"""CSV import/export for the spool inventory (#1576).

One module owns the round-trip: the same fixed column schema is used to
serialise existing spools out and to parse + validate a user-supplied CSV
back in. Validation reuses the `SpoolCreate` Pydantic model so the CSV path
and the form path share a single source of truth — anything the form rejects,
the import rejects too, with the same rules.

The import flow is two-phase by design: `parse_and_validate()` never writes.
The route calls it once for the dry-run preview (so the user sees per-row
valid/error/skipped before committing) and again on confirm, then persists
only the rows that came back `valid`.
"""

import csv
import io
from datetime import datetime

from pydantic import BaseModel, ValidationError
from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession

from backend.app.models.color_catalog import ColorCatalogEntry
from backend.app.models.spool import Spool
from backend.app.schemas.spool import SpoolCreate

# Fixed CSV header, in output order. Round-trips cleanly: export writes these
# columns, import expects them. `material` is the only required field; the rest
# are optional. Keep aligned with the SpoolCreate fields referenced below.
#
# `remaining` is a derived, export-only column (= label_weight - weight_used).
# It's written out for human readability and round-trip clarity, but ignored on
# import — `weight_used` is the source of truth, and accepting both would let
# them contradict. `last_used` is a timestamp the model carries but SpoolCreate
# does not, so import applies it to the ORM object directly (see persist path).
# `storage_location`, `category` and `low_stock_threshold_pct` are SpoolCreate
# fields included so a round-trip preserves them (they'd otherwise be lost).
CSV_COLUMNS = [
    "material",
    "brand",
    "subtype",
    "color_name",
    "rgba",
    "extra_colors",
    "effect_type",
    "label_weight",
    "weight_used",
    "remaining",
    "cost_per_kg",
    "nozzle_temp_min",
    "nozzle_temp_max",
    "last_used",
    "note",
    "storage_location",
    "category",
    "low_stock_threshold_pct",
]

# Upload ceiling for the import endpoint. A spool inventory CSV is a few KB
# even with thousands of rows; 5 MB is a generous cap that still refuses an
# OOM-sized body before it's read into memory.
MAX_CSV_IMPORT_BYTES = 5 * 1024 * 1024

# Spreadsheet formula-injection guard. A cell whose first character is one of
# these is treated as a formula by Excel / LibreOffice / Sheets; we prefix it
# with a single quote on export so the value renders as literal text.
_FORMULA_INJECTION_PREFIXES = ("=", "+", "-", "@", "\t", "\r")

# Columns whose CSV cell must be coerced to a number before SpoolCreate sees it.
# DictReader hands us strings; SpoolCreate wants int/float. Empty cell → omit
# the field (falls back to the schema default / None).
_INT_COLUMNS = {"label_weight", "nozzle_temp_min", "nozzle_temp_max", "low_stock_threshold_pct"}
_FLOAT_COLUMNS = {"cost_per_kg", "weight_used"}

# label_weight default, pulled from the schema so the weight_used bounds check
# stays in sync if the schema default ever changes.
_DEFAULT_LABEL_WEIGHT = SpoolCreate.model_fields["label_weight"].default


class ImportRowResult(BaseModel):
    """Per-row outcome of a parse+validate pass.

    `spool` carries the validated, SpoolCreate-shaped dict for `valid` rows so
    the route can persist without re-parsing. `resolved_color` flags rows whose
    rgba/extra_colors/effect_type were filled in from the Color Catalog rather
    than supplied in the CSV — surfaced in the preview so the user knows a
    colour was inferred.
    """

    row_number: int  # 1-based data row (header is not counted)
    status: str  # "valid" | "error" | "skipped"
    reason: str | None = None
    material: str | None = None
    brand: str | None = None
    color_name: str | None = None
    rgba: str | None = None
    resolved_color: bool = False
    # True when the colour was resolved from a catalog entry of a DIFFERENT
    # material (no exact material match existed). Surfaced so the preview can
    # warn the user the colour came from another material's variant.
    cross_material_color: bool = False
    # True when an active spool with the same material+brand+color_name already
    # exists. Informational only — the import still creates the row (there's no
    # unique constraint); the preview warns so a double-click / re-upload of the
    # same CSV doesn't silently duplicate the inventory.
    duplicate_of_existing: bool = False
    spool: dict | None = None


class ImportPreview(BaseModel):
    """Result of a dry-run (or the pre-write pass of a real import)."""

    columns: list[str]
    total: int
    valid_count: int
    error_count: int
    skipped_count: int
    rows: list[ImportRowResult]
    warnings: list[str] = []


class ImportResult(BaseModel):
    """Summary returned after a real (non-dry-run) import."""

    created: int
    skipped: int
    errors: int
    error_rows: list[ImportRowResult] = []


def _normalize_header(name: str) -> str:
    """Map a CSV header cell to a canonical field name.

