factor out common code for lazy mappings

1 files changed, 123 insertions, 0 deletions

diff --git a/incria/src/lazy_mapping.rs b/incria/src/lazy_mapping.rs
new file mode 100644
index 0000000..bef3a3d
--- /dev/null
+++ b/incria/src/lazy_mapping.rs
@@ -0,0 +1,123 @@
+use std::{collections::HashMap, future::Future, hash::Hash, sync::Arc};
+
+use tokio::sync::{Mutex, Notify, RwLock, RwLockReadGuard};
+
+use crate::{
+    deps::{self, NodeId},
+    Mapper,
+};
+
+pub trait LazyMappingBackend<K, V> {
+    async fn is_dirty(&self, key: &K) -> bool;
+    fn compute(&self, key: K) -> impl Future<Output = V> + Send + '_;
+}
+
+#[derive(Debug, Default)]
+pub struct LazyMapper<K, V, B> {
+    /// The backend we use to compute values and check dirtiness
+    backend: B,
+
+    /// Map of all values we've calculated already
+    calculated: RwLock<HashMap<K, (V, NodeId)>>,
+
+    /// Values we're currently calculating
+    /// The Notify will be triggered when computation is done.
+    waiting: Mutex<HashMap<K, Arc<Notify>>>,
+}
+
+impl<K: Clone + Eq + Hash + Send + Sync, V: Send + Sync, B: LazyMappingBackend<K, V>>
+    LazyMapper<K, V, B>
+{
+    /// Create a new instance of the lazy mapper.
+    pub fn new(backend: B) -> Self {
+        Self {
+            backend,
+            calculated: RwLock::default(),
+            waiting: Mutex::default(),
+        }
+    }
+
+    /// Returns a reference to the backend of this [`LazyMapper<K, V, B>`].
+    pub fn backend(&self) -> &B {
+        &self.backend
+    }
+
+    /// Get the node ID for the given key
+    pub async fn dep_id(&self, key: &K) -> Option<NodeId> {
+        self.calculated.read().await.get(key).map(|(_, node)| *node)
+    }
+}
+
+impl<K, V, B> Mapper for LazyMapper<K, V, B>
+where
+    K: Clone + Eq + Hash + Send + Sync,
+    V: Send + Sync,
+    B: LazyMappingBackend<K, V>,
+{
+    type Key = K;
+    type Value = V;
+    type Wrapper<'a> = RwLockReadGuard<'a, V> where V: 'a;
+
+    async fn get<'a>(&'a self, key: &Self::Key) -> Self::Wrapper<'a> {
+        // Attempt to reuse or evict the existing value
+        let mut reuse_dep_id = None;
+        {
+            let finished = self.calculated.read().await;
+            if let Some((_, node)) = finished.get(key) {
+                let dirty = deps::is_dirty(*node) || self.backend.is_dirty(key).await;
+                if !dirty {
+                    deps::add_dep(*node);
+                    return RwLockReadGuard::map(finished, |hm| &hm.get(key).unwrap().0);
+                } else {
+                    reuse_dep_id = Some(*node);
+                    drop(finished);
+                    // Dirty, so we'll recompute below but we should remove it now
+                    if self.calculated.write().await.remove(key).is_none() {
+                        // Someone else already noticed it was dirty and removed it before us, so we need to deal with that
+                        todo!("dirty value removed between us noticing and us doing something")
+                    }
+                }
+            }
+        }
+
+        let barrier = self.waiting.lock().await.get(key).cloned();
+        if let Some(barrier) = barrier {
+            // Waiting for completion
+            barrier.notified().await;
+
+            let val = RwLockReadGuard::map(self.calculated.read().await, |hm| hm.get(key).unwrap());
+            deps::add_dep(val.1);
+
+            return RwLockReadGuard::map(val, |inf| &inf.0);
+        } else {
+            // Needs calculated
+            let notify = Arc::new(Notify::new());
+            self.waiting
+                .lock()
+                .await
+                .insert(key.clone(), notify.clone());
+
+            let dep = if let Some(x) = reuse_dep_id {
+                deps::clear(x);
+                x
+            } else {
+                deps::next_node_id()
+            };
+
+            let val = deps::with_node_id(dep, self.backend.compute(key.clone())).await;
+            deps::add_dep(dep);
+
+            self.calculated
+                .write()
+                .await
+                .insert(key.clone(), (val, dep));
+            self.waiting.lock().await.remove(key);
+
+            notify.notify_waiters();
+
+            return RwLockReadGuard::map(self.calculated.read().await, |hm| {
+                &hm.get(key).unwrap().0
+            });
+        }
+    }
+}