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
//! Sparse mapping of entity references to larger value types.
//!
//! This module provides a `SparseMap` data structure which implements a sparse mapping from an
//! `EntityRef` key to a value type that may be on the larger side. This implementation is based on
//! the paper:
//!
//! > Briggs, Torczon, *An efficient representation for sparse sets*,
//!   ACM Letters on Programming Languages and Systems, Volume 2, Issue 1-4, March-Dec. 1993.

use crate::map::SecondaryMap;
use crate::EntityRef;
use alloc::vec::Vec;
use core::mem;
use core::slice;
use core::u32;

/// Trait for extracting keys from values stored in a `SparseMap`.
///
/// All values stored in a `SparseMap` must keep track of their own key in the map and implement
/// this trait to provide access to the key.
pub trait SparseMapValue<K> {
    /// Get the key of this sparse map value. This key is not allowed to change while the value
    /// is a member of the map.
    fn key(&self) -> K;
}

/// A sparse mapping of entity references.
///
/// A `SparseMap<K, V>` map provides:
///
/// - Memory usage equivalent to `SecondaryMap<K, u32>` + `Vec<V>`, so much smaller than
///   `SecondaryMap<K, V>` for sparse mappings of larger `V` types.
/// - Constant time lookup, slightly slower than `SecondaryMap`.
/// - A very fast, constant time `clear()` operation.
/// - Fast insert and erase operations.
/// - Stable iteration that is as fast as a `Vec<V>`.
///
/// # Compared to `SecondaryMap`
///
/// When should we use a `SparseMap` instead of a secondary `SecondaryMap`? First of all,
/// `SparseMap` does not provide the functionality of a `PrimaryMap` which can allocate and assign
/// entity references to objects as they are pushed onto the map. It is only the secondary entity
/// maps that can be replaced with a `SparseMap`.
///
/// - A secondary entity map assigns a default mapping to all keys. It doesn't distinguish between
///   an unmapped key and one that maps to the default value. `SparseMap` does not require
///   `Default` values, and it tracks accurately if a key has been mapped or not.
/// - Iterating over the contents of an `SecondaryMap` is linear in the size of the *key space*,
///   while iterating over a `SparseMap` is linear in the number of elements in the mapping. This
///   is an advantage precisely when the mapping is sparse.
/// - `SparseMap::clear()` is constant time and super-fast. `SecondaryMap::clear()` is linear in
///   the size of the key space. (Or, rather the required `resize()` call following the `clear()`
///   is).
/// - `SparseMap` requires the values to implement `SparseMapValue<K>` which means that they must
///   contain their own key.
pub struct SparseMap<K, V>
where
    K: EntityRef,
    V: SparseMapValue<K>,
{
    sparse: SecondaryMap<K, u32>,
    dense: Vec<V>,
}

impl<K, V> SparseMap<K, V>
where
    K: EntityRef,
    V: SparseMapValue<K>,
{
    /// Create a new empty mapping.
    pub fn new() -> Self {
        Self {
            sparse: SecondaryMap::new(),
            dense: Vec::new(),
        }
    }

    /// Returns the number of elements in the map.
    pub fn len(&self) -> usize {
        self.dense.len()
    }

    /// Returns true is the map contains no elements.
    pub fn is_empty(&self) -> bool {
        self.dense.is_empty()
    }

    /// Remove all elements from the mapping.
    pub fn clear(&mut self) {
        self.dense.clear();
    }

    /// Returns a reference to the value corresponding to the key.
    pub fn get(&self, key: K) -> Option<&V> {
        if let Some(idx) = self.sparse.get(key).cloned() {
            if let Some(entry) = self.dense.get(idx as usize) {
                if entry.key() == key {
                    return Some(entry);
                }
            }
        }
        None
    }

    /// Returns a mutable reference to the value corresponding to the key.
    ///
    /// Note that the returned value must not be mutated in a way that would change its key. This
    /// would invalidate the sparse set data structure.
    pub fn get_mut(&mut self, key: K) -> Option<&mut V> {
        if let Some(idx) = self.sparse.get(key).cloned() {
            if let Some(entry) = self.dense.get_mut(idx as usize) {
                if entry.key() == key {
                    return Some(entry);
                }
            }
        }
        None
    }

