"""
Verifies that no instructor is booked into two sessions at the same
day+slot (whether both are lectures, both labs, or a mix of either).

Like student_conflict_checker, this re-derives occupancy directly from the
session list rather than trusting the scheduler's internal busy-maps.
"""

from collections import defaultdict


def check_teacher_conflicts(sessions):
    """
    Returns a list of conflict dicts, one per (instructor_code, day, slot)
    where two or more sessions overlap for that instructor:
        {
            "instructor_code": str,
            "day": str,
            "slot_index": int,
            "sessions": [session, session, ...],
        }
    """
    # (instructor_code, day, slot_index) -> list of sessions occupying that cell
    occupied = defaultdict(list)

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

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

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


def format_teacher_conflicts(conflicts):
    """Human-readable lines describing each teacher conflict."""
    lines = []
    for c in conflicts:
        course_list = ", ".join(
            f'{s["course_id"]} ({s["course_type"]}) for batch(es) {"+".join(s["batch_codes"])} in {s["room_id"]}'
            for s in c["sessions"]
        )
        instructor_name = c["sessions"][0]["instructor_name"]
        lines.append(
            f'Instructor {instructor_name} ({c["instructor_code"]}) is double-booked on '
            f'{c["day"]} slot {c["slot_index"]}: {course_list}'
        )
    return lines