largest prime factor

largest_prime_factor/largest_prime_factor_test.go

@@ -0,0 +1,41 @@
+package main
+
+import "testing"
+
+func TestLargestPrimeFactor(t *testing.T) {
+	tt := []struct {
+		name  string
+		input int
+		want  int
+	}{
+		{
+			"largest prime factor of 1",
+			1,
+			1,
+		},
+		{
+			"largest prime factor of 2",
+			2,
+			2,
+		},
+		{
+			"largest prime factor of 4",
+			4,
+			2,
+		},
+		{
+			"largest prime factor of a large number",
+			2 * 2 * 2 * 3 * 3 * 5 * 17 * 37,
+			37,
+		},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			got := LargestPrimeFactor(tc.input)
+			if got != tc.want {
+				t.Errorf("LargestPrimeFactor(%d)=%d, want %d", tc.input, got, tc.want)
+			}
+		})
+	}
+}

largest_prime_factor/main.go

@@ -0,0 +1,23 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("the largest prime factor of 600851475143 is", LargestPrimeFactor(600851475143))
+}
+
+func LargestPrimeFactor(n int) int {
+	divider := 2
+	for divider < n {
+		for n%divider == 0 {
+			n /= divider
+		}
+		divider++
+	}
+	if n > 1 {
+		return n
+	} else {
+		divider--
+		return divider
+	}
+}