feat(base32ct): add const encoded_len fn (#1424)

This can be handy to avoid an heap allocation, because it is possible to
use the result of the const fn to allocate an array of the right size.

base32ct/src/encoding.rs

@@ -220,14 +220,9 @@ impl<T: Alphabet> Encoding for T {
         }
     }
 
+    #[inline]
     fn encoded_len(bytes: &[u8]) -> usize {
-        if bytes.is_empty() {
-            0
-        } else if Self::PADDED {
-            ((bytes.len() - 1) / 5 + 1) * 8
-        } else {
-            (bytes.len() * 8 + 4) / 5
-        }
+        encoded_len::<Self>(bytes.len())
     }
 }
 
@@ -259,6 +254,17 @@ fn remove_padding(mut input: &[u8]) -> Result<&[u8]> {
     Ok(input)
 }
 
+/// Get the length of Base32 produced by encoding the given amount of bytes.
+pub const fn encoded_len<T: Encoding>(length: usize) -> usize {
+    if length == 0 {
+        0
+    } else if T::PADDED {
+        ((length - 1) / 5 + 1) * 8
+    } else {
+        (length * 8 + 4) / 5
+    }
+}
+
 #[cfg(all(test, feature = "alloc"))]
 mod tests {
     use crate::{Base32, Base32Unpadded, Encoding};

base32ct/src/lib.rs

@@ -59,6 +59,6 @@ mod error;
 
 pub use crate::{
     alphabet::rfc4648::{Base32, Base32Unpadded, Base32Upper, Base32UpperUnpadded},
-    encoding::Encoding,
+    encoding::{encoded_len, Encoding},
     error::{Error, Result},
 };