From 966002db2269cb3ecf51308b53a9c1cd3f0f70ba Mon Sep 17 00:00:00 2001
From: Gregory Eremin <g@erem.in>
Date: Sun, 11 Oct 2015 21:55:50 +0300
Subject: [PATCH] Implemented binary search algorithm

---
 binary_search.go      | 43 +++++++++++++++++++++++++++++++++++++
 binary_search_test.go | 49 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 92 insertions(+)
 create mode 100644 binary_search.go
 create mode 100644 binary_search_test.go

diff --git a/binary_search.go b/binary_search.go
new file mode 100644
index 0000000..d0704b3
--- /dev/null
+++ b/binary_search.go
@@ -0,0 +1,43 @@
+package algorithms
+
+// BinarySearch is an implementation of binary search algorithm.
+// Wikipedia: https://en.wikipedia.org/wiki/Binary_search_algorithm
+//
+// The binary search algorithm begins by comparing the target value to the value
+// of the middle element of the sorted array. If the target value is equal to
+// the middle element's value, then the position is returned and the search is
+// finished. If the target value is less than the middle element's value, then
+// the search continues on the lower half of the array; or if the target value
+// is greater than the middle element's value, then the search continues on the
+// upper half of the array. This process continues, eliminating half of the
+// elements, and comparing the target value to the value of the middle element
+// of the remaining elements - until the target value is either found (and its
+// associated element position is returned), or until the entire array has been
+// searched (and "not found" is returned).
+func BinarySearch(haystack []int, needle int) (pos int, ok bool) {
+	if len(haystack) == 0 {
+		return 0, false
+	}
+	if haystack[0] > needle {
+		return 0, false
+	}
+	if haystack[len(haystack)-1] < needle {
+		return 0, false
+	}
+
+	first := 0
+	last := len(haystack)
+	for first < last {
+		mid := first + (last-first)/2
+
+		if haystack[mid] == needle {
+			return mid, true
+		} else if haystack[mid] > needle {
+			last = mid
+		} else {
+			first = mid + 1
+		}
+	}
+
+	return 0, false
+}
diff --git a/binary_search_test.go b/binary_search_test.go
new file mode 100644
index 0000000..436f361
--- /dev/null
+++ b/binary_search_test.go
@@ -0,0 +1,49 @@
+package algorithms
+
+import (
+	"testing"
+)
+
+func TestBinarySearchEmptyArray(t *testing.T) {
+	if _, ok := BinarySearch([]int{}, 1); ok {
+		t.Error("Position should not be found")
+	}
+}
+
+func TestBinarySearchLowerThanFirstValue(t *testing.T) {
+	if _, ok := BinarySearch([]int{1, 2, 3, 4, 5}, 0); ok {
+		t.Error("Position should not be found")
+	}
+}
+
+func TestBinarySearchHigherThanLastValue(t *testing.T) {
+	if _, ok := BinarySearch([]int{1, 2, 3, 4, 5}, 6); ok {
+		t.Error("Position should not be found")
+	}
+}
+
+func TestBinarySearchNotFound(t *testing.T) {
+	if _, ok := BinarySearch([]int{1, 2, 4, 5}, 3); ok {
+		t.Error("Position should not be found")
+	}
+}
+
+func TestBinarySearchOddSize(t *testing.T) {
+	pos, ok := BinarySearch([]int{1, 2, 3, 4, 5}, 1)
+	if !ok {
+		t.Fatalf("Position should be found")
+	}
+	if pos != 0 {
+		t.Errorf("Expected position %d, got %d", 0, pos)
+	}
+}
+
+func TestBinarySearchEvenSize(t *testing.T) {
+	pos, ok := BinarySearch([]int{1, 2, 3, 4, 5, 6}, 6)
+	if !ok {
+		t.Fatalf("Position should be found")
+	}
+	if pos != 5 {
+		t.Errorf("Expected position %d, got %d", 5, pos)
+	}
+}