"""Fetch historical + recent daily weather from Open-Meteo and cache to parquet.

The full multi-decade daily record for a grid cell is fetched once and stored as
a zstd-compressed parquet file keyed by cell id. Percentiles are computed on the
fly from this raw record (see grading.py), which keeps the cache small and lets
us handle ties (e.g. many zero-precip days) correctly.
"""
import datetime
import os
import threading
import time

import httpx
import numpy as np
import polars as pl

from core import audit
from core import metrics
import paths
from data import store

CACHE_DIR = os.path.join(paths.DATA_DIR, "cache")
MAX_ATTEMPTS = 3                   # per upstream call, before giving up
START_DATE = "1980-01-01"          # ERA5 reaches back to 1940; 1980 = 45 yrs, fast + robust
ARCHIVE_LATENCY_DAYS = 6           # ERA5 archive lags real time by a few days
# A real archive spans decades (~16k daily rows from START_DATE). A self-hosted
# Open-Meteo instance that isn't backfilled yet answers with all-null values or only
# its recent sync window, which _finalize_frame reduces to a near-empty frame. Below
# this many days we treat the archive as "not ready" — fall through to the NASA backup
# and do NOT cache the short result as a complete, indefinitely-held record.
MIN_ARCHIVE_DAYS = 3650            # ~10 yrs: far above any sync window, far below a full archive
# History rarely changes, so the full multi-decade archive is fetched once and
# cached indefinitely (refetched only to add new metric columns). Only the recent
# tail is refreshed — a small incremental fetch, at most hourly.
HISTORY_TOPUP_INTERVAL = 3600      # seconds between recent-tail refresh attempts
FORECAST_TTL_HOURS = 1             # refetch the forward forecast hourly to track updates

# The archive (historical) endpoint. Overridable so it can point at a self-hosted
# Open-Meteo instance (ERA5 in object storage) instead of the rate-limited public
# API; the response shape is identical either way.
ARCHIVE_URL = os.environ.get("THERMOGRAPH_ARCHIVE_URL",
                             "https://archive-api.open-meteo.com/v1/archive")
# The historical model: the ERA5 "seamless" blend — 0.1° ERA5-Land for
# temperature/precip/humidity/wind, 0.25° ERA5 for wind gusts (which ERA5-Land
# lacks). This is the public API's default; sent explicitly so a self-hosted
# instance serves the same 0.1° resolution. Archive requests only, not forecast.
ARCHIVE_MODEL = "era5_seamless"
FORECAST_URL = "https://api.open-meteo.com/v1/forecast"
# Backup archive when Open-Meteo is unavailable (e.g. its daily rate limit). NASA
# POWER is free + keyless, global, daily from 1981. It lacks gusts + apparent temp,
# so gusts read as unavailable and "feels like" is computed from heat index/chill.
NASA_POWER_URL = "https://power.larc.nasa.gov/api/temporal/daily/point"
NASA_START = "19810101"
NASA_FILL = -900.0   # POWER's missing-value sentinel is ~-999

# Backup forward forecast when Open-Meteo's forecast API is unavailable. MET Norway
# (yr.no) is free + keyless + global, mirroring NASA POWER's role for history. It
# returns a sub-daily timeseries in metric units with no gusts or apparent temp, so
# we aggregate to daily, convert units, and derive feels-like like the NASA path.
# It is forecast-only — no recent past days — so it's a degraded-but-working fallback.
METNO_URL = "https://api.met.no/weatherapi/locationforecast/2.0/complete"
# MET Norway's ToS requires an identifying User-Agent (a missing/generic one is
# 403'd); include the app and a contact URL so they can reach us if usage misbehaves.
METNO_UA = "Thermograph/0.2 (+https://thermograph.org)"

DAILY_VARS = (
    "temperature_2m_max,temperature_2m_min,precipitation_sum,"
    "wind_speed_10m_max,wind_gusts_10m_max,"
    "apparent_temperature_max,apparent_temperature_min,"
    "relative_humidity_2m_mean"
)

# Columns added after the original tmax/tmin/precip schema. A cached parquet
# missing any of these predates the wind/feels/humidity features (or the split
# apparent high/low) and is refetched so the new metrics show up immediately
# instead of after the 30-day cache TTL.
NEW_COLS = ("wind", "gust", "feels", "humid", "fmax", "fmin")

# Thermoneutral baseline (°F). The combined "feels like" metric reports whichever
# apparent-temperature extreme — the heat-index-driven daily max or the
# wind-chill-driven daily min — sits further from this comfort point, so a single
# daily value captures both heat index (hot side) and wind chill (cold side).
COMFORT_F = 65.0


def _cache_path(cell_id: str) -> str:
    return os.path.join(CACHE_DIR, f"{cell_id}.parquet")


