package com.company;

public class Stack {
    private Double[] _array; // inner array to store information
    private int _top;

    public Stack() {
        this(0);
    }

    public Stack(int size) {
        _array = new Double[size];
        _top = -1; // initial index
        // if we use this initial index then _top will show us exactly the last element
    }

    private void resize() {
        int capacity = ((_top+1) == 0) ? 4 : (_top) * 2; // ascending rule of array
        Double[] array = new Double[capacity];
        if (_top != -1) { // there are something to copy

            for (int i = 0; i < _top + 1; i++) { // manual array copy. O(n) complexity
                array[i] = _array[i];
            }
        }
        _array = array; // new link for the array
    }

    public void push(Double item) { // in worst case O(n) complexity because of resize
        if (_top == -1 || this.isFull()) { // if the array is full
            this.resize();
        }

        _array[++_top] = item; // storing the new element at the top of array

    }

    public Double pop() { // returns last element and deletes it
        if (!this.isEmpty())
            return _array[_top--];
        else
            return null;
    }

    public Double peek() {
        return _array[_top];
    } // view top element without deleting

    public int getCount() {
        return _top + 1;
    }

    public boolean isEmpty(){
        return _top == -1;
    }

    private boolean isFull(){
        return _top == _array.length - 1;
    }
}