"""
Filters and transforms raw AEMO FPPDAILY CSV bytes using Polars.
"""
import io
import logging
from datetime import date, datetime, time as dtime, timedelta

import polars as pl
import pyarrow.parquet as pq
import pyarrow as pa

from app.config import REQUIRED_COLUMNS

logger = logging.getLogger(__name__)


class DataProcessingError(Exception):
    pass


# NEM market day boundary: 04:00 AEST (UTC+10, no daylight saving).
# All AEMO NEMWEB timestamps are in AEST.
_DAY_START_HOUR = 4


def _segment_to_df(header: list[str], data_lines: list[list[str]]) -> pl.DataFrame:
    """Build a Polars DataFrame from one parsed AEMO CSV segment."""
    n_cols = len(header)
    padded = []
    for row in data_lines:
        if len(row) >= n_cols:
            padded.append(row[:n_cols])
        else:
            padded.append(row + [""] * (n_cols - len(row)))
    csv_content = ",".join(header) + "\n" + "\n".join(",".join(r) for r in padded)
    return pl.read_csv(io.StringIO(csv_content), infer_schema_length=None)


def _parse_aemo_csv(csv_bytes: bytes) -> pl.DataFrame:
    """
    AEMO CSV files have a non-standard header format:
      - "C,..." rows: comment / metadata (skipped)
      - "I,..." row:  column headers (format: I, TABLE, SUBTABLE, col1, col2, …)
      - "D,..." rows: data

    When csv_bytes is the concatenation of two half-day files, there will be
    TWO "I," lines.  The two files may have different column schemas (e.g.
    FPPMW first-half vs. FPPMW_2 second-half formats differ).  To avoid
    silently misaligning rows from the first file under the second file's
    header, we treat every "I," line as the start of a new segment, parse
    each segment independently, and then reconcile the schemas via a
    diagonal concat (missing columns are filled with null).
    """
    text = csv_bytes.decode("utf-8", errors="replace")
    lines = text.splitlines()

    segments: list[tuple[list[str], list[list[str]]]] = []
    current_header: list[str] | None = None
    current_data: list[list[str]] = []

    for line in lines:
        if not line.strip():
            continue
        if line.startswith("I,"):
            parts = line.split(",")
            # Strip the leading "I, TABLE_NAME, SUBTABLE" — columns start at index 3
            if len(parts) > 3:
                # Flush accumulated data for the previous header before switching
                if current_header is not None and current_data:
                    segments.append((current_header, current_data))
                    current_data = []
                current_header = parts[3:]
        elif line.startswith("D,"):
            parts = line.split(",")
            if len(parts) > 3:
                current_data.append(parts[3:])

    # Flush the final segment
    if current_header is not None and current_data:
        segments.append((current_header, current_data))

    if not segments:
        if current_header is None:
            raise DataProcessingError("Could not find column header row in CSV file.")
        raise DataProcessingError("CSV file contained no data rows.")

    if len(segments) == 1:
        return _segment_to_df(*segments[0])

    # Multiple segments (concatenated files): parse each independently so that
    # each file's own column header governs its own data rows.
    dfs = []
    for i, (hdr, rows) in enumerate(segments):
        logger.debug("CSV segment %d: %d columns, %d rows — %s", i, len(hdr), len(rows), hdr)
        dfs.append(_segment_to_df(hdr, rows))

    col_sets = [set(df.columns) for df in dfs]
    if len(set(frozenset(s) for s in col_sets)) > 1:
        logger.warning(
            "Multi-segment CSV has differing schemas across %d segments; "
            "using diagonal concat (missing columns filled with null). "
            "Segment column sets: %s",
            len(dfs),
            [sorted(s) for s in col_sets],
        )

    try:
        return pl.concat(dfs, how="diagonal")
    except Exception as exc:
        logger.warning("diagonal concat failed (%s); falling back to first segment only", exc)
        return dfs[0]


def _find_duid_col(df: pl.DataFrame) -> str:
    """
    Return the column name used as the unit identifier.

    FPP format (up to Jan 10 2026) uses 'FPP_UNITID'.
    FPPMW format (from Jan 11 2026 / Mar 2025 archive) uses 'DUID'.
    """
    for candidate in ("FPP_UNITID", "DUID"):
        if candidate in df.columns:
            return candidate
    raise DataProcessingError(
        "Cannot find unit identifier column (expected 'FPP_UNITID' or 'DUID'). "
        "AEMO may have changed the file format."
    )


def _prefilter_bytes_by_duid(csv_bytes: bytes, duid: str) -> bytes:
    """
    Pre-filter raw AEMO CSV bytes, keeping only header/comment lines and
    data rows that contain the target DUID.