# One lock per cell so concurrent requests/refreshes don't each pull the full
# multi-decade archive (which quickly trips the upstream rate limit).
_LOCKS_GUARD = threading.Lock()
_CELL_LOCKS: dict[str, threading.Lock] = {}

# When the archive rate-limits us (429), back off globally for a bit rather than
# re-hitting it on every refresh — that only prolongs the limit.
ARCHIVE_COOLDOWN = 120  # seconds
_archive_cooldown_until = 0.0


_archive_limit_daily = False   # is the current cooldown a daily-quota exhaustion?


class WeatherUnavailable(RuntimeError):
    """Weather data can't be fetched right now (rate limit, upstream outage with
    no cached fallback). Carries user-facing text; ``daily`` marks a daily-quota
    exhaustion (resets after UTC midnight) rather than a transient burst limit.
    The API layer maps this to a retryable 503 — it never needs to parse the
    message, and new upstream sources classify their own failures here."""

    def __init__(self, message: str, daily: bool = False):
        super().__init__(message)
        self.daily = daily


def limit_message(daily: bool) -> str:
    """The user-facing rate-limit copy, in one place."""
    if daily:
        return ("Open-Meteo's daily request limit is exhausted, so new locations will work again "
                "tomorrow. Places you've already viewed still work.")
    return "The weather service is rate-limited right now. Please try again in a minute."


def is_rate_limit(e) -> bool:
    return getattr(getattr(e, "response", None), "status_code", None) == 429


def _rate_limit_reason(e) -> str:
    """Upstream's human-readable 429 reason, if present in the response body."""
    resp = getattr(e, "response", None)
    if resp is not None:
        try:
            j = resp.json()
            if isinstance(j, dict) and j.get("reason"):
                return str(j["reason"])
        except Exception:  # noqa: BLE001
            pass
    return ""


def _seconds_to_utc_reset() -> float:
    """Seconds until just after the next UTC midnight (when daily quotas reset)."""
    now = datetime.datetime.now(datetime.timezone.utc)
    reset = (now + datetime.timedelta(days=1)).replace(hour=0, minute=10, second=0, microsecond=0)
    return max((reset - now).total_seconds(), 600)


def _note_rate_limit(e) -> None:
    """Record a rate limit: back off ~2 min, or until tomorrow for a daily quota."""
    global _archive_cooldown_until, _archive_limit_daily
    reason = _rate_limit_reason(e).lower()
    _archive_limit_daily = "daily" in reason or "tomorrow" in reason
    _archive_cooldown_until = time.time() + (
        _seconds_to_utc_reset() if _archive_limit_daily else ARCHIVE_COOLDOWN)


def _cell_lock(cell_id: str) -> threading.Lock:
    with _LOCKS_GUARD:
        lk = _CELL_LOCKS.get(cell_id)
        if lk is None:
            lk = _CELL_LOCKS[cell_id] = threading.Lock()
        return lk


def _request(url, params, timeout, *, phase, headers=None, attempts=MAX_ATTEMPTS):
    """GET with bounded retries; every retry and the final failure are logged to
    the errors folder (tagged ``retry`` / ``error``). A 429 (rate limit) fails fast
    without retrying, so we don't hammer the limit and make it worse."""
    last = None
    for attempt in range(1, attempts + 1):
        try:
            r = httpx.get(url, params=params, timeout=timeout, headers=headers)
            r.raise_for_status()
            metrics.record_outbound(phase, "ok")
            return r
        except Exception as e:  # noqa: BLE001 - upstream/network failures are expected
            last = e
            status = getattr(getattr(e, "response", None), "status_code", None)
            rate_limited = status == 429
            final = rate_limited or attempt == attempts
            metrics.record_outbound(
                phase, "rate_limited" if rate_limited else ("error" if final else "retry"))
            audit.log_event(
                "error" if final else "retry",
                {"phase": phase, "attempt": attempt, "max_attempts": attempts,
                 "url": url, "status": status, "error": repr(e)},
            )
            if final:
                break
            time.sleep(min(0.5 * 2 ** (attempt - 1), 4.0))
    raise last


def _normalize_read(df: pl.DataFrame) -> pl.DataFrame:
    """Normalize a frame read from the parquet cache: the daily `date` column is a
    calendar date, so coerce it to ``pl.Date`` (older files were written by pandas
    as a nanosecond ``Datetime``). Downstream date math and comparisons all key on
    ``pl.Date`` / stdlib ``datetime.date``."""
    if df.schema["date"] != pl.Date:
        df = df.with_columns(pl.col("date").cast(pl.Date))
    return df


def _combined_feels_expr(hi: str = "fmax", lo: str = "fmin") -> pl.Expr:
    """Expression for one daily "feels like" value: the apparent-temperature extreme
    furthest from the comfort baseline — the heat-index high on warm days, the
    wind-chill low on cold ones. Falls back to whichever side is present if one is
    missing (the coalesce picks the non-null side when a comparison is null)."""
    amax, amin = pl.col(hi), pl.col(lo)
    hot, cold = amax - COMFORT_F, COMFORT_F - amin
    chosen = pl.when(hot >= cold).then(amax).otherwise(amin)
    return pl.coalesce([chosen, amax, amin])


def _derive_humidity(df: pl.DataFrame) -> pl.DataFrame:
    """Replace the raw mean *relative* humidity column with *absolute* humidity
    (grams of water vapor per m³).

