diff --git a/src/lib.rs b/src/lib.rs
index 2173705..c239685 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,88 +1,86 @@
+pub const BASE64_ALPHABET : &str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=-_";
+pub const HEX_ALPHABET : &str = "ABCDEFabcdef0123456789";
 
-// Calculate the shannon entropy of a given string for a given alphabeth
-pub fn shannon_entropy(s: &str) -> f32 {
-    let mut entropy = 0f32;
+// Calculate the shannon entropy of a given byte slice
+pub fn shannon_entropy(bytes: &[u8]) -> f32 {
+    let mut entropy = 0.0;
+    let mut counts = [0; 256];
 
-    if s.is_empty() {
-        return entropy;
+    for &b in bytes {
+        counts[b as usize] += 1;
     }
 
-    for c in 0..=255u8 {
-        let count = s.matches(char::from(c)).count() as f32;
-        let slen = s.len() as f32;
-        let p : f32 = count / slen;
-        println!("{:?} {:?} {:?}", count, slen, p);
-        if p > 0.0 {
-            entropy += - p * p.log2();
-        }
+    for &count in counts.iter() {
+        if count == 0 { continue }
+
+        let p: f32 = (count as f32) / (bytes.len() as f32);
+        entropy -= p * p.log(2.0);
     }
 
     entropy
 }
 
-// Calculate the shannon entropy of a given string detecting the used alphabeth
-pub fn str_shannon_entropy(s: &str) -> f32 {
-    let mut entropy = 0f32;
+// Calculate the shannon entropy of a given byte slice for a specific charset
+pub fn shannon_entropy_charset(bytes: &[u8], charset: &str) -> f32 {
+    let mut entropy = 0.0;
 
-    if s.is_empty() {
-        return entropy;
-    } 
-    entropy = 1f32;
+    for single_char in charset.chars() {
+        let count = bytes.iter().filter(|&&n| n as char == single_char).count();
+        if count == 0 { continue }
+        let p: f32 = (count as f32) / (bytes.len() as f32);
+        entropy -= p * p.log(2.0);
+    }
 
-    entropy 
+    entropy
 }
 
+// Determine if a string is made up only of hexdigits
+pub fn is_hex_str(s: &str) -> bool {
+    s.chars().filter(|&n| !(n.is_digit(16))).count() == 0
+}
+
+// Determine if a string is made up only of base64/base64url digits
+pub fn is_base64_str(s: &str) -> bool {
+    let alphabet: Vec<_> = BASE64_ALPHABET.chars().collect();
+    s.chars().filter(|n| !(alphabet.contains(n))).count() == 0
+}
+
+// Calculate the shannon entropy of a given string
+pub fn str_entropy(s: &str) -> f32 {
+    if is_hex_str(s) {
+        return shannon_entropy_charset(s.as_bytes(), HEX_ALPHABET);
+    }
+
+    if is_base64_str(s) {
+        return shannon_entropy_charset(s.as_bytes(), BASE64_ALPHABET);
+    }
+
+    shannon_entropy(s.as_bytes())
+}
 
 // Tests
 #[cfg(test)]
 mod tests {
-    use super::shannon_entropy;
+    use super::{shannon_entropy, str_entropy, is_hex_str, is_base64_str};
 
     #[test]
-    fn it_works() {
+    fn test_entropy() {
         let test_strings = vec![
             ("hello world", 2.8453512),
             ("hello worldd", 2.8553884),
             ("a", 0.0),
+            ("aaaaa", 0.0),
+            ("ab", 1.0),
+            ("aab", 0.9182958),
             ("", 0.0),
         ];
 
         for (test, answer) in test_strings {
-            let entropy: f32 = shannon_entropy(test);
+            let entropy: f32 = str_entropy(test);
             assert_eq!(entropy, answer);
         }  
     }
 
-    #[test]
-    fn test_entropy_empty() {
-        let h = shannon_entropy("");
-        assert_eq!(h, 0.0);
-    }
-
-    #[test]
-    fn test_entropy_a() {
-        let h = shannon_entropy("a");
-        assert_eq!(h, 0.0);
-    }
-
-    #[test]
-    fn test_entropy_aaaaa() {
-        let h = shannon_entropy("aaaaa");
-        assert_eq!(h, 0.0);
-    }
-
-    #[test]
-    fn test_entropy_ab() {
-        let h = shannon_entropy("ab");
-        assert_eq!(h, 1.0);
-    }
-
-    #[test]
-    fn test_entropy_aab() {
-        let h = shannon_entropy("aab");
-        assert_eq!(h, 0.9182958);
-    }
-
     #[test]
     fn test_entropy_equal_distribution1() {
         let mut bytes = [0u8; 256];
@@ -90,7 +88,7 @@ mod tests {
             bytes[i] = i as u8;
         }
 
-        let h = shannon_entropy(&String::from_utf8_lossy(&bytes));
+        let h = shannon_entropy(&bytes);
         assert_eq!(h, 8.0);
     }
 
@@ -101,17 +99,33 @@ mod tests {
             bytes[i] = (i % 256) as u8;
         }
 
-        let pippo = String::from(&bytes);
-        println!("{:?} {:?}", pippo, pippo.len());
-        let h = shannon_entropy(&pippo);
+        let h = shannon_entropy(&bytes);
         assert_eq!(h, 8.0);
     }
 
     #[test]
     fn test_entropy_helloworld() {
-        let h = shannon_entropy("hello, world");
+        let h = str_entropy("hello, world");
         assert_eq!(h, 3.0220551);
-        let h = shannon_entropy("hello world");
+        let h = str_entropy("hello world");
         assert_eq!(h, 2.8453512);
     }
+
+    #[test]
+    fn test_hex_str_reconizer() {
+        let s = is_hex_str("0123456789abcdef0123456789abcdef");
+        assert_eq!(s, true);
+        let s = is_hex_str("68656c6c6f20776f726c64");
+        assert_eq!(s, true);
+        let s = is_hex_str("g");
+        assert_eq!(s, false);
+    }
+
+    #[test]
+    fn test_base64_str_reconizer() {
+        let s = is_base64_str("aGVsbG8gd29ybGQ=");
+        assert_eq!(s, true);
+        let s = is_base64_str("#@$");
+        assert_eq!(s, false);
+    }
 }