Adding to_chars fallback (#117)

* adding `to_chars` fallback

* tweak suggested by @thinlang

* fixing test

adobe/string/to_string.hpp

@@ -24,6 +24,19 @@
 
 #include <adobe/cassert.hpp>
 
+// `to_chars` floating-point support was added to macOS 13.3. Some ASL
+// clients have deployment targets that go back earlier than that.
+// In such case, `adobe::to_string` falls back to an implementation
+// that relies on the output-iterator variant of `adobe::to_string`,
+// pivoting on precise/short based on the exponent of the number
+// being serialized. This gets us close to the output of `to_chars`
+// without overcomplicating the fallback.
+#if defined(_LIBCPP_AVAILABILITY_HAS_NO_TO_CHARS_FLOATING_POINT)
+    #define ADOBE_HAS_TO_CHARS_FP() 0
+#else
+    #define ADOBE_HAS_TO_CHARS_FP() 1
+#endif
+
 /**************************************************************************************************/
 
 namespace adobe {
@@ -161,6 +174,7 @@ inline std::string to_string(double x) {
     if (x == std::numeric_limits<double>::infinity()) return "Infinity";
     if (x == -std::numeric_limits<double>::infinity()) return "-Infinity";
 
+#if ADOBE_HAS_TO_CHARS_FP()
     std::array<char, 64> str;
     char* first = &str[0];
     char* last = first + str.size();
@@ -169,6 +183,14 @@ inline std::string to_string(double x) {
     return tcr.ec == std::errc() ?
                std::string(first, tcr.ptr - first) :
                std::make_error_code(tcr.ec).message();
+#else
+    std::string result;
+    double f3;
+    double f2 = std::modf(x, &f3);
+    const auto use_precise = std::abs(std::log10(f2)) > 7;
+    to_string(x, std::back_inserter(result), use_precise);
+    return result;
+#endif
 }
 
 /**************************************************************************************************/

test/to_string/main.cpp

@@ -109,13 +109,17 @@ std::string asl_to_string(const test_t& test) {
 }
 
 std::string std_to_chars(const test_t& test) {
+#if ADOBE_HAS_TO_CHARS_FP()
     std::array<char, 64> str;
     if (auto [ptr, ec] = std::to_chars(str.data(), str.data() + str.size(), test.value_m); ec == std::errc()) {
         const auto sz = ptr - str.data();
         return std::string(str.data(), sz);
     } else {
         return std::make_error_code(ec).message();
     }
+#else
+    return "unavailable";
+#endif
 }
 
 std::string any_regular_serialization(const test_t& test) {
@@ -147,15 +151,23 @@ std::size_t test_suite(std::string (*convert)(const test_t& test), const char* l
 /******************************************************************************/
 
 BOOST_AUTO_TEST_CASE(serialization_test_suite) {
+#if ADOBE_HAS_TO_CHARS_FP()
+    constexpr auto to_string_match_count_k = 10;
+    constexpr auto to_chars_match_count_k = 6;
+#else
+    constexpr auto to_string_match_count_k = 9;
+    constexpr auto to_chars_match_count_k = 0;
+#endif
+
     BOOST_CHECK_EQUAL(test_suite(&asl_to_string_v1<true>, "adobe::to_string (precise)"), 3);
 
     BOOST_CHECK_EQUAL(test_suite(&asl_to_string_v1<false>, "adobe::to_string (short)"), 2);
 
-    BOOST_CHECK_EQUAL(test_suite(&asl_to_string, "adobe::to_string (v2)"), 10);
+    BOOST_CHECK_EQUAL(test_suite(&asl_to_string, "adobe::to_string (v2)"), to_string_match_count_k);
 
-    BOOST_CHECK_EQUAL(test_suite(&std_to_chars, "std::to_chars"), 6);
+    BOOST_CHECK_EQUAL(test_suite(&std_to_chars, "std::to_chars"), to_chars_match_count_k);
 
-    BOOST_CHECK_EQUAL(test_suite(&any_regular_serialization, "adobe::any_regular_t"), 10);
+    BOOST_CHECK_EQUAL(test_suite(&any_regular_serialization, "adobe::any_regular_t"), to_string_match_count_k);
 }
 
 /******************************************************************************/