    Absolute humidity is derived from the day's mean RH and its mean temperature
    ((tmax+tmin)/2) via the Magnus saturation-vapor-pressure formula. It's a far
    more informative "how muggy was it" signal than relative humidity, which mostly
    tracks the day/night temperature swing (cold nights read ~100% RH regardless of
    actual moisture). Applied at the read boundary so the parquet cache keeps the
    raw RH the archive returns — no refetch needed for existing cells."""
    if "humid" not in df.columns:
        return df
    tmean_c = ((pl.col("tmax") + pl.col("tmin")) / 2.0 - 32.0) * 5.0 / 9.0
    rh = pl.col("humid").cast(pl.Float64, strict=False)
    es = 6.112 * (17.67 * tmean_c / (tmean_c + 243.5)).exp()   # sat. vapor pressure, hPa
    return df.with_columns(
        (es * rh * 2.1674 / (273.15 + tmean_c)).round(1).alias("humid"))   # abs. humidity, g/m³


def _derive_wetbulb(df: pl.DataFrame) -> pl.DataFrame:
    """Add a `wetbulb` column (°F): the daytime-peak wet-bulb temperature via the
    Stull (2011) single-value approximation from the day's high (`tmax`) and mean
    relative humidity.

    Wet bulb is the temperature a parcel reaches by evaporative cooling to
    saturation — the ceiling on how much the body can shed heat by sweating, so it
    is the sharper heat-stress signal than dry-bulb temperature or humidity alone.
    Must run BEFORE ``_derive_humidity`` (which replaces the raw RH column with
    absolute humidity, the input this needs). Read-time only, like the humidity
    derivation, so the parquet cache is untouched. Stull's fit is valid for
    RH 5-99% and −20..50 °C; days outside that range grade as null."""
    if "humid" not in df.columns:
        return df
    t = (pl.col("tmax") - 32.0) * 5.0 / 9.0          # daily high, °C
    rh = pl.col("humid").cast(pl.Float64, strict=False)
    tw = (t * (0.151977 * (rh + 8.313659).sqrt()).arctan()
          + (t + rh).arctan()
          - (rh - 1.676331).arctan()
          + 0.00391838 * rh.pow(1.5) * (0.023101 * rh).arctan()
          - 4.686035)                                # wet bulb, °C
    tw_f = (tw * 9.0 / 5.0 + 32.0).round(1)
    valid = (rh >= 5) & (rh <= 99) & (t >= -20) & (t <= 50)
    return df.with_columns(pl.when(valid).then(tw_f).otherwise(None).alias("wetbulb"))


def _derive_metrics(df: pl.DataFrame) -> pl.DataFrame:
    """Read-boundary derivations that depend on the raw relative-humidity column.
    Wet bulb must be computed before ``_derive_humidity`` replaces raw RH with
    absolute humidity, so both live behind this single wrapper to keep the order
    right at every read site."""
    return _derive_humidity(_derive_wetbulb(df))


def _with_doy(df: pl.DataFrame) -> pl.DataFrame:
    """(Re)attach the int16 day-of-year column the grading windows key on."""
    return df.with_columns(pl.col("date").dt.ordinal_day().cast(pl.Int16).alias("doy"))


def _write_cache(df: pl.DataFrame, path: str) -> None:
    """Persist a daily record to the parquet cache — the raw record only, never
    the derived doy column (it's recomputed at read time)."""
    os.makedirs(CACHE_DIR, exist_ok=True)
    df.drop("doy", strict=False).write_parquet(path, compression="zstd")


def _om_daily_params(cell: dict, **window) -> dict:
    """The Open-Meteo daily-request params every fetch shares — location, the
    variable set, imperial units. The date/window selectors (start_date/end_date
    or past_days/forecast_days) come in as kwargs."""
    return {
        "latitude": cell["center_lat"],
        "longitude": cell["center_lon"],
        "daily": DAILY_VARS,
        "timezone": "auto",
        "temperature_unit": "fahrenheit",
        "precipitation_unit": "inch",
        "wind_speed_unit": "mph",
        **window,
    }


