Optimise the JSON parser (#2241)

Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Author: jviotti

src/core/json/include/sourcemeta/core/json_object.h

@@ -183,6 +183,11 @@ template <typename Key, typename Value, typename Hash> class JSONObject {
   /// Check if the object is empty
   [[nodiscard]] inline auto empty() const -> bool { return this->data.empty(); }
 
+  /// Reserve capacity for a given number of entries
+  inline auto reserve(const size_type capacity) -> void {
+    this->data.reserve(capacity);
+  }
+
   /// Access an object entry by its underlying positional index
   [[nodiscard]] inline auto at(const size_type index) const -> const Entry & {
     return this->data.at(index);

src/core/json/json.cc

@@ -13,6 +13,7 @@
 #include <fstream>      // std::ifstream
 #include <istream>      // std::basic_istream
 #include <ostream>      // std::basic_ostream
+#include <sstream>      // std::basic_ostringstream
 #include <system_error> // std::make_error_code, std::errc
 
 namespace sourcemeta::core {
@@ -21,7 +22,20 @@ namespace sourcemeta::core {
 auto parse_json(std::basic_istream<JSON::Char, JSON::CharTraits> &stream,
                 std::uint64_t &line, std::uint64_t &column,
                 const JSON::ParseCallback &callback) -> JSON {
-  return internal_parse_json(stream, line, column, callback);
+  const auto start_position{stream.tellg()};
+  std::basic_ostringstream<JSON::Char, JSON::CharTraits> buffer;
+  buffer << stream.rdbuf();
+  const auto input{buffer.str()};
+  const char *cursor{input.data()};
+  const char *end{input.data() + input.size()};
+  auto result{internal_parse_json(cursor, end, line, column, callback)};
+  if (start_position != static_cast<std::streampos>(-1)) {
+    const auto consumed{static_cast<std::streamoff>(cursor - input.data())};
+    stream.clear();
+    stream.seekg(start_position + consumed);
+  }
+
+  return result;
 }
 
 auto parse_json(const std::basic_string<JSON::Char, JSON::CharTraits> &input,