[hex-literal] Add new hex_custom_padding!() macro; enables custom padding between hex characters

hex-literal/src/lib.rs

@@ -5,7 +5,7 @@
     html_favicon_url = "https://raw.githubusercontent.com/RustCrypto/media/6ee8e381/logo.svg"
 )]
 
-const fn next_hex_char(string: &[u8], mut pos: usize) -> Option<(u8, usize)> {
+const fn next_hex_char(string: &[u8], mut pos: usize, extra_padding: &[u8]) -> Option<(u8, usize)> {
     while pos < string.len() {
         let raw_val = string[pos];
         pos += 1;
@@ -14,6 +14,7 @@ const fn next_hex_char(string: &[u8], mut pos: usize) -> Option<(u8, usize)> {
             b'A'..=b'F' => raw_val - 55,
             b'a'..=b'f' => raw_val - 87,
             b' ' | b'\r' | b'\n' | b'\t' => continue,
+            b if byte_slice_contains(extra_padding, b) => continue,
             0..=127 => panic!("Encountered invalid ASCII character"),
             _ => panic!("Encountered non-ASCII character"),
         };
@@ -22,12 +23,12 @@ const fn next_hex_char(string: &[u8], mut pos: usize) -> Option<(u8, usize)> {
     None
 }
 
-const fn next_byte(string: &[u8], pos: usize) -> Option<(u8, usize)> {
-    let (half1, pos) = match next_hex_char(string, pos) {
+const fn next_byte(string: &[u8], pos: usize, extra_padding: &[u8]) -> Option<(u8, usize)> {
+    let (half1, pos) = match next_hex_char(string, pos, extra_padding) {
         Some(v) => v,
         None => return None,
     };
-    let (half2, pos) = match next_hex_char(string, pos) {
+    let (half2, pos) = match next_hex_char(string, pos, extra_padding) {
         Some(v) => v,
         None => panic!("Odd number of hex characters"),
     };
@@ -38,12 +39,12 @@ const fn next_byte(string: &[u8], pos: usize) -> Option<(u8, usize)> {
 ///
 /// This function is an implementation detail and SHOULD NOT be called directly!
 #[doc(hidden)]
-pub const fn len(strings: &[&[u8]]) -> usize {
+pub const fn len(strings: &[&[u8]], extra_padding: &[u8]) -> usize {
     let mut i = 0;
     let mut len = 0;
     while i < strings.len() {
         let mut pos = 0;
-        while let Some((_, new_pos)) = next_byte(strings[i], pos) {
+        while let Some((_, new_pos)) = next_byte(strings[i], pos, extra_padding) {
             len += 1;
             pos = new_pos;
         }
@@ -56,7 +57,10 @@ pub const fn len(strings: &[&[u8]]) -> usize {
 ///
 /// This function is an implementation detail and SHOULD NOT be called directly!
 #[doc(hidden)]
-pub const fn decode<const LEN: usize>(strings: &[&[u8]]) -> Option<[u8; LEN]> {
+pub const fn decode<const LEN: usize>(
+    strings: &[&[u8]],
+    extra_padding: &[u8],
+) -> Option<[u8; LEN]> {
     let mut string_pos = 0;
     let mut buf = [0u8; LEN];
     let mut buf_pos = 0;
@@ -65,7 +69,7 @@ pub const fn decode<const LEN: usize>(strings: &[&[u8]]) -> Option<[u8; LEN]> {
         let string = &strings[string_pos];
         string_pos += 1;
 
-        while let Some((byte, new_pos)) = next_byte(string, pos) {
+        while let Some((byte, new_pos)) = next_byte(string, pos, extra_padding) {
             buf[buf_pos] = byte;
             buf_pos += 1;
             pos = new_pos;
@@ -79,11 +83,30 @@ pub const fn decode<const LEN: usize>(strings: &[&[u8]]) -> Option<[u8; LEN]> {
 /// See the crate-level docs for more information.
 #[macro_export]
 macro_rules! hex {
-    ($($s:literal)*) => {{
+    ($($s:literal)*) => {
+        ::hex_literal::hex_custom_padding!("", $($s)*)
+    }
+}
+
+#[macro_export]
+macro_rules! hex_custom_padding {
+    ($padding:literal, $($s:literal)*) => {{
+        const PADDING: &'static [u8] = $padding.as_bytes();
         const STRINGS: &[&'static [u8]] = &[$($s.as_bytes(),)*];
         const {
-            $crate::decode::<{ $crate::len(STRINGS) }>(STRINGS)
+            $crate::decode::<{ $crate::len(STRINGS, PADDING) }>(STRINGS, PADDING)
                 .expect("Output array length should be correct")
         }
     }};
 }
+
+const fn byte_slice_contains(haystack: &[u8], needle: u8) -> bool {
+    let mut i = 0;
+    while i < haystack.len() {
+        if haystack[i] == needle {
+            return true;
+        }
+        i += 1;
+    }
+    false
+}