2 files changed, 58 insertions, 0 deletions
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/test/content/hint-key-producer.test.js b/test/content/hint-key-producer.test.js
new file mode 100644
index 0000000..74fb462
--- /dev/null
+++ b/test/content/hint-key-producer.test.js
@@ -0,0 +1,25 @@
+import { expect } from "chai";
+import HintKeyProducer from '../../src/content/hint-key-producer';
+
+describe('HintKeyProducer class', () => {
+  describe('#constructor', () => {
+    it('throws an exception on empty charset', () => {
+      expect(() => new HintKeyProducer([])).to.throw(TypeError);
+    });
+  });
+
+  describe('#produce', () => {
+    it('produce incremented keys', () => {
+      let charset = 'abc';
+      let sequences = [
+        'a', 'b', 'c',
+        'aa', 'ab', 'ac', 'ba', 'bb', 'bc', 'ca', 'cb', 'cc',
+        'aaa', 'aab', 'aac', 'aba']
+
+      let producer = new HintKeyProducer(charset);
+      for (let i = 0; i < sequences.length; ++i) {
+        expect(producer.produce()).to.equal(sequences[i]);
+      }
+    });
+  });
+});