Clarify/optimize to_ascii_lower

Author: Joy-less

jsonh_cpp/jsonh_reader.hpp

@@ -1058,7 +1058,7 @@ class jsonh_reader : utf8_reader {
             }
 
             // Digit
-            if (base_digits.find(to_lower(next.value().data())) != std::string::npos) {
+            if (base_digits.find(to_ascii_lower(next.value().data())) != std::string::npos) {
                 read();
                 number_builder += next.value();
             }
@@ -1398,10 +1398,12 @@ class jsonh_reader : utf8_reader {
     static bool is_utf16_high_surrogate(unsigned int code_point) noexcept {
         return code_point >= 0xD800 && code_point <= 0xDBFF;
     }
-    static std::string to_lower(const char* string) noexcept {
+    static std::string to_ascii_lower(const char* string) noexcept {
         std::string result(string);
         for (char& next : result) {
-            next = std::tolower(next);
+            if (next <= 'Z' && next >= 'A') {
+                next -= ('Z' - 'z');
+            }
         }
         return result;
     }