Workaround for ECMWF x1, y1, x2, y2 unsigned bug.

Author: rhattersley

lib/iris/fileformats/grib/_load_convert.py

@@ -233,6 +233,21 @@ def fixup_float32_from_int32(value):
     return float(value_as_float32)
 
 
+def fixup_int32_from_uint32(value):
+    """
+    Workaround for use when reading a signed, 4-byte integer which the
+    ECMWF GRIB API has erroneously treated as an unsigned, 4-byte
+    integer.
+
+    NB. This workaround is safe to use with values which are already
+    treated as signed, 4-byte integers.
+
+    """
+    if value >= 0x80000000:
+        value = 0x80000000 - value
+    return value
+
+
 ###############################################################################
 #
 # Identification Section 1
@@ -694,17 +709,24 @@ def grid_definition_template_12(section, metadata):
     cs = icoord_systems.TransverseMercator(lat, lon, easting, northing,
                                            scale, geog_cs)
 
+    # Deal with bug in ECMWF GRIB API (present at 1.12.1) where these
+    # values are treated as unsigned, 4-byte integers.
+    x1 = fixup_int32_from_uint32(section['x1'])
+    y1 = fixup_int32_from_uint32(section['y1'])
+    x2 = fixup_int32_from_uint32(section['x2'])
+    y2 = fixup_int32_from_uint32(section['y2'])
+
     # Rather unhelpfully this grid definition template seems to be
     # overspecified, and thus open to inconsistency.
-    last_x = section['x1'] + (section['Ni'] - 1) * section['Di']
-    last_y = section['y1'] + (section['Nj'] - 1) * section['Dj']
-    if (last_x != section['x2'] or last_y != section['y2']):
+    last_x = x1 + (section['Ni'] - 1) * section['Di']
+    last_y = y1 + (section['Nj'] - 1) * section['Dj']
+    if (last_x != x2 or last_y != y2):
         raise TranslationError('Inconsistent grid definition')
 
-    x1 = section['x1'] * CM_TO_M
+    x1 = x1 * CM_TO_M
     dx = section['Di'] * CM_TO_M
     x_points = x1 + np.arange(section['Ni']) * dx
-    y1 = section['y1'] * CM_TO_M
+    y1 = y1 * CM_TO_M
     dy = section['Dj'] * CM_TO_M
     y_points = y1 + np.arange(section['Nj']) * dy
 

lib/iris/tests/unit/fileformats/grib/load_convert/test_fixup_int32_from_uint32.py

@@ -0,0 +1,56 @@
+# (C) British Crown Copyright 2014, Met Office
+#
+# This file is part of Iris.
+#
+# Iris 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 3 of the License, or
+# (at your option) any later version.
+#
+# Iris 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 Iris.  If not, see <http://www.gnu.org/licenses/>.
+"""
+Unit tests for `iris.fileformats.grib._load_convert.fixup_int32_from_uint32`.
+
+"""
+
+from __future__ import (absolute_import, division, print_function)
+
+# Import iris.tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests
+
+from iris.fileformats.grib._load_convert import fixup_int32_from_uint32
+
+
+class Test(tests.IrisTest):
+    def test_negative(self):
+        result = fixup_int32_from_uint32(0x80000005)
+        self.assertEqual(result, -5)
+
+    def test_negative_zero(self):
+        result = fixup_int32_from_uint32(0x80000000)
+        self.assertEqual(result, 0)
+
+    def test_zero(self):
+        result = fixup_int32_from_uint32(0)
+        self.assertEqual(result, 0)
+
+    def test_positive(self):
+        result = fixup_int32_from_uint32(200000)
+        self.assertEqual(result, 200000)
+
+    def test_already_negative(self):
+        # If we *already* have a negative value the fixup routine should
+        # leave it alone.
+        result = fixup_int32_from_uint32(-7)
+        self.assertEqual(result, -7)
+
+
+if __name__ == '__main__':
+    tests.main()