MNT: Ensure float dtype conversion to avoid int32 overflow in met con…

…version

imap_processing/cdf/utils.py

@@ -53,7 +53,7 @@ def met_to_j2000ns(
     #       float input and we want to keep ns precision in those floats
     # NOTE: We need int64 here when running on 32bit systems as plain int will default
     #       to 32bit and overflow due to the nanosecond multiplication
-    time_array = (np.asarray(met) * 1e9).astype(np.int64)
+    time_array = (np.asarray(met, dtype=float) * 1e9).astype(np.int64)
     # Calculate the time difference between our reference system and J2000
     j2000_offset = (
         (reference_epoch - J2000_EPOCH).astype("timedelta64[ns]").astype(np.int64)

imap_processing/tests/cdf/test_utils.py

@@ -53,6 +53,9 @@ def test_met_to_j2000ns():
     imap_epoch_offset = (IMAP_EPOCH - J2000_EPOCH).astype(np.int64)
     assert met_to_j2000ns(0) == imap_epoch_offset
     assert met_to_j2000ns(1) == imap_epoch_offset + 1e9
+    # Large input should work (avoid overflow with int32 SHCOARSE inputs)
+    assert met_to_j2000ns(np.int32(2**30)) == imap_epoch_offset + 2**30 * 1e9
+    assert met_to_j2000ns(0).dtype == np.int64
     # Float input should work
     assert met_to_j2000ns(0.0) == imap_epoch_offset
     assert met_to_j2000ns(1.2) == imap_epoch_offset + 1.2e9