thermograph/backend/data/climate.py

"""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 concurrent.futures
import datetime
import os
import queue
import tempfile
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 climate_store
from data import era5lake
from data import meteostat
from data import store

# Reused across every fetch (archive/forecast/geocode/revgeo) instead of a bare
# httpx.get per call opening a fresh TCP+TLS connection every time -- mirrors
# web/app.py's reused _frontend_client. Every call still passes its own explicit
# per-call timeout (see _request), so this only changes connection reuse, never
# retry/backoff semantics. Never explicitly closed: this is a long-lived module
# used by both the server process and one-shot scripts (warm_cities.py,
# migrate_cache_to_pg.py), and the OS reclaims the sockets at process exit either
# way.
_client = httpx.Client()

CACHE_DIR = os.path.join(paths.DATA_DIR, "cache")
MAX_ATTEMPTS = 3                   # per upstream call, before giving up

# _fetch_history / _fetch_history_nasa run under _load_history's per-cell lock
# (see _cell_lock), so every attempt's timeout adds directly to how long a
# same-cell waiter queues behind this thread. A flat 180s x MAX_ATTEMPTS could
# pin the thread for up to 9 minutes on one slow-but-eventually-ok upstream. The
# full archive response is genuinely large (~16k daily rows from START_DATE, see
# below), so it can legitimately need more than a topup fetch's 60s -- but that
# headroom belongs on the LAST attempt only: the first attempts fail fast so a
# truly wedged upstream frees the lock for a waiter sooner, and only the final,
# worth-waiting-out attempt gets the longer allowance. Worst case under the lock
# drops from 540s (9 min) to 60+60+150=270s (4.5 min).
ARCHIVE_FETCH_TIMEOUTS = (60, 60, 150)
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 = 4             # refetch the recent+forecast bundle every ~4h (lower IO;
                                   # daily-resolution forecast normals move slowly)

# 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

# Forward-forecast source (primary for the forward days). MET Norway (yr.no) is free
# + keyless + global. It returns a sub-daily timeseries in metric units with no gusts
# or apparent temp, so we aggregate to daily, convert units, derive feels-like like
# the NASA path, and fill gusts from Meteostat. It is forecast-only (no recent past),
# so the recent observed days come from a NASA POWER range instead. The /compact
# endpoint carries every field we use at a smaller payload than /complete.
METNO_URL = "https://api.met.no/weatherapi/locationforecast/2.0/compact"
# 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,"
    "sunshine_duration,daylight_duration"
)

# Optional metric columns beyond the required date/tmax/tmin/precip schema. A
# source that lacks one gets an all-null column in _finalize_frame, so every
# finalized frame has the same schema without each builder stubbing missing
# columns by hand. `sun` (the sunshine fraction) only comes from Open-Meteo, so
# the NASA/MET fallbacks pick up its null column here too.
OPTIONAL_COLS = ("wind", "gust", "humid", "fmax", "fmin", "sun")

# 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, or the sunshine fraction) and is refetched so the new
# metrics show up immediately instead of after the 30-day cache TTL. `feels` is
# derived in _finalize_frame; the rest are the optional raw inputs, so this stays
# in lockstep with them.
NEW_COLS = (*OPTIONAL_COLS, "feels")

# 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?

# The forecast (recent+forward) endpoint's own cooldown, tracked separately from
# the archive one above: Open-Meteo's archive and forecast APIs have independent
# quotas, so a 429 on one must not gate (or be masked by) a cooldown meant for the
# other. Without this, a forecast brownout let every subscribed cell AND every
# live /cell request independently retry-storm the endpoint -- each paying
# MAX_ATTEMPTS x up to 60s plus the MET Norway fallback's own retry cycle, with no
# circuit breaker (see _load_recent_forecast).
FORECAST_COOLDOWN = 120  # seconds
_forecast_cooldown_until = 0.0
_forecast_limit_daily = False   # is the current forecast 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 an archive 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 _note_forecast_rate_limit(e) -> None:
    """Record a forecast-endpoint rate limit. Mirrors _note_rate_limit exactly,
    but against the separate _forecast_cooldown_until (see its module comment)."""
    global _forecast_cooldown_until, _forecast_limit_daily
    reason = _rate_limit_reason(e).lower()
    _forecast_limit_daily = "daily" in reason or "tomorrow" in reason
    _forecast_cooldown_until = time.time() + (
        _seconds_to_utc_reset() if _forecast_limit_daily else FORECAST_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.

