Update web-crawler-multithreaded.cpp

Author: kamyu104

C++/web-crawler-multithreaded.cpp

@@ -77,3 +77,75 @@ class Solution {
     mutex m_;
     condition_variable cv_;
 };
+
+// Time:  O(|V| + |E|)
+// Space: O(|V|)
+class Solution2 {
+public:
+    vector<string> crawl(string startUrl, HtmlParser htmlParser) {
+        q_.emplace(startUrl);
+        unordered_set<string> lookup = {startUrl};
+        vector<thread> workers;
+        for (int i = 0; i < NUMBER_OF_WORKERS; ++i) {
+            workers.emplace_back(bind(&Solution::worker, this, &htmlParser, &lookup));
+        }
+        {
+            unique_lock<mutex> lock{m_};
+            cv_.wait(lock, [this]() { return q_.empty() && !working_count_; });
+            for (const auto& t : workers) {
+                q_.emplace();
+            }
+            cv_.notify_all();
+        }
+        for (auto& t : workers) {
+            t.join();
+        }
+        return vector<string>(lookup.cbegin(), lookup.cend());
+    }
+
+private:
+    void worker(HtmlParser *htmlParser, unordered_set<string> *lookup) {
+        while (true) {
+            string from_url;
+            {
+                unique_lock<mutex> lock{m_};
+                cv_.wait(lock, [this]() { return !q_.empty(); });
+                from_url = q_.front(); q_.pop();
+                if (from_url.empty()) {
+                    break;
+                }
+                ++working_count_;
+            }
+            const auto& name = hostname(from_url);
+            for (const auto& to_url: htmlParser->getUrls(from_url)) {
+                if (name != hostname(to_url)) {
+                    continue;
+                }
+                unique_lock<mutex> lock{m_};
+                if (!lookup->count(to_url)) {
+                    lookup->emplace(to_url);
+                    q_.emplace(to_url);
+                    cv_.notify_all();
+                }
+            }
+            {
+                unique_lock<mutex> lock{m_};
+                --working_count_;
+                if (q_.empty() && !working_count_) {
+                    cv_.notify_all();
+                }
+            }
+        }
+    }
+    
+    string hostname(const string& url) {
+        static const string scheme = "http://";
+        return url.substr(0, url.find('/', scheme.length()));
+    }
+
+    static const int NUMBER_OF_WORKERS = 4;
+    queue<string> q_;
+    int working_count_ = 0;
+    mutex m_;
+    condition_variable cv_;
+};