next.js/packages/next/src/client/components/segment-cache-impl/tuple-map.ts

// Utility type. Prefix<[A, B, C, D]> matches [A], [A, B], [A, B, C] etc.
export type Prefix<T extends any[]> = T extends [infer First, ...infer Rest]
  ? [] | [First] | [First, ...Prefix<Rest>]
  : []

export type TupleMap<Keypath extends Array<any>, V> = {
  set(keys: Prefix<Keypath>, value: V): void
  get(keys: Prefix<Keypath>): V | null
  delete(keys: Prefix<Keypath>): void
}

/**
 * Creates a map whose keys are tuples. Tuples are compared per-element. This
 * is useful when a key has multiple parts, but you don't want to concatenate
 * them into a single string value.
 *
 * In the Segment Cache, we use this to store cache entries by both their href
 * and their Next-URL.
 *
 * Example:
 *   map.set(['https://localhost', 'foo/bar/baz'], 'yay');
 *   map.get(['https://localhost', 'foo/bar/baz']); // returns 'yay'
 */
export function createTupleMap<Keypath extends Array<any>, V>(): TupleMap<
  Keypath,
  V
> {
  type MapEntryShared = {
    parent: MapEntry | null
    key: any
    map: Map<any, MapEntry> | null
  }

  type EmptyMapEntry = MapEntryShared & {
    value: null
    hasValue: false
  }

  type FullMapEntry = MapEntryShared & {
    value: V
    hasValue: true
  }

  type MapEntry = EmptyMapEntry | FullMapEntry

  let rootEntry: MapEntry = {
    parent: null,
    key: null,
    hasValue: false,
    value: null,
    map: null,
  }

  // To optimize successive lookups, we cache the last accessed keypath.
  // Although it's not encoded in the type, these are both null or
  // both non-null. It uses object equality, so to take advantage of this
  // optimization, you must pass the same array instance to each successive
  // method call, and you must also not mutate the array between calls.
  let lastAccessedEntry: MapEntry | null = null
  let lastAccessedKeys: Prefix<Keypath> | null = null

  function getOrCreateEntry(keys: Prefix<Keypath>): MapEntry {
    if (lastAccessedKeys === keys) {
      return lastAccessedEntry!
    }

    // Go through each level of keys until we find the entry that matches,
    // or create a new one if it doesn't already exist.
    let entry = rootEntry
    for (let i = 0; i < keys.length; i++) {
      const key = keys[i]
      let map = entry.map
      if (map !== null) {
        const existingEntry = map.get(key)
        if (existingEntry !== undefined) {
          // Found a match. Keep going.
          entry = existingEntry
          continue
        }
      } else {
        map = new Map()
        entry.map = map
      }
      // No entry exists yet at this level. Create a new one.
      const newEntry: MapEntry = {
        parent: entry,
        key,
        value: null,
        hasValue: false,
        map: null,
      }
      map.set(key, newEntry)
      entry = newEntry
    }

    lastAccessedKeys = keys
    lastAccessedEntry = entry

    return entry
  }

  function getEntryIfExists(keys: Prefix<Keypath>): MapEntry | null {
    if (lastAccessedKeys === keys) {
      return lastAccessedEntry
    }

    // Go through each level of keys until we find the entry that matches, or
    // return null if no match exists.
    let entry = rootEntry
    for (let i = 0; i < keys.length; i++) {
      const key = keys[i]
      let map = entry.map
      if (map !== null) {
        const existingEntry = map.get(key)
        if (existingEntry !== undefined) {
          // Found a match. Keep going.
          entry = existingEntry
          continue
        }
      }
      // No entry exists at this level.
      return null
    }

    lastAccessedKeys = keys
    lastAccessedEntry = entry

    return entry
  }

  function set(keys: Prefix<Keypath>, value: V): void {
    const entry = getOrCreateEntry(keys)
    entry.hasValue = true
    entry.value = value
  }

  function get(keys: Prefix<Keypath>): V | null {
    const entry = getEntryIfExists(keys)
    if (entry === null || !entry.hasValue) {
      return null
    }
    return entry.value
  }

  function deleteEntry(keys: Prefix<Keypath>): void {
    const entry = getEntryIfExists(keys)
    if (entry === null || !entry.hasValue) {
      return
    }

    // Found a match. Delete it from the cache.
    const deletedEntry: EmptyMapEntry = entry as any
    deletedEntry.hasValue = false
    deletedEntry.value = null

    // Check if we can garbage collect the entry.
    if (deletedEntry.map === null) {
      // Since this entry has no value, and also no child entries, we can
      // garbage collect it. Remove it from its parent, and keep garbage
      // collecting the parents until we reach a non-empty entry.

      // Unlike a `set` operation, these are no longer valid because the entry
      // itself is being modified, not just the value it contains.
      lastAccessedEntry = null
      lastAccessedKeys = null

      let parent = deletedEntry.parent
      let key = deletedEntry.key
      while (parent !== null) {
        const parentMap = parent.map
        if (parentMap !== null) {
          parentMap.delete(key)
          if (parentMap.size === 0) {
            // We just removed the last entry in the parent map.
            parent.map = null
            if (parent.value === null) {
              // The parent node has no child entries, nor does it have a value
              // on itself. It can be garbage collected. Keep going.
              key = parent.key
              parent = parent.parent
              continue
            }
          }
        }
        // The parent is not empty. Stop garbage collecting.
        break
      }
    }
  }

  return {
    set,
    get,
    delete: deleteEntry,
  }
}