    ``timeout`` is either one number applied to every attempt (the common case),
    or a sequence of per-attempt values (see ARCHIVE_FETCH_TIMEOUTS) so a caller
    that holds a lock across every attempt can fail fast on the early ones and
    reserve a longer wait for the last."""
    last = None
    for attempt in range(1, attempts + 1):
        attempt_timeout = (timeout[min(attempt - 1, len(timeout) - 1)]
                           if isinstance(timeout, (tuple, list)) else timeout)
        try:
            r = _client.get(url, params=params, timeout=attempt_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).

    Written to a tempfile in the same directory, then renamed into place
    (os.replace is atomic on the same filesystem) — mirrors the pattern in
    api/homepage.py's refresh() and data/places.py's _fetch. A reader (or a
    second writer — warm_cities.py guards against overlapping runs, but a live
    request racing a topup is normal) must never see a partially-written parquet
    file; writing straight to ``path`` (the old behavior) could hand a truncated
    file to a concurrent read."""
    directory = os.path.dirname(path)
    os.makedirs(directory, exist_ok=True)
    fd, tmp = tempfile.mkstemp(dir=directory, suffix=".tmp")
    try:
        os.close(fd)
        df.drop("doy", strict=False).write_parquet(tmp, compression="zstd")
        os.replace(tmp, path)
    except BaseException:
        try:
            os.unlink(tmp)
        except OSError:
            pass
        raise


# --- cache backend dispatch --------------------------------------------------
# The raw daily record lives in TimescaleDB hypertables on Postgres (prod) and in
# per-cell parquet files otherwise (dev / tests / offline). These helpers hide the
# choice so the loaders below stay backend-agnostic: they return the same shapes a
# parquet read produced — a frame WITHOUT the derived `doy` column, plus an age in
# seconds since the last write (the mtime age the topup/TTL logic keyed on). The
# active backend is climate_store.is_postgres() (same THERMOGRAPH_DATABASE_URL
# switch as accounts/db.py and data/store.py).


def _read_history_backed(cell_id: str):
    """(schema-complete history frame without doy, age_s) or None."""
    if climate_store.is_postgres():
        hit = climate_store.read_history(cell_id)
        if hit is None:
            return None
        df, synced_at = hit
        return df, time.time() - synced_at
    return _read_history_cache(_cache_path(cell_id))


def _stale_history_backed(cell_id: str):
    """Any cached history rows regardless of schema completeness (for the
    all-fetches-failed stale serve), or None."""
    if climate_store.is_postgres():
        return climate_store.read_history_raw(cell_id)
    path = _cache_path(cell_id)
    return _normalize_read(pl.read_parquet(path)) if os.path.exists(path) else None


def _write_history_backed(cell_id: str, full_df: pl.DataFrame,
                          delta_df: "pl.DataFrame | None" = None) -> None:
    """Persist a cell's history. Parquet rewrites the whole file from ``full_df``;
    Postgres upserts ``delta_df`` when given (just the topped-up tail) else the full
    frame — the upsert makes a delta write equivalent to a full rewrite."""
    if climate_store.is_postgres():
        climate_store.write_history(
            cell_id, delta_df if delta_df is not None else full_df, time.time())
    else:
        _write_cache(full_df, _cache_path(cell_id))


def _touch_history_backed(cell_id: str) -> None:
    """Reset the hourly topup timer when the tail is already current, without a
    rewrite — bump history_synced_at on Postgres, or the file mtime on parquet."""
    if climate_store.is_postgres():
        climate_store.touch_history(cell_id, time.time())
    else:
        try:
            os.utime(_cache_path(cell_id), None)
        except OSError:
            pass


def _read_recent_backed(cell_id: str):
    """(recent+forecast frame without doy, age_s) or None."""
    if climate_store.is_postgres():
        hit = climate_store.read_recent(cell_id)
        if hit is None:
            return None
        df, synced_at = hit
        return df, time.time() - synced_at
    path = _rf_cache_path(cell_id)
    if not os.path.exists(path):
        return None
    try:
        return _normalize_read(pl.read_parquet(path)), time.time() - os.path.getmtime(path)
    except Exception:  # noqa: BLE001 - a truncated/corrupt cache file is a miss
        return None


def _write_recent_backed(cell_id: str, df: pl.DataFrame) -> None:
    """Persist a cell's recent+forecast bundle (full replace on both backends)."""
    if climate_store.is_postgres():
        climate_store.write_recent(cell_id, df, time.time())
    else:
        _write_cache(df, _rf_cache_path(cell_id))


