Use ring buffer to keep track of previous trades

lib/rex/book/trade_tracker.rb

@@ -1,28 +1,28 @@
 module Rex
   module Book
+    # Trade Tracker implemented as a simple ring buffer
     class TradeTracker
+      include Enumerable
       DEFAULT_TRADE_HISTORY_LIMIT = 200
 
       attr_reader :trades
 
       def initialize(limit: DEFAULT_TRADE_HISTORY_LIMIT)
         @limit = limit
-        @trades = []
+        @trades = Array.new(limit)
+        @current_id = -1
       end
 
       def add(trade)
-        @trades.push(trade)
-        cap
+        @current_id += 1
+        idx = (@current_id % @limit)
+        @trades[idx] = trade
       end
 
-      def fetch_trades(limit)
-        @trades.last(limit)
+      def each(&block)
+        (([@current_id - @limit, 0].max)..@current_id ).each do |id|
+          yield @trades[id % @limit]
         end
-
-      private
-
-      def cap
-        @trades = @trades.last(@limit)
       end
     end
   end

lib/rex/server/matching_engine.rb

@@ -50,7 +50,7 @@ module Rex
       end
 
       def fetch_trades(user_id)
-        @trade_tracker.fetch_trades(50).each do |trade|
+        @trade_tracker.each do |trade|
           @outbox.push(
             Messages::TradeFetchEvent.new(
               trade.id,

spec/book/trade_tracker_spec.rb

@@ -15,10 +15,10 @@ RSpec.describe Rex::Book::TradeTracker do
     end
   end
 
-  describe "#fetch_trades" do
+  describe "#each" do
     let(:n) { 3 }
     it "returns only the last n trades" do
-      expect(instance.fetch_trades(1)).to eq([11])
+      expect(instance.to_a).to eq([10, 11])
     end
   end
 end