BF - high value comparisons hit float error

Found using the round trip tester - high values for min or max were
hitting floating point error in comparisons, and comparing equal when
they were not.

Author: matthew-brett

nibabel/arraywriters.py

@@ -304,7 +304,11 @@ def _do_scaling(self):
         # (u)int to (u)int
         info = np.iinfo(out_dtype)
         out_max, out_min = info.max, info.min
-        if mx <= out_max and mn >= out_min: # already in range
+        # If left as int64, uint64, comparisons will default to floats, and
+        # these are inexact for > 2**53 - so convert to int
+        if (as_int(mx) <= as_int(out_max) and
+            as_int(mn) >= as_int(out_min)):
+            # already in range
             return
         # (u)int to (u)int scaling
         self._iu2iu()

nibabel/tests/test_arraywriters.py

@@ -194,6 +194,16 @@ def test_special_rt():
                     assert_array_equal(round_trip(aw), 0)
 
 
+def test_high_int2uint():
+    # Need to take care of high values when testing whether values are already
+    # in range.  There was a bug here were the comparison was in floating point,
+    # and therefore not exact, and 2**63 appeared to be in range for np.int64
+    arr = np.array([2**63], dtype=np.uint64)
+    out_type = np.int64
+    aw = SlopeInterArrayWriter(arr, out_type)
+    assert_equal(aw.inter, 2**64)
+
+
 def test_slope_inter_castable():
     # Test scaling for arraywriter instances
     # Test special case of all zeros