Add float fuzzer and cleanup

test/fuzzing/float.cc

@@ -0,0 +1,34 @@
+// A fuzzer for floating-point formatter.
+// For the license information refer to format.h.
+
+#include <cstdint>
+#include <cstdlib>
+#include <stdexcept>
+#include <limits>
+#include <fmt/format.h>
+
+#include "fuzzer-common.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
+  if (size <= sizeof(double) || !std::numeric_limits<double>::is_iec559)
+    return 0;
+
+  auto value = assign_from_buf<double>(data);
+  auto buffer = fmt::memory_buffer();
+  fmt::format_to(buffer, "{}", value);
+
+  // Check a round trip.
+  if (std::isnan(value)) {
+    auto nan = std::signbit(value) ? "-nan" : "nan";
+    if (fmt::string_view(buffer.data(), buffer.size()) != nan)
+      throw std::runtime_error("round trip failure");
+    return 0;
+  }
+  buffer.push_back('\0');
+  char* ptr = nullptr;
+  if (std::strtod(buffer.data(), &ptr) != value)
+    throw std::runtime_error("round trip failure");
+  if (ptr != buffer.end())
+    throw std::runtime_error("unparsed output");
+  return 0;
+}