summaryrefslogtreecommitdiff
path: root/js/src/ds/Fifo.h
blob: 4946c8452cc35babc1b245fc7e365d043bef08d6 (plain)
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
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef js_Fifo_h
#define js_Fifo_h

#include "mozilla/Move.h"

#include "js/Utility.h"
#include "js/Vector.h"

namespace js {

// A first-in-first-out queue container type. Fifo calls constructors and
// destructors of all elements added so non-PODs may be used safely. |Fifo|
// stores the first |MinInlineCapacity| elements in-place before resorting to
// dynamic allocation.
//
// T requirements:
//  - Either movable or copyable.
// MinInlineCapacity requirements:
//  - Must be even.
// AllocPolicy:
//  - see "Allocation policies" in AllocPolicy.h
template <typename T,
          size_t MinInlineCapacity = 0,
          class AllocPolicy = TempAllocPolicy>
class Fifo
{
    static_assert(MinInlineCapacity % 2 == 0, "MinInlineCapacity must be even!");

  protected:
    // An element A is "younger" than an element B if B was inserted into the
    // |Fifo| before A was.
    //
    // Invariant 1: Every element within |front_| is older than every element
    // within |rear_|.
    // Invariant 2: Entries within |front_| are sorted from younger to older.
    // Invariant 3: Entries within |rear_| are sorted from older to younger.
    // Invariant 4: If the |Fifo| is not empty, then |front_| is not empty.
    Vector<T, MinInlineCapacity / 2, AllocPolicy> front_;
    Vector<T, MinInlineCapacity / 2, AllocPolicy> rear_;

  private:
    // Maintain invariants after adding or removing entries.
    bool fixup() {
        if (!front_.empty())
            return true;

        if (!front_.reserve(rear_.length()))
            return false;

        while (!rear_.empty()) {
            front_.infallibleAppend(mozilla::Move(rear_.back()));
            rear_.popBack();
        }

        return true;
    }

  public:
    explicit Fifo(AllocPolicy alloc = AllocPolicy())
        : front_(alloc)
        , rear_(alloc)
    { }

    Fifo(Fifo&& rhs)
        : front_(mozilla::Move(rhs.front_))
        , rear_(mozilla::Move(rhs.rear_))
    { }

    Fifo& operator=(Fifo&& rhs) {
        MOZ_ASSERT(&rhs != this, "self-move disallowed");
        this->~Fifo();
        new (this) Fifo(mozilla::Move(rhs));
        return *this;
    }

    Fifo(const Fifo&) = delete;
    Fifo& operator=(const Fifo&) = delete;

    size_t length() const {
        MOZ_ASSERT_IF(rear_.length() > 0, front_.length() > 0); // Invariant 4.
        return front_.length() + rear_.length();
    }

    bool empty() const {
        MOZ_ASSERT_IF(rear_.length() > 0, front_.length() > 0); // Invariant 4.
        return front_.empty();
    }

    // Push an element to the back of the queue. This method can take either a
    // |const T&| or a |T&&|.
    template <typename U>
    MOZ_MUST_USE bool pushBack(U&& u) {
        if (!rear_.append(mozilla::Forward<U>(u)))
            return false;
        if (!fixup()) {
            rear_.popBack();
            return false;
        }
        return true;
    }

    // Construct a T in-place at the back of the queue.
    template <typename... Args>
    MOZ_MUST_USE bool emplaceBack(Args&&... args) {
        if (!rear_.emplaceBack(mozilla::Forward<Args>(args)...))
            return false;
        if (!fixup()) {
            rear_.popBack();
            return false;
        }
        return true;
    }

    // Access the element at the front of the queue.
    T& front() {
        MOZ_ASSERT(!empty());
        return front_.back();
    }
    const T& front() const {
        MOZ_ASSERT(!empty());
        return front_.back();
    }

    // Remove the front element from the queue.
    MOZ_MUST_USE bool popFront() {
        MOZ_ASSERT(!empty());
        T t(mozilla::Move(front()));
        front_.popBack();
        if (!fixup()) {
            // Attempt to remain in a valid state by reinserting the element
            // back at the front. If we can't remain in a valid state in the
            // face of OOMs, crash.
            AutoEnterOOMUnsafeRegion oomUnsafe;
            if (!front_.append(mozilla::Move(t)))
                oomUnsafe.crash("js::Fifo::popFront");
            return false;
        }
        return true;
    }

    // Clear all elements from the queue.
    void clear() {
        front_.clear();
        rear_.clear();
    }
};

} // namespace js

#endif /* js_Fifo_h */