externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c598a9a517d7bdec8ba1a0bace3ef10' as 'externals/catch'

externals/catch/fuzzing/fuzz_textflow.cpp

@@ -0,0 +1,47 @@
+//License: Boost 1.0
+//By Paul Dreik 2020
+
+#include <catch2/internal/catch_textflow.hpp>
+
+#include "NullOStream.h"
+
+#include <string>
+#include <string_view>
+
+
+template<class Callback>
+void split(const char *Data, size_t Size, Callback callback) {
+
+    using namespace std::literals;
+    constexpr auto sep="\n~~~\n"sv;
+
+    std::string_view remainder(Data,Size);
+    for (;;) {
+        auto pos=remainder.find(sep);
+        if(pos==std::string_view::npos) {
+            //not found. use the remainder and exit
+            callback(remainder);
+            return;
+        } else {
+            //found. invoke callback on the first part, then proceed with the rest.
+            callback(remainder.substr(0,pos));
+            remainder=remainder.substr(pos+sep.size());
+        }
+    }
+}
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+
+    Catch::TextFlow::Columns columns;
+
+    // break the input on separator
+    split((const char*)Data,Size,[&](std::string_view word) {
+        columns+=Catch::TextFlow::Column(std::string(word));
+    });
+
+    NullOStream nul;
+    nul << columns;
+
+    return 0;
+}
+