src/librustc_data_structures/unify/mod.rs

   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 use std::marker;
  12 use std::fmt::Debug;
  13 use std::marker::PhantomData;
  14 use snapshot_vec as sv;
  15
  16 #[cfg(test)]
  17 mod tests;
  18
  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
  22 /// variables.
  23 ///
  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.
  27 ///
  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;
  32
  33     fn index(&self) -> u32;
  34
  35     fn from_index(u: u32) -> Self;
  36
  37     fn tag(k: Option<Self>) -> &'static str;
  38 }
  39
  40 /// This trait is implemented for unify values that can be
  41 /// combined. This relation should be a monoid.
  42 pub trait Combine {
  43     fn combine(&self, other: &Self) -> Self;
  44 }
  45
  46 impl Combine for () {
  47     fn combine(&self, _other: &()) {}
  48 }
  49
  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)
  63 }
  64
  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>>,
  69 }
  70
  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,
  77 }
  78
  79 #[derive(Copy, Clone)]
  80 struct Delegate<K>(PhantomData<K>);
  81
  82 impl<K: UnifyKey> VarValue<K> {
  83     fn new_var(key: K, value: K::Value) -> VarValue<K> {
  84         VarValue::new(key, value, 0)
  85     }
  86
  87     fn new(parent: K, value: K::Value, rank: u32) -> VarValue<K> {
  88         VarValue {
  89             parent: parent, // this is a root
  90             value: value,
  91             rank: rank,
  92         }
  93     }
  94
  95     fn redirect(self, to: K) -> VarValue<K> {
  96         VarValue { parent: to, ..self }
  97     }
  98
  99     fn root(self, rank: u32, value: K::Value) -> VarValue<K> {
 100         VarValue {
 101             rank: rank,
 102             value: value,
 103             ..self
 104         }
 105     }
 106
 107     /// Returns the key of this node. Only valid if this is a root
 108     /// node, which you yourself must ensure.
 109     fn key(&self) -> K {
 110         self.parent
 111     }
 112
 113     fn parent(&self, self_key: K) -> Option<K> {
 114         self.if_not_self(self.parent, self_key)
 115     }
 116
 117     fn if_not_self(&self, key: K, self_key: K) -> Option<K> {
 118         if key == self_key {
 119             None
 120         } else {
 121             Some(key)
 122         }
 123     }
 124 }
 125
 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.
 130
 131 impl<K: UnifyKey> UnificationTable<K> {
 132     pub fn new() -> UnificationTable<K> {
 133         UnificationTable { values: sv::SnapshotVec::new() }
 134     }
 135
 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> {
 139         Snapshot {
 140             marker: marker::PhantomData::<K>,
 141             snapshot: self.values.start_snapshot(),
 142         }
 143     }
 144
 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);
 150     }
 151
 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);
 157     }
 158
 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);
 164         key
 165     }
 166
 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>.
 170     ///
 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) {
 177             Some(redirect) => {
 178                 let root: VarValue<K> = self.get(redirect);
 179                 if root.key() != redirect {
 180                     // Path compression
 181                     value.parent = root.key();
 182                     self.values.set(index, value);
 183                 }
 184                 root
 185             }
 186             None => value,
 187         }
 188     }
 189
 190     fn is_root(&self, key: K) -> bool {
 191         let index = key.index() as usize;
 192         self.values.get(index).parent(key).is_none()
 193     }
 194
 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));
 199
 200         debug!("Updating variable {:?} to {:?}", key, new_value);
 201
 202         let index = key.index() as usize;
 203         self.values.set(index, new_value);
 204     }
 205
 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`.
 209     ///
 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) -> K {
 215         debug!("unify(root_a(id={:?}, rank={:?}), root_b(id={:?}, rank={:?}))",
 216                root_a.key(),
 217                root_a.rank,
 218                root_b.key(),
 219                root_b.rank);
 220
 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)
 228         } else {
 229             // If equal, redirect one to the other and increment the
 230             // other's rank.
