Experimental analytics. (#170)

.gitignore

@@ -3,3 +3,4 @@
 admin/node_modules
 public/test.txt
 comment/node_modules
+node_modules

analytics/visitor.js

@@ -0,0 +1,136 @@
+(() => {
+    const startTimestamp = Date.now();
+    let pageviewId = null;
+
+    // Reads the previously stored user id.
+    const readUserId = () => {
+        const match = document.cookie.match(/visitor_user\s*=\s*([a-z0-9\-]+)/);
+        return match ? match[1] : null;
+    };
+
+    // User ids use permanent cookies.
+    const writeUserId = (userId) => {
+        const expires = new Date();
+        expires.setFullYear(expires.getFullYear() + 1);
+        document.cookie = 'visitor_user=' + userId + '; path=/; expires=' + expires.toUTCString();
+    };
+
+    const readSessionId = () => {
+        const match = document.cookie.match(/visitor_session\s*=\s*([a-z0-9\-]+)/);
+        return match ? match[1] : null;
+    };
+
+    const writeSessionId = (sessionId) => {
+        document.cookie = 'visitor_session=' + sessionId + '; path=/';
+    };
+
+    // Helper to run HTTP POST request to the backend.
+    const postJSON = (url, data, cb) => {
+        const supported = typeof XMLHttpRequest !== 'undefined' &&
+            typeof JSON !== 'undefined';
+        if (supported) {
+            const xmlhttp = new XMLHttpRequest();
+            xmlhttp.open('POST', url);
+            xmlhttp.setRequestHeader('Content-Type', 'application/json');
+            xmlhttp.send(JSON.stringify(data));
+            xmlhttp.onreadystatechange = () => {
+                if (xmlhttp.readyState === 4 && xmlhttp.status === 200) {
+                    cb(JSON.parse(xmlhttp.responseText));
+                }
+            };
+        }
+    };
+
+    // Tries to use either the stored user id
+    // or generates a new one on the server.
+    const doWithUser = (cb) => {
+        const userId = readUserId();
+        if (userId) {
+            // Use the stored user id.
+            cb(userId);
+        } else {
+            // Record the new user.
+            postJSON('/api/visitor/user', {}, (response) => {
+                if (response.status === 'success') {
+                    const userId = response.data;
+                    // Remember for later.
+                    writeUserId(userId);
+                    // Use the generated user id.
+                    cb(userId);
+                }
+            });
+        }
+    };
+
+    // Tries to use either the stored session id
+    // or generates a new one on the server.
+    const doWithSession = (userId, cb) => {
+        const sessionId = readSessionId();
+        if (sessionId) {
+            // Use the stored session id.
+            cb(userId, sessionId);
+        } else {
+            // Record the new session.
+            const data = {
+                user_id: userId,
+                agent: navigator.userAgent || null,
+                platform: navigator.platform || null
+            };
+            postJSON('/api/visitor/session', data, (response) => {
+                if (response.status === 'success') {
+                    const sessionId = response.data;
+                    // Remember for later.
+                    writeSessionId(sessionId);
+                    // Use the generated id.
+                    cb(userId, sessionId);
+                }
+            });
+        }
+    };
+
+    // Starts reporting the current page.
+    const doWithPageview = (userId, sessionId, cb) => {
+        const elapsed = Math.floor((Date.now() - startTimestamp) / 1000);
+        if (pageviewId) {
+            // Repeated pageview. Just send elapsed time.
+            const data = {
+                pageview_id: pageviewId,
+                elapsed: elapsed
+            }
+            postJSON('/api/visitor/pageview_extend', data, (response) => {
+                if (response.status === 'success') {
+                    cb(userId, sessionId);
+                }
+            });
+        } else {
+            const data = {
+                session_id: sessionId,
+                location: window.location.toString(),
+                referrer: document.referrer || null,
+                elapsed: elapsed
+            };
+            // First pageview record.
+            postJSON('/api/visitor/pageview', data, (response) => {
+                if (response.status === 'success') {
+                    // Update the current pageview id.
+                    pageviewId = response.data;
+                    cb(userId, sessionId);
+                }
+            });
+        }
+    };
+
+    // Reports the current page periodically with
+    // exponential interval.
+    const reportPageviewLoop = (userId, sessionId, timeout) => {
+        doWithPageview(sessionId, sessionId, () =>
+            setTimeout(() =>
+                reportPageviewLoop(userId, sessionId, 2 * timeout), timeout));
+    };
+
+    doWithUser((userId) => {
+        doWithSession(userId, (userId, sessionId) => {
+            reportPageviewLoop(userId, sessionId, 1000);
+        });
+    });    
+})();