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+/*
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import {getInteractionCount} from './polyfills/interactionCountPolyfill.js';
+
+interface Interaction {
+ id: number;
+ latency: number;
+ entries: PerformanceEventTiming[];
+}
+
+interface EntryPreProcessingHook {
+ (entry: PerformanceEventTiming): void;
+}
+
+// A list of longest interactions on the page (by latency) sorted so the
+// longest one is first. The list is at most MAX_INTERACTIONS_TO_CONSIDER long.
+export const longestInteractionList: Interaction[] = [];
+
+// A mapping of longest interactions by their interaction ID.
+// This is used for faster lookup.
+export const longestInteractionMap: Map<number, Interaction> = new Map();
+
+// The default `durationThreshold` used across this library for observing
+// `event` entries via PerformanceObserver.
+export const DEFAULT_DURATION_THRESHOLD = 40;
+
+// Used to store the interaction count after a bfcache restore, since p98
+// interaction latencies should only consider the current navigation.
+let prevInteractionCount = 0;
+
+/**
+ * Returns the interaction count since the last bfcache restore (or for the
+ * full page lifecycle if there were no bfcache restores).
+ */
+const getInteractionCountForNavigation = () => {
+ return getInteractionCount() - prevInteractionCount;
+};
+
+export const resetInteractions = () => {
+ prevInteractionCount = getInteractionCount();
+ longestInteractionList.length = 0;
+ longestInteractionMap.clear();
+};
+
+/**
+ * Returns the estimated p98 longest interaction based on the stored
+ * interaction candidates and the interaction count for the current page.
+ */
+export const estimateP98LongestInteraction = () => {
+ const candidateInteractionIndex = Math.min(
+ longestInteractionList.length - 1,
+ Math.floor(getInteractionCountForNavigation() / 50),
+ );
+
+ return longestInteractionList[candidateInteractionIndex];
+};
+
+// To prevent unnecessary memory usage on pages with lots of interactions,
+// store at most 10 of the longest interactions to consider as INP candidates.
+const MAX_INTERACTIONS_TO_CONSIDER = 10;
+
+/**
+ * A list of callback functions to run before each entry is processed.
+ * Exposing this list allows the attribution build to hook into the
+ * entry processing pipeline.
+ */
+export const entryPreProcessingCallbacks: EntryPreProcessingHook[] = [];
+
+/**
+ * Takes a performance entry and adds it to the list of worst interactions
+ * if its duration is long enough to make it among the worst. If the
+ * entry is part of an existing interaction, it is merged and the latency
+ * and entries list is updated as needed.
+ */
+export const processInteractionEntry = (entry: PerformanceEventTiming) => {
+ entryPreProcessingCallbacks.forEach((cb) => cb(entry));
+
+ // Skip further processing for entries that cannot be INP candidates.
+ if (!(entry.interactionId || entry.entryType === 'first-input')) return;
+
+ // The least-long of the 10 longest interactions.
+ const minLongestInteraction =
+ longestInteractionList[longestInteractionList.length - 1];
+
+ const existingInteraction = longestInteractionMap.get(entry.interactionId!);
+
+ // Only process the entry if it's possibly one of the ten longest,
+ // or if it's part of an existing interaction.
+ if (
+ existingInteraction ||
+ longestInteractionList.length < MAX_INTERACTIONS_TO_CONSIDER ||
+ entry.duration > minLongestInteraction.latency
+ ) {
+ // If the interaction already exists, update it. Otherwise create one.
+ if (existingInteraction) {
+ // If the new entry has a longer duration, replace the old entries,
+ // otherwise add to the array.
+ if (entry.duration > existingInteraction.latency) {
+ existingInteraction.entries = [entry];
+ existingInteraction.latency = entry.duration;
+ } else if (
+ entry.duration === existingInteraction.latency &&
+ entry.startTime === existingInteraction.entries[0].startTime
+ ) {
+ existingInteraction.entries.push(entry);
+ }
+ } else {
+ const interaction = {
+ id: entry.interactionId!,
+ latency: entry.duration,
+ entries: [entry],
+ };
+ longestInteractionMap.set(interaction.id, interaction);
+ longestInteractionList.push(interaction);
+ }
+
+ // Sort the entries by latency (descending) and keep only the top ten.
+ longestInteractionList.sort((a, b) => b.latency - a.latency);
+ if (longestInteractionList.length > MAX_INTERACTIONS_TO_CONSIDER) {
+ longestInteractionList
+ .splice(MAX_INTERACTIONS_TO_CONSIDER)
+ .forEach((i) => longestInteractionMap.delete(i.id));
+ }
+ }
+};