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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::{
collections::{HashMap, VecDeque},
fmt::Write,
future::Future,
};
use once_cell::sync::OnceCell;
use parking_lot::Mutex;
/// Identifier of a node, unique across a program run.
pub type NodeId = usize;
tokio::task_local! {
/// The ID of the node in the dependency tree for the currently running computation
static NODE_ID: NodeId;
}
/// 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>>,
/// Next node ID to issue
next_node_id: Mutex<NodeId>,
}
impl DepTracker {
/// See [`self::add_dep`]
fn add_dep(&self, dep: NodeId) {
self.deps
.lock()
.entry(NODE_ID.get())
.and_modify(|v| v.deps.push(dep))
.or_insert(NodeInfo {
deps: vec![dep],
dirty: false,
});
}
/// See [`self::next_node_id`]
fn next_node_id(&self) -> usize {
let mut lock = self.next_node_id.lock();
*lock += 1;
*lock - 1
}
/// See [`self::mark_dirty`]
fn mark_dirty(&self, node: NodeId) {
let mut lock = self.deps.lock();
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, dependent_info) in lock.iter() {
if dependent_info.deps.contains(&node_id) {
frontier.push_back(*dependent);
}
}
}
}
/// See [`self::is_dirty`]
fn is_dirty(&self, node: NodeId) -> bool {
self.deps
.lock()
.get(&node)
.map(|v| v.dirty)
.unwrap_or(false)
}
/// See [`self::clear`]
fn clear(&self, node: NodeId) {
let mut lock = self.deps.lock();
let node = match lock.get_mut(&node) {
Some(x) => x,
None => return,
};
node.dirty = false;
node.deps = vec![];
}
}
/// Info about a single dependency node
struct NodeInfo {
deps: Vec<NodeId>,
dirty: bool,
}
/// The global instance of the dependency tracker
static DEP_TRACKER: OnceCell<DepTracker> = OnceCell::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();
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.deps.iter() {
writeln!(&mut out, "\t{} -> {}", id, dep).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
}
|
