Add Hamming.

README.md

@@ -40,7 +40,7 @@
   * [Fisher–Yates Shuffle](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/fisher-yates) - random permutation of a finite sequence
 * **String**
   * [Levenshtein Distance](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/string/levenshtein-distance) - minimum edit distance between two sequences
-  * Hamming
+  * [Hamming Distance](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/string/hamming-distance) - number of positions at which the symbols are different
   * Huffman
   * Knuth Morris Pratt
   * Longest common subsequence

src/algorithms/string/hamming-distance/README.md

@@ -0,0 +1,23 @@
+# Hamming Distance
+
+the Hamming distance between two strings of equal length is the 
+number of positions at which the corresponding symbols are 
+different. In other words, it measures the minimum number of
+substitutions required to change one string into the other, or 
+the minimum number of errors that could have transformed one 
+string into the other. In a more general context, the Hamming 
+distance is one of several string metrics for measuring the 
+edit distance between two sequences.
+
+## Examples
+
+The Hamming distance between:
+
+- "ka**rol**in" and "ka**thr**in" is **3**.
+- "k**a**r**ol**in" and "k**e**r**st**in" is **3**.
+- 10**1**1**1**01 and 10**0**1**0**01 is **2**.
+- 2**17**3**8**96 and 2**23**3**7**96 is **3**.
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Hamming_distance)

src/algorithms/string/hamming-distance/__test__/hammingDistance.test.js

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+import hammingDistance from '../hammingDistance';
+
+describe('hammingDistance', () => {
+  it('should throw an error when trying to compare the strings of different lengths', () => {
+    const compareStringsOfDifferentLength = () => {
+      hammingDistance('a', 'aa');
+    };
+
+    expect(compareStringsOfDifferentLength).toThrowError();
+  });
+
+  it('should calculate difference between two strings', () => {
+    expect(hammingDistance('a', 'a')).toBe(0);
+    expect(hammingDistance('a', 'b')).toBe(1);
+    expect(hammingDistance('abc', 'add')).toBe(2);
+    expect(hammingDistance('karolin', 'kathrin')).toBe(3);
+    expect(hammingDistance('karolin', 'kerstin')).toBe(3);
+    expect(hammingDistance('1011101', '1001001')).toBe(2);
+    expect(hammingDistance('2173896', '2233796')).toBe(3);
+  });
+});

src/algorithms/string/hamming-distance/hammingDistance.js

@@ -0,0 +1,20 @@
+/**
+ * @param {string} a
+ * @param {string} b
+ * @return {number}
+ */
+export default function hammingDistance(a, b) {
+  if (a.length !== b.length) {
+    throw new Error('Strings must be of the same length');
+  }
+
+  let distance = 0;
+
+  for (let i = 0; i < a.length; i += 1) {
+    if (a[i] !== b[i]) {
+      distance += 1;
+    }
+  }
+
+  return distance;
+}