From ad55a4b05ac35cc21c64c1dd62bff9faec775eb6 Mon Sep 17 00:00:00 2001
From: tcmal <tcmal>
Date: Fri, 16 Jun 2023 14:19:58 +0000
Subject: factor out common code for lazy mappings

---
 incria/src/thunk.rs | 114 +++++++++-------------------------------------------
 1 file changed, 20 insertions(+), 94 deletions(-)

(limited to 'incria/src/thunk.rs')

diff --git a/incria/src/thunk.rs b/incria/src/thunk.rs
index f076ba1..937f109 100644
--- a/incria/src/thunk.rs
+++ b/incria/src/thunk.rs
@@ -1,111 +1,37 @@
 //! A mapper based on a function from keys to values.
-use std::{collections::HashMap, hash::Hash, pin::Pin, sync::Arc};
+use std::pin::Pin;
 
-use tokio::sync::{Mutex, Notify, RwLock, RwLockReadGuard};
+use crate::lazy_mapping::{LazyMapper, LazyMappingBackend};
 
-use crate::{
-    deps::{self, NodeId},
-    Mapper,
-};
+/// A function from keys to values.
+///
+/// Should be pure, except for use of other mappings. This ensures recomputation is done when needed.
+pub trait Thunk<K, V>: Send + 'static {
+    fn compute(&self, key: K) -> Pin<Box<dyn std::future::Future<Output = V> + Send + '_>>;
+}
 
-/// A mapping that lazily computes values with a given async thunk and memoizes them.
 #[derive(Debug, Default)]
-pub struct ThunkMapper<K, V, T> {
-    /// The thunk we use to compute values
+pub struct ThunkBackend<T> {
     thunk: T,
-
-    /// 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, T: Thunk<K, V>> ThunkMapper<K, V, T> {
-    /// Create a new instance of the computing store.
+impl<T> ThunkBackend<T> {
     pub fn new(thunk: T) -> Self {
-        Self {
-            thunk,
-            calculated: RwLock::default(),
-            waiting: Mutex::default(),
-        }
+        Self { thunk }
     }
 }
 
-impl<K: Clone + Eq + Hash + Send + Sync, V: Send + Sync, C: Thunk<K, V>> Mapper
-    for ThunkMapper<K, V, C>
+impl<K, V: 'static, T> LazyMappingBackend<K, V> for ThunkBackend<T>
+where
+    T: Thunk<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((_, dep)) = finished.get(key) {
-                if !deps::is_dirty(*dep) {
-                    deps::add_dep(*dep);
-                    return RwLockReadGuard::map(finished, |hm| &hm.get(key).unwrap().0);
-                } else {
-                    reuse_dep_id = Some(*dep);
-                    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.thunk.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();
+    async fn is_dirty(&self, _: &K) -> bool {
+        false // only dirty if marked dirty by someting else
+    }
 
-            return RwLockReadGuard::map(self.calculated.read().await, |hm| {
-                &hm.get(key).unwrap().0
-            });
-        }
+    fn compute(&self, key: K) -> impl std::future::Future<Output = V> + Send + '_ {
+        self.thunk.compute(key)
     }
 }
 
-/// A function from keys to values.
-///
-/// Should be pure, except for use of other mappings. This ensures recomputation is done when needed.
-pub trait Thunk<K, V>: Send + 'static {
-    fn compute(&self, key: K) -> Pin<Box<dyn std::future::Future<Output = V> + Send + '_>>;
-}
+pub type ThunkMapper<K, V, T> = LazyMapper<K, V, ThunkBackend<T>>;
-- 
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