Add resource exception

src/command.cr

@@ -62,12 +62,14 @@ class HarvestCrystalCommand < Command
 end
 
 class BuildCrystalRestorerCommand < Command
-  BUILD_TIME = 50
+  CRYSTALS_COST = 100
+  BUILD_TIME    =  50
 
   def initialize(@point : Point)
   end
 
   def start(world : World) : Int32
+    world.resources.dec(ResourceType::Crystal, CRYSTALS_COST)
     BUILD_TIME
   end
 
@@ -112,12 +114,14 @@ class RestoreCrystalCommand < Command
 end
 
 class BuildTerraformerCommand < Command
-  BUILD_TIME = 120
+  CRYSTALS_COST = 100
+  BUILD_TIME    = 120
 
   def initialize(@point : Point)
   end
 
   def start(world : World) : Int32
+    world.resources.dec(ResourceType::Crystal, CRYSTALS_COST)
     BUILD_TIME
   end
 

src/exception.cr

@@ -0,0 +1,2 @@
+class NotEnoughtResources < Exception
+end

src/resources.cr

@@ -1,3 +1,5 @@
+require "./exception"
+
 enum ResourceType
   Crystal
   Terraformation
@@ -16,6 +18,14 @@ class Resources
   end
 
   def inc(t : ResourceType, value : Int32)
-    @values[t] = @values[t] + value
+    new_value = @values[t] + value
+    if new_value < 0
+      raise NotEnoughtResources.new
+    end
+    @values[t] = new_value
+  end
+
+  def dec(t : ResourceType, value : Int32)
+    inc(t, -value)
   end
 end