Optimize spicy::bytes_to_hexstring and spicy::bytes_to_mac.

spicy/runtime/src/util.cc

@@ -20,23 +20,41 @@ std::string spicy::rt::version() {
 #endif
 }
 
+
+static inline void byte_to_hex(unsigned char byte, char* hex_out) {
+    static constexpr char hex_chars[] = "0123456789ABCDEF";
+    hex_out[0] = hex_chars[(byte & 0xf0) >> 4];
+    hex_out[1] = hex_chars[byte & 0x0f];
+}
+
 std::string spicy::rt::bytes_to_hexstring(const hilti::rt::Bytes& value) {
+    const auto& data = value.str();
+
+    if ( data.empty() )
+        return "";
+
     std::string result;
+    result.resize(data.size() * 2); // 2 digits per hex byte
 
-    for ( auto x : value )
-        result += hilti::rt::fmt("%02X", x);
+    for ( unsigned long i = 0; i < data.size(); i++ )
+        byte_to_hex(data[i], &result[i * 2]);
 
     return result;
 }
 
 std::string spicy::rt::bytes_to_mac(const hilti::rt::Bytes& value) {
-    std::vector<std::string> bytes;
-    bytes.reserve(value.size());
+    const auto& data = value.str();
+
+    if ( data.empty() )
+        return "";
 
-    for ( auto x : value )
-        bytes.emplace_back(hilti::rt::fmt("%02X", x));
+    // Two digits per hex byte, plus one colon per byte except the last.
+    std::string result((data.size() * 2) + (data.size() - 1), ':');
 
-    return hilti::rt::join(bytes, ":");
+    for ( unsigned long i = 0; i < data.size(); i++ )
+        byte_to_hex(data[i], &result[i * 3]);
+
+    return result;
 }
 
 const hilti::rt::Vector<