Memcache

Memcache

Implement a memcache which support the following features:

  1. get(curtTime, key). Get the key's value, return 2147483647 if key does not exist.
  2. set(curtTime, key, value, ttl). Set the key-value pair in memcache with a time to live (ttl). The key will be valid from curtTime to curtTime + ttl - 1 and it will be expired after ttl seconds. if ttl is 0, the key lives forever until out of memory.
  3. delete(curtTime, key). Delete the key.
  4. incr(curtTime, key, delta). Increase the key's value by delta return the new value. Return 2147483647 if key does not exist.
  5. decr(curtTime, key, delta). Decrease the key's value by delta return the new value. Return 2147483647 if key does not exist.
    It's guaranteed that the input is given with increasingcurtTime.

Clarification
Actually, a real memcache server will evict keys if memory is not sufficient, and it also supports variety of value types like string and integer. In our case, let's make it simple, we can assume that we have enough memory and all of the values are integers.

Search "LRU" & "LFU" on google to get more information about how memcache evict data.

Try the following problem to learn LRU cache:
http://www.lintcode.com/problem/lru-cache

Example

get(1, 0)  
>> 2147483647
set(2, 1, 1, 2)  
get(3, 1)  
>> 1
get(4, 1)  
>> 2147483647
incr(5, 1, 1)  
>> 2147483647
set(6, 1, 3, 0)  
incr(7, 1, 1)  
>> 4
decr(8, 1, 1)  
>> 3
get(9, 1)  
>> 3
delete(10, 1)  
get(11, 1)  
>> 2147483647
incr(12, 1, 1)  
>> 2147483647
class Resource {  
    public int value;
    public int expired;

    public Resource(int value, int expired) {
        this.value = value;
        this.expired = expired;
    }
}

public class Memcache {  
    HashMap<Integer, Resource> map;

    public Memcache() {
        map = new HashMap<>();
    }

    public int get(int curtTime, int key) {
        if (map.containsKey(key)) {
            Resource res = map.get(key);

            if (res.expired == -1 || res.expired >= curtTime) {
                return res.value;
            }
        }

        return Integer.MAX_VALUE;
    }

    public void set(int curtTime, int key, int value, int ttl) {
        Resource res = new Resource(value, curtTime + ttl - 1);

        if (ttl == 0) {
            res.expired = -1;
        }

        map.put(key, res);
    }

    public void delete(int curtTime, int key) {
        if (map.containsKey(key)) {
            map.remove(key);
        }
    }

    public int incr(int curtTime, int key, int delta) {
        if (get(curtTime, key) == Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        }

        Resource res = map.get(key);
        res.value += delta;

        return res.value;
    }

    public int decr(int curtTime, int key, int delta) {
        if (get(curtTime, key) == Integer.MAX_VALUE) {
            return Integer.MAX_VALUE;
        }

        Resource res = map.get(key);
        res.value -= delta;

        return res.value;
    }
}

Hope this helps,
Michael