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package hassemble.core.observing

import kotlinx.atomicfu.atomic

/**
 * A ring buffer of a fixed size.
 *
 * From https://gist.github.com/ToxicBakery/05d3d98256aaae50bfbde04ae0c62dbd
 */
class CircularArray : Iterable {

    /**
     * Creates a new instance of the array with the given size.
     */
    constructor(bufferSize: Int) {
        this.arr = arrayOfNulls(bufferSize)
        this.tail = -1
    }

    /**
     * Creates a new instance of the array as a copy.
     */
    constructor(circularArray: CircularArray) {
        this.arr = circularArray.arr.copyOf()
        this._size = circularArray._size
        this.tail = circularArray.tail
    }

    private val arr: Array
    private var _size: Int = 0
    private var tail: Int

    private val head: Int
        get() = if (_size == arr.size) (tail + 1) % _size else 0

    /**
     * Number of elements currently stored in the array.
     */
    val size: Int
        get() = _size

    /**
     * Add an element to the array.
     */
    fun add(item: T) {
        tail = (tail + 1) % arr.size
        arr[tail] = item
        if (_size < arr.size) _size++
    }

    /**
     * Get an element from the array.
     */
    @Suppress("UNCHECKED_CAST")
    operator fun get(index: Int): T =
        when {
            _size == 0 || index > _size || index < 0 -> throw IndexOutOfBoundsException("$index")
            _size == arr.size -> arr[(head + index) % arr.size]
            else -> arr[index]
        } as T

    /**
     * This array as a list.
     */
    @Suppress("UNCHECKED_CAST")
    fun toList(): List = iterator().asSequence().toList()

    public fun clone(): CircularArray = CircularArray(this)

    override fun iterator(): Iterator = object : Iterator {
        private val index = atomic(0)

        override fun hasNext(): Boolean = index.value < size

        override fun next(): T = get(index.getAndIncrement())
    }
}