Quote the dict key in exceptions

(This is really mainly important because we'll be able to stringify other
character types, e.g. bytes.)

Author: jimporter

include/bencode.hpp

@@ -344,6 +344,24 @@ namespace bencode {
       requires stringish<typename T::key_type>;
     };
 
+    template<typename T>
+    std::string quoted_key(const T &key) {
+      std::string s;
+      s.reserve(key.size() + 2);
+      s.push_back('"');
+      for(auto &&i : key) {
+        char c = static_cast<char>(i);
+        switch(c) {
+        case '\0': s.append("\\0");  break;
+        case '\n': s.append("\\n");  break;
+        case '"':  s.append("\\\""); break;
+        default:   s.push_back(c);
+        }
+      }
+      s.push_back('"');
+      return s;
+    }
+
     template<std::integral Integer>
     inline void check_overflow(Integer value, Integer digit) {
       using limits = std::numeric_limits<Integer>;
@@ -523,7 +541,7 @@ namespace bencode {
           auto i = p->emplace(std::move(dict_key), std::move(thing));
           if(!i.second) {
             throw syntax_error(
-              "duplicated key in dict: " + std::string(i.first->first)
+              "duplicated key in dict: " + quoted_key(i.first->first)
             );
           }
           return &i.first->second;

test/test_decode.cpp

@@ -415,10 +415,10 @@ suite<> test_decode("test decoder", [](auto &_) {
     });
 
     _.test("duplicated key", []() {
-      expect(
-        []() { bencode::decode("d3:fooi1e3:fooi1ee"); },
-        decode_error<bencode::syntax_error>("duplicated key in dict: foo", 17)
-      );
+      expect([]() { bencode::decode("d3:fooi1e3:fooi1ee"); },
+             decode_error<bencode::syntax_error>(
+               "duplicated key in dict: \"foo\"", 17
+             ));
     });
   });