def _finalize_frame(df: pl.DataFrame) -> pl.DataFrame:
    """Shared tail of every source→frame mapping: unify missing values as null, add
    the combined feels-like, apply the valid-day filter, and attach day-of-year. A
    usable climate day needs a real high/low; other columns may be missing and
    simply grade as None. Float NaN (from numpy-derived columns) is folded into null
    so the whole pipeline models "missing" one way — the grading boundary drops it."""
    df = df.with_columns(pl.col(pl.Float32, pl.Float64).fill_nan(None))
    df = df.with_columns(_combined_feels_expr().alias("feels"))
    df = df.drop_nulls(subset=["tmax", "tmin"])
    return _with_doy(df)


def _to_frame(daily: dict) -> pl.DataFrame:
    n = len(daily["time"])
    # Older upstream responses (or a narrowed variable set) may omit a series; fall
    # back to an all-null column of the right length so the frame shape is stable.
    def col(key):
        vals = daily.get(key)
        return vals if vals else [None] * n

    df = pl.DataFrame(
        {
            "date": daily["time"],
            "tmax": daily["temperature_2m_max"],
            "tmin": daily["temperature_2m_min"],
            "precip": daily["precipitation_sum"],
            "wind": col("wind_speed_10m_max"),
            "gust": col("wind_gusts_10m_max"),
            "humid": col("relative_humidity_2m_mean"),
            # The apparent (felt) high and low are kept as their own columns — graded
            # independently — alongside the combined `feels` (whichever side is further
            # from the fixed comfort baseline), which the weekly/day views still use.
            "fmax": col("apparent_temperature_max"),
            "fmin": col("apparent_temperature_min"),
        }
    )
    df = df.with_columns(
        pl.col("date").str.to_date(),
        *[pl.col(c).cast(pl.Float64, strict=False)
          for c in ("tmax", "tmin", "precip", "wind", "gust", "fmax", "fmin")],
    )
    return _finalize_frame(df)


def _fetch_history(cell: dict) -> pl.DataFrame:
    end = (datetime.date.today() - datetime.timedelta(days=ARCHIVE_LATENCY_DAYS)).isoformat()
    params = _om_daily_params(cell, start_date=START_DATE, end_date=end, models=ARCHIVE_MODEL)
    r = _request(ARCHIVE_URL, params, 180, phase="history_fetch")
    return _to_frame(r.json()["daily"])


def _heat_index(t_f, rh):
    """NWS heat index (°F); below ~80°F it's just the air temperature."""
    T, R = t_f, rh
    hi = (-42.379 + 2.04901523 * T + 10.14333127 * R - 0.22475541 * T * R
          - 0.00683783 * T * T - 0.05481717 * R * R + 0.00122874 * T * T * R
          + 0.00085282 * T * R * R - 0.00000199 * T * T * R * R)
    return np.where(np.asarray(T, dtype="float64") >= 80, hi, T)


def _wind_chill(t_f, v_mph):
    """NWS wind chill (°F); applies only when cold + breezy, else the air temp."""
    T = np.asarray(t_f, dtype="float64")
    V = np.clip(np.asarray(v_mph, dtype="float64"), 0, None)
    Vp = np.power(V, 0.16)
    wc = 35.74 + 0.6215 * T - 35.75 * Vp + 0.4275 * T * Vp
    return np.where((T <= 50) & (V >= 3), wc, T)


def _nasa_to_frame(param: dict) -> pl.DataFrame:
    """Map a NASA POWER daily response to our (tmax/tmin/precip/wind/gust/humid/feels)
    schema, converting units (°C→°F, mm→in, m/s→mph) and computing feels-like."""
    dates = sorted(param.get("T2M_MAX", {}).keys())

    def col(key, transform):
        d = param.get(key, {})
        return [None if (v is None or v <= NASA_FILL) else transform(v)
                for v in (d.get(k) for k in dates)]

    c2f = lambda c: c * 9.0 / 5.0 + 32.0
    df = pl.DataFrame({
        "date": dates,
        "tmax": col("T2M_MAX", c2f),
        "tmin": col("T2M_MIN", c2f),
        "precip": col("PRECTOTCORR", lambda mm: mm / 25.4),
        "wind": col("WS10M_MAX", lambda ms: ms * 2.2369362920544),
        "gust": [None] * len(dates),          # POWER has no gusts
        "humid": col("RH2M", lambda v: v),
    })
    df = df.with_columns(
        pl.col("date").str.to_date("%Y%m%d"),
        *[pl.col(c).cast(pl.Float64, strict=False)
          for c in ("tmax", "tmin", "precip", "wind", "gust")],
    )
    # POWER has no apparent temperature; approximate the felt high with the NWS
    # heat index and the felt low with NWS wind chill (each falls back to the air
    # temperature outside its regime), mirroring the Open-Meteo columns. numpy NaN
    # from these lands in float columns and is folded to null in _finalize_frame.
    df = df.with_columns(
        pl.Series("fmax", _heat_index(df["tmax"].to_numpy(), df["humid"].to_numpy())),
        pl.Series("fmin", _wind_chill(df["tmin"].to_numpy(), df["wind"].to_numpy())),
    )
    return _finalize_frame(df)


