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//! The main code used for tracking dependencies (mostly) invisibly to the user.
//!
//! Each mapping that could be memoised is represented by a Node. Each Node can have one or more dependencies, forming a directed acyclic graph.
//!
//! [`self::add_dep`] should be called whenever any mapping is retrieved, to record the dependency.
//!
//! [`self::next_node_id`] and [`self::with_node_id`] should be used to create new nodes in the graph when a new mapping is being evaluated.
//!
//! [`self::mark_dirty`] should be called from outside the evaluation of any mapping. In many cases, this requires the dependency ID to be stored externally alongside the key, which can be accomplished with [`self::current_node_id`]
//!
//! [`self::is_dirty`] should be used when a mapping is retrieved to determine if it needs to be re-evaluated. If it does, [`self::clear`] should be used to reset the mapping's dependencies and dirty state.
use std::{
cell::RefCell,
collections::{HashMap, VecDeque},
fmt::Write,
future::Future,
sync::{Mutex, OnceLock},
thread,
};
use tracing::instrument;
/// Identifier of a node, unique across a program run.
pub type NodeId = u64;
tokio::task_local! {
/// The ID of the node in the dependency tree for the currently running computation
static NODE_ID: NodeId;
}
thread_local! {
static NEXT_NODE_ID_LSB: RefCell<u64> = RefCell::new(5);
}
/// Responsible for tracking dependencies.
/// There is one global instance of this, [`DEP_TRACKER`], with convenience functions for most of its functionality.
#[derive(Default)]
struct DepTracker {
/// Neighbour-List representation of dependency tree
deps: Mutex<HashMap<NodeId, NodeInfo>>,
}
impl DepTracker {
/// See [`self::add_dep`]
#[instrument(level = "trace", skip(self))]
fn add_dep(&self, dep: NodeId) {
self.deps
.lock()
.unwrap()
.entry(dep)
.and_modify(|v| v.dependents.push(NODE_ID.get()))
.or_insert(NodeInfo {
dependents: vec![NODE_ID.get()],
dirty: false,
});
}
/// See [`self::next_node_id`]
fn next_node_id(&self) -> NodeId {
let tid = thread::current().id().as_u64();
let lsb = NEXT_NODE_ID_LSB.with(|lsb| {
let old = *lsb.borrow();
*lsb.borrow_mut() = old + 1;
old
});
(tid.get() << 32) | (lsb & 0xffffffff)
}
/// See [`self::mark_dirty`]
#[instrument(level = "trace", skip(self))]
fn mark_dirty(&self, node: NodeId) {
let mut lock = self.deps.lock().unwrap();
let mut frontier = VecDeque::new();
frontier.push_back(node);
while let Some(node_id) = frontier.pop_front() {
let node = match lock.get_mut(&node_id) {
Some(x) => x,
None => continue,
};
if node.dirty {
continue;
}
node.dirty = true;
for dependent in node.dependents.iter() {
frontier.push_back(*dependent);
}
}
}
/// See [`self::is_dirty`]
#[instrument(level = "trace", skip(self))]
fn is_dirty(&self, node: NodeId) -> bool {
self.deps
.lock()
.unwrap()
.get(&node)
.map(|v| v.dirty)
.unwrap_or(false)
}
/// See [`self::clear`]
#[instrument(level = "trace", skip(self))]
fn clear(&self, node: NodeId) {
let mut lock = self.deps.lock().unwrap();
let node = match lock.get_mut(&node) {
Some(x) => x,
None => return,
};
node.dirty = false;
node.dependents = vec![];
}
}
/// Info about a single dependency node
struct NodeInfo {
dependents: Vec<NodeId>,
dirty: bool,
}
/// The global instance of the dependency tracker
static DEP_TRACKER: OnceLock<DepTracker> = OnceLock::new();
fn dep_tracker() -> &'static DepTracker {
DEP_TRACKER.get_or_init(DepTracker::default)
}
/// Register a dependency for the current node
pub fn add_dep(dep: NodeId) {
dep_tracker().add_dep(dep);
}
/// Get the next node id to use
pub fn next_node_id() -> NodeId {
dep_tracker().next_node_id()
}
/// Get the ID of the current node
pub fn current_node_id() -> NodeId {
NODE_ID.get()
}
/// Mark the given node and all nodes that depend on it 'dirty'.
///
/// This should be used in conjunction with [`self::is_dirty`] to recompute values when necessary.
pub fn mark_dirty(dep: NodeId) {
dep_tracker().mark_dirty(dep)
}
/// Check if the given node is dirty, indicating it should be recomputed.
pub fn is_dirty(dep: NodeId) -> bool {
dep_tracker().is_dirty(dep)
}
/// Clear all information about a given node.
///
/// This should be used when a node is recomputed, to ensure stale dependencies are removed.
pub fn clear(dep: NodeId) {
dep_tracker().clear(dep)
}
/// Generate a graphviz representation of the current dependency graph, for debugging.
///
/// Dirty nodes will be coloured grey.
pub fn dep_graphviz() -> String {
let mut out = String::new();
let deps = dep_tracker().deps.lock().unwrap();
writeln!(&mut out, "digraph G {{").unwrap();
for (id, info) in deps.iter() {
if *id == 0 {
writeln!(&mut out, "\t{} [label=Root]", id).unwrap();
} else if info.dirty {
writeln!(&mut out, "\t{} [style=filled,color=lightgrey]", id).unwrap();
}
for dep in info.dependents.iter() {
writeln!(&mut out, "\t{} -> {}", dep, id).unwrap();
}
}
writeln!(&mut out, "}}").unwrap();
out
}
/// Decorate the given future to have the given dependency ID.
pub async fn with_node_id<F>(dep: NodeId, f: F) -> F::Output
where
F: Future + Send,
F::Output: Send,
{
NODE_ID.scope(dep, f).await
}
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