chore: improve old value implementation performance

.changeset/early-ties-study.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: improve old value implementation performance

packages/svelte/src/internal/client/reactivity/sources.js

@@ -1,4 +1,4 @@
-/** @import { Derived, Effect, Source, Value } from '#client' */
+/** @import { Derived, Effect, Source, Value, ValueNode } from '#client' */
 import { DEV } from 'esm-env';
 import {
 	active_reaction,
@@ -37,7 +37,70 @@ import { proxy } from '../proxy.js';
 import { execute_derived } from './deriveds.js';
 
 export let inspect_effects = new Set();
-export const old_values = new Map();
+/** @type { null | ValueNode } */
+export let old_value_node = null;
+/** @type { null | ValueNode } */
+export let last_old_value_node = null;
+
+export function clear_old_value_node() {
+	old_value_node = null;
+	last_old_value_node = null;
+}
+
+/**
+ * @param {Source} signal
+ */
+export function get_old_value_node(signal) {
+	let current = old_value_node;
+	while (current !== null) {
+		if (current.s === signal) {
+			return current;
+		}
+		current = current.n;
+	}
+	return null;
+}
+
+/**
+ * @param {Source} signal
+ * @param {any} value
+ */
+function create_value_node(signal, value) {
+	return {
+		s: signal,
+		v: value,
+		n: null
+	};
+}
+
+/**
+ * @param {Source} signal
+ * @param {any} value
+ */
+function set_old_value(signal, value) {
+	const value_node = get_old_value_node(signal);
+	if (value_node !== null) {
+		value_node.v = value;
+		return;
+	}
+	add_old_value(signal, value);
+}
+
+/**
+ * @param {Source} signal
+ * @param {any} value
+ */
+function add_old_value(signal, value) {
+	const value_node = create_value_node(signal, value);
+
+	if (last_old_value_node === null) {
+		last_old_value_node = value_node;
+		old_value_node = value_node;
+	} else {
+		last_old_value_node.n = value_node;
+		last_old_value_node = value_node;
+	}
+}
 
 /**
  * @param {Set<any>} v
@@ -155,9 +218,9 @@ export function internal_set(source, value) {
 		var old_value = source.v;
 
 		if (is_destroying_effect) {
-			old_values.set(source, value);
+			set_old_value(source, value);
 		} else {
-			old_values.set(source, old_value);
+			add_old_value(source, old_value);
 		}
 
 		source.v = value;

packages/svelte/src/internal/client/reactivity/types.d.ts

@@ -74,3 +74,5 @@ export interface Effect extends Reaction {
 export type Source<V = unknown> = Value<V>;
 
 export type MaybeSource<T = unknown> = T | Source<T>;
+
+export type ValueNode = { s: Source; n: null | ValueNode; v: any };

packages/svelte/src/internal/client/runtime.js

@@ -26,7 +26,7 @@ import {
 	EFFECT_IS_UPDATING
 } from './constants.js';
 import { flush_tasks } from './dom/task.js';
-import { internal_set, old_values } from './reactivity/sources.js';
+import { clear_old_value_node, internal_set, get_old_value_node } from './reactivity/sources.js';
 import { destroy_derived_effects, update_derived } from './reactivity/deriveds.js';
 import * as e from './errors.js';
 import { FILENAME } from '../../constants.js';
@@ -676,7 +676,7 @@ function flush_queued_root_effects() {
 				var collected_effects = process_effects(root_effects[i]);
 				flush_queued_effects(collected_effects);
 			}
-			old_values.clear();
+			clear_old_value_node();
 		}
 	} finally {
 		is_flushing = false;
@@ -931,8 +931,11 @@ export function get(signal) {
 		}
 	}
 
-	if (is_destroying_effect && old_values.has(signal)) {
-		return old_values.get(signal);
+	if (is_destroying_effect) {
+		const value_node = get_old_value_node(signal);
+		if (value_node !== null) {
+			return value_node.v;
+		}
 	}
 
 	return signal.v;