5 files changed, 210 insertions, 0 deletions
diff --git a/src/content/follow.js b/src/content/follow.js
new file mode 100644
index 0000000..ffa16b9
--- /dev/null
+++ b/src/content/follow.js
@@ -0,0 +1,123 @@
+import Hint from './hint';
+import HintKeyProducer from './hint-key-producer';
+
+const DEFAULT_HINT_CHARSET = 'abcdefghijklmnopqrstuvwxyz'
+
+export default class Follow {
+  constructor(doc) {
+    this.doc = doc;
+    this.hintElements = {};
+    this.keys = [];
+
+    // TODO activate input elements and push button elements
+    let links = Follow.getTargetElements(doc);
+
+    this.addHints(links);
+
+    this.boundKeydown = this.handleKeydown.bind(this);
+    doc.addEventListener('keydown', this.boundKeydown);
+  }
+
+  addHints(elements) {
+    let producer = new HintKeyProducer(DEFAULT_HINT_CHARSET);
+    Array.prototype.forEach.call(elements, (ele) => {
+      let keys = producer.produce();
+      let hint = new Hint(ele, keys)
+
+      this.hintElements[keys] = hint;
+    });
+  }
+
+  handleKeydown(e) {
+    let keyCode = e.keyCode;
+    if (keyCode === KeyboardEvent.DOM_VK_ESCAPE) {
+      this.remove();
+      return;
+    } else if (keyCode === KeyboardEvent.DOM_VK_ENTER ||
+               keyCode === KeyboardEvent.DOM_VK_RETURN) {
+      let chars = Follow.codeChars(this.keys);
+      this.hintElements[chars].activate();
+      return;
+    } else if (Follow.availableKey(keyCode)) {
+      this.keys.push(keyCode);
+    } else if (keyCode === KeyboardEvent.DOM_VK_BACK_SPACE ||
+               keyCode === KeyboardEvent.DOM_VK_DELETE) {
+      this.keys.pop();
+    }
+
+    this.refreshKeys();
+  }
+
+  refreshKeys() {
+    let chars = Follow.codeChars(this.keys);
+    let shown = Object.keys(this.hintElements).filter((key) => {
+      return key.startsWith(chars);
+    });
+    let hidden = Object.keys(this.hintElements).filter((key) => {
+      return !key.startsWith(chars);
+    });
+    if (shown.length == 0) {
+      this.remove();
+      return;
+    } else if (shown.length == 1) {
+      this.remove();
+      this.hintElements[chars].activate();
+    }
+
+    shown.forEach((key) => {
+      this.hintElements[key].show();
+    });
+    hidden.forEach((key) => {
+      this.hintElements[key].hide();
+    });
+  }
+
+
+  remove() {
+    this.doc.removeEventListener("keydown", this.boundKeydown);
+    Object.keys(this.hintElements).forEach((key) => {
+      this.hintElements[key].remove();
+    });
+  }
+
+  static availableKey(keyCode) {
+    return (
+      KeyboardEvent.DOM_VK_0 <= keyCode && keyCode <= KeyboardEvent.DOM_VK_9 ||
+      KeyboardEvent.DOM_VK_A <= keyCode && keyCode <= KeyboardEvent.DOM_VK_Z
+    );
+  }
+
+  static codeChars(codes) {
+    const CHARCODE_ZERO = '0'.charCodeAt(0);
+    const CHARCODE_A = 'a'.charCodeAt(0);
+
+    let chars = '';
+
+    for (let code of codes) {
+      if (KeyboardEvent.DOM_VK_0 <= code && code <= KeyboardEvent.DOM_VK_9) {
+        chars += String.fromCharCode(code - KeyboardEvent.DOM_VK_0 + CHARCODE_ZERO);
+      } else if (KeyboardEvent.DOM_VK_A <= code && code <= KeyboardEvent.DOM_VK_Z) {
+        chars += String.fromCharCode(code - KeyboardEvent.DOM_VK_A + CHARCODE_A);
+      }
+    }
+    return chars;
+  }
+
+  static getTargetElements(doc) {
+    let all = doc.querySelectorAll('a');
+    let filtered = Array.prototype.filter.call(all, (e) => {
