Adding userDB

UserDb.js

@@ -0,0 +1,48 @@
+// Persisting the User Map to disk
+
+const murmurhash = require('murmurhash')
+const fs = require('fs').promises
+const User = require("./user.js")
+
+class UserDb extends Map {
+  constructor(file) {
+    super()
+    this.seed = 42
+    this.file = file
+    this.lastHash = 0
+  }
+  load() {
+    let data = []
+    return fs.readFile(this.file).then((d) => {
+      data = JSON.parse(d)
+      data.forEach(u => {
+        const user = new User(u.userId, u.experimentId)
+        user.redirectedUrl = u.redirectedUrl
+        user.oTreeId = u.oTreeId
+        user.tokenParams = u.tokenParams
+        user.state = (u.redirectedUrl) ? u.state : user.state
+        this.set(user.userId, user)
+      })
+    }).catch((err) => {
+      console.error(`[WARNING] UserDb file ${this.file} file not found will be recreated!`)
+    })
+  }
+  save() {
+    const data = []
+    this.forEach((v, k) => {
+      data.push(v.serialize())
+    })
+
+    const dump = JSON.stringify(data)
+    const h = murmurhash(dump, this.seed)
+    if (this.lastHash !== h) {
+      fs.writeFile(this.file, dump).then(() => {
+        this.lastHash = h
+      }).catch((err) => {
+        console.error("[ERROR] Writing UserDb to file.")
+      })
+    }
+  }
+}
+
+module.exports = UserDb