    /// Return the index into `dense` of the value corresponding to `key`.
    fn index(&self, key: K) -> Option<usize> {
        if let Some(idx) = self.sparse.get(key).cloned() {
            let idx = idx as usize;
            if let Some(entry) = self.dense.get(idx) {
                if entry.key() == key {
                    return Some(idx);
                }
            }
        }
        None
    }

    /// Return `true` if the map contains a value corresponding to `key`.
    pub fn contains_key(&self, key: K) -> bool {
        self.get(key).is_some()
    }

    /// Insert a value into the map.
    ///
    /// If the map did not have this key present, `None` is returned.
    ///
    /// If the map did have this key present, the value is updated, and the old value is returned.
    ///
    /// It is not necessary to provide a key since the value knows its own key already.
    pub fn insert(&mut self, value: V) -> Option<V> {
        let key = value.key();

        // Replace the existing entry for `key` if there is one.
        if let Some(entry) = self.get_mut(key) {
            return Some(mem::replace(entry, value));
        }

        // There was no previous entry for `key`. Add it to the end of `dense`.
        let idx = self.dense.len();
        debug_assert!(idx <= u32::MAX as usize, "SparseMap overflow");
        self.dense.push(value);
        self.sparse[key] = idx as u32;
        None
    }

    /// Remove a value from the map and return it.
    pub fn remove(&mut self, key: K) -> Option<V> {
        if let Some(idx) = self.index(key) {
            let back = self.dense.pop().unwrap();

            // Are we popping the back of `dense`?
            if idx == self.dense.len() {
                return Some(back);
            }

            // We're removing an element from the middle of `dense`.
            // Replace the element at `idx` with the back of `dense`.
            // Repair `sparse` first.
            self.sparse[back.key()] = idx as u32;
            return Some(mem::replace(&mut self.dense[idx], back));
        }

        // Nothing to remove.
        None
    }

    /// Remove the last value from the map.
    pub fn pop(&mut self) -> Option<V> {
        self.dense.pop()
    }

    /// Get an iterator over the values in the map.
    ///
    /// The iteration order is entirely determined by the preceding sequence of `insert` and
    /// `remove` operations. In particular, if no elements were removed, this is the insertion
    /// order.
    pub fn values(&self) -> slice::Iter<V> {
        self.dense.iter()
    }

    /// Get the values as a slice.
    pub fn as_slice(&self) -> &[V] {
        self.dense.as_slice()
    }
}

/// Iterating over the elements of a set.
impl<'a, K, V> IntoIterator for &'a SparseMap<K, V>
where
    K: EntityRef,
    V: SparseMapValue<K>,
{
    type Item = &'a V;
    type IntoIter = slice::Iter<'a, V>;

    fn into_iter(self) -> Self::IntoIter {
        self.values()
    }
}

/// Any `EntityRef` can be used as a sparse map value representing itself.
impl<T> SparseMapValue<T> for T
where
    T: EntityRef,
{
    fn key(&self) -> Self {
        *self
    }
}

/// A sparse set of entity references.
///
/// Any type that implements `EntityRef` can be used as a sparse set value too.
pub type SparseSet<T> = SparseMap<T, T>;

#[cfg(test)]
mod tests {
    use super::*;
    use crate::EntityRef;

    /// An opaque reference to an instruction in a function.
    #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
    pub struct Inst(u32);
    entity_impl!(Inst, "inst");

    // Mock key-value object for testing.
    #[derive(PartialEq, Eq, Debug)]
    struct Obj(Inst, &'static str);

    impl SparseMapValue<Inst> for Obj {
        fn key(&self) -> Inst {
            self.0
        }
    }

    #[test]
    fn empty_immutable_map() {
        let i1 = Inst::new(1);
        let map: SparseMap<Inst, Obj> = SparseMap::new();

        assert!(map.is_empty());
        assert_eq!(map.len(), 0);
        assert_eq!(map.get(i1), None);
        assert_eq!(map.values().count(), 0);
    }