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."""
    # Backfill any optional metric a source didn't provide as an all-null column, so
    # the finalized schema (and the NEW_COLS cache check) is uniform across sources.
    absent = [pl.lit(None, dtype=pl.Float64).alias(c)
              for c in OPTIONAL_COLS if c not in df.columns]
    if absent:
        df = df.with_columns(absent)
    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 _finalize_approximated(df: pl.DataFrame) -> pl.DataFrame:
    """Finalize a source that lacks apparent temperature (NASA POWER, MET Norway):
    approximate the felt high with the NWS heat index and the felt low with wind
    chill (each falls back to the air temperature outside its regime), mirroring the
    Open-Meteo fmax/fmin columns, then finalize. The numpy NaN these produce lands
    in float columns and is folded to null by _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 _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"),
            # Raw seconds of sunshine and of daylight; combined below into `sun`.
            "sunsec": col("sunshine_duration"),
            "daysec": col("daylight_duration"),
        }
    )
    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",
                    "sunsec", "daysec")],
    )
    # `sun` = fraction of the day's daylight that was sunshine, clamped to [0, 1]
    # (sunshine_duration can slightly exceed daylight near the poles). Null when
    # either series is missing or daylight is zero (polar night).
    df = df.with_columns(
        pl.when((pl.col("daysec") > 0) & pl.col("sunsec").is_not_null())
        .then((pl.col("sunsec") / pl.col("daysec")).clip(0.0, 1.0))
        .otherwise(None)
        .alias("sun")
    ).drop("sunsec", "daysec")
    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, ARCHIVE_FETCH_TIMEOUTS, 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),
        "humid": col("RH2M", lambda v: v),
    })                                        # POWER has no gusts (backfilled null)
    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")],
    )
    return _finalize_approximated(df)


def _fetch_history_nasa(cell: dict, start: str = NASA_START,
                        end: "str | None" = None) -> pl.DataFrame:
    """Primary history fetch from NASA POWER (keyless, independent of Open-Meteo).

    `start`/`end` accept NASA's YYYYMMDD or ISO YYYY-MM-DD; `end` defaults to the
    archive-latency cutoff. Serves both the full multi-decade pull and the tail
    top-up range. NASA carries no gusts, so they're filled from Meteostat below."""
    if end is None:
        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": start.replace("-", ""),
        "end": end.replace("-", ""),
        "format": "JSON",
    }
    r = _request(NASA_POWER_URL, params, ARCHIVE_FETCH_TIMEOUTS, phase="history_nasa")
    df = _nasa_to_frame(r.json()["properties"]["parameter"])
    # NASA POWER carries no gusts; fill from the nearest Meteostat station, falling
    # back to an estimate from sustained wind where no station is in range.
    return meteostat.fill_gusts(cell["center_lat"], cell["center_lon"], df)


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],
        "humid": [sum(agg[d]["rh"]) / len(agg[d]["rh"]) if agg[d]["rh"] else None
                  for d in dates],
    })                                        # MET Norway has no gusts (backfilled null)
    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")],
    )
    return _finalize_approximated(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 _fetch_history_tail(cell: dict, start_date: str, end_date: str) -> pl.DataFrame:
    """Fetch a recent history range to top up the cached tail — NASA POWER primary,
    Open-Meteo archive fallback (mirroring the full-history source order). The
    Open-Meteo fallback is skipped while it's in a rate-limit cooldown."""
    try:
        return _fetch_history_nasa(cell, start=start_date, end=end_date)
    except Exception:  # noqa: BLE001 - NASA unavailable; try Open-Meteo unless it's cooling down
        if time.time() < _archive_cooldown_until:
            raise
        return _fetch_history_range(cell, start_date, end_date)


def _topup_tail(cell: dict, df: pl.DataFrame) -> 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."""
    cell_id = cell["id"]
    with _cell_lock(cell_id):
        fresh = _read_history_backed(cell_id)   # another thread may have just refreshed it
        if fresh is not None:
            df, age_s = fresh
            if age_s < HISTORY_TOPUP_INTERVAL:
                return df
        expected = datetime.date.today() - datetime.timedelta(days=ARCHIVE_LATENCY_DAYS)
        cached_max = df["date"].max()
        if cached_max >= expected:
            _touch_history_backed(cell_id)   # tail already current — reset the hourly timer
            return df
        try:
            recent = _fetch_history_tail(
                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.
        tail = recent.drop("doy", strict=False)
        merged = (pl.concat([df, tail], how="diagonal_relaxed")
                  .unique(subset="date", keep="last", maintain_order=True)
                  .sort("date"))
        # Postgres only needs the new/refetched tail rows upserted (ON CONFLICT gives
        # the same keep="last" precedence); parquet rewrites the whole file.
        _write_history_backed(cell_id, merged, delta_df=tail)
        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 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_backed(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."""
    cell_id = cell["id"]

    hit = _read_history_backed(cell_id)
    if hit is not None:
        df, age_s = hit
        if age_s > HISTORY_TOPUP_INTERVAL:
            df = _topup_tail(cell, df)   # 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_backed(cell_id)   # 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 = _stale_history_backed(cell_id)

