Return feedback instead of crashing on submissions with no notes

[peterbjohnson]

Closes #13

Problem

An empty note list raised IndexError, which reaches the student as a 500 rather than a feedback message.

compare_performance_ED({"notes": []}, reference)
# IndexError: list index out of range

This is reachable in production: a student submits nothing, uploads a silent or failed recording, or plays so quietly that transcription returns no notes at all.

Approach

Tests first, added to evaluation_test.py as section 11, alongside the existing tests for compare_performance_ED (section 7) and evaluation_function (section 8). Before the change:

9 failed, 4 passed

The four passes are the deliberate control: one- and two-note submissions already worked, and they are covered so they stay working. All three empty combinations crashed.

What was actually broken

Two distinct faults, the second only visible once the first was fixed.

1. Indexing element zero of an empty list. event_alignment_ED inspected response_events[0] to decide whether its input was already grouped into events. The edit-distance boundary conditions already handle a zero-length side correctly, so the check just needed to be skipped when the list is empty.

2. Float arrays from empty lists. With the first fault fixed, build_cost_matrix failed differently:

TypeError: ufunc 'bitwise_and' not supported for the input types

np.array([]) defaults to float64, so the chord flags came back as floats and the & mask raised. The dtypes are now given explicitly.

Reporting the degenerate cases

Structurally the pipeline now produces the right numbers: an empty response marks all four reference notes missing. But the standard wording would be misleading, so both degenerate cases get their own message.

Empty response:

Practice Summary
No notes were detected in your submission, so there was nothing to compare against the reference.

What to check
If you submitted a recording, check that it is not silent and that your instrument
can be heard clearly. If you submitted MIDI, check that it contains notes.

Have another go when you are ready.

Empty reference (a misconfigured question, not a student error):

Practice Summary
This question has no reference notes to compare your performance against, so it
could not be evaluated. Please let your teacher know.

is_correct is now also guarded, because an empty response against an empty reference satisfied every count and was being reported as correct.

Verification

Check	Result
New tests, before	9 failed, 4 passed
New tests, after	13 passed
Full suite	86 passed
CI lint gate (E9,F63,F7,F82)	0

🤖 Generated with Claude Code