` does not implement `UndoLogs`
236 pub fn with_log<'a, L>(
237 &'a mut self,
238 undo_log: L,
239 ) -> UnificationTable<InPlace<K, &'a mut UnificationStorage<K>, L>>
240 where
241 L: UndoLogs<sv::UndoLog<Delegate<K>>>,
242 {
243 UnificationTable {
244 values: InPlace {
245 values: self.values.values.with_log(undo_log),
246 },
247 }
248 }
249}
250
251// We can't use V:LatticeValue, much as I would like to,
252// because frequently the pattern is that V=Option<U> for some
253// other type parameter U, and we have no way to say
254// Option<U>:LatticeValue.
255
256impl<S: UnificationStoreBase + Default> UnificationTable<S> {
257 pub fn new() -> Self {
258 Self::default()
259 }
260}
261
262impl<S: UnificationStore> UnificationTable<S> {
263 /// Starts a new snapshot. Each snapshot must be either
264 /// rolled back or committed in a "LIFO" (stack) order.
265 pub fn snapshot(&mut self) -> Snapshot<S> {
266 Snapshot {
267 marker: marker::PhantomData::<S>,
268 snapshot: self.values.start_snapshot(),
269 }
270 }
271
272 /// Reverses all changes since the last snapshot. Also
273 /// removes any keys that have been created since then.
274 pub fn rollback_to(&mut self, snapshot: Snapshot<S>) {
275 debug!("{}: rollback_to()", S::tag());
276 self.values.rollback_to(snapshot.snapshot);
277 }
278
279 /// Commits all changes since the last snapshot. Of course, they
280 /// can still be undone if there is a snapshot further out.
281 pub fn commit(&mut self, snapshot: Snapshot<S>) {
282 debug!("{}: commit()", S::tag());
283 self.values.commit(snapshot.snapshot);
284 }
285
286 /// Returns the keys of all variables created since the `snapshot`.
287 pub fn vars_since_snapshot(&self, snapshot: &Snapshot<S>) -> Range<S::Key> {
288 let range = self.values.values_since_snapshot(&snapshot.snapshot);
289 S::Key::from_index(range.start as u32)..S::Key::from_index(range.end as u32)
290 }
291}
292
293impl<S: UnificationStoreBase> UnificationTable<S> {
294 /// Returns the number of keys created so far.
295 pub fn len(&self) -> usize {
296 self.values.len()
297 }
298
299 /// Obtains the current value for a particular key.
300 /// Not for end-users; they can use `probe_value`.
301 fn value(&self, key: S::Key) -> &VarValue<S::Key> {
302 &self.values[key.index() as usize]
303 }
304}
305
306impl<S: UnificationStoreMut> UnificationTable<S> {
307 /// Creates a fresh key with the given value.
308 pub fn new_key(&mut self, value: S::Value) -> S::Key {
309 let len = self.values.len();
310 let key: S::Key = UnifyKey::from_index(len as u32);
311 self.values.push(VarValue::new_var(key, value));
312 debug!("{}: created new key: {:?}", S::tag(), key);
313 key
314 }
315
316 /// Reserve memory for `num_new_keys` to be created. Does not
317 /// actually create the new keys; you must then invoke `new_key`.
318 pub fn reserve(&mut self, num_new_keys: usize) {
319 self.values.reserve(num_new_keys);
320 }
321
322 /// Clears all unifications that have been performed, resetting to
323 /// the initial state. The values of each variable are given by
324 /// the closure.
325 pub fn reset_unifications(&mut self, mut value: impl FnMut(S::Key) -> S::Value) {
326 self.values.reset_unifications(|i| {
327 let key = UnifyKey::from_index(i as u32);
328 let value = value(key);
329 VarValue::new_var(key, value)
330 });
331 }
332
333 /// Find the root node for `vid`. This uses the standard
334 /// union-find algorithm with path compression:
335 /// <http://en.wikipedia.org/wiki/Disjoint-set_data_structure>.
336 ///
337 /// NB. This is a building-block operation and you would probably
338 /// prefer to call `probe` below.
339 ///
340 /// This is an always-inlined version of this function for the hot
341 /// callsites. `uninlined_get_root_key` is the never-inlined version.