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

        # Fetch. The ERA5 lake is first when configured (true ERA5 with measured
        # gusts, served from our own bucket — no third-party API in the path);
        # unconfigured or missing-point cells fall through at zero cost. NASA
        # POWER is next (keyless, independent of Open-Meteo; its missing gusts
        # are filled from Meteostat inside _fetch_history_nasa), then Open-Meteo
        # — still guarded by its rate-limit cooldown. Every source must return a
        # plausibly-full span before being accepted and cached as a complete
        # record (a short/partial response is rejected rather than held
        # indefinitely).
        df = None
        source = None
        try:
            fetched = era5lake.fetch_history_lake(cell, start=START_DATE)
            if fetched.height >= MIN_ARCHIVE_DAYS:
                df = fetched
                source = "era5-lake"
        except Exception:  # noqa: BLE001 - lake unconfigured/miss; fall through to NASA
            pass
        if df is None:
            try:
                fetched = _fetch_history_nasa(cell)
                if fetched.height >= MIN_ARCHIVE_DAYS:
                    df = fetched
                    source = "nasa-power"
            except Exception:  # noqa: BLE001 - primary unavailable; fall through to Open-Meteo
                pass
        if df is None and time.time() >= _archive_cooldown_until:
            try:
                fetched = _fetch_history(cell)
                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:
            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_history_backed(cell_id, df)
        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
# NASA POWER near-real-time lags a couple of days, so the recent observed window ends
# here; the small today-1/today-2 gap before the MET Norway forecast begins grades as
# missing (drop_nulls), bracketed by history behind and forecast ahead.
RECENT_END_LAG_DAYS = 2


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 (whole seconds) of the cell's cached recent+forecast bundle;
    0 when absent. On Postgres it's int(recent_synced_at) from climate_sync; on
    parquet it's the file mtime. Either way it changes exactly when the
    recent/forecast data is rewritten (and NOT on a stale serve), so it's the
    freshness token for every payload that grades recent or future days."""
    if climate_store.is_postgres():
        return int(climate_store.recent_synced_at(cell_id))
    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 cache ONLY — never fetches upstream, and unlike the
    loader below it does not care how stale the record 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."""
    hit = _read_recent_backed(cell["id"])
    if hit is None:
        return None
    try:
        return _derive_metrics(_with_doy(hit[0]))
    except Exception:  # noqa: BLE001 - a truncated/corrupt cache record is a miss, not a crash
        return None


def _fetch_recent_forecast(cell: dict) -> pl.DataFrame:
    """Recent observations + forward forecast WITHOUT Open-Meteo: the recent observed
    window from a NASA POWER range (measured), the forward days from MET Norway,
    merged by date. Gusts — which neither source carries — are filled from Meteostat:
    measured for the observed days, estimated from wind for the forecast days."""
    today = datetime.date.today()
    start = (today - datetime.timedelta(days=RECENT_PAST_DAYS)).isoformat()
    end = (today - datetime.timedelta(days=RECENT_END_LAG_DAYS)).isoformat()
    past = _fetch_history_nasa(cell, start=start, end=end)   # measured + Meteostat gusts
    fwd = meteostat.fill_gusts(
        cell["center_lat"], cell["center_lon"], _fetch_forecast_metno(cell))
    # Forecast wins on any overlapping day (freshest model value for today); the NASA
    # observed days fill the recent past MET Norway lacks.
    return (pl.concat([past, fwd], how="diagonal_relaxed")
            .unique(subset="date", keep="last", maintain_order=True)
            .sort("date"))


def _fetch_recent_forecast_om(cell: dict) -> pl.DataFrame:
    """Fallback recent+forecast bundle from the Open-Meteo forecast API (the former
    primary): recent past + forward days in one call."""
    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,
    }
    r = _request(FORECAST_URL, params, 60, phase="recent_forecast_fetch")
    return _to_frame(r.json()["daily"])


def _load_recent_forecast(cell: dict) -> pl.DataFrame:
    """Recent observations AND the forward forecast for a cell.

