"""Velocity-anomaly detection per ЖК (Issue #17).

Detects жилые комплексы whose recent sales velocity has dropped значительно
versus their own trailing history. Deterministic (pure statistics — z-score +
percent-change over the per-object monthly series). No ML, no external calls.

Data source
-----------
`domrf_kn_sale_graph` — monthly per-object sale series (type='apartments').
Columns used: obj_id, report_month, realised (monthly units sold — FLOW, not
cumulative — verified on prod 2026-06: values rise and fall month-to-month),
snapshot_date.

Object metadata (comm_name / dev_name / region_cd) is joined from
`domrf_kn_objects` (latest snapshot per obj_id).

Real-data gotchas (verified on prod)
-------------------------------------
1. **Snapshot coverage is uneven.** `MAX(snapshot_date)` is NOT safe: the two
   newest snapshots on prod (2026-06-03, 2026-04-27) are partial scrapes with
   only 2–7 objects. The richest snapshots carry ~440 objects. We therefore
   pick the snapshot with the MOST distinct objects (ties broken by recency)
   and run the whole analysis within that single consistent snapshot.
2. **All sale_graph objects are region 66** today, but we still filter by
   region via domrf_kn_objects.region_cd so the endpoint stays correct if
   other regions are scraped later.
3. **History is ~9 months** in the richest snapshot, so `lookback_months`
   defaults to 9 and we require ≥6 months (issue acceptance) before scoring.

Algorithm (per object, within the chosen snapshot)
--------------------------------------------------
- Order months descending; take the most recent 3 as the "recent" window and
  everything older (within lookback) as the "prior" window.
- recent_velocity = mean(realised) over recent 3 months.
- prior_velocity  = mean(realised) over prior months.
- prior_std       = sample stddev of prior months.
- z = (recent_velocity - prior_velocity) / prior_std   (negative ⇒ slowdown)
- drop_pct = (recent_velocity - prior_velocity) / prior_velocity * 100
- An object is an alert when z <= -min_zscore AND drop_pct <= -min_drop_pct
  (both gates: statistical significance AND material magnitude — keeps the
  false-positive rate down on noisy low-volume objects, per acceptance ≤10%).

Severity
--------
- 'high'   : z <= -3.0  OR drop_pct <= -60
- 'medium' : otherwise (still past the alert gates)
"""

from __future__ import annotations

import logging
from decimal import Decimal
from typing import Any

from sqlalchemy import text
from sqlalchemy.orm import Session

logger = logging.getLogger(__name__)

# Minimum months of history before an object is eligible (issue: ignore <6 mo).
_MIN_MONTHS_HISTORY = 6

# Recent window size (rolling 3-mo mean), per issue spec.
_RECENT_WINDOW = 3

# Minimum prior observations needed for a meaningful stddev / z-score.
_MIN_PRIOR_OBS = 3

# Default magnitude gate alongside the z-score gate. A pure z-score alert on a
# low-volume object (prior mean ~2/mo, stddev ~1) fires on tiny absolute moves;
# requiring a real percent drop suppresses those false positives.
_DEFAULT_MIN_DROP_PCT = 30.0

# Severity thresholds.
_HIGH_ZSCORE = 3.0
_HIGH_DROP_PCT = 60.0


def _f(value: Any) -> float | None:
    if value is None:
        return None
    if isinstance(value, Decimal):
        return float(value)
    return float(value)


def _best_snapshot(db: Session, *, sale_type: str = "apartments") -> Any | None:
    """Pick the sale_graph snapshot with the most distinct objects.

    NOT ``MAX(snapshot_date)`` — the freshest snapshots on prod are frequently
    partial scrapes (2–7 objects). The richest snapshot gives the broadest,
    most consistent cohort. Ties broken by recency.
    """
    return db.execute(
        text(
            """
            SELECT snapshot_date
              FROM domrf_kn_sale_graph
             WHERE type = :sale_type
             GROUP BY snapshot_date
             ORDER BY COUNT(DISTINCT obj_id) DESC, snapshot_date DESC
             LIMIT 1
            """
        ),
        {"sale_type": sale_type},
    ).scalar()


def detect_velocity_anomalies(
    db: Session,
    region_code: int = 66,
    lookback_months: int = 9,
    min_zscore: float = 2.0,
    min_drop_pct: float = _DEFAULT_MIN_DROP_PCT,
    limit: int = 100,
) -> dict[str, Any]:
    """Return ЖК whose recent velocity dropped significantly vs their history.

    Args:
        db: SQLAlchemy session (sync).
        region_code: filter via domrf_kn_objects.region_cd (66 = Свердловская обл.).
        lookback_months: months of history to consider (recent + prior windows).
        min_zscore: absolute z-score threshold; alert when z <= -min_zscore.
        min_drop_pct: absolute percent-drop threshold; alert when drop_pct <= -value.
        limit: max alerts returned (ordered by z-score, most severe first).

