44: pentagon numbers

44_pentagon_numbers/main.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+func main() {
+	j := 1
+	f := true
+	for f {
+		a := j * (3*j - 1) / 2
+		for k := j + 1; k < 5000; k++ {
+			b := k * (3*k - 1) / 2
+			d := b - a
+			if isPentagonalNumber(d) && isPentagonalNumber(a+b) {
+				fmt.Println(j, k, a, b, d, a+b)
+				f = false
+				break
+			}
+		}
+		j++
+	}
+}
+
+func pentagonalNumberN(n int) int {
+	return n * (3*n - 1) / 2
+}
+
+func isPentagonalNumber(n int) bool {
+	r := math.Sqrt(float64(1 + 24*n))
+	if r-float64(int(r)) != 0 {
+		return false
+	}
+	return int(r)%6 == 5
+}

44_pentagon_numbers/main_test.go

@@ -0,0 +1,81 @@
+package main
+
+import "testing"
+
+func Test_pentagonalNumberN(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want int
+	}{
+		{
+			"1",
+			args{1},
+			1,
+		},
+		{
+			"2",
+			args{2},
+			5,
+		},
+		{
+			"10",
+			args{10},
+			145,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := pentagonalNumberN(tt.args.n); got != tt.want {
+				t.Errorf("pentagonalNumberN() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_isPentagonalNumber(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			"1",
+			args{1},
+			true,
+		},
+		{
+			"5",
+			args{5},
+			true,
+		},
+		{
+			"6",
+			args{6},
+			false,
+		},
+		{
+			"120",
+			args{120},
+			false,
+		},
+		{
+			"145",
+			args{145},
+			true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isPentagonalNumber(tt.args.n); got != tt.want {
+				t.Errorf("isPentagonalNumber() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}