/**
 * The maximum number of items that can be stored in an ArraySet in array mode before switching to Set mode.
 *
 * @public
 * @example
 * ```ts
 * import { ARRAY_SIZE_THRESHOLD } from '@tldraw/state'
 *
 * console.log(ARRAY_SIZE_THRESHOLD) // 8
 * ```
 */
export const ARRAY_SIZE_THRESHOLD = 8

/**
 * An ArraySet operates as an array until it reaches a certain size, after which a Set is used
 * instead. In either case, the same methods are used to get, set, remove, and visit the items.
 * @internal
 */
export class ArraySet<T> {
	private arraySize = 0

	private array: (T | undefined)[] | null = Array(ARRAY_SIZE_THRESHOLD)

	private set: Set<T> | null = null

	/**
	 * Get whether this ArraySet has any elements.
	 *
	 * @returns True if this ArraySet has any elements, false otherwise.
	 */
	// eslint-disable-next-line tldraw/no-setter-getter
	get isEmpty() {
		if (this.array) {
			return this.arraySize === 0
		}

		if (this.set) {
			return this.set.size === 0
		}

		throw new Error('no set or array')
	}

	/**
	 * Add an element to the ArraySet if it is not already present.
	 *
	 * @param elem - The element to add to the set
	 * @returns `true` if the element was added, `false` if it was already present
	 * @example
	 * ```ts
	 * const arraySet = new ArraySet<string>()
	 *
	 * console.log(arraySet.add('hello')) // true
	 * console.log(arraySet.add('hello')) // false (already exists)
	 * ```
	 */
	add(elem: T) {
		if (this.array) {
			const idx = this.array.indexOf(elem)

			// Return false if the element is already in the array.
			if (idx !== -1) {
				return false
			}

			if (this.arraySize < ARRAY_SIZE_THRESHOLD) {
				// If the array is below the size threshold, push items into the array.

				// Insert the element into the array's next available slot.
				this.array[this.arraySize] = elem
				this.arraySize++

				return true
			} else {
				// If the array is full, convert it to a set and remove the array.
				this.set = new Set(this.array as any)
				this.array = null
				this.set.add(elem)

				return true
			}
		}

		if (this.set) {
			// Return false if the element is already in the set.
			if (this.set.has(elem)) {
				return false
			}

			this.set.add(elem)
			return true
		}

		throw new Error('no set or array')
	}

	/**
	 * Remove an element from the ArraySet if it is present.
	 *
	 * @param elem - The element to remove from the set
	 * @returns `true` if the element was removed, `false` if it was not present
	 * @example
	 * ```ts
	 * const arraySet = new ArraySet<string>()
	 * arraySet.add('hello')
	 *
	 * console.log(arraySet.remove('hello')) // true
	 * console.log(arraySet.remove('hello')) // false (not present)
	 * ```
	 */
	remove(elem: T) {
		if (this.array) {
			const idx = this.array.indexOf(elem)

			// If the item is not in the array, return false.
			if (idx === -1) {
				return false
			}

			this.array[idx] = undefined
			this.arraySize--

			if (idx !== this.arraySize) {
				// If the item is not the last item in the array, move the last item into the
				// removed item's slot.
				this.array[idx] = this.array[this.arraySize]
				this.array[this.arraySize] = undefined
			}

			return true
		}

		if (this.set) {
			// If the item is not in the set, return false.
			if (!this.set.has(elem)) {
				return false
			}

			this.set.delete(elem)

			return true
		}

		throw new Error('no set or array')
	}

	/**
	 * Execute a callback function for each element in the ArraySet.
	 *
	 * @param visitor - A function to call for each element in the set
	 * @example
	 * ```ts
	 * const arraySet = new ArraySet<string>()
	 * arraySet.add('hello')
	 * arraySet.add('world')
	 *
	 * arraySet.visit((item) => {
	 *   console.log(item) // 'hello', 'world'
	 * })
	 * ```
	 */
	visit(visitor: (item: T) => void) {
		if (this.array) {
			for (let i = 0; i < this.arraySize; i++) {
				const elem = this.array[i]

				if (typeof elem !== 'undefined') {
					visitor(elem)
				}
			}

			return
		}

		if (this.set) {
			this.set.forEach(visitor)

			return
		}

		throw new Error('no set or array')
	}

	/**
	 * Make the ArraySet iterable, allowing it to be used in for...of loops and with spread syntax.
	 *
	 * @returns An iterator that yields each element in the set
	 * @example
	 * ```ts
	 * const arraySet = new ArraySet<number>()
	 * arraySet.add(1)
	 * arraySet.add(2)
	 *
	 * for (const item of arraySet) {
	 *   console.log(item) // 1, 2
	 * }
	 *
	 * const items = [...arraySet] // [1, 2]
	 * ```
	 */
	*[Symbol.iterator]() {
		if (this.array) {
			for (let i = 0; i < this.arraySize; i++) {
				const elem = this.array[i]

				if (typeof elem !== 'undefined') {
					yield elem
				}
			}
		} else if (this.set) {
			yield* this.set
		} else {
			throw new Error('no set or array')
		}
	}

	/**
	 * Check whether an element is present in the ArraySet.
	 *
	 * @param elem - The element to check for
	 * @returns `true` if the element is present, `false` otherwise
	 * @example
	 * ```ts
	 * const arraySet = new ArraySet<string>()
	 * arraySet.add('hello')
	 *
	 * console.log(arraySet.has('hello')) // true
	 * console.log(arraySet.has('world')) // false
	 * ```
	 */
	has(elem: T) {
		if (this.array) {
			return this.array.indexOf(elem) !== -1
		} else {
			return this.set!.has(elem)
		}
	}

	/**
	 * Remove all elements from the ArraySet.
	 *
	 * @example
	 * ```ts
	 * const arraySet = new ArraySet<string>()
	 * arraySet.add('hello')
	 * arraySet.add('world')
	 *
	 * arraySet.clear()
	 * console.log(arraySet.size()) // 0
	 * ```
	 */
	clear() {
		if (this.set) {
			this.set.clear()
		} else {
			this.arraySize = 0
			this.array = []
		}
	}

	/**
	 * Get the number of elements in the ArraySet.
	 *
	 * @returns The number of elements in the set
	 * @example
	 * ```ts
	 * const arraySet = new ArraySet<string>()
	 * console.log(arraySet.size()) // 0
	 *
	 * arraySet.add('hello')
	 * console.log(arraySet.size()) // 1
	 * ```
	 */
	size() {
		if (this.set) {
			return this.set.size
		} else {
			return this.arraySize
		}
	}
}