kraken: engine: add trading on cross behaviour

It is limited, and mostly untested, I would need more explicit semantics
for the border cases to make it more robust.

src/engine/engine.cc

@@ -87,8 +87,9 @@ private:
     TopInfo top_info_{};
 };
 
-Engine::Engine(std::shared_ptr<EngineListener> listener)
+Engine::Engine(std::shared_ptr<EngineListener> listener,
-    : listener_(listener) {}
+               CrossBehaviour cross_behaviour)
+    : listener_(listener), cross_behaviour_(cross_behaviour) {}
 
 void Engine::process_orders(std::vector<Order> const& orders) {
     for (auto const& order : orders) {
@@ -109,8 +110,31 @@ void Engine::operator()(TradeOrder const& trade_order) {
             auto& [_, bid_map] = *bid_map_it;
             if (bid_map.size() > 0
                 && bid_map.begin()->first >= trade_order.price) {
-                // FIXME: handle matching if enabled
+                if (cross_behaviour_ == CrossBehaviour::REJECT) {
-                listener_->on_rejection(trade_order.user, trade_order.id);
+                    listener_->on_rejection(trade_order.user, trade_order.id);
+                } else {
+                    // FIXME: assumes a single trade for the order
+                    // FIXME: assumes no remaining orders on both sides
+                    auto matching = bid_map.begin();
+                    auto const& bid_order = matching->second;
+
+                    auto const matching_quantity
+                        = std::min(bid_order.quantity, trade_order.quantity);
+                    // NOTE: Pick the asking price for matching
+                    auto const matching_price = trade_order.price;
+
+                    listener_->on_acknowledgement(trade_order.user,
+                                                  trade_order.id);
+                    listener_->on_match(bid_order.user, bid_order.id,
+                                        trade_order.user, trade_order.id,
+                                        matching_price, matching_quantity);
+                    assert(matching_quantity == bid_order.quantity
+                           && "multiple matches not implemented");
+                    assert(matching_quantity == trade_order.quantity
+                           && "multiple matches not implemented");
+
+                    bid_map.erase(matching);
+                }
                 return;
             }
         }
@@ -124,8 +148,30 @@ void Engine::operator()(TradeOrder const& trade_order) {
             auto& [_, ask_map] = *ask_map_it;
             if (ask_map.size() > 0
                 && ask_map.begin()->first <= trade_order.price) {
-                // FIXME: handle matching if enabled
+                if (cross_behaviour_ == CrossBehaviour::REJECT) {
-                listener_->on_rejection(trade_order.user, trade_order.id);
+                    listener_->on_rejection(trade_order.user, trade_order.id);
+                } else {
+                    // FIXME: assumes a single trade for the order
+                    // FIXME: assumes no remaining orders on both sides
+                    auto matching = ask_map.begin();
+                    // NOTE: Pick the asking price for matching
+                    auto const& [matching_price, ask_order] = *matching;
+
+                    auto const matching_quantity
+                        = std::min(ask_order.quantity, trade_order.quantity);
+
+                    listener_->on_acknowledgement(trade_order.user,
+                                                  trade_order.id);
+                    listener_->on_match(trade_order.user, trade_order.id,
+                                        ask_order.user, ask_order.id,
+                                        matching_price, matching_quantity);
+                    assert(matching_quantity == ask_order.quantity
+                           && "multiple matches not implemented");
+                    assert(matching_quantity == trade_order.quantity
+                           && "multiple matches not implemented");
+
+                    ask_map.erase(matching);
+                }
                 return;
             }
         }

src/engine/engine.hh

@@ -9,12 +9,21 @@
 
 namespace kraken::engine {
 
+/// Which behaviour on cross orders.
+enum class CrossBehaviour {
+    /// Reject the crossing order.
+    REJECT,
+    /// Make the trade with the matching order(s).
+    MATCH,
+};
+
 struct CallbackOnTopOfBookChange;
 struct EngineListener;
 
 /// Matching engine which processes orders and keeps the book up-to-date.
 struct Engine {
-    Engine(std::shared_ptr<EngineListener> listener);
+    Engine(std::shared_ptr<EngineListener> listener,
+           CrossBehaviour cross_behaviour = CrossBehaviour::REJECT);
 
     /// Process orders, triggerring the listener on each event.
     void process_orders(std::vector<Order> const& orders);
@@ -25,6 +34,7 @@ private:
     void operator()(FlushOrder const& flush_order);
 
     std::shared_ptr<EngineListener> listener_;
+    CrossBehaviour cross_behaviour_;
 
     // Symbol, price, side are implicit given the way the book is represented
     struct OrderMetaData {