Introduce get_checksum_bytes method and improvements

`get_checksum_bytes` returns a descriptor checksum as `[u8; 8]` instead
of `String`, potentially improving performance and memory usage.

In addition to this, since descriptors only use charaters that fit
within a UTF-8 8-bit code unit, there is no need to use the `char` type
(which is 4 bytes). This can also potentially bring in some performance
and memory-usage benefits.

CHANGELOG.md

@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- Add `descriptor::checksum::get_checksum_bytes` method.
 
 ## [v0.20.0] - [v0.19.0]
 

src/descriptor/checksum.rs

@@ -14,12 +14,10 @@
 //! This module contains a re-implementation of the function used by Bitcoin Core to calculate the
 //! checksum of a descriptor
 
-use std::iter::FromIterator;
-
 use crate::descriptor::DescriptorError;
 
-const INPUT_CHARSET: &str =  "0123456789()[],'/*abcdefgh@:$%{}IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";
-const CHECKSUM_CHARSET: &str = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
+const INPUT_CHARSET: &[u8] = b"0123456789()[],'/*abcdefgh@:$%{}IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";
+const CHECKSUM_CHARSET: &[u8] = b"qpzry9x8gf2tvdw0s3jn54khce6mua7l";
 
 fn poly_mod(mut c: u64, val: u64) -> u64 {
     let c0 = c >> 35;
@@ -43,15 +41,17 @@ fn poly_mod(mut c: u64, val: u64) -> u64 {
     c
 }
 
-/// Compute the checksum of a descriptor
-pub fn get_checksum(desc: &str) -> Result<String, DescriptorError> {
+/// Computes the checksum bytes of a descriptor
+pub fn get_checksum_bytes(desc: &str) -> Result<[u8; 8], DescriptorError> {
     let mut c = 1;
     let mut cls = 0;
     let mut clscount = 0;
-    for ch in desc.chars() {
+
+    for ch in desc.as_bytes() {
         let pos = INPUT_CHARSET
-            .find(ch)
-            .ok_or(DescriptorError::InvalidDescriptorCharacter(ch))? as u64;
+            .iter()
+            .position(|b| b == ch)
+            .ok_or(DescriptorError::InvalidDescriptorCharacter(*ch))? as u64;
         c = poly_mod(c, pos & 31);
         cls = cls * 3 + (pos >> 5);
         clscount += 1;
@@ -67,17 +67,18 @@ pub fn get_checksum(desc: &str) -> Result<String, DescriptorError> {
     (0..8).for_each(|_| c = poly_mod(c, 0));
     c ^= 1;
 
-    let mut chars = Vec::with_capacity(8);
+    let mut checksum = [0_u8; 8];
     for j in 0..8 {
-        chars.push(
-            CHECKSUM_CHARSET
-                .chars()
-                .nth(((c >> (5 * (7 - j))) & 31) as usize)
-                .unwrap(),
-        );
+        checksum[j] = CHECKSUM_CHARSET[((c >> (5 * (7 - j))) & 31) as usize];
     }
 
-    Ok(String::from_iter(chars))
+    Ok(checksum)
+}
+
+/// Compute the checksum of a descriptor
+pub fn get_checksum(desc: &str) -> Result<String, DescriptorError> {
+    // unsafe is okay here as the checksum only uses bytes in `CHECKSUM_CHARSET`
+    get_checksum_bytes(desc).map(|b| unsafe { String::from_utf8_unchecked(b.to_vec()) })
 }
 
 #[cfg(test)]
@@ -97,17 +98,12 @@ mod test {
 
     #[test]
     fn test_get_checksum_invalid_character() {
-        let sparkle_heart = vec![240, 159, 146, 150];
-        let sparkle_heart = std::str::from_utf8(&sparkle_heart)
-            .unwrap()
-            .chars()
-            .next()
-            .unwrap();
+        let sparkle_heart = unsafe { std::str::from_utf8_unchecked(&[240, 159, 146, 150]) };
         let invalid_desc = format!("wpkh(tprv8ZgxMBicQKsPdpkqS7Eair4YxjcuuvDPNYmKX3sCniCf16tHEVrjjiSXEkFRnUH77yXc6ZcwHHcL{}fjdi5qUvw3VDfgYiH5mNsj5izuiu2N/1/2/*)", sparkle_heart);
 
         assert!(matches!(
             get_checksum(&invalid_desc).err(),
-            Some(DescriptorError::InvalidDescriptorCharacter(invalid_char)) if invalid_char == sparkle_heart
+            Some(DescriptorError::InvalidDescriptorCharacter(invalid_char)) if invalid_char == sparkle_heart.as_bytes()[0]
         ));
     }
 }

src/descriptor/error.rs

@@ -28,8 +28,8 @@ pub enum Error {
     /// Error while extracting and manipulating policies
     Policy(crate::descriptor::policy::PolicyError),
 
-    /// Invalid character found in the descriptor checksum
-    InvalidDescriptorCharacter(char),
+    /// Invalid byte found in the descriptor checksum
+    InvalidDescriptorCharacter(u8),
 
     /// BIP32 error
     Bip32(bitcoin::util::bip32::Error),