342 #[inline(always)]
343 fn inlined_get_root_key(&mut self, vid: S::Key) -> S::Key {
344 let v = self.value(vid);
345 if v.parent == vid {
346 return vid;
347 }
348
349 let redirect = v.parent;
350 let root_key: S::Key = self.uninlined_get_root_key(redirect);
351 if root_key != redirect {
352 // Path compression
353 self.update_value(vid, |value| value.parent = root_key);
354 }
355
356 root_key
357 }
358
359 // This is a never-inlined version of this function for cold callsites.
360 // 'inlined_get_root_key` is the always-inlined version.
361 #[inline(never)]
362 fn uninlined_get_root_key(&mut self, vid: S::Key) -> S::Key {
363 self.inlined_get_root_key(vid)
364 }
365
366 fn update_value<OP>(&mut self, key: S::Key, op: OP)
367 where
368 OP: FnOnce(&mut VarValue<S::Key>),
369 {
370 self.values.update(key.index() as usize, op);
371 debug!("Updated variable {:?} to {:?}", key, self.value(key));
372 }
373
374 /// Either redirects `node_a` to `node_b` or vice versa, depending
375 /// on the relative rank. The value associated with the new root
376 /// will be `new_value`.
377 ///
378 /// NB: This is the "union" operation of "union-find". It is
379 /// really more of a building block. If the values associated with
380 /// your key are non-trivial, you would probably prefer to call
381 /// `unify_var_var` below.
382 fn unify_roots(&mut self, key_a: S::Key, key_b: S::Key, new_value: S::Value) {
383 debug!("unify(key_a={:?}, key_b={:?})", key_a, key_b);
384
385 let rank_a = self.value(key_a).rank;
386 let rank_b = self.value(key_b).rank;
387 if let Some((new_root, redirected)) = S::Key::order_roots(
388 key_a,
389 &self.value(key_a).value,
390 key_b,
391 &self.value(key_b).value,
392 ) {
393 // compute the new rank for the new root that they chose;
394 // this may not be the optimal choice.
395 let new_rank = if new_root == key_a {
396 debug_assert!(redirected == key_b);
397 if rank_a > rank_b {
398 rank_a
399 } else {
400 rank_b + 1
401 }
402 } else {
403 debug_assert!(new_root == key_b);
404 debug_assert!(redirected == key_a);
405 if rank_b > rank_a {
406 rank_b
407 } else {
408 rank_a + 1
409 }
410 };
411 self.redirect_root(new_rank, redirected, new_root, new_value);
412 } else if rank_a > rank_b {
413 // a has greater rank, so a should become b's parent,
414 // i.e., b should redirect to a.
415 self.redirect_root(rank_a, key_b, key_a, new_value);
416 } else if rank_a < rank_b {
417 // b has greater rank, so a should redirect to b.
418 self.redirect_root(rank_b, key_a, key_b, new_value);
419 } else {
420 // If equal, redirect one to the other and increment the
421 // other's rank.
422 self.redirect_root(rank_a + 1, key_a, key_b, new_value);
423 }
424 }
425
426 /// Internal method to redirect `old_root_key` (which is currently
427 /// a root) to a child of `new_root_key` (which will remain a
428 /// root). The rank and value of `new_root_key` will be updated to
429 /// `new_rank` and `new_value` respectively.
430 fn redirect_root(
431 &mut self,
432 new_rank: u32,
433 old_root_key: S::Key,
434 new_root_key: S::Key,
435 new_value: S::Value,
436 ) {
437 self.update_value(old_root_key, |old_root_value| {
438 old_root_value.redirect(new_root_key);
439 });
440 self.update_value(new_root_key, |new_root_value| {
441 new_root_value.root(new_rank, new_value);
442 });
443 }
444}
445
446/// ////////////////////////////////////////////////////////////////////////
447/// Public API
448
449impl<S, K, V> UnificationTable<S>
450where
451 S: UnificationStoreBase<Key = K, Value = V>,
452 K: UnifyKey<Value = V>,
453 V: UnifyValue,
454{
455 /// Obtains current value for key without any pointer chasing; may return `None` if key has been union'd.
456 #[inline]
457 pub fn try_probe_value<'a, K1>(&'a self, id: K1) -> Option<&'a V>
458 where
459 K1: Into<K>,
460 K: 'a,
461 {
462 let id: K = id.into();
463 let v: &VarValue = self.value(key:id);
464 if v.parent == id {
465 return Some(&v.value);
466 }
467 None
468 }
469}
470
471impl<S, K, V> UnificationTable<S>
472where
473 S: UnificationStoreMut<Key = K, Value = V>,
474 K: UnifyKey<Value = V>,
475 V: UnifyValue,
476{
477 /// Unions two keys without the possibility of failure; only
478 /// applicable when unify values use `NoError` as their error
479 /// type.
