diff --git a/lib/versioned-map.js b/lib/versioned-map.js
new file mode 100644
index 0000000..83e5251
--- /dev/null
+++ b/lib/versioned-map.js
@@ -0,0 +1,253 @@
+/**
+ * Copyright 2013-2016  Zaid Abdulla
+ *
+ * GenieACS is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * GenieACS is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with GenieACS.  If not, see <http://www.gnu.org/licenses/>.
+ */
+"use strict";
+
+const NONEXISTENT = new Object();
+
+
+function compareEquality(a, b) {
+  var t = typeof a;
+  if (t === 'number' || (a == null) || t === 'boolean' || t === 'string' || t === 'symbol')
+    return a === b;
+
+  return JSON.stringify(a) === JSON.stringify(b);
+}
+
+
+class VersionedMap {
+
+  constructor() {
+    this._sizeDiff = [0];
+    this._revision = 0;
+    this.map = new Map();
+  }
+
+
+  get size() {
+    return this.map.size + this._sizeDiff[this.revision];
+  }
+
+
+  get revision() {
+    return this._revision;
+  }
+
+
+  set revision(rev) {
+    for (let i = this._sizeDiff.length; i <= rev; ++ i)
+      this._sizeDiff[i] = this._sizeDiff[i - 1];
+
+    this._revision = rev;
+  }
+
+
+  get(key, rev) {
+    if (rev == null)
+      rev = this._revision
+
+    var v = this.map.get(key);
+    if (!v)
+      return;
+
+    for (let i = Math.min(v.length - 1, rev); i >= 0; -- i)
+      if (i in v)
+        if (v[i] === NONEXISTENT)
+          return undefined;
+        else
+          return v[i];
+  }
+
+
+  has(key, rev) {
+    if (rev == null)
+      rev = this._revision
+
+    var v = this.map.get(key);
+
+    if (v == null)
+      return false;
+
+    for (let i = Math.min(v.length - 1, rev); i >= 0; -- i)
+      if (i in v)
+        if (v[i] === NONEXISTENT)
+          return false;
+        else
+          return true;
+
+    return false;
+  }
+
+
+  set(key, value, rev) {
+    if (rev == null)
+      rev = this._revision;
+
+    var v = this.map.get(key);
+    if (!v) {
+      for (let i = 0; i < rev; ++ i)
+        this._sizeDiff[i] -= 1;
+
+      v = [];
+      v[rev] = value;
+      this.map.set(key, v);
+      return this;
+    }
+
+    var old = v[Math.min(rev, v.length - 1)];
+
+    if (rev < v.length - 1) {
+      if (compareEquality(value, old))
+        return
+      else
+        throw new Error('Cannot modify old revisions');
+    }
+
+    if (old === NONEXISTENT)
+      ++ this._sizeDiff[rev];
+
+    v[rev] = value;
+  }
+
+  delete(key, rev) {
+    if (rev == null)
+      rev = this._revision;
+
+    var v = this.map.get(key);
+    if (!v)
+      return false;
+
+    if (rev < v.length - 1)
+      throw new Error('Cannot modify old revisions');
+
+    var old = v[v.length - 1];
+    if (old === NONEXISTENT)
+      return false;
+
+    -- this._sizeDiff[rev];
+    v[rev] = NONEXISTENT;
+
+    return old;
+  }
+
+
+  getRevisions(key) {
+    var v = this.map.get(key);
+    if (!v)
+      return null;
+
+    var res = {};
+
+    for (let i in v)
+      if (v[i] === NONEXISTENT)
+        res.delete |= 1 << i;
+      else
+        res[i] = v[i];
+
+    return res;
+  }
+
+
+  setRevisions(key, revisions) {
+    var del = 0;
+    var rev = [];
+    var minKey = 999;
+    for (let k in revisions) {
+      if (k === 'delete')
+        del = revisions[k];
+      else {
+        let r = parseInt(k);
+        minKey = Math.min(minKey, r);
+        for (let i = this._sizeDiff.length; i <= r; ++ i)
+          this._sizeDiff[i] = this._sizeDiff[i - 1];
+
+        rev[r] = revisions[k];
+      }
+    }
+
+    for (let i = 0; i < minKey; ++ i)
+      -- this._sizeDiff[i];
+
+    for (let i = 0; del > 0; del >>= 1, ++ i)
+      if (del & 1)
+        rev[i] = NONEXISTENT;
+
+    this.map.set(key, rev);
+  }
+
+
+  getDiff(key) {
+    var revisions = this.map.get(key);
+    if (!revisions)
+      return [];
+
+    var current = revisions[revisions.length - 1];
+    if (current === NONEXISTENT) {
+      if (0 in revisions)
+        return [key, revisions[0]];
+      else
+        return [key];
+    } else if (0 in revisions)
+      return [key, revisions[0], current];
+    else
+      return [key, , current];
+  }
+
+
+  *diff() {
+    for (let pair of this.map) {
+      let current = pair[1][pair[1].length - 1];
+      if (current === NONEXISTENT) {
+        if (0 in pair[1])
+          yield [pair[0], pair[1][0]];
+        else
+          yield [pair[0]];
+      } else if (0 in pair[1])
+        yield [pair[0], pair[1][0], current];
+      else
+        yield [pair[0], , current];
+    }
+  }
+
+
+  collapse(revision) {
+    if (this._sizeDiff.length <= revision)
+      return;
+
+    this._sizeDiff[revision] = this._sizeDiff[this._sizeDiff.length - 1];
+    this._sizeDiff.splice(revision + 1, this._sizeDiff.length);
+
+    for (let pair of this.map) {
+      let k = pair[0];
+      let v = pair[1];
+      let last = v[v.length - 1];
+      if (last === NONEXISTENT) {
+        v.splice(revision, v.length);
+        if (v.some(function(val) { return true; }))
+          v[revision] = NONEXISTENT;
+        else
+          this.map.delete(k);
+      }
+      else {
+        v[revision] = last;
+        v.splice(revision + 1, v.length);
+      }
+    }
+  }
+
+}
+
+module.exports = VersionedMap;