Add GRIB2 workaround helper for int32->uint32.

lib/iris/fileformats/grib/_save_rules.py

@@ -38,6 +38,26 @@
 from iris.fileformats.grib import grib_phenom_translation as gptx
 
 
+def fixup_int32_as_uint32(value):
+    """
+    Workaround for use when the ECMWF GRIB API treats a signed, 4-byte
+    integer value as an unsigned, 4-byte integer.
+
+    Returns the unsigned integer value which will result in the on-disk
+    representation corresponding to the signed, 4-byte integer value.
+
+    """
+    value = int(value)
+    if -0x7fffffff <= value <= 0x7fffffff:
+        if value < 0:
+            # Convert from two's-complement to sign-and-magnitude.
+            value = 0x80000000 - value
+    else:
+        msg = '{} out of range -2147483647 to 2147483647.'.format(value)
+        raise ValueError(msg)
+    return value
+
+
 ###############################################################################
 #
 # Constants

lib/iris/tests/unit/fileformats/grib/save_rules/test_fixup_int32_as_uint32.py

@@ -0,0 +1,54 @@
+# (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._save_rules.fixup_int32_as_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._save_rules import fixup_int32_as_uint32
+
+
+class Test(tests.IrisTest):
+    def test_very_negative(self):
+        with self.assertRaises(ValueError):
+            fixup_int32_as_uint32(-0x80000000)
+
+    def test_negative(self):
+        result = fixup_int32_as_uint32(-3)
+        self.assertEqual(result, 0x80000003)
+
+    def test_zero(self):
+        result = fixup_int32_as_uint32(0)
+        self.assertEqual(result, 0)
+
+    def test_positive(self):
+        result = fixup_int32_as_uint32(5)
+        self.assertEqual(result, 5)
+
+    def test_very_positive(self):
+        with self.assertRaises(ValueError):
+            fixup_int32_as_uint32(0x80000000)
+
+
+if __name__ == '__main__':
+    tests.main()