From 84685c9bc3d8cfb32e6536132062eb29603966b9 Mon Sep 17 00:00:00 2001
From: paboyle <paboyle@ph.ed.ac.uk>
Date: Mon, 4 Jun 2018 13:42:07 +0100
Subject: [PATCH] Overflow fix

---
 lib/lattice/Lattice_rng.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/lib/lattice/Lattice_rng.h b/lib/lattice/Lattice_rng.h
index edf9dd23..654c5dd5 100644
--- a/lib/lattice/Lattice_rng.h
+++ b/lib/lattice/Lattice_rng.h
@@ -158,10 +158,19 @@ namespace Grid {
       // tens of seconds per trajectory so this is clean in all reasonable cases,
       // and margin of safety is orders of magnitude.
       // We could hack Sitmo to skip in the higher order words of state if necessary
+      //
+      // Replace with 2^30 ; avoid problem on large volumes
+      //
       /////////////////////////////////////////////////////////////////////////////////////
       //      uint64_t skip = site+1;  //   Old init Skipped then drew.  Checked compat with faster init
+      const int shift = 30;
+
       uint64_t skip = site;
-      skip = skip<<40;
+
+      skip = skip<<shift;
+
+      assert((skip >> shift)==site); // check for overflow
+
       eng.discard(skip);
       //      std::cout << " Engine  " <<site << " state " <<eng<<std::endl;
     }