## Thursday, July 7, 2016

### LeetCode OJ - Design Twitter

Problem:

Analysis:

We need at most 10 tweets, so we can keep, for each user, the 10 most recent tweets, ordered by time (from most recent to least recent). Apparently, we should use a deque for it. The problem is how can we give the right news feed for a particular user fast enough. The key idea is that it is just a k-way merge.

Solution:

For a practical standpoint, we expect a lot more read than write for tweets. It makes sense for us to cache. As long as the following relationship does not change, we can have the tweet poster to take to hit to update his followers.

Code:

#include "stdafx.h"

#include <unordered_map>
#include <unordered_set>
#include <iostream>
#include <deque>
#include <vector>
#include <string>

using namespace std;

{
const int MAX_TWEETS = 10;

private:
unordered_set<int> m_users;
unordered_map<int, deque<pair<int, int>>> m_user_tweets;
unordered_map<int, unordered_set<int>> m_following;
unordered_map<int, unordered_set<int>> m_follower;
unordered_map<int, deque<int>> m_news_feed_cache;
int m_time;
public:
/** Initialize your data structure here. */
{
this->m_time = 0;
}

/** Compose a new tweet. */
void postTweet(int userId, int tweetId)
{
if (this->m_users.find(userId) == this->m_users.end())
{
this->m_users.insert(userId);
this->m_news_feed_cache.insert(make_pair(userId, deque<int>()));
}

auto probe = this->m_user_tweets.find(userId);
if (probe == this->m_user_tweets.end())
{
deque<pair<int, int>> new_user_tweets;
new_user_tweets.push_front(pair<int, int>(tweetId, this->m_time));
this->m_user_tweets.insert(pair<int, deque<pair<int, int>>>(userId, new_user_tweets));
}
else
{
deque<pair<int, int>>& old_user_tweets = probe->second;
if (old_user_tweets.size() == MAX_TWEETS)
{
old_user_tweets.pop_back();
}
old_user_tweets.push_front(pair<int, int>(tweetId, this->m_time));
}

auto selfCacheProbe = this->m_news_feed_cache.find(userId);
if (selfCacheProbe != this->m_news_feed_cache.end())
{
if (selfCacheProbe->second.size() == MAX_TWEETS)
{
selfCacheProbe->second.pop_back();
}

selfCacheProbe->second.push_front(tweetId);
}

auto followerSetProbe = this->m_follower.find(userId);
if (followerSetProbe != this->m_follower.end())
{
for (auto fi = followerSetProbe->second.begin(); fi != followerSetProbe->second.end(); fi++)
{
auto followerCacheProbe = this->m_news_feed_cache.find(*fi);
if (followerCacheProbe != this->m_news_feed_cache.end())
{
if (followerCacheProbe->second.size() == MAX_TWEETS)
{
followerCacheProbe->second.pop_back();
}

followerCacheProbe->second.push_front(tweetId);
}
}
}

this->m_time++;
}

/** Retrieve the 10 most recent tweet ids in the user's news feed. Each item in the news feed must be posted by users who the user followed or by the user herself. Tweets must be ordered from most recent to least recent. */
vector<int> getNewsFeed(int userId)
{
vector<int> result;

auto cacheProbe = this->m_news_feed_cache.find(userId);
if (cacheProbe != this->m_news_feed_cache.end())
{
for (auto ci = cacheProbe->second.begin(); ci != cacheProbe->second.end(); ci++)
{
result.push_back(*ci);
}

return result;
}

vector<pair<deque<pair<int, int>>::iterator, deque<pair<int, int>>::iterator>> following_tweets;
deque<int> cache;
auto selfProbe = this->m_user_tweets.find(userId);
if (selfProbe != this->m_user_tweets.end())
{
following_tweets.push_back(make_pair(selfProbe->second.begin(), selfProbe->second.end()));
}

auto followingSetProbe = this->m_following.find(userId);
if (followingSetProbe != this->m_following.end())
{
for (auto fi = followingSetProbe->second.begin(); fi != followingSetProbe->second.end(); fi++)
{
auto followingProbe = this->m_user_tweets.find(*fi);
if (followingProbe != this->m_user_tweets.end())
{
following_tweets.push_back(make_pair(followingProbe->second.begin(), followingProbe->second.end()));
}
}
}

while (result.size() < MAX_TWEETS)
{
size_t best = -1;
for (size_t i = 0; i < following_tweets.size(); i++)
{
if (following_tweets[i].first != following_tweets[i].second)
{
// We have an item in a following tweet
if (best == -1)
{
best = i;
}
else if (following_tweets[i].first->second > following_tweets[best].first->second)
{
best = i;
}
}
}
if (best == -1)
{
break;
}
else
{
result.push_back(following_tweets[best].first->first);
cache.push_back(following_tweets[best].first->first);
following_tweets[best].first++;
}
}

this->m_news_feed_cache.insert(make_pair(userId, cache));

return result;
}

/** Follower follows a followee. If the operation is invalid, it should be a no-op. */
void follow(int followerId, int followeeId)
{
if (this->m_users.find(followerId) == this->m_users.end())
{
this->m_users.insert(followerId);
this->m_news_feed_cache.insert(make_pair(followerId, deque<int>()));
}

if (followerId == followeeId)
{
return;
}

auto probeFollowing = this->m_following.find(followerId);
if (probeFollowing == this->m_following.end())
{
unordered_set<int> newFollowing;
newFollowing.insert(followeeId);
this->m_following.insert(pair<int, unordered_set<int>>(followerId, newFollowing));
}
else
{
probeFollowing->second.insert(followeeId);
}

auto probeFollower = this->m_follower.find(followeeId);
if (probeFollower == this->m_follower.end())
{
unordered_set<int> newFollower;
newFollower.insert(followerId);
this->m_follower.insert(pair<int, unordered_set<int>>(followeeId, newFollower));
}
else
{
probeFollower->second.insert(followerId);
}

// Invalidate my cache since I changed my following relationship
if (this->m_news_feed_cache.find(followerId) != this->m_news_feed_cache.end())
{
this->m_news_feed_cache.erase(followerId);
}
}

/** Follower unfollows a followee. If the operation is invalid, it should be a no-op. */
void unfollow(int followerId, int followeeId)
{
auto probeFollowing = this->m_following.find(followerId);
if (probeFollowing != this->m_following.end())
{
if (probeFollowing->second.find(followeeId) != probeFollowing->second.end())
{
probeFollowing->second.erase(followeeId);
}
}

auto probeFollower = this->m_follower.find(followeeId);
if (probeFollower != this->m_follower.end())
{
if (probeFollower->second.find(followerId) != probeFollower->second.end())
{
probeFollower->second.erase(followerId);
}
}

// Invalidate my cache since I changed my following relationship
if (this->m_news_feed_cache.find(followerId) != this->m_news_feed_cache.end())
{
this->m_news_feed_cache.erase(followerId);
}
}
};
};

{
vector<int> result;
}