Add GCD to src/algorithms/math.go

docs/algorithms/math.md

@@ -2,7 +2,11 @@
 
 Ahh, math problems. I like them the least! :D
 
-## Factorization
+### Greatest common divisor (GCD)
+GCD helps to find number `c` that both input numbers `a` and `b` can be divided. Takes `O(logn)` time if you use [Euclid's algorithm](https://en.wikipedia.org/wiki/Euclidean_algorithm).
+You can also get GCD using [primes factorization](#primes-factorization) bu getting primes for the first and the second number, and finding the greatest one.
+
+### Primes factorization
 
 Prime number only has two factors - one and the number itself.
 Opposite of prime is composite.

src/algorithms/math.go

@@ -1,6 +1,19 @@
 package algorithms
 
-////////////////////////// Prime factors //////////////////////////
+////////////////////////// Greatest common divider (GCD) //////////////////////////
+// You can find this algorithms overview in docs/algorithms/math.md
+
+func GCD(a, b int) int {
+	for b != 0 {
+		t := b
+		b = a % b
+		a = t
+	}
+
+	return a
+}
+
+////////////////////////// Primes factorization //////////////////////////
 // You can find this algorithms overview in docs/algorithms/math.md
 
 func GetPrimeFactors(number int, maxNumber int) []int {

src/algorithms/math_test.go

@@ -5,6 +5,13 @@ import (
 	"testing"
 )
 
+func TestGCD(t *testing.T) {
+	assert.Equal(t, 4, GCD(8, 12))
+	assert.Equal(t, 6, GCD(54, 24))
+	assert.Equal(t, 14, GCD(42, 56))
+	assert.Equal(t, 1, GCD(13, 48))
+}
+
 func TestGetPrimeFactors(t *testing.T) {
 	maxNumber := 10000