Add support for afl-fuzz testing

"make fuzz" creates a simple executable that de-serialises stdin
and re-serialises to stdout.
"make fuzz_testcases" extracts the smaller json test cases into
a testcases directory.

The library can then be fuzzed as follows:
    CC=afl-clang-fast make fuzz
    make fuzz_testcases
    mkdir out
    afl-fuzz -i testcases -o out ./fuzz

Makefile

@@ -9,7 +9,7 @@ all: json_unit
 
 # clean up
 clean:
-	rm -f json_unit json_benchmarks
+	rm -f json_unit json_benchmarks fuzz
 
 
 ##########################################################################
@@ -24,6 +24,16 @@ json_unit: test/unit.cpp src/json.hpp test/catch.hpp
 	$(CXX) -std=c++11 $(CXXFLAGS) $(FLAGS) $(CPPFLAGS) -I src -I test $< $(LDFLAGS) -o $@
 
 
+##########################################################################
+# fuzzing
+##########################################################################
+
+fuzz: test/fuzz.cpp src/json.hpp
+	$(CXX) -std=c++11 $(CXXFLAGS) $(FLAGS) $(CPPFLAGS) -I src -I test $< $(LDFLAGS) -lstdc++ -lm -o $@
+fuzz_testcases:
+	mkdir -p testcases && find test/ -size -5k -name *json | xargs -I{} cp "{}" testcases
+	@echo "Test cases suitable for fuzzing have been copied into the testcases directory"
+
 ##########################################################################
 # static analyzer
 ##########################################################################
@@ -48,7 +58,7 @@ pretty:
 	   --indent-col1-comments --pad-oper --pad-header --align-pointer=type \
 	   --align-reference=type --add-brackets --convert-tabs --close-templates \
 	   --lineend=linux --preserve-date --suffix=none \
-	   src/json.hpp src/json.hpp.re2c test/unit.cpp benchmarks/benchmarks.cpp doc/examples/*.cpp
+	   src/json.hpp src/json.hpp.re2c test/unit.cpp test/fuzz.cpp benchmarks/benchmarks.cpp doc/examples/*.cpp
 
 
 ##########################################################################

test/fuzz.cpp

@@ -0,0 +1,42 @@
+/*
+    __ _____ _____ _____
+ __|  |   __|     |   | |  JSON for Modern C++ (fuzz test support)
+|  |  |__   |  |  | | | |  version 2.0.0
+|_____|_____|_____|_|___|  https://github.com/nlohmann/json
+
+To run under afl:
+  afl-fuzz -i testcases -o output ./fuzz
+
+Licensed under the MIT License <http://opensource.org/licenses/MIT>.
+*/
+
+#include <json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+	json *jp;
+
+#ifdef __AFL_HAVE_MANUAL_CONTROL
+	while (__AFL_LOOP(1000)) {
+#endif
+		jp = new json();
+		json j = *jp;
+		try {
+			j << std::cin;
+		} catch (std::invalid_argument e) {
+			std::cout << "Invalid argument in parsing" << e.what() << '\n';
+		}
+
+		if (j.find("foo") != j.end()) {
+			std::cout << "Found a foo";
+		}
+
+		std::cout << j.type() << j << std::endl;
+
+		delete jp;
+#ifdef __AFL_HAVE_MANUAL_CONTROL
+	}
+#endif
+}