commit 1991d2075ac81aa9c799c140cda35784a73c969d
Author: Taeyeon Mori <orochimarufan.x3@gmail.com>
Date:   Mon Jul 30 23:00:05 2018 +0200

    youtube_speed: Add YouTube playback rate script

diff --git a/youtube_speed.user.js b/youtube_speed.user.js
new file mode 100644
index 0000000..2651d18
--- /dev/null
+++ b/youtube_speed.user.js
@@ -0,0 +1,135 @@
+// ==UserScript==
+// @name YouTube Playback speed adjust
+// @version 0.9
+// @namespace userjs.osm
+// @match *://www.youtube.com/*
+// @grant none
+// ==/UserScript==
+
+"use strict";
+
+
+class PlaySpeed {
+  // Dom accessors
+  get dom_video() {
+    return document.querySelector("video");
+  };
+  
+  get dom_container() {
+    return document.getElementById("movie_player");
+  };
+
+  // Constructor
+  constructor() {
+    let self = this;
+
+    this.overlay = null;
+    this.rate = this.saved_rate;
+
+    this.domevent = window.addEventListener("DOMContentLoaded", () => {
+      let videos = document.getElementsByTagName("video");
+
+      if (videos[0])
+        videos[0].addEventListener("playing", () => {
+          // Refresh
+          self.set_rate(self.rate);
+        });
+    });
+    
+    // Handle keypresses
+    this.keyevent = window.addEventListener("keydown", e => {
+      // [ :: Decrease speed
+      // ] :: Increase speed
+      // Shift- :: Smaller steps
+      // Alt-   :: Coarser steps
+      if (e.keyCode === 219) // [
+        self.set_rate(self.rate - self.modifier_step(e));
+      else if (e.keyCode === 221) // ]
+        self.set_rate(self.rate + self.modifier_step(e));
+      // \       :: Restore speed
+      // Shift-\ :: Save speed
+      // Alt-\   :: Set speed to 1
+      else if (e.keyCode === 220) { // \
+        if (e.altKey)
+          self.set_rate(1);
+        else if (e.shiftKey)
+          self.save();
+        else
+          self.restore();
+      }
+    });
+  };
+
+  // Get step size from modifier keys
+  modifier_step(e) {
+    if (e.shiftKey == e.altKey)
+      return 0.1;
+    if (e.shiftKey)
+      return 0.01;
+    else if (e.altKey)
+      return 1;
+  };
+
+  // Speed overlay
+  do_overlay(rate) {
+    let div = this.dom_container;
+    let ol = this.overlay;
+
+    if (div) {
+      let need_overlay = Math.abs(rate - 1) > 0.001;
+
+      if (!need_overlay) {
+        if (ol) {
+          div.removeChild(ol);
+          ol = this.overlay = null;
+        }
+
+        return;
+      }
+
+      if (!ol) {
+        ol = this.overlay = document.createElement("div");
+        ol.style.position = "absolute";
+        ol.style.top = 0;
+        ol.style.left = 0;
+        ol.style.zIndex = 10;
+        div.appendChild(ol);
+      }
+
+      ol.textContent = rate.toPrecision(3);
+    }
+  };
+
+  // set rates
+  set_rate(rate) {
+    this.dom_video.playbackRate = rate;
+    this.do_overlay(rate);
+    this.rate = rate;
+  };
+
+  // Restore from localstorage
+  get lst_key() {
+    return "yt_playspeed_adjust";
+  };
+
+  get saved_rate() {
+    let saved = window.localStorage.getItem(this.lst_key);
+    
+    if (saved)
+      return parseFloat(saved);
+    else
+      return 1;
+  }
+  
+  restore() {
+    this.set_rate(this.saved_rate);
+  }
+    
+  save() {
+    window.localStorage.setItem(this.lst_key, this.rate.toPrecision(5));
+  }
+};
+
+
+const yt_playspeed = new PlaySpeed();
+