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Added support for NAVGEM files which include a text header #115

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Jan 5, 2024
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Added support for NAVGEM files which include a text header #115

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17 changes: 16 additions & 1 deletion grib2io/_grib2io.py
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Original file line number Diff line number Diff line change
Expand Up @@ -197,7 +197,22 @@ def _build_index(self, no_data=False):
try:
# Read first 4 bytes and decode...looking for "GRIB"
pos = self._filehandle.tell()
header = struct.unpack('>i',self._filehandle.read(4))[0]

# Ignore headers (usually text) that are not part of the GRIB2
# file. For example, NAVGEM files have a http header at the
# beginning that needs to be ignored.

# Read a byte at a time until "GRIB" is found. Using
# "wgrib2" on a NAVGEM file, the header was 421 bytes and
# decided to go to 2048 bytes to be safe. For normal GRIB2
# files this should be quick and break out of the first
# loop when "test_offset" is 0.
for test_offset in range(2048):
self._filehandle.seek(pos + test_offset)
header = struct.unpack(">i", self._filehandle.read(4))[0]
if header.to_bytes(4, "big") == b"GRIB":
pos = pos + test_offset
break

# Test header. Then get information from GRIB2 Section 0: the discipline
# number, edition number (should always be 2), and GRIB2 message size.
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