480 pub fn union<K1, K2>(&mut self, a_id: K1, b_id: K2)
481 where
482 K1: Into<K>,
483 K2: Into<K>,
484 V: UnifyValue<Error = NoError>,
485 {
486 self.unify_var_var(a_id, b_id).unwrap();
487 }
488
489 /// Unions a key and a value without the possibility of failure;
490 /// only applicable when unify values use `NoError` as their error
491 /// type.
492 pub fn union_value<K1>(&mut self, id: K1, value: V)
493 where
494 K1: Into<K>,
495 V: UnifyValue<Error = NoError>,
496 {
497 self.unify_var_value(id, value).unwrap();
498 }
499
500 /// Given two keys, indicates whether they have been unioned together.
501 pub fn unioned<K1, K2>(&mut self, a_id: K1, b_id: K2) -> bool
502 where
503 K1: Into<K>,
504 K2: Into<K>,
505 {
506 self.find(a_id) == self.find(b_id)
507 }
508
509 /// Given a key, returns the (current) root key.
510 pub fn find<K1>(&mut self, id: K1) -> K
511 where
512 K1: Into<K>,
513 {
514 let id = id.into();
515 self.uninlined_get_root_key(id)
516 }
517
518 /// Unions together two variables, merging their values. If
519 /// merging the values fails, the error is propagated and this
520 /// method has no effect.
521 pub fn unify_var_var<K1, K2>(&mut self, a_id: K1, b_id: K2) -> Result<(), V::Error>
522 where
523 K1: Into<K>,
524 K2: Into<K>,
525 {
526 let a_id = a_id.into();
527 let b_id = b_id.into();
528
529 let root_a = self.uninlined_get_root_key(a_id);
530 let root_b = self.uninlined_get_root_key(b_id);
531
532 if root_a == root_b {
533 return Ok(());
534 }
535
536 let combined = V::unify_values(&self.value(root_a).value, &self.value(root_b).value)?;
537
538 Ok(self.unify_roots(root_a, root_b, combined))
539 }
540
541 /// Sets the value of the key `a_id` to `b`, attempting to merge
542 /// with the previous value.
543 pub fn unify_var_value<K1>(&mut self, a_id: K1, b: V) -> Result<(), V::Error>
544 where
545 K1: Into<K>,
546 {
547 let a_id = a_id.into();
548 let root_a = self.uninlined_get_root_key(a_id);
549 let value = V::unify_values(&self.value(root_a).value, &b)?;
550 self.update_value(root_a, |node| node.value = value);
551 Ok(())
552 }
553
554 /// Returns the current value for the given key. If the key has
555 /// been union'd, this will give the value from the current root.
556 pub fn probe_value<K1>(&mut self, id: K1) -> V
557 where
558 K1: Into<K>,
559 {
560 self.inlined_probe_value(id)
561 }
562
563 // An always-inlined version of `probe_value`, for hot callsites.
564 #[inline(always)]
565 pub fn inlined_probe_value<K1>(&mut self, id: K1) -> V
566 where
567 K1: Into<K>,
568 {
569 let id = id.into();
570 let id = self.inlined_get_root_key(id);
571 self.value(id).value.clone()
572 }
573}
574
575///////////////////////////////////////////////////////////////////////////
576
577impl UnifyValue for () {
578 type Error = NoError;
579
580 fn unify_values(_: &(), _: &()) -> Result<(), NoError> {
581 Ok(())
582 }
583}
584
585impl<V: UnifyValue> UnifyValue for Option<V> {
586 type Error = V::Error;
587
588 fn unify_values(a: &Option<V>, b: &Option<V>) -> Result<Self, V::Error> {
589 match (a, b) {
590 (&None, &None) => Ok(None),
591 (&Some(ref v: &V), &None) | (&None, &Some(ref v: &V)) => Ok(Some(v.clone())),
592 (&Some(ref a: &V), &Some(ref b: &V)) => match V::unify_values(value1:a, value2:b) {
593 Ok(v: V) => Ok(Some(v)),
594 Err(err: ::Error) => Err(err),
595 },
596 }
597 }
598}
599