def _fetch_history_nasa(cell: dict) -> pl.DataFrame:
    """Backup history fetch from NASA POWER (used when Open-Meteo is unavailable)."""
    end = (datetime.date.today() - datetime.timedelta(days=ARCHIVE_LATENCY_DAYS)).strftime("%Y%m%d")
    params = {
        "parameters": "T2M_MAX,T2M_MIN,PRECTOTCORR,WS10M_MAX,RH2M",
        "community": "RE",
        "latitude": cell["center_lat"],
        "longitude": cell["center_lon"],
        "start": NASA_START,
        "end": end,
        "format": "JSON",
    }
    r = _request(NASA_POWER_URL, params, 180, phase="history_nasa")
    return _nasa_to_frame(r.json()["properties"]["parameter"])


def _metno_to_frame(props: dict) -> pl.DataFrame:
    """Map a MET Norway Locationforecast (yr.no) response to our daily schema.

    MET Norway returns a per-timestep timeseries (hourly near-term, then 6-hourly)
    in metric units, with no gusts or apparent temperature, so we aggregate to daily
    extremes/means, convert units (°C→°F, mm→in, m/s→mph), and approximate feels-like
    from the NWS heat index / wind chill — the same treatment as the NASA POWER path."""
    # Aggregate the sub-daily steps into per-day values, keyed by (UTC) calendar day.
    agg: dict[str, dict] = {}
    for step in props.get("timeseries", []):
        day = step["time"][:10]
        data = step.get("data", {})
        inst = data.get("instant", {}).get("details", {})
        a = agg.setdefault(day, {"t": [], "rh": [], "wind": [], "precip": None})
        if (t := inst.get("air_temperature")) is not None:
            a["t"].append(t)
        if (rh := inst.get("relative_humidity")) is not None:
            a["rh"].append(rh)
        if (w := inst.get("wind_speed")) is not None:
            a["wind"].append(w)
        # Precip: prefer the 1-hour block (hourly near-term), else the 6-hour block
        # (6-hourly far-term). Never add both — they overlap — so summing each step's
        # chosen block avoids double counting across the resolution switch.
        p = (data.get("next_1_hours") or {}).get("details", {}).get("precipitation_amount")
        if p is None:
            p = (data.get("next_6_hours") or {}).get("details", {}).get("precipitation_amount")
        if p is not None:
            a["precip"] = (a["precip"] or 0.0) + p

    dates = sorted(agg)
    c2f = lambda c: c * 9.0 / 5.0 + 32.0
    df = pl.DataFrame({
        "date": dates,
        "tmax": [c2f(max(agg[d]["t"])) if agg[d]["t"] else None for d in dates],
        "tmin": [c2f(min(agg[d]["t"])) if agg[d]["t"] else None for d in dates],
        "precip": [agg[d]["precip"] / 25.4 if agg[d]["precip"] is not None else None
                   for d in dates],
        "wind": [max(agg[d]["wind"]) * 2.2369362920544 if agg[d]["wind"] else None
                 for d in dates],
        "gust": [None] * len(dates),          # MET Norway has no gusts
        "humid": [sum(agg[d]["rh"]) / len(agg[d]["rh"]) if agg[d]["rh"] else None
                  for d in dates],
    })
    df = df.with_columns(
        pl.col("date").str.to_date(),
        *[pl.col(c).cast(pl.Float64, strict=False)
          for c in ("tmax", "tmin", "precip", "wind", "gust")],
    )
    df = df.with_columns(
        pl.Series("fmax", _heat_index(df["tmax"].to_numpy(), df["humid"].to_numpy())),
        pl.Series("fmin", _wind_chill(df["tmin"].to_numpy(), df["wind"].to_numpy())),
    )
    return _finalize_frame(df)


def _fetch_forecast_metno(cell: dict) -> pl.DataFrame:
    """Backup forward-forecast fetch from MET Norway / yr.no (used when Open-Meteo's
    forecast API is unavailable). Coordinates are rounded to 4 decimals per MET's
    ToS (improves their cache hit rate)."""
    r = _request(
        METNO_URL,
        {"lat": round(cell["center_lat"], 4), "lon": round(cell["center_lon"], 4)},
        60,
        phase="forecast_metno",
        headers={"User-Agent": METNO_UA},
    )
    return _metno_to_frame(r.json().get("properties", {}))


def _fetch_history_range(cell: dict, start_date: str, end_date: str) -> pl.DataFrame:
    """Fetch just a date range of archive history (used to top up the recent tail)."""
    params = _om_daily_params(cell, start_date=start_date, end_date=end_date, models=ARCHIVE_MODEL)
    r = _request(ARCHIVE_URL, params, 60, phase="history_topup")
    return _to_frame(r.json()["daily"])


def _read_history_cache(path):
    """Schema-complete cached frame (no doy) + its file age in seconds, or None.