+      return Follow.isVisibleElement(e);
+    });
+    return filtered;
+  }
+
+  static isVisibleElement(element) {
+    var style = window.getComputedStyle(element);
+    if (style.display === 'none') {
+      return false;
+    } else if (style.visibility === 'hidden') {
+      return false;
+    }
+    return true;
+  }
+}
diff --git a/src/content/hint-key-producer.js b/src/content/hint-key-producer.js
new file mode 100644
index 0000000..8064afb
--- /dev/null
+++ b/src/content/hint-key-producer.js
@@ -0,0 +1,33 @@
+export default class HintKeyProducer {
+  constructor(charset) {
+    if (charset.length === 0) {
+      throw new TypeError('charset is empty');
+    }
+
+    this.charset = charset;
+    this.counter = [];
+  }
+
+  produce() {
+    this.increment();
+
+    return this.counter.map((x) => this.charset[x]).join('');
+  }
+
+  increment() {
+    let max = this.charset.length - 1;
+    if (this.counter.every((x) => x == max)) {
+      this.counter = new Array(this.counter.length + 1).fill(0);
+      return;
+    }
+
+    this.counter.reverse();
+    let len = this.charset.length;
+    let num = this.counter.reduce((x,y,index) => x + y * (len ** index)) + 1;
+    for (let i = 0; i < this.counter.length; ++i) {
+      this.counter[i] = num % len;
+      num = ~~(num / len);
+    }
+    this.counter.reverse();
+  }
+}
diff --git a/src/content/hint.css b/src/content/hint.css
new file mode 100644
index 0000000..a0f1233
--- /dev/null
+++ b/src/content/hint.css
@@ -0,0 +1,7 @@
+.vimvixen-hint {
+  background-color: yellow;
+  border: 1px solid gold;
+  font-weight: bold;
+  position: absolute;
+  text-transform: uppercase;
+}
diff --git a/src/content/hint.js b/src/content/hint.js
new file mode 100644
index 0000000..fabf725
--- /dev/null
+++ b/src/content/hint.js
@@ -0,0 +1,43 @@
+import './hint.css';
+
+export default class Hint {
+  constructor(target, tag) {
+    if (!(document.body instanceof HTMLElement)) {
+      throw new TypeError('target is not an HTMLElement');
+    }
+
+    this.target = target;
+
+    let doc = target.ownerDocument
+    let { top, left } = target.getBoundingClientRect();
+    let scrollX = window.scrollX;
+    let scrollY = window.scrollY;
+
+    this.element = doc.createElement('span');
+    this.element.className = 'vimvixen-hint';
+    this.element.textContent = tag;
+    this.element.style.left = left + scrollX + 'px';
+    this.element.style.top = top + scrollY + 'px';
+
+    this.show();
+    doc.body.append(this.element);
+  }
+
+  show() {
+    this.element.style.display = 'inline';
+  }
+
+  hide() {
+    this.element.style.display = 'none';
+  }
+
+  remove() {
+    this.element.remove();
+  }
+
+  activate() {
+    if (this.target.tagName.toLowerCase() === 'a') {
+      this.target.click();
+    }
+  }
+}
diff --git a/src/content/index.js b/src/content/index.js
index 17ab308..78389fd 100644
--- a/src/content/index.js
+++ b/src/content/index.js
@@ -1,5 +1,6 @@
 import * as scrolls from './scrolls';
 import FooterLine from './footer-line';
+import Follow from './follow';
 import * as actions from '../shared/actions';
 
 var footer = null;
@@ -52,6 +53,9 @@ const invokeEvent = (action) => {
   case actions.SCROLL_BOTTOM:
     scrolls.scrollBottom(window, action[1]);
     break;
+  case actions.FOLLOW_START:
+    new Follow(window.document, action[1] || false);
+    break;
   }
 }