#!/usr/bin/env python3
"""
凯撒加密算法示例
一个教学用的凯撒密码实现
"""

def caesar_encrypt(plaintext, shift):
    """凯撒加密函数"""
    ciphertext = ""
    
    for char in plaintext:
        if char.isalpha():
            base = ord('A') if char.isupper() else ord('a')
            new_char = chr((ord(char) - base + shift) % 26 + base)
            ciphertext += new_char
        else:
            ciphertext += char
    
    return ciphertext

def caesar_decrypt(ciphertext, shift):
    """凯撒解密函数"""
    return caesar_encrypt(ciphertext, -shift)

def main():
    """主函数，用户交互界面"""
    print("凯撒密码示例")
    
    while True:
        print("\n1.加密\n2.解密\n3.退出")
        choice = input("选择: ").strip()
        
        if choice == "1":
            text = input("明文: ")
            try:
                shift = int(input("移位值: "))
                result = caesar_encrypt(text, shift)
                print(f"密文: {result}")
            except ValueError:
                print("移位值必须是整数!")
                
        elif choice == "2":
            text = input("密文: ")
            try:
                shift = int(input("移位值: "))
                result = caesar_decrypt(text, shift)
                print(f"明文: {result}")
            except ValueError:
                print("移位值必须是整数!")
                
        elif choice == "3":
            print("再见!")
            break
        else:
            print("无效选择")

if __name__ == "__main__":
    main()