buddy-system 0.2.0

Simple buddy system allocator for real-time usecases
Documentation 
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

//! Buddy system implementations.
//!
//! Note: everything in this module is unchecked. It shouldn't panic (the only `unwraps`
//! should be unreachable unless an internal invariant is broken), but it won't behave as
//! expected if it's given the wrong inputs or state.

use std::{cmp::Ordering, ops::Range, time::Instant};

use generational_arena::{Arena, Index};

pub(crate) fn is_pow_of_two(x: usize) -> bool {
    (x != 0) && ((x & (x - 1)) == 0)
}

pub(crate) enum BlockState {
    Split(Index, Index),
    Available,
    Occupied,
}

pub struct Block {
    pub(crate) range: Range<usize>,
    pub(crate) state: BlockState,
}

/// Assumes `desired_size` is a power of 2
pub fn alloc(arena: &mut Arena<Block>, block_index: Index, desired_size: usize) -> Option<Index> {
    debug_assert!(is_pow_of_two(desired_size));

    let block = &arena[block_index];

    match block.range.len().cmp(&desired_size) {
        Ordering::Less => None,
        Ordering::Equal => {
            if let BlockState::Available = block.state {
                arena[block_index].state = BlockState::Occupied;

                Some(block_index)
            } else {
                None
            }
        }
        Ordering::Greater => match block.state {
            BlockState::Occupied => None,
            BlockState::Available => {
                let first_range = (block.range.start)..(block.range.start + block.range.len() / 2);
                let second_range = (block.range.start + block.range.len() / 2)..(block.range.end);

                let first = arena.insert(Block {
                    range: first_range,
                    state: BlockState::Available,
                });

                let second = arena.insert(Block {
                    range: second_range,
                    state: BlockState::Available,
                });

                arena[block_index].state = BlockState::Split(first, second);

                alloc(arena, first, desired_size)
            }
            BlockState::Split(first_index, second_index) => {
                if let Some(result) = alloc(arena, first_index, desired_size) {
                    Some(result)
                } else if let Some(result) = alloc(arena, second_index, desired_size) {
                    Some(result)
                } else {
                    None
                }
            }
        },
    }
}

pub(crate) fn dealloc(arena: &mut Arena<Block>, block_index: Index) {
    arena[block_index].state = BlockState::Available;
}

#[repr(transparent)]
pub struct IsAvailable(bool);

pub fn tidy(arena: &mut Arena<Block>, block_index: Index) -> IsAvailable {
    // go through and merge
    let block = &arena[block_index];

    match block.state {
        BlockState::Split(first, second) => {
            let first_available = tidy(arena, first).0;
            let second_available = tidy(arena, second).0;

            if first_available && second_available {
                arena.remove(first).unwrap();
                arena.remove(second).unwrap();

                arena[block_index].state = BlockState::Available;

                IsAvailable(true)
            } else {
                IsAvailable(false)
            }
        }
        BlockState::Available => IsAvailable(true),
        BlockState::Occupied => IsAvailable(false),
    }
}

pub fn tidy_gas(arena: &mut Arena<Block>, block_index: Index, gas: &mut usize) -> IsAvailable {
    if *gas == 0 {
        return IsAvailable(false);
    }

    *gas -= 1;

    // go through and merge
    let block = &arena[block_index];

    match block.state {
        BlockState::Split(first, second) => {
            let first_available = tidy_gas(arena, first, gas).0;
            let second_available = tidy_gas(arena, second, gas).0;

            if first_available && second_available {
                arena.remove(first).unwrap();
                arena.remove(second).unwrap();

                arena[block_index].state = BlockState::Available;

                IsAvailable(true)
            } else {
                IsAvailable(false)
            }
        }
        BlockState::Available => IsAvailable(true),
        BlockState::Occupied => IsAvailable(false),
    }
}

pub fn tidy_timed(arena: &mut Arena<Block>, block_index: Index, deadline: Instant) -> IsAvailable {
    if Instant::now() >= deadline {
        // return not available so the recursion chain stops
        return IsAvailable(false);
    }

    // go through and merge
    let block = &arena[block_index];

    match block.state {
        BlockState::Split(first, second) => {
            let first_available = tidy_timed(arena, first, deadline).0;
            let second_available = tidy_timed(arena, second, deadline).0;

            if first_available && second_available {
                arena.remove(first).unwrap();
                arena.remove(second).unwrap();

                arena[block_index].state = BlockState::Available;

                IsAvailable(true)
            } else {
                IsAvailable(false)
            }
        }
        BlockState::Available => IsAvailable(true),
        BlockState::Occupied => IsAvailable(false),
    }
}