feat: 优化默认字符集与字符串构建

- 引入默认字符集常量并优化字符串构建及生成功能以提升效率

src/generate.rs

@@ -1,5 +1,8 @@
 use rand::Rng;
 
+const DEFAULT_CHARSET: &[u8] =
+  b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
+
 /// 随机Token生成器结构体
 pub struct TokenGenerator {
   uppercase: bool,
@@ -38,48 +41,47 @@ impl TokenGenerator {
   /// 生成随机token
   pub fn generate(&self, length: usize) -> String {
     let charset = self.build_charset();
+    let mut result = String::with_capacity(length);
+
     if charset.is_empty() {
       // 如果没有选择任何字符类型，使用默认字符集
       return self.generate_with_default_charset(length);
     }
 
     let mut rng = rand::rng();
-    (0..length)
-      .map(|_| {
+    for _ in 0..length {
       let idx = rng.random_range(0..charset.len());
-        charset[idx] as char
-      })
-      .collect()
+      result.push(charset[idx] as char);
+    }
+    result
   }
 
   /// 构建字符集
   fn build_charset(&self) -> Vec<u8> {
-    let mut charset = Vec::new();
+    let mut charset_string = String::new();
 
     if self.uppercase {
-      charset.extend_from_slice(b"ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+      charset_string.push_str("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
     }
     if self.lowercase {
-      charset.extend_from_slice(b"abcdefghijklmnopqrstuvwxyz");
+      charset_string.push_str("abcdefghijklmnopqrstuvwxyz");
     }
     if self.numbers {
-      charset.extend_from_slice(b"0123456789");
+      charset_string.push_str("0123456789");
     }
 
-    charset
+    charset_string.into_bytes()
   }
 
   /// 使用默认字符集生成token
   fn generate_with_default_charset(&self, length: usize) -> String {
-    let charset =
-      b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
     let mut rng = rand::rng();
+    let mut result = String::with_capacity(length);
 
-    (0..length)
-      .map(|_| {
-        let idx = rng.random_range(0..charset.len());
-        charset[idx] as char
-      })
-      .collect()
+    for _ in 0..length {
+      let idx = rng.random_range(0..DEFAULT_CHARSET.len());
+      result.push(DEFAULT_CHARSET[idx] as char);
+    }
+    result
   }
 }