import { swap } from '../../core/iterable.ts'

/**
 * A minimum binary heap.
 * 
 * A binary heap always satisfies the following properties:
 * - The root node is the minimum element in the heap.
 * - The children of a node are always greater than or equal to that node.
 * 
 * @author MindfulMinun
 * @since 2023-03-10
 */
export class BinaryHeap<T> implements Iterable<T> {
    #comparator: (a: T, b: T) => number
    #elements: T[]

    constructor(
        comparator: (a: T, b: T) => number,
        initials: Iterable<T> = []
    ) {
        this.#comparator = comparator
        this.#elements = []

        for (const el of initials) this.push(el)
    }

    *[Symbol.iterator](): Generator<NonNullable<T>, void, unknown> {
        while (this.length !== 0) yield this.pop()!
    }

    /** Insert an element into the heap. */
    push(element: T): void {
        this.#elements.push(element)
        this.#bubbleUp(this.length - 1)
    }

    /** Remove the minimum element from the heap and return it. */
    pop(): T | undefined {
        if (this.length === 0) return undefined

        // Swap the last element with the root
        swap(this.#elements, 0, this.length - 1)
        // Remove the last element
        const el = this.#elements.pop()
        // Bubble the new root down
        this.#bubbleDown(0)

        return el
    }

    /** Preview the minimum element in the heap without removing it. */
    peek(): T | undefined {
        if (this.length === 0) return undefined
        return this.#elements[0]
    }

    /** Bubble an element up the heap until it is in the correct position. */
    #bubbleUp(index: number): void {
        // If the element is the root, it is in the correct position
        if (index === 0) return

        // Bubble the element up the heap until it is in the correct position
        let current = index
        let parent = BinaryHeap.parent(current)
        const val = this.#elements[current]

        // While the element is less than its parent, swap it with its parent
        while (0 < current && this.#comparator(val, this.#elements[parent]) < 0) {
            this.#elements[current] = this.#elements[parent]
            current = parent
            parent = BinaryHeap.parent(current)
        }
        this.#elements[current] = val
    }

    /** Bubble an element down the heap until it is in the correct position. */
    #bubbleDown(index: number): void {
        // If the element is a leaf, it is in the correct position
        if (index >= this.length) return

        // Bubble the element down the heap until it is in the correct position
        let current = index
        let [left, right] = BinaryHeap.leftRight(current)
        const val = this.#elements[current]

        // While the element is greater than its children, swap it with its smallest child
        while (left < this.length) {
            // Find the smallest child
            let smallest = left
            if (right < this.length && 0 < this.#comparator(this.#elements[left], this.#elements[right])) {
                smallest = right
            }

            // If the element is smaller than its smallest child, it is in the correct position
            if (this.#comparator(val, this.#elements[smallest]) <= 0) break

            // Swap the element with its smallest child
            this.#elements[current] = this.#elements[smallest]
            current = smallest
            ;[left, right] = BinaryHeap.leftRight(current)
        }

        this.#elements[current] = val
    }

    private static leftRight(index: number) {
        return [index * 2 + 1, index * 2 + 2]
    }

    private static parent(index: number) {
        return Math.floor((index - 1) / 2)
    }

    /** The number of elements in the heap */
    get length(): number { return this.#elements.length }
}