From 95a894e53f9b7ae62aa439a413e552ed4684877a Mon Sep 17 00:00:00 2001
From: dudaodong <lanliddd.2007@163.com>
Date: Tue, 19 Nov 2024 10:12:07 +0800
Subject: [PATCH] feat: add Permutation and Combination

---
 mathutil/mathutil.go              | 44 +++++++++++++++++++++++++----
 mathutil/mathutil_exmaple_test.go | 24 ++++++++++++++++
 mathutil/mathutil_test.go         | 46 +++++++++++++++++++++++++++++++
 3 files changed, 108 insertions(+), 6 deletions(-)

diff --git a/mathutil/mathutil.go b/mathutil/mathutil.go
index 0f4c2d7..148d3f3 100644
--- a/mathutil/mathutil.go
+++ b/mathutil/mathutil.go
@@ -44,14 +44,19 @@ func Fibonacci(first, second, n int) int {
 	}
 }
 
-// Factorial calculate x!.
+// Factorial calculate n!.
 // Play: https://go.dev/play/p/tt6LdOK67Nx
-func Factorial(x uint) uint {
-	var f uint = 1
-	for ; x > 1; x-- {
-		f *= x
+func Factorial(n uint) uint {
+	if n == 0 || n == 1 {
+		return 1
 	}
-	return f
+
+	result := uint(1)
+	for i := uint(2); i <= n; i++ {
+		result *= i
+	}
+
+	return result
 }
 
 // Percent calculate the percentage of value to total.
@@ -417,3 +422,30 @@ func Variance[T constraints.Float | constraints.Integer](numbers []T) float64 {
 func StdDev[T constraints.Float | constraints.Integer](numbers []T) float64 {
 	return math.Sqrt(Variance(numbers))
 }
+
+// Permutation calculate P(n, k).
+// Play: todo
+func Permutation(n, k uint) uint {
+	if n < k {
+		return 0
+	}
+
+	nFactorial := Factorial(n)
+	nMinusKFactorial := Factorial(n - k)
+
+	return nFactorial / nMinusKFactorial
+}
+
+// Combination calculate C(n, k).
+// Play: todo
+func Combination(n, k uint) uint {
+	if n < k {
+		return 0
+	}
+
+	nFactorial := Factorial(n)
+	kFactorial := Factorial(k)
+	nMinusKFactorial := Factorial(n - k)
+
+	return nFactorial / (kFactorial * nMinusKFactorial)
+}
diff --git a/mathutil/mathutil_exmaple_test.go b/mathutil/mathutil_exmaple_test.go
index 1408aab..ed840ac 100644
--- a/mathutil/mathutil_exmaple_test.go
+++ b/mathutil/mathutil_exmaple_test.go
@@ -502,3 +502,27 @@ func ExampleStdDev() {
 	// 1.41
 	// 1.55
 }
+
+func ExamplePermutation() {
+	result1 := Permutation(5, 3)
+	result2 := Permutation(5, 5)
+
+	fmt.Println(result1)
+	fmt.Println(result2)
+
+	// Output:
+	// 60
+	// 120
+}
+
+func ExampleCombination() {
+	result1 := Combination(5, 3)
+	result2 := Combination(5, 5)
+
+	fmt.Println(result1)
+	fmt.Println(result2)
+
+	// Output:
+	// 10
+	// 1
+}
diff --git a/mathutil/mathutil_test.go b/mathutil/mathutil_test.go
index b79fb24..0252490 100644
--- a/mathutil/mathutil_test.go
+++ b/mathutil/mathutil_test.go
@@ -491,3 +491,49 @@ func TestStdDev(t *testing.T) {
 		assert.Equal(tt.expected, TruncRound(StdDev(tt.numbers), 3))
 	}
 }
+
+func TestPermutation(t *testing.T) {
+	t.Parallel()
+
+	assert := internal.NewAssert(t, "TestPermutation")
+
+	tests := []struct {
+		n        uint
+		k        uint
+		expected uint
+	}{
+		{1, 1, 1},
+		{1, 0, 1},
+		{0, 1, 0},
+		{0, 0, 1},
+		{3, 2, 6},
+		{4, 2, 12},
+	}
+
+	for _, tt := range tests {
+		assert.Equal(tt.expected, Permutation(tt.n, tt.k))
+	}
+}
+
+func TestCombination(t *testing.T) {
+	t.Parallel()
+
+	assert := internal.NewAssert(t, "TestCombination")
+
+	tests := []struct {
+		n        uint
+		k        uint
+		expected uint
+	}{
+		{1, 1, 1},
+		{1, 0, 1},
+		{0, 1, 0},
+		{0, 0, 1},
+		{3, 2, 3},
+		{4, 2, 6},
+	}
+
+	for _, tt := range tests {
+		assert.Equal(tt.expected, Combination(tt.n, tt.k))
+	}
+}