From ef428809b5d0567572f68f68e1621a0a015a10f3 Mon Sep 17 00:00:00 2001
From: Bruno Windels <brunow@matrix.org>
Date: Tue, 6 Oct 2020 18:01:56 +0200
Subject: [PATCH] implement navigation stack

---
 src/domain/navigation/Navigation.js | 99 +++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)
 create mode 100644 src/domain/navigation/Navigation.js

diff --git a/src/domain/navigation/Navigation.js b/src/domain/navigation/Navigation.js
new file mode 100644
index 00000000..5e482c87
--- /dev/null
+++ b/src/domain/navigation/Navigation.js
@@ -0,0 +1,99 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {ObservableValue} from "../../observable/ObservableValue.js";
+
+export class Navigation {
+    constructor(allowsChild) {
+        this._allowsChild = allowsChild;
+        this._path = new Path([], allowsChild);
+        this._observables = new Map();
+    }
+
+    get path() {
+        return this._path;
+    }
+
+    applyPath(path) {
+        this._path = path;
+        for (const [type, observable] of this._observables) {
+            // if the value did not change, this won't emit
+            observable.set(this._path.get(type)?.value);
+        }
+    }
+
+    observe(type) {
+        let observable = this._observables.get(type);
+        if (!observable) {
+            observable = new ObservableValue(this._path.get(type)?.value);
+            this._observables.set(type, observable);
+        }
+        return observable;
+    }
+
+    pathFrom(segments) {
+        let parent;
+        let i;
+        for (i = 0; i < segments.length; i += 1) {
+            if (!this._allowsChild(parent, segments[i])) {
+                return new Path(segments.slice(0, i), this._allowsChild);
+            }
+            parent = segments[i];
+        }
+        return new Path(segments, this._allowsChild);
+    }
+}
+
+export class Segment {
+    constructor(type, value = true) {
+        this.type = type;
+        this.value = value;
+    }
+}
+
+class Path {
+    constructor(segments = [], allowsChild) {
+        this._segments = segments;
+        this._allowsChild = allowsChild;
+    }
+
+    clone() {
+        return new Path(this._segments.slice(), this._allowsChild);
+    }
+
+    with(segment) {
+        let index = this._segments.length - 1;
+        do {
+            if (this._allowsChild(this._segments[index], segment)) {
+                // pop the elements that didn't allow the new segment as a child
+                const newSegments = this._segments.slice(0, index + 1);
+                newSegments.push(segment);
+                return new Path(newSegments, this._allowsChild);
+            }
+            index -= 1;
+        } while(index >= -1);
+        // allow -1 as well so we check if the segment is allowed as root
+        return null;
+    }
+
+    get(type) {
+        return this._segments.find(s => s.type === type);
+    }
+
+    get segments() {
+        return this._segments;
+    }
+}