    Era 1 files are ~1 GB with 9.3M rows for ALL units.  Parsing the full
    file into a DataFrame before filtering is extremely expensive (~6 GB
    peak RAM).  By scanning raw bytes first we reduce to ~1-2 MB, making
    subsequent parsing fast and lightweight.
    """
    duid_bytes = duid.encode("utf-8")
    kept: list[bytes] = []
    for line in csv_bytes.split(b"\n"):
        if not line:
            continue
        if line[:2] in (b"I,", b"C,"):
            kept.append(line)
        elif line[:2] == b"D," and duid_bytes in line:
            kept.append(line)
    return b"\n".join(kept)


def _parse_and_filter_duid(csv_bytes: bytes, duid: str) -> pl.DataFrame:
    """Parse CSV bytes, filter to the given DUID, return raw (uncast) DataFrame."""
    # Pre-filter raw bytes to drastically reduce memory for large Era 1 files
    filtered_bytes = _prefilter_bytes_by_duid(csv_bytes, duid)
    df = _parse_aemo_csv(filtered_bytes)

    missing = [c for c in REQUIRED_COLUMNS if c not in df.columns]
    if missing:
        raise DataProcessingError(
            f"CSV missing expected columns: {missing}. "
            "AEMO may have changed the file format."
        )

    duid_col = _find_duid_col(df)
    return df.filter(pl.col(duid_col) == duid).select(REQUIRED_COLUMNS)


def filter_and_process(
    csv_bytes_or_chunks: bytes | list[bytes],
    duid: str,
    target_date: date,
    csv_bytes_next: bytes | None = None,
) -> pl.DataFrame:
    """
    Parse and filter CSV data to the requested DUID, covering exactly one NEM
    market day: 04:00 AEST on target_date → 04:00 AEST on target_date + 1 day.

    csv_bytes_or_chunks can be a single bytes object or a list of byte chunks.
    When a list is provided, each chunk is parsed and DUID-filtered independently
    so that large files (Era 1: ~1 GB each) are never all in memory at once.

    AEMO NEMWEB timestamps are in AEST (UTC+10, no daylight saving).
    The strict [04:00 AEST D, 04:00 AEST D+1) boundary filter is applied,
    so we never include data from a neighbouring market day.
    """
    # Normalise input to a list of byte chunks
    if isinstance(csv_bytes_or_chunks, bytes):
        chunks = [csv_bytes_or_chunks]
        if csv_bytes_next is not None:
            chunks.append(csv_bytes_next)
    else:
        chunks = csv_bytes_or_chunks

    # Parse and DUID-filter each chunk independently to limit peak memory
    dfs: list[pl.DataFrame] = []
    for i, chunk in enumerate(chunks):
        try:
            df_chunk = _parse_and_filter_duid(chunk, duid)
            logger.info("Chunk %d: %d rows after DUID filter (%s)", i, len(df_chunk), duid)
            if not df_chunk.is_empty():
                dfs.append(df_chunk)
        except Exception as exc:
            logger.warning("Could not process chunk %d; skipping: %s", i, exc)

    if not dfs:
        raise DataProcessingError(
            f"No data found for DUID '{duid}' on this date. "
            "This unit may not have been operational or eligible for FPP on this date."
        )

    df = pl.concat(dfs) if len(dfs) > 1 else dfs[0]

    # Cast to proper types
    df = df.with_columns([
        pl.col("INTERVAL_DATETIME").str.to_datetime(
            format="%Y/%m/%d %H:%M:%S", strict=False
        ),
        pl.col("MEASUREMENT_DATETIME").str.to_datetime(
            format="%Y/%m/%d %H:%M:%S", strict=False
        ),
        pl.col("MEASURED_MW").cast(pl.Float64, strict=False),
        pl.col("MW_QUALITY_FLAG").cast(pl.Int32, strict=False),
    ])

    # Log how many MEASUREMENT_DATETIME values parsed successfully
    n_parsed = df["MEASUREMENT_DATETIME"].drop_nulls().len()
    logger.info(
        "MEASUREMENT_DATETIME parsed: %d/%d rows (format '%%Y/%%m/%%d %%H:%%M:%%S')",
        n_parsed, len(df),
    )

