Fix events.tsv onset off by one in bids_preproc

[arnodelorme]

🤖

Problem

bids_preproc rewrote the derivative events.tsv with ev_time = EEG['times'][e['latency']] / 1000.0. Event latencies are 1-based samples (latency 1.0 = first sample, EEGLAB convention) while EEG['times'] is a 0-based array, so:

• Every onset was one sample late: latency 1 mapped to times[1] = 7.8125 ms at 128 Hz instead of 0.
• An event at the last sample (latency == pnts) raised IndexError and was dropped.
• Any fractional latency (e.g. 129.00875 in sample_data/eeglab_data.set, or anything after resampling) raised IndexError and was dropped. On eeglab_data.set the old code kept 0 of 154 events.

The dropped events also masked a second failure: pop_loadset yields duration=None for empty MATLAB fields, so e.get('duration', 0.0) / srate would have raised TypeError once an event got past the onset lookup.

Fix

• Onset is now (latency - 1) / srate, the same conversion already used by plugins/EEG_BIDS/pop_exportbids.py. No times[...] lookup, so no IndexError and no dropped events. Duration stays duration / srate (EEGLAB stores duration in samples); None is treated as 0.
• The writer is extracted into a top-level _write_events_tsv(EEG, fpath) so it can be tested on real data without running the pipeline. grep -rn "times\[" src/eegprep/plugins/EEG_BIDS/ shows this was the only such lookup under EEG_BIDS.

Epoched data

bids_preproc writes events.tsv after the optional pop_epoch stage, so epoched datasets are exported too. pop_epoch produces 1-based latencies concatenated across epochs (... + 1 + pnts * epoch_index), so (latency - 1) / srate gives the onset relative to the start of the concatenated epoched array, consistent with pop_exportbids. No behaviour beyond that is introduced; the helper docstring states this.

Verification

EEGPREP_SKIP_MATLAB=1 python -m pytest tests/test_bids_preproc.py tests/test_bids_preproc_old.py \
    tests/test_bids_gen_derived_fpath.py tests/test_bids_load_frombids_helpers.py -q
# 8 passed, 2 skipped (MATLAB end-to-end), 3 subtests passed
ruff check / ruff format --check on the two changed files: clean

New TestWriteEventsTsv (2 tests, real sample_data/eeglab_data.set, no torch/MATLAB): all 154 events written with onset (latency-1)/srate (fractional latencies included, duration 0, type preserved), and latency 1 / latency pnts map to 0.0 s and (pnts-1)/srate.

[claude]

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[claude]

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