    No age expiry — history is cached indefinitely; a pre-wind/humidity schema is
    the only reason to refetch (to add the new metric columns)."""
    if not os.path.exists(path):
        return None
    df = _normalize_read(pl.read_parquet(path))
    if not all(c in df.columns for c in NEW_COLS):
        return None
    return df, time.time() - os.path.getmtime(path)


def _topup_tail(cell: dict, df: pl.DataFrame, path: str) -> pl.DataFrame:
    """Append newly-available archive days to a cached record. Best-effort and
    serialized per cell; a small incremental fetch, not the full multi-decade pull."""
    global _archive_cooldown_until
    with _cell_lock(cell["id"]):
        fresh = _read_history_cache(path)   # another thread may have just refreshed it
        if fresh is not None:
            df, age_s = fresh
            if age_s < HISTORY_TOPUP_INTERVAL:
                return df
        if time.time() < _archive_cooldown_until:
            return df
        expected = datetime.date.today() - datetime.timedelta(days=ARCHIVE_LATENCY_DAYS)
        cached_max = df["date"].max()
        if cached_max >= expected:
            try: os.utime(path, None)   # tail already current — reset the hourly timer
            except OSError: pass
            return df
        try:
            recent = _fetch_history_range(
                cell, (cached_max + datetime.timedelta(days=1)).isoformat(), expected.isoformat())
        except Exception as e:  # noqa: BLE001 - keep the cached record on failure
            if is_rate_limit(e):
                _note_rate_limit(e)
            return df
        # Concatenate archive-first, recent-last so `keep="last"` prefers a freshly
        # fetched day over its cached duplicate; maintain_order keeps that precedence
        # before the final chronological sort.
        merged = (pl.concat([df, recent.drop("doy", strict=False)], how="diagonal_relaxed")
                  .unique(subset="date", keep="last", maintain_order=True)
                  .sort("date"))
        _write_cache(merged, path)
        return merged


def get_history(cell: dict) -> tuple[pl.DataFrame, dict]:
    """Return (daily history frame, cache metadata) for a cell, with humidity as
    absolute humidity (g/m³). Thin wrapper over the raw loader (see below)."""
    df, meta = _load_history(cell)
    return _derive_metrics(df), meta


def load_cached_history(cell: dict) -> pl.DataFrame | None:
    """History from the parquet cache ONLY — never fetches upstream and never tops
    up the tail. Powers the warm-only prefetch path (which must not spend upstream
    quota) and the offline migrate script. None when the cell has no
    (schema-complete) cached record."""
    hit = _read_history_cache(_cache_path(cell["id"]))
    if hit is None:
        return None
    return _derive_metrics(_with_doy(hit[0]))


def _load_history(cell: dict) -> tuple[pl.DataFrame, dict]:
    """Return (daily history frame, cache metadata) for a cell.

    The full archive is cached indefinitely (fetched once); only the recent tail is
    topped up, at most hourly. Concurrent callers are serialized so only one archive
    fetch happens; on an upstream failure (e.g. 429) any existing cache is served."""
    path = _cache_path(cell["id"])

    hit = _read_history_cache(path)
    if hit is not None:
        df, age_s = hit
        if age_s > HISTORY_TOPUP_INTERVAL:
            df = _topup_tail(cell, df, path)   # refresh just the recent days
        return _with_doy(df), {"cached": True, "cache_age_days": round(age_s / 86400.0, 1)}

    global _archive_cooldown_until

    with _cell_lock(cell["id"]):
        hit = _read_history_cache(path)   # another thread may have populated it while we waited
        if hit is not None:
            df, age_s = hit
            return _with_doy(df), {"cached": True, "cache_age_days": round(age_s / 86400.0, 1)}

        stale = _normalize_read(pl.read_parquet(path)) if os.path.exists(path) else None

        def _serve_stale():
            return _with_doy(stale), {"cached": True, "stale": True, "cache_age_days": None}

