Add the new utils plugin, change entity spawning and the lookup command plugins to use it.

data/plugins/util/item-on-item-intercept.rb

@@ -0,0 +1,46 @@
+
+# Adds an interception for the ItemOnItem message
+add_interception :item_on_item do |used, target, reversible, block|
+  interception = ItemOnItemPair.new(used, target)
+
+  ITEM_PAIRS[interception] = block
+  ITEM_PAIRS[interception.reverse] = block if reversible == :reversible
+end
+
+private
+
+# A hash of ItemOnItemPairs to blocks.
+ITEM_PAIRS = {}
+
+# A pair of items that will cause a block to be executed if one (the 'used' item) is used on the other (the 'target' item).
+class ItemOnItemPair
+  attr_reader :used, :target
+
+  def initialize(used, target)
+    @used = used
+    @target = target
+  end
+
+  # Returns a new ItemOnItemPair that is the reverse of this.
+  def reverse
+    return ItemOnItemPair.new(@target, @used)
+  end
+
+  def eql?(other)
+    return (other.kind_of?(ItemOnItemPair) && @used == other.used && @target == other.target)
+  end
+
+  def hash
+    return @used << 16 | @target
+  end
+
+end
+
+# Adds a message handler to the item on item message.
+on :message, :item_on_item do |ctx, player, message|
+  used, target = message.id, message.target_id
+  pair = ItemOnItemPair.new(used, target)
+  block = ITEM_PAIRS[pair]
+
+  block.call(ctx, player, message) unless block == nil
+end