    # Apply the NEM market day boundary in AEST.
    # Timestamps in the CSVs are naive AEST datetimes; we compare directly.
    day_start = datetime.combine(target_date, dtime(_DAY_START_HOUR, 0, 0))
    day_end   = datetime.combine(target_date + timedelta(days=1), dtime(_DAY_START_HOUR, 0, 0))
    df = df.filter(
        (pl.col("MEASUREMENT_DATETIME") >= day_start) &
        (pl.col("MEASUREMENT_DATETIME") <  day_end)
    )
    logger.info(
        "After date-window filter [%s, %s): %d rows",
        day_start.strftime("%Y-%m-%d %H:%M"),
        day_end.strftime("%Y-%m-%d %H:%M"),
        len(df),
    )

    if df.is_empty():
        raise DataProcessingError(
            f"No data found for DUID '{duid}' within the NEM market day "
            f"({day_start.strftime('%d %b %Y %H:%M')}–"
            f"{day_end.strftime('%d %b %Y %H:%M')} AEST). "
            "This unit may not have been operational or eligible for FPP on this date."
        )

    # Deduplicate on MEASUREMENT_DATETIME (boundary rows may appear in both
    # the D and D+1 files) then sort chronologically.
    return (
        df.unique(subset=["MEASUREMENT_DATETIME"], keep="first")
          .sort("MEASUREMENT_DATETIME")
    )


def compute_summary(df: pl.DataFrame) -> dict:
    """Compute summary statistics for the filtered data."""
    # fill_nan converts float NaN → null before drop_nulls so that NaN values
    # (which Polars treats as distinct from null) don't silently corrupt stats.
    mw = df["MEASURED_MW"].fill_nan(None).drop_nulls()
    flags = df["MW_QUALITY_FLAG"].value_counts().sort("MW_QUALITY_FLAG")
    total = len(df)

    flag_breakdown = {}
    for row in flags.iter_rows(named=True):
        flag = str(row["MW_QUALITY_FLAG"])
        count = row["count"]
        flag_breakdown[flag] = {
            "count": count,
            "pct": round(100 * count / total, 1) if total > 0 else 0,
        }

    def _safe(val) -> float | None:
        return round(float(val), 3) if val is not None else None

    return {
        "total_rows": total,
        "min_mw":  _safe(mw.min()  if len(mw) > 0 else None),
        "max_mw":  _safe(mw.max()  if len(mw) > 0 else None),
        "mean_mw": _safe(mw.mean() if len(mw) > 0 else None),
        "std_mw":  _safe(mw.std()  if len(mw) > 0 else None),
        "flag_breakdown": flag_breakdown,
    }


def to_csv_bytes(df: pl.DataFrame) -> bytes:
    """Serialize DataFrame to CSV bytes with space-separated datetimes (no T)."""
    out = df.with_columns([
        pl.col("INTERVAL_DATETIME").dt.strftime("%Y-%m-%d %H:%M:%S"),
        pl.col("MEASUREMENT_DATETIME").dt.strftime("%Y-%m-%d %H:%M:%S"),
    ])
    return out.write_csv().encode("utf-8")


def to_parquet_bytes(df: pl.DataFrame) -> bytes:
    """Serialize DataFrame to Parquet bytes."""
    buf = io.BytesIO()
    df.write_parquet(buf)
    return buf.getvalue()


def to_json_records(df: pl.DataFrame, max_rows: int = 25_000) -> list[dict]:
    """
    Return data as a list of dicts for JSON response.
    Datetimes formatted as 'YYYY-MM-DD HH:MM:SS' (no T, no microseconds).
    Default cap of 25 000 rows covers a full 24-hour NEM day at 4-second
    resolution (21 600 rows) with headroom.
    """
    display_df = df.head(max_rows).with_columns([
        pl.col("INTERVAL_DATETIME").dt.strftime("%Y-%m-%d %H:%M:%S"),
        pl.col("MEASUREMENT_DATETIME").dt.strftime("%Y-%m-%d %H:%M:%S"),
        # Polars float NaN is not JSON-serializable; normalise to null (→ JSON null).
        pl.col("MEASURED_MW").fill_nan(None),
    ])
    return display_df.to_dicts()