        # Fetch. Open-Meteo is primary (richer: gusts + apparent temp); NASA POWER is
        # the backup when Open-Meteo is unavailable (network error or its daily limit).
        # Skip Open-Meteo entirely while it's in a rate-limit cooldown.
        df = None
        source = None
        if time.time() >= _archive_cooldown_until:
            try:
                fetched = _fetch_history(cell)
                # Guard a self-hosted archive that isn't backfilled yet: an all-null or
                # recent-only response reduces to a near-empty frame, which would
                # otherwise be cached indefinitely as a complete record and never
                # fall to NASA. Require a plausibly-full span before accepting it.
                if fetched.height >= MIN_ARCHIVE_DAYS:
                    df = fetched
                    source = "open-meteo"
            except Exception as e:  # noqa: BLE001
                if is_rate_limit(e):
                    _note_rate_limit(e)
        if df is None:
            try:
                df = _fetch_history_nasa(cell)
                source = "nasa-power"
            except Exception:  # noqa: BLE001 - backup unavailable too
                df = None
        if df is None:
            if stale is not None:
                return _serve_stale()
            raise WeatherUnavailable(limit_message(_archive_limit_daily),
                                     daily=_archive_limit_daily)

        # Store the raw record; percentiles are derived at request time.
        _write_cache(df, path)
        return df, {"cached": False, "cache_age_days": 0, "source": source}


RECENT_PAST_DAYS = 25   # recent observations window (covers the ~2-week graded view)
FORECAST_DAYS = 8       # today + 7 days ahead


def _rf_cache_path(cell_id: str) -> str:
    return os.path.join(CACHE_DIR, f"{cell_id}_rf.parquet")


def recent_stamp(cell_id: str) -> int:
    """Identity stamp (mtime, whole seconds) of the cell's cached recent+forecast
    parquet; 0 when absent. Changes exactly when the recent/forecast data does, so
    it's the freshness token for every payload that grades recent or future days."""
    try:
        return int(os.path.getmtime(_rf_cache_path(cell_id)))
    except OSError:
        return 0


def get_recent_forecast(cell: dict) -> pl.DataFrame:
    """Recent observations + forward forecast, with humidity as absolute humidity
    (g/m³). Thin wrapper over the raw loader (see below)."""
    return _derive_metrics(_load_recent_forecast(cell))


def load_cached_recent_forecast(cell: dict) -> "pl.DataFrame | None":
    """Recent+forecast from the parquet cache ONLY — never fetches upstream, and
    unlike the loader below it does not care how stale the file is. The sibling of
    load_cached_history, for the same reason: the homepage precompute sweeps every
    cached city and must not spend a single upstream request doing it. None when
    the cell has no cached recent/forecast record."""
    path = _rf_cache_path(cell["id"])
    if not os.path.exists(path):
        return None
    try:
        return _derive_metrics(_with_doy(_normalize_read(pl.read_parquet(path))))
    except Exception:  # noqa: BLE001 - a truncated/corrupt cache file is a miss, not a crash
        return None


def _load_recent_forecast(cell: dict) -> pl.DataFrame:
    """Recent observations AND the forward forecast in ONE forecast-API call.

    Both the recent (past) view and the forecast (future) view slice from this, so
    a cell needs just one upstream forecast request per hour (plus the ~monthly
    archive fetch). Cached per cell for one hour so it still follows model updates.
    """
    path = _rf_cache_path(cell["id"])
    if os.path.exists(path):
        age_h = (time.time() - os.path.getmtime(path)) / 3600.0
        if age_h < FORECAST_TTL_HOURS:
            return _with_doy(_normalize_read(pl.read_parquet(path)))

    params = {
        "latitude": cell["center_lat"],
        "longitude": cell["center_lon"],
        "daily": DAILY_VARS,
        "timezone": "auto",
        "temperature_unit": "fahrenheit",
        "precipitation_unit": "inch",
        "wind_speed_unit": "mph",
        "past_days": RECENT_PAST_DAYS,
        "forecast_days": FORECAST_DAYS,
    }
    try:
        r = _request(FORECAST_URL, params, 60, phase="recent_forecast_fetch")
        df = _to_frame(r.json()["daily"])
    except Exception as e:  # noqa: BLE001
        # Open-Meteo forecast unavailable (rate limit or outage). Fall back to MET
        # Norway (yr.no) — global + keyless — mirroring the archive's NASA POWER
        # backup. MET Norway is forecast-only (no recent past days), so this keeps
        # the forecast / day-ahead views working in a degraded form.
        try:
            df = _fetch_forecast_metno(cell)
        except Exception:  # noqa: BLE001 - backup unavailable too
            # Last resort: serve the stale cache if we have one. An hours-old bundle
            # (which still carries the recent observed days MET Norway lacks) beats a
            # hard failure, mirroring _load_history's stale-serve. Return WITHOUT
            # rewriting it, so its mtime stays old and the next request still retries
            # upstream first rather than serving this as if it were fresh.
            if os.path.exists(path):
                return _with_doy(_normalize_read(pl.read_parquet(path)))
            # No cache either: surface the original error, classifying an Open-Meteo
            # rate limit as the typed, daily-aware WeatherUnavailable (the archive
            # path classifies its own inside _load_history).
            if is_rate_limit(e):
                daily = "daily" in _rate_limit_reason(e).lower()
                raise WeatherUnavailable(limit_message(daily), daily=daily) from e
            raise
    _write_cache(df, path)
    return df


