Add a fallback fuzz binary if libfuzzer not available

CMakeLists.txt

@@ -173,6 +173,11 @@ if(HAS_LIB_FUZZER)
   target_link_libraries(fuzz PRIVATE simdjson)
   target_compile_options(fuzz PRIVATE -fsanitize=fuzzer ${TEST_FLAGS})
   target_link_options(fuzz PRIVATE -fsanitize=fuzzer)
+else()
+  add_executable(fuzz src/fuzz.cpp src/lib.cpp src/fuzz_driver.cpp)
+  target_include_directories(fuzz PRIVATE include)
+  target_link_libraries(fuzz PRIVATE simdjson)
+  target_compile_options(fuzz PRIVATE ${TEST_FLAGS})
 endif()
 
 add_executable(validate src/validate.cpp)

src/fuzz_driver.cpp

@@ -0,0 +1,19 @@
+#include <cstddef>
+#include <cstdint>
+#include <fstream>
+#include <sstream>
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size);
+
+int main(int argc, char **argv) {
+  auto doTest = [&]() {
+    for (int i = 1; i < argc; ++i) {
+      std::ifstream t(argv[i], std::ios::binary);
+      std::stringstream buffer;
+      buffer << t.rdbuf();
+      auto str = buffer.str();
+      LLVMFuzzerTestOneInput((const uint8_t *)str.data(), str.size());
+    }
+  };
+  doTest();
+}