Add missing revision store

src/libs/revisions.js

@@ -0,0 +1,35 @@
+export class RevisionStore {
+  constructor(initialRevisions=[]) {
+    this.revisions = initialRevisions
+    this.currentIdx = initialRevisions.length - 1
+  }
+
+  get latest() {
+    return this.revisions[this.revisions.length - 1]
+  }
+
+  get current() {
+    return this.revisions[this.currentIdx]
+  }
+
+  addRevision(revision) {
+    //TODO: compare new revision style id with old ones
+    //and ensure that it is always the same id
+    this.revisions.push(revision)
+    this.currentIdx++
+  }
+
+  undo() {
+    if(this.currentIdx > 0) {
+      this.currentIdx--
+    }
+    return this.current
+  }
+
+  redo() {
+    if(this.currentIdx < this.revisions.length - 1) {
+      this.currentIdx++
+    }
+    return this.current
+  }
+}