 231             self.redirect_root(root_a.rank + 1, root_a, root_b, new_value)
 232         }
 233     }
 234
 235     fn redirect_root(&mut self,
 236                      new_rank: u32,
 237                      old_root: VarValue<K>,
 238                      new_root: VarValue<K>,
 239                      new_value: K::Value) -> K {
 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));
 244         new_root_key
 245     }
 246 }
 247
 248 impl<K: UnifyKey> sv::SnapshotVecDelegate for Delegate<K> {
 249     type Value = VarValue<K>;
 250     type Undo = ();
 251
 252     fn reverse(_: &mut Vec<VarValue<K>>, _: ()) {}
 253 }
 254
 255 // # Base union-find algorithm, where we are just making sets
 256
 257 impl<'tcx, K: UnifyKey> UnificationTable<K>
 258     where K::Value: Combine
 259 {
 260     pub fn union(&mut self, a_id: K, b_id: K) -> K {
 261         let node_a = self.get(a_id);
 262         let node_b = self.get(b_id);
 263         let a_id = node_a.key();
 264         let b_id = node_b.key();
 265         if a_id != b_id {
 266             let new_value = node_a.value.combine(&node_b.value);
 267             self.unify(node_a, node_b, new_value)
 268         } else {
 269             a_id
 270         }
 271     }
 272
 273     pub fn find(&mut self, id: K) -> K {
 274         self.get(id).key()
 275     }
 276
 277     pub fn find_value(&mut self, id: K) -> K::Value {
 278         self.get(id).value
 279     }
 280
 281     pub fn unioned(&mut self, a_id: K, b_id: K) -> bool {
 282         self.find(a_id) == self.find(b_id)
 283     }
 284 }
 285
 286 // # Non-subtyping unification
 287 //
 288 // Code to handle keys which carry a value, like ints,
 289 // floats---anything that doesn't have a subtyping relationship we
 290 // need to worry about.
 291
 292 impl<'tcx, K, V> UnificationTable<K>
 293     where K: UnifyKey<Value = Option<V>>,
 294           V: Clone + PartialEq + Debug
 295 {
 296     pub fn unify_var_var(&mut self, a_id: K, b_id: K) -> Result<K, (V, V)> {
 297         let node_a = self.get(a_id);
 298         let node_b = self.get(b_id);
 299         let a_id = node_a.key();
 300         let b_id = node_b.key();
 301
 302         if a_id == b_id {
 303             return Ok(a_id);
 304         }
 305
 306         let combined = {
 307             match (&node_a.value, &node_b.value) {
 308                 (&None, &None) => None,
 309                 (&Some(ref v), &None) | (&None, &Some(ref v)) => Some(v.clone()),
 310                 (&Some(ref v1), &Some(ref v2)) => {
 311                     if *v1 != *v2 {
 312                         return Err((v1.clone(), v2.clone()));
 313                     }
 314                     Some(v1.clone())
 315                 }
 316             }
 317         };
 318
 319         Ok(self.unify(node_a, node_b, combined))
 320     }
 321
 322     /// Sets the value of the key `a_id` to `b`. Because simple keys do not have any subtyping
 323     /// relationships, if `a_id` already has a value, it must be the same as `b`.
 324     pub fn unify_var_value(&mut self, a_id: K, b: V) -> Result<(), (V, V)> {
 325         let mut node_a = self.get(a_id);
 326
 327         match node_a.value {
 328             None => {
 329                 node_a.value = Some(b);
 330                 self.set(node_a.key(), node_a);
 331                 Ok(())
 332             }
 333
 334             Some(ref a_t) => {
 335                 if *a_t == b {
 336                     Ok(())
 337                 } else {
 338                     Err((a_t.clone(), b))
 339                 }
 340             }
 341         }
 342     }
 343
 344     pub fn has_value(&mut self, id: K) -> bool {
 345         self.get(id).value.is_some()
 346     }
 347
 348     pub fn probe(&mut self, a_id: K) -> Option<V> {
 349         self.get(a_id).value.clone()
 350     }
 351
 352     pub fn unsolved_variables(&mut self) -> Vec<K> {
 353         self.values
 354             .iter()
 355             .filter_map(|vv| {
 356                 if vv.value.is_some() {
 357                     None
 358                 } else {
 359                     Some(vv.key())
 360                 }
 361             })
 362             .collect()
 363     }
 364 }