Use locale-independent toLower

We generally do not even try to pretend to support more than plain ASCII,
because doing unicode in old C++ standards is a pain and a half. For
`toLower`, we used to wrap `std::tolower` with multiple casts to avoid UB,
but calling into it is relatively slow, and it tries to handle different
locales, which could cause unexpected results.

In addition, the new implementation is significantly faster, which helps
speed up the initial process bring up (as comparing tags, and thus test
cases is now faster).

2 files changed, 6 insertions, 5 deletions

diff --git a/src/catch2/internal/catch_string_manip.cpp b/src/catch2/internal/catch_string_manip.cpp
index 68983787..ac88be2e 100644
--- a/src/catch2/internal/catch_string_manip.cpp
+++ b/src/catch2/internal/catch_string_manip.cpp
@@ -11,7 +11,6 @@
 
 #include <ostream>
 #include <cstring>
-#include <cctype>
 #include <vector>
 
 namespace Catch {
@@ -41,9 +40,6 @@ namespace Catch {
         toLowerInPlace( lc );
         return lc;
     }
-    char toLower(char c) {
-        return static_cast<char>(std::tolower(static_cast<unsigned char>(c)));
-    }
 
     std::string trim( std::string const& str ) {
         static char const* whitespaceChars = "\n\r\t ";
diff --git a/src/catch2/internal/catch_string_manip.hpp b/src/catch2/internal/catch_string_manip.hpp
index 4251f83d..ce317c60 100644
--- a/src/catch2/internal/catch_string_manip.hpp
+++ b/src/catch2/internal/catch_string_manip.hpp
@@ -25,7 +25,12 @@ namespace Catch {
     bool contains( std::string const& s, std::string const& infix );
     void toLowerInPlace( std::string& s );
     std::string toLower( std::string const& s );
-    char toLower( char c );
+    //! ASCII only.
+    constexpr char toLower( char c ) {
+        const uint32_t as_number = static_cast<unsigned char>( c );
+        const bool isUpper = ( as_number - static_cast<uint32_t>( 'A' ) ) < 26u;
+        return static_cast<char>( as_number | ( isUpper << 5 ) );
+    }
     //! Returns a new string without whitespace at the start/end
     std::string trim( std::string const& str );
     //! Returns a substring of the original ref without whitespace. Beware lifetimes!