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.
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.
13 use std::marker::PhantomData;
14 use snapshot_vec as sv;
19 /// This trait is implemented by any type that can serve as a type
20 /// variable. We call such variables *unification keys*. For example,
21 /// this trait is implemented by `IntVid`, which represents integral
24 /// Each key type has an associated value type `V`. For example, for
25 /// `IntVid`, this is `Option<IntVarValue>`, representing some
26 /// (possibly not yet known) sort of integer.
28 /// Clients are expected to provide implementations of this trait; you
29 /// can see some examples in the `test` module.
30 pub trait UnifyKey : Copy + Clone + Debug + PartialEq {
31 type Value: Clone + PartialEq + Debug;
33 fn index(&self) -> u32;
35 fn from_index(u: u32) -> Self;
37 fn tag(k: Option<Self>) -> &'static str;
40 /// This trait is implemented for unify values that can be
41 /// combined. This relation should be a monoid.
43 fn combine(&self, other: &Self) -> Self;
47 fn combine(&self, _other: &()) {}
50 /// Value of a unification key. We implement Tarjan's union-find
51 /// algorithm: when two keys are unified, one of them is converted
52 /// into a "redirect" pointing at the other. These redirects form a
53 /// DAG: the roots of the DAG (nodes that are not redirected) are each
54 /// associated with a value of type `V` and a rank. The rank is used
55 /// to keep the DAG relatively balanced, which helps keep the running
56 /// time of the algorithm under control. For more information, see
57 /// <http://en.wikipedia.org/wiki/Disjoint-set_data_structure>.
58 #[derive(PartialEq,Clone,Debug)]
59 pub struct VarValue<K: UnifyKey> {
60 parent: K, // if equal to self, this is a root
61 value: K::Value, // value assigned (only relevant to root)
62 rank: u32, // max depth (only relevant to root)
65 /// Table of unification keys and their values.
66 pub struct UnificationTable<K: UnifyKey> {
67 /// Indicates the current value of each key.
68 values: sv::SnapshotVec<Delegate<K>>,
71 /// At any time, users may snapshot a unification table. The changes
72 /// made during the snapshot may either be *committed* or *rolled back*.
73 pub struct Snapshot<K: UnifyKey> {
74 // Link snapshot to the key type `K` of the table.
75 marker: marker::PhantomData<K>,
76 snapshot: sv::Snapshot,
79 #[derive(Copy, Clone)]
80 struct Delegate<K>(PhantomData<K>);
82 impl<K: UnifyKey> VarValue<K> {
83 fn new_var(key: K, value: K::Value) -> VarValue<K> {
84 VarValue::new(key, value, 0)
87 fn new(parent: K, value: K::Value, rank: u32) -> VarValue<K> {
89 parent: parent, // this is a root
95 fn redirect(self, to: K) -> VarValue<K> {
96 VarValue { parent: to, ..self }
99 fn root(self, rank: u32, value: K::Value) -> VarValue<K> {
107 /// Returns the key of this node. Only valid if this is a root
108 /// node, which you yourself must ensure.
113 fn parent(&self, self_key: K) -> Option<K> {
114 self.if_not_self(self.parent, self_key)
117 fn if_not_self(&self, key: K, self_key: K) -> Option<K> {
126 // We can't use V:LatticeValue, much as I would like to,
127 // because frequently the pattern is that V=Option<U> for some
128 // other type parameter U, and we have no way to say
129 // Option<U>:LatticeValue.
131 impl<K: UnifyKey> UnificationTable<K> {
132 pub fn new() -> UnificationTable<K> {
133 UnificationTable { values: sv::SnapshotVec::new() }
136 /// Starts a new snapshot. Each snapshot must be either
137 /// rolled back or committed in a "LIFO" (stack) order.
138 pub fn snapshot(&mut self) -> Snapshot<K> {
140 marker: marker::PhantomData::<K>,
141 snapshot: self.values.start_snapshot(),
145 /// Reverses all changes since the last snapshot. Also
146 /// removes any keys that have been created since then.
147 pub fn rollback_to(&mut self, snapshot: Snapshot<K>) {
148 debug!("{}: rollback_to()", UnifyKey::tag(None::<K>));
149 self.values.rollback_to(snapshot.snapshot);
152 /// Commits all changes since the last snapshot. Of course, they
153 /// can still be undone if there is a snapshot further out.
154 pub fn commit(&mut self, snapshot: Snapshot<K>) {
155 debug!("{}: commit()", UnifyKey::tag(None::<K>));
156 self.values.commit(snapshot.snapshot);
159 pub fn new_key(&mut self, value: K::Value) -> K {
160 let len = self.values.len();
161 let key: K = UnifyKey::from_index(len as u32);
162 self.values.push(VarValue::new_var(key, value));
163 debug!("{}: created new key: {:?}", UnifyKey::tag(None::<K>), key);
167 /// Find the root node for `vid`. This uses the standard
168 /// union-find algorithm with path compression:
169 /// <http://en.wikipedia.org/wiki/Disjoint-set_data_structure>.
