From e1fd77f4a2fdb0eb439263a6f6ce8147db134d92 Mon Sep 17 00:00:00 2001
From: VicRen <vic.ren@me.com>
Date: Fri, 2 Oct 2020 20:21:27 +0800
Subject: [PATCH] largest prime factor

---
 .../largest_prime_factor_test.go              | 41 +++++++++++++++++++
 largest_prime_factor/main.go                  | 23 +++++++++++
 2 files changed, 64 insertions(+)
 create mode 100644 largest_prime_factor/largest_prime_factor_test.go
 create mode 100644 largest_prime_factor/main.go

diff --git a/largest_prime_factor/largest_prime_factor_test.go b/largest_prime_factor/largest_prime_factor_test.go
new file mode 100644
index 0000000..6f4832e
--- /dev/null
+++ b/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)
+			}
+		})
+	}
+}
diff --git a/largest_prime_factor/main.go b/largest_prime_factor/main.go
new file mode 100644
index 0000000..c025bb7
--- /dev/null
+++ b/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
+	}
+}