    #[test]
    fn single_entry() {
        let i0 = Inst::new(0);
        let i1 = Inst::new(1);
        let i2 = Inst::new(2);
        let mut map = SparseMap::new();

        assert!(map.is_empty());
        assert_eq!(map.len(), 0);
        assert_eq!(map.get(i1), None);
        assert_eq!(map.get_mut(i1), None);
        assert_eq!(map.remove(i1), None);

        assert_eq!(map.insert(Obj(i1, "hi")), None);
        assert!(!map.is_empty());
        assert_eq!(map.len(), 1);
        assert_eq!(map.get(i0), None);
        assert_eq!(map.get(i1), Some(&Obj(i1, "hi")));
        assert_eq!(map.get(i2), None);
        assert_eq!(map.get_mut(i0), None);
        assert_eq!(map.get_mut(i1), Some(&mut Obj(i1, "hi")));
        assert_eq!(map.get_mut(i2), None);

        assert_eq!(map.remove(i0), None);
        assert_eq!(map.remove(i2), None);
        assert_eq!(map.remove(i1), Some(Obj(i1, "hi")));
        assert_eq!(map.len(), 0);
        assert_eq!(map.get(i1), None);
        assert_eq!(map.get_mut(i1), None);
        assert_eq!(map.remove(i0), None);
        assert_eq!(map.remove(i1), None);
        assert_eq!(map.remove(i2), None);
    }

    #[test]
    fn multiple_entries() {
        let i0 = Inst::new(0);
        let i1 = Inst::new(1);
        let i2 = Inst::new(2);
        let i3 = Inst::new(3);
        let mut map = SparseMap::new();

        assert_eq!(map.insert(Obj(i2, "foo")), None);
        assert_eq!(map.insert(Obj(i1, "bar")), None);
        assert_eq!(map.insert(Obj(i0, "baz")), None);

        // Iteration order = insertion order when nothing has been removed yet.
        assert_eq!(
            map.values().map(|obj| obj.1).collect::<Vec<_>>(),
            ["foo", "bar", "baz"]
        );

        assert_eq!(map.len(), 3);
        assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
        assert_eq!(map.get(i1), Some(&Obj(i1, "bar")));
        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
        assert_eq!(map.get(i3), None);

        // Remove front object, causing back to be swapped down.
        assert_eq!(map.remove(i1), Some(Obj(i1, "bar")));
        assert_eq!(map.len(), 2);
        assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
        assert_eq!(map.get(i1), None);
        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
        assert_eq!(map.get(i3), None);

        // Reinsert something at a previously used key.
        assert_eq!(map.insert(Obj(i1, "barbar")), None);
        assert_eq!(map.len(), 3);
        assert_eq!(map.get(i0), Some(&Obj(i0, "baz")));
        assert_eq!(map.get(i1), Some(&Obj(i1, "barbar")));
        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
        assert_eq!(map.get(i3), None);

        // Replace an entry.
        assert_eq!(map.insert(Obj(i0, "bazbaz")), Some(Obj(i0, "baz")));
        assert_eq!(map.len(), 3);
        assert_eq!(map.get(i0), Some(&Obj(i0, "bazbaz")));
        assert_eq!(map.get(i1), Some(&Obj(i1, "barbar")));
        assert_eq!(map.get(i2), Some(&Obj(i2, "foo")));
        assert_eq!(map.get(i3), None);

        // Check the reference `IntoIter` impl.
        let mut v = Vec::new();
        for i in &map {
            v.push(i.1);
        }
        assert_eq!(v.len(), map.len());
    }

    #[test]
    fn entity_set() {
        let i0 = Inst::new(0);
        let i1 = Inst::new(1);
        let mut set = SparseSet::new();

        assert_eq!(set.insert(i0), None);
        assert_eq!(set.insert(i0), Some(i0));
        assert_eq!(set.insert(i1), None);
        assert_eq!(set.get(i0), Some(&i0));
        assert_eq!(set.get(i1), Some(&i1));
    }
}