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
//! Densely numbered entity references as set keys.

use crate::keys::Keys;
use crate::EntityRef;
use alloc::vec::Vec;
use core::marker::PhantomData;

/// A set of `K` for densely indexed entity references.
///
/// The `EntitySet` data structure uses the dense index space to implement a set with a bitvector.
/// Like `SecondaryMap`, an `EntitySet` is used to associate secondary information with entities.
#[derive(Debug, Clone)]
pub struct EntitySet<K>
where
    K: EntityRef,
{
    elems: Vec<u8>,
    len: usize,
    unused: PhantomData<K>,
}

impl<K: EntityRef> Default for EntitySet<K> {
    fn default() -> Self {
        Self {
            elems: Vec::new(),
            len: 0,
            unused: PhantomData,
        }
    }
}

/// Shared `EntitySet` implementation for all value types.
impl<K> EntitySet<K>
where
    K: EntityRef,
{
    /// Create a new empty set.
    pub fn new() -> Self {
        Self::default()
    }

    /// Creates a new empty set with the specified capacity.
    pub fn with_capacity(capacity: usize) -> Self {
        Self {
            elems: Vec::with_capacity((capacity + 7) / 8),
            ..Self::new()
        }
    }

    /// Get the element at `k` if it exists.
    pub fn contains(&self, k: K) -> bool {
        let index = k.index();
        if index < self.len {
            (self.elems[index / 8] & (1 << (index % 8))) != 0
        } else {
            false
        }
    }

    /// Is this set completely empty?
    pub fn is_empty(&self) -> bool {
        if self.len != 0 {
            false
        } else {
            self.elems.iter().all(|&e| e == 0)
        }
    }

    /// Returns the cardinality of the set.  More precisely, it returns the number of calls to
    /// `insert` with different key values, that have happened since the the set was most recently
    /// `clear`ed or created with `new`.
    pub fn cardinality(&self) -> usize {
        let mut n: usize = 0;
        for byte_ix in 0..self.len / 8 {
            n += self.elems[byte_ix].count_ones() as usize;
        }
        for bit_ix in (self.len / 8) * 8..self.len {
            if (self.elems[bit_ix / 8] & (1 << (bit_ix % 8))) != 0 {
                n += 1;
            }
        }
        n
    }

    /// Remove all entries from this set.
    pub fn clear(&mut self) {
        self.len = 0;
        self.elems.clear()
    }

    /// Iterate over all the keys in this set.
    pub fn keys(&self) -> Keys<K> {
        Keys::with_len(self.len)
    }

    /// Resize the set to have `n` entries by adding default entries as needed.
    pub fn resize(&mut self, n: usize) {
        self.elems.resize((n + 7) / 8, 0);
        self.len = n
    }

    /// Insert the element at `k`.
    pub fn insert(&mut self, k: K) -> bool {
        let index = k.index();
        if index >= self.len {
            self.resize(index + 1)
        }
        let result = !self.contains(k);
        self.elems[index / 8] |= 1 << (index % 8);
        result
    }

    /// Removes and returns the entity from the set if it exists.
    pub fn pop(&mut self) -> Option<K> {
        if self.len == 0 {
            return None;
        }

        // Clear the last known entity in the list.
        let last_index = self.len - 1;
        self.elems[last_index / 8] &= !(1 << (last_index % 8));

        // Set the length to the next last stored entity or zero if we pop'ed
        // the last entity.
        self.len = self
            .elems
            .iter()
            .enumerate()
            .rev()
            .find(|(_, &byte)| byte != 0)
            // Map `i` from byte index to bit level index.
            // `(i + 1) * 8` = Last bit in byte.
            // `last - byte.leading_zeros()` = last set bit in byte.
            // `as usize` won't ever truncate as the potential range is `0..=8`.
            .map_or(0, |(i, byte)| ((i + 1) * 8) - byte.leading_zeros() as usize);

        Some(K::new(last_index))
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use core::u32;

    // `EntityRef` impl for testing.
    #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
    struct E(u32);

    impl EntityRef for E {
        fn new(i: usize) -> Self {
            E(i as u32)
        }
        fn index(self) -> usize {
            self.0 as usize
        }
    }

    #[test]
    fn basic() {
        let r0 = E(0);
        let r1 = E(1);
        let r2 = E(2);
        let mut m = EntitySet::new();

        let v: Vec<E> = m.keys().collect();
        assert_eq!(v, []);
        assert!(m.is_empty());

        m.insert(r2);
        m.insert(r1);

        assert!(!m.contains(r0));
        assert!(m.contains(r1));
        assert!(m.contains(r2));
        assert!(!m.contains(E(3)));
        assert!(!m.is_empty());

        let v: Vec<E> = m.keys().collect();
        assert_eq!(v, [r0, r1, r2]);

        m.resize(20);
        assert!(!m.contains(E(3)));
        assert!(!m.contains(E(4)));
        assert!(!m.contains(E(8)));
        assert!(!m.contains(E(15)));
        assert!(!m.contains(E(19)));

        m.insert(E(8));
        m.insert(E(15));
        assert!(!m.contains(E(3)));
        assert!(!m.contains(E(4)));
        assert!(m.contains(E(8)));
        assert!(!m.contains(E(9)));
        assert!(!m.contains(E(14)));
        assert!(m.contains(E(15)));
        assert!(!m.contains(E(16)));
        assert!(!m.contains(E(19)));
        assert!(!m.contains(E(20)));
        assert!(!m.contains(E(u32::MAX)));

        m.clear();
        assert!(m.is_empty());
    }

    #[test]
    fn pop_ordered() {
        let r0 = E(0);
        let r1 = E(1);
        let r2 = E(2);
        let mut m = EntitySet::new();
        m.insert(r0);
        m.insert(r1);
        m.insert(r2);

        assert_eq!(r2, m.pop().unwrap());
        assert_eq!(r1, m.pop().unwrap());
        assert_eq!(r0, m.pop().unwrap());
        assert!(m.pop().is_none());
        assert!(m.pop().is_none());
    }

    #[test]
    fn pop_unordered() {
        let mut blocks = [
            E(0),
            E(1),
            E(6),
            E(7),
            E(5),
            E(9),
            E(10),
            E(2),
            E(3),
            E(11),
            E(12),
        ];

        let mut m = EntitySet::new();
        for &block in &blocks {
            m.insert(block);
        }
        assert_eq!(m.len, 13);
        blocks.sort();

        for &block in blocks.iter().rev() {
            assert_eq!(block, m.pop().unwrap());
        }

        assert!(m.is_empty());
    }
}