"""
Verifies that no batch (and therefore no student in that batch) is booked
into two courses — lecture or lab — at the same day+slot.

This is an independent verification pass over the scheduler's output: it
does not trust the scheduler's own busy-maps, it re-derives every batch's
occupied (day, slot_index) cells directly from the session list and flags
any cell used by more than one session.
"""

from collections import defaultdict


def check_student_conflicts(sessions):
    """
    Returns a list of conflict dicts, one per (batch, day, slot) where two
    or more sessions overlap for that batch:
        {
            "batch_code": str,
            "day": str,
            "slot_index": int,
            "sessions": [session, session, ...],
        }
    Sessions that span multiple slot_indices (labs) occupy every one of
    their slot_indices, so a lab overlapping a lecture in either of its two
    slots is caught too.
    """
    # (batch_code, day, slot_index) -> list of sessions occupying that cell
    occupied = defaultdict(list)

    for session in sessions:
        day = session["day"]
        for slot_index in session["slot_indices"]:
            for batch_code in session["batch_codes"]:
                occupied[(batch_code, day, slot_index)].append(session)

    conflicts = []
    for (batch_code, day, slot_index), cell_sessions in occupied.items():
        if len(cell_sessions) > 1:
            conflicts.append({
                "batch_code": batch_code,
                "day": day,
                "slot_index": slot_index,
                "sessions": cell_sessions,
            })

    conflicts.sort(key=lambda c: (c["batch_code"], c["day"], c["slot_index"]))
    return conflicts


def format_student_conflicts(conflicts):
    """Human-readable lines describing each student/batch conflict."""
    lines = []
    for c in conflicts:
        course_list = ", ".join(
            f'{s["course_id"]} ({s["course_type"]}) with {s["instructor_name"]} in {s["room_id"]}'
            for s in c["sessions"]
        )
        lines.append(
            f'Batch {c["batch_code"]} has {len(c["sessions"])} overlapping courses on '
            f'{c["day"]} slot {c["slot_index"]}: {course_list}'
        )
    return lines