ADDED: '$first_non_octet'/2, for much more efficient domain checks in library(crypto).

'$first_non_octet'(Cs, C) <=> C is the leftmost character in the
string Cs whose character code is not in 0..255.

src/clause_types.rs

@@ -296,6 +296,7 @@ pub(crate) enum SystemClauseType {
     Ed25519NewKeyPair,
     Ed25519KeyPairPublicKey,
     Curve25519ScalarMult,
+    FirstNonOctet,
     LoadHTML,
     LoadXML,
     GetEnv,
@@ -588,6 +589,7 @@ impl SystemClauseType {
                 clause_name!("$ed25519_keypair_public_key")
             }
             &SystemClauseType::Curve25519ScalarMult => clause_name!("$curve25519_scalar_mult"),
+            &SystemClauseType::FirstNonOctet => clause_name!("$first_non_octet"),
             &SystemClauseType::LoadHTML => clause_name!("$load_html"),
             &SystemClauseType::LoadXML => clause_name!("$load_xml"),
             &SystemClauseType::GetEnv => clause_name!("$getenv"),
@@ -810,6 +812,7 @@ impl SystemClauseType {
             ("$ed25519_new_keypair", 1) => Some(SystemClauseType::Ed25519NewKeyPair),
             ("$ed25519_keypair_public_key", 2) => Some(SystemClauseType::Ed25519KeyPairPublicKey),
             ("$curve25519_scalar_mult", 3) => Some(SystemClauseType::Curve25519ScalarMult),
+            ("$first_non_octet", 2) => Some(SystemClauseType::FirstNonOctet),
             ("$load_html", 3) => Some(SystemClauseType::LoadHTML),
             ("$load_xml", 3) => Some(SystemClauseType::LoadXML),
             ("$getenv", 2) => Some(SystemClauseType::GetEnv),

src/machine/system_calls.rs

@@ -5340,6 +5340,18 @@ impl MachineState {
                 let cstr = self.heap.put_complete_string(&string);
                 (self.unify_fn)(self, self[temp_v!(3)], cstr);
             }
+            &SystemClauseType::FirstNonOctet => {
+                for c in self.heap_pstr_iter(self[temp_v!(1)]).to_string().chars() {
+                    if c as u32 > 255 {
+                        let chars = clause_name!(String::from(c.to_string()), self.atom_tbl);
+                        let non_octet = self.heap.to_unifiable(HeapCellValue::Atom(chars, None));
+                        (self.unify_fn)(self, self[temp_v!(2)], non_octet);
+                        return return_from_clause!(self.last_call, self);
+                    }
+                }
+                self.fail = true;
+                return Ok(());
+            }
             &SystemClauseType::LoadHTML => {
                 let string = self.heap_pstr_iter(self[temp_v!(1)]).to_string();
                 let doc = select::document::Document::from_read(string.as_bytes()).unwrap();