// validating/CircularQueue.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Demonstration of Design by Contract (DbC)
package validating;
import java.util.*;

public class CircularQueue {
 private Object[] data;
 private int
 in = 0, // Next available storage space
 out = 0; // Next gettable object
 // Has it wrapped around the circular queue?
 private boolean wrapped = false;
 public CircularQueue(int size) {
 data = new Object[size];
 // Must be true after construction:
 assert invariant();
 }
 public boolean empty() {
 return !wrapped && in == out;
 }
 public boolean full() {
 return wrapped && in == out;
 }
 public boolean isWrapped() { return wrapped; }
 public void put(Object item) {
 precondition(item != null, "put() null item");
 precondition(!full(),
 "put() into full CircularQueue");
 assert invariant();
 data[in++] = item;
 if(in >= data.length) {
 in = 0;
 wrapped = true;
 }
 assert invariant();
 }
 public Object get() {
 precondition(!empty(),
 "get() from empty CircularQueue");
 assert invariant();
 Object returnVal = data[out];
 data[out] = null;
 out++;
 if(out >= data.length) {
 out = 0;
 wrapped = false;
 }
 assert postcondition(
 returnVal != null,
 "Null item in CircularQueue");
 assert invariant();
 return returnVal;
 }
 // Design-by-contract support methods:
 private static void
 precondition(boolean cond, String msg) {
 if(!cond) throw new CircularQueueException(msg);
 }
 private static boolean
 postcondition(boolean cond, String msg) {
 if(!cond) throw new CircularQueueException(msg);
 return true;
 }
 private boolean invariant() {
 // Guarantee that no null values are in the
 // region of 'data' that holds objects:
 for(int i = out; i != in; i = (i + 1) % data.length)
 if(data[i] == null)
 throw new CircularQueueException(
 "null in CircularQueue");
 // Guarantee that only null values are outside the
 // region of 'data' that holds objects:
 if(full()) return true;
 for(int i = in; i != out; i = (i + 1) % data.length)
 if(data[i] != null)
 throw new CircularQueueException(
 "non-null outside of CircularQueue range: "
 + dump());
 return true;
 }
 public String dump() {
 return "in = " + in +
 ", out = " + out +
 ", full() = " + full() +
 ", empty() = " + empty() +
 ", CircularQueue = " + Arrays.asList(data);
 }
}