/*
 * ListStack.kt
 * 
 * Based on Dave Musicant's Stack.kt from Fall 2024,
 * which was ported by Dave from the stack code at
 * https://www.geeksforgeeks.org/introduction-to-stack-data-structure-and-algorithm-tutorials/.
 * 
 * Modified by Tanya Amert for Spring 2025.
 * 
 * Illustrates an implementation of the Stack ADT using
 * a Kotlin MutableList.
 */

/*
 * Stack interface: implements a stack of integers
 */
interface Stack {
    fun push(item: Int)
    fun pop(): Int
    fun peek(): Int
    fun isEmpty(): Boolean
}

class ListStack : Stack {
    // Initialize the data storage for our stack
    private val myList: MutableList<Int> = mutableListOf()

    // Push an item to the top of the stack
    override fun push(item: Int) {
        // The "top" of the stack is the end of the list
        myList.add(item)
    }

    // Pop the top of the stack and return it
    override fun pop(): Int {
        if (myList.count() == 0) {
            throw RuntimeException("The stack is empty!")
        }

        // Remove the last thing in the list, as that's the "top"
        return myList.removeLast() // VS Code gives red squigglies, but it works
    }

    // Peek at the top of the stack (but don't change it)
    override fun peek(): Int {
        if (myList.count() == 0) {
            throw RuntimeException("The stack is empty!")
        }
        
        // Look at the last thing in the list, as that's the "top"
        return myList.last()
    }

    // Check whether the stack is empty
    override fun isEmpty(): Boolean {
        return myList.isEmpty()
    }

    // Return a string representing the stack
    override fun toString(): String {
        var s: String = ""
        for (i:Int in myList.count()-1 downTo 0) {
            if (i == myList.count()-1) {
                s += "  top:"
            }
            s += "\t${myList[i]}\n" // the element on a line of its own
        }
        return s
    }
}

// Test out the ListStack implementation
fun main() {
    val s = ListStack()
    s.push(2025)
    s.push(4)
    s.push(16)
    
    println("Elements present in stack: ")
    println(s)
    
    println("" + s.pop() + " popped from stack")
    println("Top element is: " + s.peek())

    println("Elements present in stack: ")
    println(s)
}