算法 RingBuffer

https://en.wikipedia.org/wiki/Circular_buffer

某大外企面试问到到一个，当时有点紧张，然后用链表实现，最后写的也有些问题，要求的单元测试也没有完成。

两种实现，使用数组或者链表，相对来说不需要随机访问，使用链表会更好，实现上链表也更容易些。

 接口定义，实际使用应该定义泛型接口

package com.ljw.javatest;

// Ringbuffer is a circle, and has a head and tail.
// When add item, add item at tail, if item's quantity exceed max,
// then delete the head item.
public interface RingBuffer{

    public void addItem(int value);
    //return the oldest item
    public Integer getItem();
    //delege the oldest item
    public void deleteItem();
    //return item from head to tail
    public Integer[] getAll();
}

 链表实现，定义了一个内部类存储节点

package com.ljw.javatest;

public class RingBufferWithLinked implements RingBuffer{

    class Node{
        Integer value;
        Node next;
        Node(Integer value){
            this.value=value;
        }
    }

    private int max;
    private int count;
    private Node head;
    private Node tail;

    public RingBufferWithLinked(int max) throws Exception {
        if(max<=0){
            throw new Exception("invalid max");
        }
        this.max=max;
        this.count=0;
    }

    @Override
    public void addItem(int value) {
        if(head==null){
            head=new Node(value);
            tail=head;
            count++;
            return;
        }

        Node temp = new Node(value);

        tail.next = temp;
        tail = temp;
        if (count < max) {
            count++;
            return;
        }
        head=head.next;
        
    }

    @Override
    public Integer getItem() {
        if(head!=null){
            return head.value;
        }
        return null;
    }

    @Override
    public void deleteItem() {
        if(head==null){
            return;
        }
        head=head.next;
        count--;
        if(count==0){
            head=tail=null;
        }
    }

    @Override
    public Integer[] getAll() {
        if(head==null){
            return null;
        }
        Integer[] result=new Integer[count];
        int i=0;
        Node temp=head;
        while(temp!=null){
            result[i++]=temp.value;
            temp=temp.next;
        }
        return result;
    }

}

数组实现，如果说不好，可能就是需要分配连续空间，以及最大不能超过数组最大长度

package com.ljw.javatest;

import java.util.List;

public class RingBufferWithArray implements RingBuffer {

    public static void main(String[] args) {

    }

    Integer[] buffer;
    int head;
    int tail;
    int max;
    int count;

    public RingBufferWithArray(int max) throws Exception {
        if (max <= 0) {
            throw new Exception("invalid max");
        }
        buffer = new Integer[max];
        this.max = max;
        count = 0;
        head = -1;
        tail = -1;
    }

    @Override
    public void addItem(int value) {
        if (count + 1 > max) {
            buffer[head] = value;
            tail = head;
            if (head == max - 1) {
                head = 0;
            } else {
                head++;
            }
        } else {
            count++;
            if (count == 1) {
                head=tail=0;
                buffer[head]=value;
            } 
            else {
                if (tail == max - 1) {
                    tail = 0;
                    buffer[tail] = value;
                } else {
                    buffer[++tail] = value;
                }
            }
        }
    }

    @Override
    public Integer getItem() {
        if (head == -1) {
            return null;
        } else {
            return buffer[head];
        }
    }

    @Override
    public void deleteItem() {
        if (count == 0) {
            return;
        }
        buffer[head] = null;
        count--;
        if (count == 0) {
            head = tail = -1;
        }
        if (head++ == max) {
            head = 0;
        }
    }

    @Override
    public Integer[] getAll() {
        if (head == -1) {
            return null;
        }

        Integer[] result = new Integer[count];
        for (int i = 0, temp = head; i < count; i++, temp++) {
            if (temp == max) {
                temp = 0;
            }
            result[i] = buffer[temp];
        }
        return result;

    }

}

测试，主要的边界情况是max=1，以及增加元素超过时如何处理。

package com.ljw.javatest;


public class App {
    public static void main(String[] args) throws Exception {
        int max=3;//max=1;
        RingBuffer rb=new RingBufferWithLinked(max);
        rb.addItem(1);
        print(rb.getItem()==1);
        rb.deleteItem();
        print(rb.getItem()==null);

        int add=2;
        for(int i=0;i<max+add;i++){
            rb.addItem(i);
        }

        Integer[] result=rb.getAll();
        for(int i=0;i<max;i++){
            print(result[i]==i+add);
        }
        
    }

    public static void print(Object value){
        System.out.println(value);
    }
}