#!/usr/bin/env python3
"""Repair timestamp_utc_s in an already-generated 100M-row file.

This fixes files generated on pandas builds where DatetimeIndex.asi8 did not
have nanosecond resolution. It does not alter meteorology, pollutants, masks,
event labels, station IDs, or split labels.
"""
from __future__ import annotations

import argparse
from pathlib import Path
import numpy as np
import pandas as pd

N_STATIONS = 1000
N_HOURS = 100_000
TZ = "Asia/Kolkata"
START_LOCAL = pd.Timestamp("2015-01-01 00:00:00", tz=TZ)


def expected_epoch_seconds() -> np.ndarray:
    idx = pd.date_range(START_LOCAL, periods=N_HOURS, freq="h")
    return (
        idx.tz_convert("UTC")
        .tz_localize(None)
        .to_numpy(dtype="datetime64[s]")
        .astype(np.int64)
    )


def main() -> None:
    ap = argparse.ArgumentParser()
    ap.add_argument("--input", default="air_quality_100m.npy")
    ap.add_argument("--block-stations", type=int, default=25)
    args = ap.parse_args()

    path = Path(args.input)
    if not path.exists():
        raise SystemExit(f"Not found: {path}")

    arr = np.load(path, mmap_mode="r+")
    if arr.shape != (N_STATIONS * N_HOURS,):
        raise SystemExit(f"Unexpected shape: {arr.shape}")
    if "timestamp_utc_s" not in arr.dtype.names:
        raise SystemExit("timestamp_utc_s field not found")

    ts = expected_epoch_seconds()
    print("Expected start UTC:", pd.to_datetime(int(ts[0]), unit="s", utc=True))
    print("Expected end UTC:  ", pd.to_datetime(int(ts[-1]), unit="s", utc=True))
    print("Cadence seconds:    ", np.unique(np.diff(ts)))

    block = max(1, args.block_stations)
    for s0 in range(0, N_STATIONS, block):
        s1 = min(N_STATIONS, s0 + block)
        for sid in range(s0, s1):
            lo = sid * N_HOURS
            hi = lo + N_HOURS
            arr[lo:hi]["timestamp_utc_s"] = ts
        arr.flush()
        print(f"Patched stations {s0:04d}..{s1-1:04d}")

    # Final independent structural check.
    first = arr[:N_HOURS]["timestamp_utc_s"]
    last = arr[(N_STATIONS-1)*N_HOURS:N_STATIONS*N_HOURS]["timestamp_utc_s"]
    assert np.array_equal(first, ts)
    assert np.array_equal(last, ts)
    assert np.all(np.diff(first) == 3600)

    print("Timestamp repair: PASS")
    print("Start local:", pd.to_datetime(int(ts[0]), unit="s", utc=True).tz_convert(TZ))
    print("End local:  ", pd.to_datetime(int(ts[-1]), unit="s", utc=True).tz_convert(TZ))


if __name__ == "__main__":
    main()