from typing import Literal, TypedDict, List

Priority = Literal["low", "normal", "high"]

class Errand(TypedDict):
    errand_id: str
    priority: Priority
    day_part: str  # morning, afternoon, evening
    requires_human: bool


class ErrandDecision(TypedDict):
    errand_id: str
    scheduled_day_part: str
    reason: str


def schedule_errands(errands: List[Errand], max_per_part: int = 3) -> List[ErrandDecision]:
    """Very small heuristic scheduler that spreads errands across the day.

    - High priority errands are kept in their requested `day_part`.
    - Normal/low priority errands may be shifted to less busy parts of day.
    """
    decisions: List[ErrandDecision] = []

    def pick_slot(preferred: str) -> str:
        # Simple fallback order
        order_slots = [preferred, "morning", "afternoon", "evening"]
        for slot in order_slots:
            if len([d for d in decisions if d["scheduled_day_part"] == slot]) < max_per_part:
                return slot
        return preferred

    # Priority order
    order = {"high": 0, "normal": 1, "low": 2}
    all_errands = sorted(errands, key=lambda e: order.get(e["priority"], 1))

    for e in all_errands:
        preferred = e["day_part"]
        slot = pick_slot(preferred)
        reason = f"Scheduled in {slot}; priority={e['priority']}."
        if slot != preferred:
            reason += f" Shifted from preferred {preferred} to balance load."
        decisions.append(
            {
                "errand_id": e["errand_id"],
                "scheduled_day_part": slot,
                "reason": reason,
            }
        )

    return decisions


if __name__ == "__main__":
    sample: List[Errand] = [
        {"errand_id": "ER_GROCERY", "priority": "high", "day_part": "morning", "requires_human": True},
        {"errand_id": "ER_TRASH", "priority": "high", "day_part": "evening", "requires_human": True},
        {"errand_id": "ER_PET_FOOD", "priority": "high", "day_part": "afternoon", "requires_human": True},
        {"errand_id": "ER_DONATION", "priority": "low", "day_part": "morning", "requires_human": True},
    ]
    for d in schedule_errands(sample):
        print(d)