_REVGEO_CACHE: dict[str, str | None] = {}

# Nominatim's usage policy allows ~1 request/second and rejects bursts. The compare
# page loads several locations at once, so serialize the reverse lookups here and
# space them out — otherwise the burst is rate-limited, a null label gets cached, and
# those locations are left showing bare coordinates.
_REVGEO_LOCK = threading.Lock()
_REVGEO_MIN_INTERVAL = 1.1   # seconds between successive Nominatim reverse calls
_revgeo_last = 0.0


def reverse_geocode_cached(lat: float, lon: float) -> tuple[bool, str | None]:
    """(found, label) from the in-memory + SQLite revgeo caches only — never calls
    Nominatim. Used by paths that must not add upstream traffic (prefetch)."""
    key = store.revgeo_key(lat, lon)
    if key in _REVGEO_CACHE:
        return (True, _REVGEO_CACHE[key])
    found, label = store.get_revgeo(key)
    if found:
        _REVGEO_CACHE[key] = label
    return (found, label)


def reverse_geocode(lat: float, lon: float) -> str | None:
    """Best-effort neighbourhood/city label for a point (OpenStreetMap Nominatim).

    Cached per ~cell — in memory for the process and in SQLite across restarts —
    so panning around (or redeploying) doesn't re-hit the service, and failures
    return None so the caller can fall back to bare coordinates.
    """
    key = store.revgeo_key(lat, lon)
    found, label = reverse_geocode_cached(lat, lon)
    if found:
        return label
    # Serialize + rate-limit the upstream call. Concurrent callers (the compare page
    # loads several locations at once) would otherwise burst past Nominatim's ~1/sec
    # limit and get rate-limited, caching a null label. Under the lock we re-check the
    # cache (a peer may have just resolved this cell) and space successive calls out.
    global _revgeo_last
    with _REVGEO_LOCK:
        found, label = reverse_geocode_cached(lat, lon)
        if found:
            return label
        wait = _REVGEO_MIN_INTERVAL - (time.monotonic() - _revgeo_last)
        if wait > 0:
            time.sleep(wait)
        label = None
        try:
            r = _request(
                "https://nominatim.openstreetmap.org/reverse",
                # zoom 14 resolves to the suburb/neighbourhood level so we can lead
                # with it when OSM has one (zoom 10 only ever returns the city).
                # accept-language=en keeps labels in one script worldwide (matches
                # the forward geocoder's language=en).
                {"lat": lat, "lon": lon, "format": "jsonv2", "zoom": 14,
                 "addressdetails": 1, "accept-language": "en"},
                15,
                phase="reverse_geocode",
                headers={"User-Agent": "Thermograph/0.1 (local weather grading app)"},
            )
            a = r.json().get("address", {}) or {}
            # Lead with the neighbourhood when available, then the city, then region.
            neighborhood = (a.get("neighbourhood") or a.get("suburb")
                            or a.get("quarter") or a.get("city_district")
                            or a.get("borough"))
            city = (a.get("city") or a.get("town") or a.get("village")
                    or a.get("hamlet") or a.get("county"))
            region = a.get("state") or a.get("province") or a.get("region")
            country = a.get("country")
            # dict.fromkeys drops duplicates (e.g. neighbourhood == city) in order.
            # Country trails the neighbourhood/city/region so the label reads as a full
            # hierarchy ("West Seattle, Seattle, Washington, United States"); the
            # frontend leads with the first part and mutes the rest.
            parts = dict.fromkeys(p for p in (neighborhood, city, region, country) if p)
            label = ", ".join(parts) or None
        except Exception:  # noqa: BLE001 - reverse geocoding is a nicety, never fatal
            label = None
        _revgeo_last = time.monotonic()
        _REVGEO_CACHE[key] = label
        store.put_revgeo(key, label)   # survive restarts (a None label retries after its TTL)
        return label


def geocode(name: str, count: int = 5) -> list[dict]:
    """Look up places by name worldwide via Open-Meteo's geocoder."""
    r = _request(
        "https://geocoding-api.open-meteo.com/v1/search",
        {"name": name, "count": count, "language": "en", "format": "json"},
        30,
        phase="geocode",
    )
    results = r.json().get("results", []) or []
    return [
        {
            "name": g.get("name"),
            "admin1": g.get("admin1"),
            "country": g.get("country"),
            "country_code": g.get("country_code"),
            "lat": g.get("latitude"),
            "lon": g.get("longitude"),
            "population": g.get("population"),
        }
        for g in results
    ]