2 files changed, 14 insertions, 2 deletions
diff --git a/curling.rb b/curling.rb
index 4b03e8d..e688a01 100644
--- a/curling.rb
+++ b/curling.rb
@@ -61,7 +61,16 @@ To restart the game, run `rm k` to remove the cookie that stores your game state
 end
 
 get '/:command' do |command|
-  engine = Hex.new cookies.to_h
+  begin
+    engine = Hex.new cookies.to_h
+  rescue InvalidChecksumError
+    halt 422, <<~MSG
+      Invalid state, checksum does not match!
+      If you lost your last valid game state, you'll have to remove your current
+      cookie and start over.
+    MSG
+  end
+
   engine.step command
   engine.state_h.each_pair do |key, value|
     cookies[key] = value
diff --git a/engine.rb b/engine.rb
index b04e2f2..7b9ddf0 100644
--- a/engine.rb
+++ b/engine.rb
@@ -1,3 +1,6 @@
+class InvalidChecksumError < StandardError
+end
+
 class Engine
   def initialize hash
     validate_checksum hash
@@ -6,7 +9,7 @@ class Engine
 
   def validate_checksum hash
     if !checksum_valid? hash
-      throw 'Invalid state, checksum does not match!'
+      raise InvalidChecksumError
     end
   end