From a79d1a6d7434125529e2f17e382791c5617183b7 Mon Sep 17 00:00:00 2001
From: Dan Goodliffe <dan@randomdan.homeip.net>
Date: Sun, 22 Mar 2020 12:56:22 +0000
Subject: C++ fixed string literal CTF

---
 libadhocutil/compileTimeFormatter.h                | 50 +++++++++++++++++++++-
 libadhocutil/unittests/Jamfile.jam                 | 15 +++++++
 .../unittests/testCompileTimeFormatter.cpp         |  2 +
 3 files changed, 65 insertions(+), 2 deletions(-)

diff --git a/libadhocutil/compileTimeFormatter.h b/libadhocutil/compileTimeFormatter.h
index 0a86946..c3837fe 100644
--- a/libadhocutil/compileTimeFormatter.h
+++ b/libadhocutil/compileTimeFormatter.h
@@ -6,10 +6,17 @@
 #include <optional>
 #include <array>
 #include <boost/preprocessor/variadic/size.hpp>
-#include "unique.h"
 
 namespace AdHoc {
+#ifdef __cpp_nontype_template_parameter_class
+#define USE_FIXED_STRING
+#endif
+
+#ifdef USE_FIXED_STRING
+#define CtfString const auto
+#else
 #define CtfString const auto &
+#endif
 	// Template char utils
 	template<typename char_type>
 	constexpr bool isdigit(const char_type & ch)
@@ -236,9 +243,45 @@ namespace AdHoc {
 #endif
 	}
 
+#ifdef USE_FIXED_STRING
+	// New C++20 implementation
+	namespace support {
+		template<typename CharT, std::size_t N>
+		class basic_fixed_string : public std::array<CharT, N> {
+			public:
+				constexpr basic_fixed_string(const CharT (&str)[N + 1])
+				{
+					for (decltype(N) x = 0; x < N; x++) {
+						this->at(x) = str[x];
+					}
+				}
+				constexpr basic_fixed_string(const CharT * str, decltype(N) len)
+				{
+					for (decltype(N) x = 0; x < len; x++) {
+						this->at(x) = str[x];
+					}
+				}
+		};
+
+		template<typename CharT, std::size_t N>
+		basic_fixed_string(const CharT (&str)[N]) -> basic_fixed_string<CharT, N - 1>;
+	}
+
+	template<const support::basic_fixed_string Str>
+	class LiteralFormatter : public FormatterDetail<Str, Str.size()> { };
+
+	template <const auto & S, decltype(strlen(S)) L = strlen(S)>
+	class Formatter : public FormatterDetail<support::basic_fixed_string<
+										typename std::decay<decltype(S[0])>::type, L>(S, L), L> { };
+
+#define AdHocFormatter(name, str) \
+	using name = LiteralFormatter<str>
+
+#else
+	// Classic pre-C++20 implementation
+#include "unique.h"
 	template <const auto & S, decltype(strlen<S>()) L = strlen<S>()>
 	class Formatter : public FormatterDetail<S, L> { };
-}
 
 #define AdHocFormatterTypedef(name, str, id) \
     inline constexpr auto id = str; \
@@ -247,4 +290,7 @@ namespace AdHoc {
     AdHocFormatterTypedef(name, str, MAKE_UNIQUE(name))
 
 #endif
+}
+
+#endif
 
diff --git a/libadhocutil/unittests/Jamfile.jam b/libadhocutil/unittests/Jamfile.jam
index d62f6da..e2b1f7c 100644
--- a/libadhocutil/unittests/Jamfile.jam
+++ b/libadhocutil/unittests/Jamfile.jam
@@ -38,6 +38,21 @@ run
 	<implicit-dependency>lorem-ipsum
 	;
 
+run
+	testCompileTimeFormatter.cpp
+	: : :
+	<define>ROOT=\"$(me)\"
+	<define>BOOST_TEST_DYN_LINK
+	<library>..//adhocutil
+	<library>stdc++fs
+	<library>boost_utf
+	-<cxxstd>17
+	<cxxstd>2a
+	<implicit-dependency>lorem-ipsum
+	:
+	testCompileTimeFormatter20
+	;
+
 run
 	testUriParse.cpp
 	: : :
diff --git a/libadhocutil/unittests/testCompileTimeFormatter.cpp b/libadhocutil/unittests/testCompileTimeFormatter.cpp
index ba8a46f..369b804 100644
--- a/libadhocutil/unittests/testCompileTimeFormatter.cpp
+++ b/libadhocutil/unittests/testCompileTimeFormatter.cpp
@@ -77,6 +77,7 @@ namespace AdHoc {
 	};
 }
 
+#ifndef __cpp_nontype_template_parameter_class
 // Compile string util assertions
 static_assert(strlen<formatEdgeCaseEmpty>() == 0);
 static_assert(strlen<formatEdgeCaseSingle>() == 1);
@@ -113,6 +114,7 @@ static_assert(strchrnul<formatEdgeCaseFormatLonely, '%'>() == 0);
 static_assert(strchrnul<formatEdgeCaseFormatLonely, '?'>() == 1);
 static_assert(strchrnul<formatStringLiteral, 'e'>() == 3);
 static_assert(strchrnul<formatStringLiteral, 'f'>() == 7);
+#endif
 
 BOOST_FIXTURE_TEST_SUITE( TestStreamWrite, std::stringstream )
 
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