"""
Fill simulation: partial fills, cancel latency, adverse selection.

Models realistic fill behavior for maker orders:
- Partial fills (not all-or-nothing)
- Cancel latency (cancel arrives after fill)
- Adverse selection (getting filled right before adverse price move)
"""

from __future__ import annotations

import math
import random
from dataclasses import dataclass, field
from typing import Optional


@dataclass
class FillEvent:
    """A fill (or partial fill) of a maker order."""

    order_id: str
    side: str
    price: float
    size: float
    fee: float
    pnl_immediate: float  # PnL if closed instantly at mid
    is_toxic: bool         # was this fill followed by adverse price move?
    timestamp: float
    aggressor_side: str = ""


@dataclass
class CancelEvent:
    order_id: str
    requested_time: float
    executed_time: float
    filled_before_cancel: float
    latency_ms: float


@dataclass
class FillModelConfig:
    """Parameters for fill simulation."""

    partial_fill_prob: float = 0.3      # probability of partial (not full) fill
    min_fill_ratio: float = 0.25         # min fraction of order filled
    cancel_latency_ms: float = 50.0     # typical cancel RTT
    cancel_latency_std_ms: float = 20.0
    adverse_selection_prob: float = 0.15 # prob a fill is adverse
    adverse_move_bps: float = 3.0       # bps adverse move after toxic fill
    queue_priority_decay: float = 0.02  # per-event reduction in fill prob if not front


class FillSimulator:
    """Simulates fill behavior for maker orders in the queue model."""

    def __init__(self, config: FillModelConfig | None = None, seed: int | None = None):
        self._cfg = config or FillModelConfig()
        self._rng = random.Random(seed)
        self._events: list[FillEvent | CancelEvent] = []

    def simulate_fill(
        self,
        order_id: str,
        side: str,
        price: float,
        size: float,
        queue_position: int,
        mid_price: float,
        aggressor_size: float,
        timestamp: float,
        fee_rate: float = 0.0002,
    ) -> Optional[FillEvent]:
        """Simulate whether this trade event fills our order.

        Returns FillEvent if filled, None if our order survives.
        """
        if queue_position > 0:
            # Not at front: low probability of fill from agg size
            prob = min(aggressor_size / (size * 2), 0.1)
            if self._rng.random() > prob:
                return None

        # At front or lucky: may get partial fill
        is_partial = self._rng.random() < self._cfg.partial_fill_prob
        fill_ratio = self._rng.uniform(self._cfg.min_fill_ratio, 1.0) if is_partial else 1.0
        fill_size = round(size * fill_ratio, 8)

        # Fee
        fee = fill_size * price * fee_rate

        # Immediate PnL estimate
        aggressive = "buy" if side == "ask" else "sell"
        if side == "bid":
            pnl_immediate = fill_size * (mid_price - price) - fee
        else:
            pnl_immediate = fill_size * (price - mid_price) - fee

        # Adverse selection check
        is_toxic = self._rng.random() < self._cfg.adverse_selection_prob

        event = FillEvent(
            order_id=order_id,
            side=side,
            price=price,
            size=fill_size,
            fee=round(fee, 6),
            pnl_immediate=round(pnl_immediate, 6),
            is_toxic=is_toxic,
            timestamp=timestamp,
            aggressor_side=aggressive,
        )
        self._events.append(event)
        return event

    def simulate_cancel(
        self,
        order_id: str,
        timestamp: float,
    ) -> CancelEvent:
        """Simulate cancel with random latency."""
        lat = max(1.0, self._rng.gauss(self._cfg.cancel_latency_ms, self._cfg.cancel_latency_std_ms))
        event = CancelEvent(
            order_id=order_id,
            requested_time=timestamp,
            executed_time=timestamp + lat / 1000.0,
            filled_before_cancel=0.0,
            latency_ms=round(lat, 2),
        )
        self._events.append(event)
        return event

    @property
    def events(self) -> list:
        return self._events

    def fills(self) -> list[FillEvent]:
        return [e for e in self._events if isinstance(e, FillEvent)]

    def cancels(self) -> list[CancelEvent]:
        return [e for e in self._events if isinstance(e, CancelEvent)]


# ── Adverse selection estimator ────────────────────────────

def adverse_selection_intensity(
    fills: list[FillEvent],
    future_mids: list[float],
    horizon_events: int = 5,
) -> dict:
    """Compute how often fills are followed by adverse price moves.

    A fill is adverse if mid price moves against the maker within
    horizon_events subsequent trades.
    """
    if not fills or len(future_mids) < horizon_events:
        return {"adverse_rate": 0, "mean_cost_bps": 0, "n_fills": len(fills)}

    adverse_count = 0
    adverse_costs = []
    n = len(future_mids)

    for i, fill in enumerate(fills):
        future_idx = min(i + horizon_events, n - 1)
        mid_now = future_mids[i] if i < n else 0
        mid_future = future_mids[future_idx]
        if mid_now <= 0 or mid_future <= 0:
            continue

        move_bps = (mid_future - mid_now) / mid_now * 10000

        if (fill.side == "bid" and move_bps < 0) or (fill.side == "ask" and move_bps > 0):
            adverse_count += 1
            adverse_costs.append(abs(move_bps))

    return {
        "adverse_rate": round(adverse_count / len(fills), 4) if fills else 0,
        "mean_cost_bps": round(sum(adverse_costs) / max(len(adverse_costs), 1), 2),
        "total_cost_bps": round(sum(adverse_costs), 2),
        "n_fills": len(fills),
    }