core: Import bup's "rollsum" code, add a test case

For static deltas, one strategy that will be employed is to split each
object into chunks, and only include changed chunks in the deltas.

tests/test-rollsum.c

@@ -0,0 +1,79 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2013 Colin Walters <walters@verbum.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#include "config.h"
+
+#include "libgsystem.h"
+
+#include "bupsplit.h"
+
+#define BLOB_MAX (8192*4)
+#define BLOB_READ_SIZE (1024*1024)
+
+int
+main (int argc, char **argv)
+{
+  GCancellable *cancellable = NULL;
+  GError *local_error = NULL;
+  GError **error = &local_error;
+  gs_unref_object GFile *path = NULL;
+  GBytes *bytes = NULL;
+
+  g_setenv ("GIO_USE_VFS", "local", TRUE);
+
+  if (argc > 1)
+    {
+      const guint8 *start;
+      gsize len;
+
+      path = g_file_new_for_path (argv[1]);
+      bytes = gs_file_map_readonly (path, cancellable, error);
+      if (!bytes)
+	goto out;
+
+      start = g_bytes_get_data (bytes, &len);
+      while (TRUE)
+	{
+	  int offset, bits;
+	  offset = bupsplit_find_ofs (start, MIN(G_MAXINT32, len), &bits); 
+	  if (offset == 0)
+	    break;
+	  if (offset > BLOB_MAX)
+	    offset = BLOB_MAX;
+          g_print ("%" G_GUINT64_FORMAT "\n", (guint64)offset);
+	  start += offset;
+          len -= offset;
+        }
+    }
+  else
+    {
+      bupsplit_selftest ();
+    }
+
+ out:
+  g_clear_pointer (&bytes, g_bytes_unref);
+  if (local_error)
+    {
+      g_printerr ("%s\n", local_error->message);
+      g_error_free (local_error);
+      return 1;
+    }
+  return 0;
+}