From 64bab9cabf36185e0df5dec5e2030f2ff080ec90 Mon Sep 17 00:00:00 2001
From: VicRen <vic.ren@me.com>
Date: Sat, 7 Nov 2020 12:17:09 +0800
Subject: [PATCH] 37: truncatable primes

---
 37_truncatable_primes/main.go      | 68 ++++++++++++++++++++++++
 37_truncatable_primes/main_test.go | 83 ++++++++++++++++++++++++++++++
 2 files changed, 151 insertions(+)
 create mode 100644 37_truncatable_primes/main.go
 create mode 100644 37_truncatable_primes/main_test.go

diff --git a/37_truncatable_primes/main.go b/37_truncatable_primes/main.go
new file mode 100644
index 0000000..e5288a1
--- /dev/null
+++ b/37_truncatable_primes/main.go
@@ -0,0 +1,68 @@
+package main
+
+import (
+	"fmt"
+	"strconv"
+)
+
+func main() {
+	sum := 0
+	start := 8
+	count := 0
+	for {
+		if count == 11 {
+			break
+		}
+		if isTruncatablePrimesL2R(start) && isTruncatablePrimesR2L(start) {
+			count++
+			fmt.Println("tp:", start)
+			sum += start
+		}
+		start++
+	}
+	fmt.Println("sum:", sum)
+}
+
+func isTruncatablePrimesL2R(n int) bool {
+	s := strconv.Itoa(n)
+	for len(s) > 0 {
+		x, _ := strconv.Atoi(s)
+		if !isPrime(x) {
+			return false
+		}
+		s = s[1:]
+	}
+	return true
+}
+
+func isTruncatablePrimesR2L(n int) bool {
+	s := strconv.Itoa(n)
+	for len(s) > 0 {
+		l := len(s)
+		x, _ := strconv.Atoi(s)
+		if !isPrime(x) {
+			return false
+		}
+		s = s[:l-1]
+	}
+	return true
+}
+
+func isPrime(n int) bool {
+	if n <= 1 {
+		return false
+	}
+	res := n
+	//牛顿法求平方根
+	for res*res > n {
+		res = (res + n/res) / 2
+	}
+	divider := 2
+	for divider <= res {
+		if n%divider == 0 {
+			return false
+		}
+		divider++
+	}
+	return true
+}
diff --git a/37_truncatable_primes/main_test.go b/37_truncatable_primes/main_test.go
new file mode 100644
index 0000000..122b362
--- /dev/null
+++ b/37_truncatable_primes/main_test.go
@@ -0,0 +1,83 @@
+package main
+
+import (
+	"testing"
+)
+
+func Test_isTruncatablePrimesL2R(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			"3797",
+			args{3797},
+			true,
+		},
+		{
+			"797",
+			args{797},
+			true,
+		},
+		{
+			"97",
+			args{97},
+			true,
+		},
+		{
+			"96",
+			args{96},
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isTruncatablePrimesL2R(tt.args.n); got != tt.want {
+				t.Errorf("isTruncatablePrimesL2R() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_isTruncatablePrimesR2L(t *testing.T) {
+	type args struct {
+		n int
+	}
+	tests := []struct {
+		name string
+		args args
+		want bool
+	}{
+		{
+			"3797",
+			args{3797},
+			true,
+		},
+		{
+			"379",
+			args{379},
+			true,
+		},
+		{
+			"37",
+			args{37},
+			true,
+		},
+		{
+			"96",
+			args{96},
+			false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isTruncatablePrimesR2L(tt.args.n); got != tt.want {
+				t.Errorf("isTruncatablePrimesR2L() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}