diff --git a/finders.c b/finders.c
index 4d8af8c..4226c49 100644
--- a/finders.c
+++ b/finders.c
@@ -1192,8 +1192,8 @@ int isViableStructurePos(int structureType, Generator *g, int x, int z, uint32_t
             if (!getStructureConfig(Fortress, g->mc, &sc))
                 return 0;
             Pos rp = {
-                (int)chunkX / sc.regionSize - (x < 0),
-                (int)chunkZ / sc.regionSize - (z < 0)
+                floordiv(x, sc.regionSize << 4),
+                floordiv(z, sc.regionSize << 4)
             };
             if (!getStructurePos(Bastion, g->mc, g->seed, rp.x, rp.z, &rp))
                 return 1;
@@ -1377,10 +1377,10 @@ int isViableStructurePos(int structureType, Generator *g, int x, int z, uint32_t
             goto L_not_viable;
         int cx0 = (chunkX-10), cx1 = (chunkX+10);
         int cz0 = (chunkZ-10), cz1 = (chunkZ+10);
-        int rx0 = cx0 / vilconf.regionSize - (cx0 < 0);
-        int rx1 = cx1 / vilconf.regionSize - (cx1 < 0);
-        int rz0 = cz0 / vilconf.regionSize - (cz0 < 0);
-        int rz1 = cz1 / vilconf.regionSize - (cz1 < 0);
+        int rx0 = floordiv(cx0, vilconf.regionSize);
+        int rx1 = floordiv(cx1, vilconf.regionSize);
+        int rz0 = floordiv(cz0, vilconf.regionSize);
+        int rz1 = floordiv(cz1, vilconf.regionSize);
         int rx, rz;
         for (rz = rz0; rz <= rz1; rz++)
         {
diff --git a/rng.h b/rng.h
index 055ba47..b32d4bd 100644
--- a/rng.h
+++ b/rng.h
@@ -70,6 +70,14 @@ uint32_t rotr32(uint32_t a, uint8_t b)
     return (a >> b) | (a << (32-b));
 }
 
+/// integer floor divide
+static inline ATTR(const, always_inline)
+int32_t floordiv(int32_t a, int32_t b)
+{
+    int32_t q = a / b;
+    int32_t r = a % b;
+    return q - ((a ^ b) < 0 && !!r);
+}
 
 ///=============================================================================
 ///                    C implementation of Java Random