    The primary source is keyless and Open-Meteo-free: a NASA POWER recent-past range
    plus the MET Norway forward forecast (see _fetch_recent_forecast). The Open-Meteo
    forecast API is the fallback (skipped while it's in its own rate-limit cooldown,
    see _note_forecast_rate_limit), then a stale cache. Cached per cell for
    FORECAST_TTL_HOURS so it follows model updates without per-hour upstream IO.
    """
    cell_id = cell["id"]
    hit = _read_recent_backed(cell_id)
    if hit is not None:
        cached, age_s = hit
        if age_s / 3600.0 < FORECAST_TTL_HOURS:
            return _with_doy(cached)

    df = None
    try:
        df = _fetch_recent_forecast(cell)          # NASA recent + MET forward (primary)
    except Exception:  # noqa: BLE001 - keyless primary unavailable; try Open-Meteo
        pass

    forecast_error = None
    if df is None and time.time() >= _forecast_cooldown_until:
        # Fall back to the Open-Meteo forecast API, skipped while it's in its own
        # rate-limit cooldown so a brownout doesn't retry-storm the endpoint.
        try:
            df = _fetch_recent_forecast_om(cell)
        except Exception as e:  # noqa: BLE001
            if is_rate_limit(e):
                _note_forecast_rate_limit(e)
            forecast_error = e

    if df is None:
        # Last resort: serve the stale cache if we have one, WITHOUT rewriting it, so
        # recent_stamp stays old and the next request retries upstream first (mirrors
        # _load_history's stale-serve).
        if hit is not None:
            return _with_doy(hit[0])
        # No cache either: surface the error, classifying an Open-Meteo rate limit as
        # the typed, daily-aware WeatherUnavailable. forecast_error is None when the
        # fallback was skipped outright (cooldown active) — report the cooldown then.
        if forecast_error is not None and is_rate_limit(forecast_error):
            daily = "daily" in _rate_limit_reason(forecast_error).lower()
            raise WeatherUnavailable(limit_message(daily), daily=daily) from forecast_error
        if forecast_error is not None:
            raise forecast_error
        raise WeatherUnavailable(limit_message(_forecast_limit_daily),
                                 daily=_forecast_limit_daily)
    _write_recent_backed(cell_id, df)
    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 the actual fetch + pacing run on ONE
# dedicated worker thread (see _revgeo_worker), never on a caller's own thread —
# in the server, a caller's thread is one of Starlette's shared sync threadpool
# threads, and the old design serialized AND slept (up to _REVGEO_MIN_INTERVAL)
# right there, pinning a threadpool thread for the whole wait. A single worker
# draining a queue gets the same ~1/sec pacing for free (nothing else ever calls
# Nominatim) without blocking anything but itself.
_REVGEO_MIN_INTERVAL = 1.1   # seconds between successive Nominatim reverse calls
_revgeo_last = 0.0

_REVGEO_QUEUE: "queue.Queue[tuple[float, float, str, concurrent.futures.Future]]" = queue.Queue()
_REVGEO_WORKER_LOCK = threading.Lock()
_revgeo_worker_started = False
_REVGEO_WAIT_TIMEOUT = 10.0   # seconds a caller waits for ITS OWN request before giving up


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 _fetch_revgeo_label(lat: float, lon: float) -> str | None:
    """The actual Nominatim reverse-geocode HTTP call + label assembly. Called
    ONLY from _revgeo_worker (never on a caller's thread) — returns None (never
    raises) so a bad response degrades to bare coordinates instead of killing the
    worker thread."""
    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)
        return ", ".join(parts) or None
    except Exception:  # noqa: BLE001 - reverse geocoding is a nicety, never fatal
        return None


