From fafce27768fdb789fe82c3698d1078d865fd55b0 Mon Sep 17 00:00:00 2001
From: Oleksii Trekhleb <trehleb@gmail.com>
Date: Mon, 16 Apr 2018 19:30:29 +0300
Subject: [PATCH] Add binary search.

---
 README.md                                     |  2 ++
 src/algorithms/search/binary-search/README.md | 17 +++++++++
 .../__test__/binarySearch.test.js             | 34 ++++++++++++++++++
 .../search/binary-search/binarySearch.js      | 35 +++++++++++++++++++
 4 files changed, 88 insertions(+)
 create mode 100644 src/algorithms/search/binary-search/README.md
 create mode 100644 src/algorithms/search/binary-search/__test__/binarySearch.test.js
 create mode 100644 src/algorithms/search/binary-search/binarySearch.js

diff --git a/README.md b/README.md
index 5873a212..e8a8bcc8 100644
--- a/README.md
+++ b/README.md
@@ -30,6 +30,8 @@
 * Graph
   * [Depth-First Search (DFS)](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/depth-first-search)
   * [Breadth-First Search (BFS)](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/graph/breadth-first-search)
+* Search
+  * [Binary Search](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/search/binary-search)
 * Sorting
   * [Bubble Sort](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sorting/bubble-sort)
   * [Selection Sort](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/sorting/selection-sort)
diff --git a/src/algorithms/search/binary-search/README.md b/src/algorithms/search/binary-search/README.md
new file mode 100644
index 00000000..0b55e1aa
--- /dev/null
+++ b/src/algorithms/search/binary-search/README.md
@@ -0,0 +1,17 @@
+# Binary Search
+
+In computer science, binary search, also known as half-interval 
+search, logarithmic search, or binary chop, is a search algorithm 
+that finds the position of a target value within a sorted 
+array. Binary search compares the target value to the middle 
+element of the array; if they are unequal, the half in which 
+the target cannot lie is eliminated and the search continues 
+on the remaining half until it is successful. If the search 
+ends with the remaining half being empty, the target is not 
+in the array.
+
+![Binary Search](https://upload.wikimedia.org/wikipedia/commons/8/83/Binary_Search_Depiction.svg)
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Binary_search_algorithm)
diff --git a/src/algorithms/search/binary-search/__test__/binarySearch.test.js b/src/algorithms/search/binary-search/__test__/binarySearch.test.js
new file mode 100644
index 00000000..72c2b344
--- /dev/null
+++ b/src/algorithms/search/binary-search/__test__/binarySearch.test.js
@@ -0,0 +1,34 @@
+import binarySearch from '../binarySearch';
+
+describe('binarySearch', () => {
+  it('should search number in sorted array', () => {
+    expect(binarySearch([], 1)).toBe(-1);
+    expect(binarySearch([1], 1)).toBe(0);
+    expect(binarySearch([1, 2], 1)).toBe(0);
+    expect(binarySearch([1, 2], 2)).toBe(1);
+    expect(binarySearch([1, 5, 10, 12], 1)).toBe(0);
+    expect(binarySearch([1, 5, 10, 12, 14, 17, 22, 100], 17)).toBe(5);
+    expect(binarySearch([1, 5, 10, 12, 14, 17, 22, 100], 1)).toBe(0);
+    expect(binarySearch([1, 5, 10, 12, 14, 17, 22, 100], 100)).toBe(7);
+    expect(binarySearch([1, 5, 10, 12, 14, 17, 22, 100], 0)).toBe(-1);
+  });
+
+  it('should search object in sorted array', () => {
+    const sortedArrayOfObjects = [
+      { key: 1, value: 'value1' },
+      { key: 2, value: 'value2' },
+      { key: 3, value: 'value3' },
+    ];
+
+    const comparator = (a, b) => {
+      if (a.key === b.key) return 0;
+      return a.key < b.key ? -1 : 1;
+    };
+
+    expect(binarySearch([], { key: 1 }, comparator)).toBe(-1);
+    expect(binarySearch(sortedArrayOfObjects, { key: 4 }, comparator)).toBe(-1);
+    expect(binarySearch(sortedArrayOfObjects, { key: 1 }, comparator)).toBe(0);
+    expect(binarySearch(sortedArrayOfObjects, { key: 2 }, comparator)).toBe(1);
+    expect(binarySearch(sortedArrayOfObjects, { key: 3 }, comparator)).toBe(2);
+  });
+});
diff --git a/src/algorithms/search/binary-search/binarySearch.js b/src/algorithms/search/binary-search/binarySearch.js
new file mode 100644
index 00000000..cc85b1ff
--- /dev/null
+++ b/src/algorithms/search/binary-search/binarySearch.js
@@ -0,0 +1,35 @@
+import Comparator from '../../../utils/comparator/Comparator';
+
+/**
+ * @param {*[]} sortedArray
+ * @param {*} seekElement
+ * @param {function(a, b)} [comparatorCallback]
+ * @return {number}
+ */
+
+export default function binarySearch(sortedArray, seekElement, comparatorCallback) {
+  const comparator = new Comparator(comparatorCallback);
+
+  let startIndex = 0;
+  let endIndex = sortedArray.length - 1;
+
+  while (startIndex <= endIndex) {
+    const middleIndex = startIndex + Math.floor((endIndex - startIndex) / 2);
+
+    // If we've found the element just return its position.
+    if (comparator.equal(sortedArray[middleIndex], seekElement)) {
+      return middleIndex;
+    }
+
+    // Decide which half to choose for seeking next: left or right one.
+    if (comparator.lessThen(sortedArray[middleIndex], seekElement)) {
+      // Go to the right half of the array.
+      startIndex = middleIndex + 1;
+    } else {
+      // Go to the left half of the array.
+      endIndex = middleIndex - 1;
+    }
+  }
+
+  return -1;
+}