First few external tests with test/unit, some minor tidying of internal data

structures.

tests/test_file_writer.rb

@@ -0,0 +1,83 @@
+# Noddy test class for writing files to disc in predictable patterns
+# in order to test FlexNBD.
+#
+class TestFileWriter
+  def initialize(filename, blocksize)
+    @fh = File.open(filename, "w+")
+    @blocksize = blocksize
+    @pattern = ""
+  end
+  
+  # We write in fixed block sizes, given by "blocksize" 
+  #    _ means skip a block
+  #    0 means write a block full of zeroes
+  #    f means write a block with the file offset packed every 4 bytes
+  #    
+  def write(data)
+    @pattern += data
+    
+    data.split("").each do |code|
+      if code == "_"
+        @fh.seek(@blocksize, IO::SEEK_CUR)
+      else
+        @fh.write(data(code))
+      end
+    end
+    @fh.flush
+    self
+  end
+  
+  # Returns what the data ought to be at the given offset and length
+  #
+  def read_original(off, len)
+    r=""
+    current = 0
+    @pattern.split("").each do |block|
+      if off >= current && (off+len) < current + blocksize
+        current += data(block, current)[
+          current-off..(current+blocksize)-(off+len)
+        ]
+      end
+      current += @blocksize
+    end
+    r
+  end
+  
+  # Read what's actually in the file
+  #
+  def read(off, len)
+    @fh.seek(off, IO::SEEK_SET)
+    @fh.read(len)
+  end
+  
+  def untouched?(offset, len)
+    read(off, len) == read_original(off, len)
+  end
+  
+  def close
+    @fh.close
+    nil
+  end
+  
+  protected
+  
+  def data(code, at=@fh.tell)
+    case code
+      when "0", "_"
+        "\0" * @blocksize
+      when "X"
+        "X" * @blocksize
+      when "f"
+        r = ""
+        (@blocksize/4).times do
+          r += [at].pack("I")
+          at += 4
+        end
+        r
+      else
+        raise "Unknown character '#{block}'"
+    end    
+  end
+  
+end
+