    Returns:
        ``{"snapshot_date": str|None, "region_code": int, "params": {...},
           "alerts": [...]}``. ``alerts`` is empty when there is no usable
        snapshot or no object clears both gates.
    """
    snapshot = _best_snapshot(db)
    if snapshot is None:
        logger.info("velocity_alerts: no sale_graph snapshot available")
        return {
            "snapshot_date": None,
            "region_code": region_code,
            "params": {
                "lookback_months": lookback_months,
                "min_zscore": min_zscore,
                "min_drop_pct": min_drop_pct,
            },
            "alerts": [],
        }

    # One pass: window the per-object monthly series, compute recent/prior
    # aggregates, score, then join names + filter by region. All bind params use
    # CAST(:x AS type) per repo convention (psycopg v3 ignores :x::type).
    rows = (
        db.execute(
            text(
                """
                WITH anchor AS (
                    -- Anchor the lookback to the LATEST report_month present in
                    -- this snapshot, NOT snapshot_date. On prod the scrape date
                    -- (snapshot_date) can lag the newest data month by months,
                    -- so anchoring to snapshot_date would silently truncate the
                    -- prior window and starve the z-score (prior_n < min).
                    SELECT MAX(report_month) AS max_month
                      FROM domrf_kn_sale_graph
                     WHERE type = 'apartments'
                       AND snapshot_date = :snap
                ),
                series AS (
                    SELECT g.obj_id, g.report_month, g.realised
                      FROM domrf_kn_sale_graph g
                     CROSS JOIN anchor a
                     WHERE g.type = 'apartments'
                       AND g.snapshot_date = :snap
                       AND g.report_month > (
                               a.max_month - CAST(:lookback_interval AS interval))
                ),
                ranked AS (
                    SELECT
                        obj_id,
                        realised,
                        ROW_NUMBER() OVER (
                            PARTITION BY obj_id ORDER BY report_month DESC) AS rn,
                        COUNT(*) OVER (PARTITION BY obj_id) AS n_months
                      FROM series
                ),
                windows AS (
                    SELECT
                        obj_id,
                        n_months,
                        AVG(realised) FILTER (WHERE rn <= :recent_window) AS recent_mean,
                        AVG(realised) FILTER (WHERE rn > :recent_window)  AS prior_mean,
                        STDDEV_SAMP(realised) FILTER (WHERE rn > :recent_window) AS prior_std,
                        COUNT(realised) FILTER (WHERE rn > :recent_window) AS prior_n
                      FROM ranked
                     GROUP BY obj_id, n_months
                ),
                scored AS (
                    SELECT
                        w.obj_id,
                        w.recent_mean,
                        w.prior_mean,
                        w.prior_std,
                        CASE WHEN w.prior_std > 0
                             THEN (w.recent_mean - w.prior_mean) / w.prior_std
                        END AS zscore,
                        CASE WHEN w.prior_mean > 0
                             THEN (w.recent_mean - w.prior_mean) / w.prior_mean * 100.0
                        END AS drop_pct
                      FROM windows w
                     WHERE w.n_months >= :min_months
                       AND w.prior_n >= :min_prior_obs
                       AND w.prior_mean > 0
                       AND w.prior_std > 0
                )
                SELECT
                    s.obj_id,
                    o.comm_name,
                    o.dev_name,
                    s.recent_mean,
                    s.prior_mean,
                    s.zscore,
                    s.drop_pct
                  FROM scored s
                  JOIN LATERAL (
                        SELECT comm_name, dev_name, region_cd
                          FROM domrf_kn_objects
                         WHERE obj_id = s.obj_id
                         ORDER BY snapshot_date DESC
                         LIMIT 1
                  ) o ON TRUE
                 WHERE o.region_cd = :region_code
                   AND s.zscore <= :neg_zscore
                   AND s.drop_pct <= :neg_drop_pct
                 ORDER BY s.zscore ASC
                 LIMIT :limit
                """
            ),
            {
                "snap": snapshot,
                "lookback_interval": f"{lookback_months} months",
                "recent_window": _RECENT_WINDOW,
                "min_months": _MIN_MONTHS_HISTORY,
                "min_prior_obs": _MIN_PRIOR_OBS,
                "region_code": region_code,
                "neg_zscore": -abs(min_zscore),
                "neg_drop_pct": -abs(min_drop_pct),
                "limit": limit,
            },
        )
        .mappings()
        .all()
    )

    alerts: list[dict[str, Any]] = []
    for r in rows:
        z = _f(r["zscore"])
        drop = _f(r["drop_pct"])
        severity = (
            "high"
            if (z is not None and z <= -_HIGH_ZSCORE)
            or (drop is not None and drop <= -_HIGH_DROP_PCT)
            else "medium"
        )
        alerts.append(
            {
                "obj_id": int(r["obj_id"]),
                "comm_name": r["comm_name"],
                "dev_name": r["dev_name"],
                "recent_velocity_pm": round(_f(r["recent_mean"]) or 0.0, 2),
                "prior_velocity_pm": round(_f(r["prior_mean"]) or 0.0, 2),
                "zscore": round(z, 2) if z is not None else None,
                "drop_pct": round(drop, 1) if drop is not None else None,
                "severity": severity,
            }
        )

    logger.info(
        "velocity_alerts: snapshot=%s region=%d lookback=%dmo z>=%.1f drop>=%.0f%% -> %d alerts",
        snapshot,
        region_code,
        lookback_months,
        min_zscore,
        min_drop_pct,
        len(alerts),
    )

    return {
        "snapshot_date": snapshot.isoformat() if snapshot else None,
        "region_code": region_code,
        "params": {
            "lookback_months": lookback_months,
            "min_zscore": min_zscore,
            "min_drop_pct": min_drop_pct,
        },
        "alerts": alerts,
    }