add HintKeyProducer

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();
+  }
+}