Resolve #43

lib/diff/lcs/hunk.rb

@@ -16,9 +16,15 @@ class Diff::LCS::Hunk
   def initialize(data_old, data_new, piece, flag_context, file_length_difference)
     # At first, a hunk will have just one Block in it
     @blocks = [Diff::LCS::Block.new(piece)]
+
+    if @blocks[0].remove.empty? && @blocks[1].insert.empty?
+      fail "Cannot build a hunk from #{piece.inspect}; has no add or remove actions"
+    end
+
     if String.method_defined?(:encoding)
       @preferred_data_encoding = data_old.fetch(0, data_new.fetch(0, '')).encoding
     end
+
     @data_old = data_old
     @data_new = data_new
 
@@ -27,10 +33,11 @@ def initialize(data_old, data_new, piece, flag_context, file_length_difference)
     @file_length_difference = after # The caller must get this manually
     @max_diff_size = @blocks.map { |e| e.diff_size.abs }.max
 
+
     # Save the start & end of each array. If the array doesn't exist (e.g.,
-    # we're only adding items in this block), then figure out the line
-    # number based on the line number of the other file and the current
-    # difference in file lengths.
+    # we're only adding items in this block), then figure out the line number
+    # based on the line number of the other file and the current difference in
+    # file lengths.
     if @blocks[0].remove.empty?
       a1 = a2 = nil
     else