def _revgeo_worker() -> None:
    """Drains _REVGEO_QUEUE one request at a time, forever. The sole caller of
    _fetch_revgeo_label, so the ~1/sec pacing below is enforced just by doing the
    work serially — no lock needed, since nothing else ever touches Nominatim.
    Re-checks the cache before fetching (a request queued behind an identical one
    is answered from what the earlier request just cached, no duplicate call),
    and always finishes the fetch + persists the result even if the original
    caller already gave up waiting (see reverse_geocode's timeout)."""
    global _revgeo_last
    while True:
        lat, lon, key, fut = _REVGEO_QUEUE.get()
        try:
            found, label = reverse_geocode_cached(lat, lon)
            if not found:
                wait = _REVGEO_MIN_INTERVAL - (time.monotonic() - _revgeo_last)
                if wait > 0:
                    time.sleep(wait)
                label = _fetch_revgeo_label(lat, lon)
                _revgeo_last = time.monotonic()
                _REVGEO_CACHE[key] = label
                store.put_revgeo(key, label)   # survive restarts (a None label retries after its TTL)
            if not fut.done():
                fut.set_result(label)
        except Exception:  # noqa: BLE001 - never let a bad request kill the worker
            if not fut.done():
                fut.set_result(None)
        finally:
            _REVGEO_QUEUE.task_done()


def _start_revgeo_worker() -> None:
    """Start the dedicated reverse-geocode worker thread, once per process
    (idempotent, thread-safe). Lazy (on first use) rather than at import, since
    climate.py has no app-lifespan hook of its own — mirrors data/places.py's
    start_loading()."""
    global _revgeo_worker_started
    with _REVGEO_WORKER_LOCK:
        if _revgeo_worker_started:
            return
        _revgeo_worker_started = True
    threading.Thread(target=_revgeo_worker, name="revgeo-worker", daemon=True).start()


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.

    The actual fetch (and Nominatim's pacing) happens on the dedicated
    _revgeo_worker thread, not here — this just enqueues the request and waits up
    to _REVGEO_WAIT_TIMEOUT for its result, so a burst of concurrent uncached
    lookups (the compare page fires several at once) queues behind the ~1/sec
    limit without pinning a caller's (in the server, a shared threadpool) thread
    for the wait. A timed-out wait returns None like any other failed lookup —
    the caller falls back to bare coordinates — but the worker keeps going and
    still caches the answer for the next call.
    """
    key = store.revgeo_key(lat, lon)
    found, label = reverse_geocode_cached(lat, lon)
    if found:
        return label
    _start_revgeo_worker()
    fut: "concurrent.futures.Future[str | None]" = concurrent.futures.Future()
    _REVGEO_QUEUE.put((lat, lon, key, fut))
    try:
        return fut.result(timeout=_REVGEO_WAIT_TIMEOUT)
    except concurrent.futures.TimeoutError:
        return None


def geocode_nominatim(name: str, count: int = 5) -> list[dict]:
    """Forward place-name lookup via OpenStreetMap Nominatim (keyless).

    Replaces the former Open-Meteo geocoder. Nominatim covers the long tail the
    local GeoNames index can't — neighbourhoods, postcodes, sub-1000-population
    villages, and alternate/native-language spellings — so /geocode falls back to
    it on a local miss. It carries no population, so results keep Nominatim's own
    relevance order; name/admin/country map straight across.

    Shares the reverse geocoder's lock and ~1/sec pacing: both hit the same host,
    so serializing them together keeps total Nominatim traffic under the usage
    policy. Only the low-volume /geocode miss path reaches here — autocomplete
    (/suggest) is served purely from the local index and never calls out.
    """
    global _revgeo_last
    with _REVGEO_LOCK:
        wait = _REVGEO_MIN_INTERVAL - (time.monotonic() - _revgeo_last)
        if wait > 0:
            time.sleep(wait)
        try:
            r = _request(
                "https://nominatim.openstreetmap.org/search",
                {"q": name, "format": "jsonv2", "addressdetails": 1,
                 "limit": count, "accept-language": "en"},
                30,
                phase="geocode",
                headers={"User-Agent": "Thermograph/0.1 (local weather grading app)"},
            )
            rows = r.json() or []
        finally:
            _revgeo_last = time.monotonic()
    out = []
    for g in rows:
        a = g.get("address", {}) or {}
        # jsonv2 gives a `name` for named features; for addresses/areas fall back to
        # the most specific place component, then the head of display_name.
        label = (g.get("name")
                 or a.get("city") or a.get("town") or a.get("village")
                 or a.get("hamlet") or a.get("suburb") or a.get("neighbourhood")
                 or (g.get("display_name") or "").split(",")[0].strip() or None)
        cc = (a.get("country_code") or "").upper() or None
        out.append({
            "name": label,
            "admin1": a.get("state") or a.get("province") or a.get("region"),
            "country": a.get("country"),
            "country_code": cc,
            "lat": float(g["lat"]) if g.get("lat") is not None else None,
            "lon": float(g["lon"]) if g.get("lon") is not None else None,
            "population": None,   # Nominatim has no population; order is relevance-based
        })
    return out