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

//! Computing the same thing each frame can be expensive,
//! so often you want to save the result from the previous frame and reuse it.
//!
//! Enter [`FrameCache`]: it caches the results of a computation for one frame.
//! If it is still used next frame, it is not recomputed.
//! If it is not used next frame, it is evicted from the cache to save memory.

/// Something that does an expensive computation that we want to cache
/// to save us from recomputing it each frame.
pub trait ComputerMut<Key, Value>: 'static + Send + Sync {
    fn compute(&mut self, key: Key) -> Value;
}

/// Caches the results of a computation for one frame.
/// If it is still used next frame, it is not recomputed.
/// If it is not used next frame, it is evicted from the cache to save memory.
pub struct FrameCache<Value, Computer> {
    generation: u32,
    computer: Computer,
    cache: nohash_hasher::IntMap<u64, (u32, Value)>,
}

impl<Value, Computer> Default for FrameCache<Value, Computer>
where
    Computer: Default,
{
    fn default() -> Self {
        Self::new(Computer::default())
    }
}

impl<Value, Computer> FrameCache<Value, Computer> {
    pub fn new(computer: Computer) -> Self {
        Self {
            generation: 0,
            computer,
            cache: Default::default(),
        }
    }

    /// Must be called once per frame to clear the cache.
    pub fn evice_cache(&mut self) {
        let current_generation = self.generation;
        self.cache.retain(|_key, cached| {
            cached.0 == current_generation // only keep those that were used this frame
        });
        self.generation = self.generation.wrapping_add(1);
    }
}

impl<Value, Computer> FrameCache<Value, Computer> {
    /// Get from cache (if the same key was used last frame)
    /// or recompute and store in the cache.
    pub fn get<Key>(&mut self, key: Key) -> Value
    where
        Key: Copy + std::hash::Hash,
        Value: Clone,
        Computer: ComputerMut<Key, Value>,
    {
        let hash = crate::util::hash(key);

        match self.cache.entry(hash) {
            std::collections::hash_map::Entry::Occupied(entry) => {
                let cached = entry.into_mut();
                cached.0 = self.generation;
                cached.1.clone()
            }
            std::collections::hash_map::Entry::Vacant(entry) => {
                let value = self.computer.compute(key);
                entry.insert((self.generation, value.clone()));
                value
            }
        }
    }
}

#[allow(clippy::len_without_is_empty)]
pub trait CacheTrait: 'static + Send + Sync {
    /// Call once per frame to evict cache.
    fn update(&mut self);

    /// Number of values currently in the cache.
    fn len(&self) -> usize;

    fn as_any_mut(&mut self) -> &mut dyn std::any::Any;
}

impl<Value: 'static + Send + Sync, Computer: 'static + Send + Sync> CacheTrait
    for FrameCache<Value, Computer>
{
    fn update(&mut self) {
        self.evice_cache();
    }

    fn len(&self) -> usize {
        self.cache.len()
    }

    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
        self
    }
}

/// ```
/// use egui::util::cache::{CacheStorage, ComputerMut, FrameCache};
///
/// #[derive(Default)]
/// struct CharCounter {}
/// impl ComputerMut<&str, usize> for CharCounter {
///     fn compute(&mut self, s: &str) -> usize {
///         s.chars().count()
///     }
/// }
/// type CharCountCache<'a> = FrameCache<usize, CharCounter>;
///
/// # let mut cache_storage = CacheStorage::default();
/// let mut cache = cache_storage.cache::<CharCountCache<'_>>();
/// assert_eq!(cache.get("hello"), 5);
/// ```
#[derive(Default)]
pub struct CacheStorage {
    caches: ahash::HashMap<std::any::TypeId, Box<dyn CacheTrait>>,
}

impl CacheStorage {
    pub fn cache<FrameCache: CacheTrait + Default>(&mut self) -> &mut FrameCache {
        self.caches
            .entry(std::any::TypeId::of::<FrameCache>())
            .or_insert_with(|| Box::new(FrameCache::default()))
            .as_any_mut()
            .downcast_mut::<FrameCache>()
            .unwrap()
    }

    /// Total number of cached values
    fn num_values(&self) -> usize {
        self.caches.values().map(|cache| cache.len()).sum()
    }

    /// Call once per frame to evict cache.
    pub fn update(&mut self) {
        for cache in self.caches.values_mut() {
            cache.update();
        }
    }
}

impl Clone for CacheStorage {
    fn clone(&self) -> Self {
        // We return an empty cache that can be filled in again.
        Self::default()
    }
}

impl std::fmt::Debug for CacheStorage {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(
            f,
            "FrameCacheStorage[{} caches with {} elements]",
            self.caches.len(),
            self.num_values()
        )
    }
}