171 /// NB. This is a building-block operation and you would probably
172 /// prefer to call `probe` below.
173 fn get(&mut self, vid: K) -> VarValue<K> {
174 let index = vid.index() as usize;
175 let mut value: VarValue<K> = self.values.get(index).clone();
176 match value.parent(vid) {
178 let root: VarValue<K> = self.get(redirect);
179 if root.key() != redirect {
181 value.parent = root.key();
182 self.values.set(index, value);
190 fn is_root(&self, key: K) -> bool {
191 let index = key.index() as usize;
192 self.values.get(index).parent(key).is_none()
195 /// Sets the value for `vid` to `new_value`. `vid` MUST be a root
196 /// node! This is an internal operation used to impl other things.
197 fn set(&mut self, key: K, new_value: VarValue<K>) {
198 assert!(self.is_root(key));
200 debug!("Updating variable {:?} to {:?}", key, new_value);
202 let index = key.index() as usize;
203 self.values.set(index, new_value);
206 /// Either redirects `node_a` to `node_b` or vice versa, depending
207 /// on the relative rank. The value associated with the new root
208 /// will be `new_value`.
210 /// NB: This is the "union" operation of "union-find". It is
211 /// really more of a building block. If the values associated with
212 /// your key are non-trivial, you would probably prefer to call
213 /// `unify_var_var` below.
214 fn unify(&mut self, root_a: VarValue<K>, root_b: VarValue<K>, new_value: K::Value) {
215 debug!("unify(root_a(id={:?}, rank={:?}), root_b(id={:?}, rank={:?}))",
221 if root_a.rank > root_b.rank {
222 // a has greater rank, so a should become b's parent,
223 // i.e., b should redirect to a.
224 self.redirect_root(root_a.rank, root_b, root_a, new_value);
225 } else if root_a.rank < root_b.rank {
226 // b has greater rank, so a should redirect to b.
227 self.redirect_root(root_b.rank, root_a, root_b, new_value);
229 // If equal, redirect one to the other and increment the
231 self.redirect_root(root_a.rank + 1, root_a, root_b, new_value);
235 fn redirect_root(&mut self,
237 old_root: VarValue<K>,
238 new_root: VarValue<K>,
239 new_value: K::Value) {
240 let old_root_key = old_root.key();
241 let new_root_key = new_root.key();
242 self.set(old_root_key, old_root.redirect(new_root_key));
243 self.set(new_root_key, new_root.root(new_rank, new_value));
247 impl<K: UnifyKey> sv::SnapshotVecDelegate for Delegate<K> {
248 type Value = VarValue<K>;
251 fn reverse(_: &mut Vec<VarValue<K>>, _: ()) {}
254 // # Base union-find algorithm, where we are just making sets
256 impl<'tcx, K: UnifyKey> UnificationTable<K>
257 where K::Value: Combine
259 pub fn union(&mut self, a_id: K, b_id: K) {
260 let node_a = self.get(a_id);
261 let node_b = self.get(b_id);
262 let a_id = node_a.key();
263 let b_id = node_b.key();
265 let new_value = node_a.value.combine(&node_b.value);
266 self.unify(node_a, node_b, new_value);
270 pub fn find(&mut self, id: K) -> K {
274 pub fn find_value(&mut self, id: K) -> K::Value {
278 pub fn unioned(&mut self, a_id: K, b_id: K) -> bool {
279 self.find(a_id) == self.find(b_id)
283 // # Non-subtyping unification
285 // Code to handle keys which carry a value, like ints,
286 // floats---anything that doesn't have a subtyping relationship we
287 // need to worry about.
289 impl<'tcx, K, V> UnificationTable<K>
290 where K: UnifyKey<Value = Option<V>>,
291 V: Clone + PartialEq + Debug
293 pub fn unify_var_var(&mut self, a_id: K, b_id: K) -> Result<(), (V, V)> {
294 let node_a = self.get(a_id);
295 let node_b = self.get(b_id);
296 let a_id = node_a.key();
297 let b_id = node_b.key();
304 match (&node_a.value, &node_b.value) {
305 (&None, &None) => None,
306 (&Some(ref v), &None) | (&None, &Some(ref v)) => Some(v.clone()),
307 (&Some(ref v1), &Some(ref v2)) => {
309 return Err((v1.clone(), v2.clone()));
316 Ok(self.unify(node_a, node_b, combined))
319 /// Sets the value of the key `a_id` to `b`. Because simple keys do not have any subtyping
320 /// relationships, if `a_id` already has a value, it must be the same as `b`.
321 pub fn unify_var_value(&mut self, a_id: K, b: V) -> Result<(), (V, V)> {
322 let mut node_a = self.get(a_id);
326 node_a.value = Some(b);
327 self.set(node_a.key(), node_a);
335 Err((a_t.clone(), b))
341 pub fn has_value(&mut self, id: K) -> bool {
342 self.get(id).value.is_some()
345 pub fn probe(&mut self, a_id: K) -> Option<V> {
346 self.get(a_id).value.clone()
349 pub fn unsolved_variables(&mut self) -> Vec<K> {
353 if vv.value.is_some() {