Add an interface for bspatch reader

Add a wrapper class to separate the patch read from data stream
decompression. Therefore, bspatch will be able to process the patch
that is compressed with various tools.

Test: unittest pass
Change-Id: I5214e0451bde80366e8a70b960703afb2b2a7d97
diff --git a/utils.cc b/utils.cc
new file mode 100644
index 0000000..63941f5
--- /dev/null
+++ b/utils.cc
@@ -0,0 +1,20 @@
+#include <stdint.h>
+
+#include "bsdiff/utils.h"
+
+namespace bsdiff {
+
+
+int64_t ParseInt64(const uint8_t* buf) {
+  int64_t result = buf[7] & 0x7F;
+  for (int i = 6; i >= 0; i--) {
+    result <<= 8;
+    result |= buf[i];
+  }
+
+  if (buf[7] & 0x80)
+    result = -result;
+  return result;
+}
+
+}  // namespace bsdiff
\ No newline at end of file