/**
* Stack
* @version 1.01
* @author  Sam Harris
*/
public class Stack {

    static int stackSize = 5;

    static int topOfStack = -1;
    static int[] stack = new int[stackSize];
    static boolean errorFree = true;

	/**
	* returns true if the stack is empty and false if not
	* no side effects
	* referentially transparent
	* @return boolean, true if stack is empty
	**/
    public static boolean isEmpty () {
        return topOfStack == -1;
    }
	/**
	* returns true if the stack is full and false if not
	* no side effects
	* referentially transparent
	* @return boolean, true if stack full
	**/
    public static boolean isFull () {
        return topOfStack == stackSize - 1;
    }

	/**
	* Moves the stack pointer to create an empty stack
	* side effect changes errorFree to true
	* referentially transparent
	**/
    public static void empty () {
        errorFree = true;
        topOfStack = -1;
    }
	/**
	* returns the value at the top of the stack
	* no side effects
	* referentially transparent
	* @return int, value of the top of the stack
	**/
    public static int top () {
	errorFree = ! (isEmpty ()) & errorFree;
        if (errorFree) {
	    return stack[topOfStack];
	} else {
	    return 0;
        }
    }
	/**
	* adds a value to the top of the stack
	* no side effects
	* based on
	*	int value
	* referentially transparent
	* @param int, value to be added to stack
	**/
    public static void push (int value) {
	errorFree = ! (isFull ()) & errorFree;
        if (errorFree) {
	    topOfStack = topOfStack + 1;
            stack[topOfStack] = value;
	}
    }
	/**
	* removes the value at the top of the stack by moving stack pointer
	* no side effects
	* referentially transparent
	**/
    public static void pop () {
	errorFree = ! (isEmpty ()) & errorFree;
        if (errorFree) {
	    topOfStack = topOfStack - 1;
	}
    }

public static void main (String args[]) {
    int test;

    System.out.println(" --- Begin Experiment 1 ---");


    System.out.println(" --- Empty ---");
    empty ();

    System.out.println("Build up a stack of one entry:");
    push(1);
    System.out.println("   push 1");

    System.out.println("Inspect the stack:");
    test = top();
    System.out.println("   top: " + test);

    System.out.println("Make the stack empty:");
    pop();
    System.out.println("   pop");

    System.out.println("Test how 'top' works on the empty stack:");
    test = top();

    if (errorFree) {
       System.out.println("   top: " + test);
    } else {
       System.out.println("   An error has occured.");
    }

    System.out.println(" --- End Experiment 1 ---");

    System.out.println(" --- Begin Experiment 2 ---");

    System.out.println(" --- Empty ---");
    empty ();

    System.out.println("Build up a stack of five entries:");

    push(1);
    System.out.println("   push 1");
    push(2);
    System.out.println("   push 2");
    push(3);
    System.out.println("   push 3");
    push(4);
    System.out.println("   push 4");
    push(5);
    System.out.println("   push 5");

    System.out.println("Push another entry and check if 'out of memory'-protection works:");

    push(6);
    if (errorFree) {
           System.out.println("   push 6");
    } else {
       System.out.println("   An error has occured.");
    }

    System.out.println(" --- End Experiment 2 ---");


    System.out.println(" --- Begin Experiment 3 ---");

    empty ();
    System.out.println(" --- Empty ---");

    System.out.println("Build up a stack of three entries:");

    push(1);
    System.out.println("   push 1");
    push(2);
    System.out.println("   push 2");
    push(3);
    System.out.println("   push 3");

    System.out.println("Take these three entries away.");
    while (! isEmpty ()) {
        test = top ();
        System.out.println("   top: " + test);
        pop ();
        System.out.println("   pop");
    }

    System.out.println(" --- End Experiment 3 ---");

	System.out.println(" --- Begin Experiment 4 ---");
	System.out.println("Create empty stack");
	empty ();
	push(25);
	System.out.println("   push 25");
	push(31);
    System.out.println("   push 31");
	System.out.println("   pop");
	pop ();
    test = top ();
    System.out.println("   top: " + test);

	System.out.println(" --- End Experiment 4 ---");

	System.out.println(" --- Begin Experiment 5 ---");

	empty ();

	if (isEmpty()) {
		System.out.println("Empty");
	} else {
		System.out.println("not empty");
	}
	push(42);
	System.out.println("   push 42");
	if (isEmpty()) {
		System.out.println("Empty");
	} else {
		System.out.println("not empty");
	}
	System.out.println("   pop");
	pop ();
	if (isEmpty()) {
		System.out.println("Empty");
	} else {
		System.out.println("not empty");
	}
	System.out.println(" --- End Experiment 5 ---");
}

}