    Case- and space-tolerant: "Color Name", "color-name", " COLOR_NAME "
    all collapse to "color_name".
    """
    return name.strip().lower().replace(" ", "_").replace("-", "_")


def _normalize_rgba(value: str) -> str | None:
    """Coerce a user-supplied colour cell to 8-char RRGGBBAA hex, or None.

    Accepts an optional leading `#` and a 6-char RRGGBB form (alpha defaults to
    `ff`). Returns None if the value isn't valid hex of length 6 or 8 — the
    caller turns that into a row error so it isn't silently dropped.
    """
    raw = value.strip().lstrip("#")
    if len(raw) not in (6, 8):
        return None
    try:
        int(raw, 16)
    except ValueError:
        return None
    if len(raw) == 6:
        raw += "ff"
    return raw.lower()


def _parse_datetime(value: str) -> datetime | None:
    """Parse an ISO-8601 timestamp, or None if it isn't valid.

    Accepts what `datetime.isoformat()` produces (what export writes) plus a
    trailing 'Z' for UTC, which `fromisoformat` rejects before Python 3.11.
    """
    raw = value.strip()
    if not raw:
        return None
    if raw.endswith("Z"):
        raw = raw[:-1] + "+00:00"
    try:
        return datetime.fromisoformat(raw)
    except ValueError:
        return None


async def _load_color_catalog(db: AsyncSession) -> list[ColorCatalogEntry]:
    """Load the whole Color Catalog once so per-row resolution is in-memory.

    A CSV can hold hundreds of rows; resolving each with its own SELECT would
    be an N+1 against a small, rarely-changing table. We pull it once here and
    let `_resolve_color` match against the list.
    """
    result = await db.execute(select(ColorCatalogEntry))
    return list(result.scalars().all())


def _spool_key(material: str | None, brand: str | None, color_name: str | None) -> tuple[str, str, str]:
    """Case/space-insensitive identity used for the duplicate soft-warn."""
    return (
        (material or "").strip().lower(),
        (brand or "").strip().lower(),
        (color_name or "").strip().lower(),
    )


async def _load_existing_spool_keys(db: AsyncSession) -> set[tuple[str, str, str]]:
    """Load material+brand+color_name keys of active spools for the dup warning.

    Spool has no unique constraint, so a double-click or re-upload of the same
    CSV would silently duplicate the inventory. We pull the active spools' keys
    once and let the preview flag matching rows — informational only, the import
    still creates them.
    """
    result = await db.execute(select(Spool.material, Spool.brand, Spool.color_name).where(Spool.archived_at.is_(None)))
    return {_spool_key(m, b, c) for m, b, c in result.all()}


def _resolve_color(
    catalog: list[ColorCatalogEntry], brand: str | None, color_name: str | None, material: str | None
) -> tuple[str, str | None, str | None, bool] | None:
    """Match brand + color_name against the preloaded catalog (case-insensitive).

    Returns (rgba, extra_colors, effect_type, cross_material) on a match, else
    None. Prefers an entry whose material matches the row; a catalog entry with
    a NULL material is the project's "matches any material" convention and counts
    as an exact match too. Only when neither exists does it fall back to another
    material's entry and set cross_material=True so the caller can warn that the
    colour came from a different material's variant.
    """
    if not brand or not color_name:
        return None

    brand_l = brand.strip().lower()
    name_l = color_name.strip().lower()
    material_l = material.strip().lower() if material else None

    matches = [
        entry
        for entry in catalog
        if entry.hex_color and entry.manufacturer.lower() == brand_l and entry.color_name.lower() == name_l
    ]
    if not matches:
        return None

    exact = next(
        (e for e in matches if e.material is None or (material_l and e.material.lower() == material_l)),
        None,
    )
    row = exact or matches[0]
    cross_material = exact is None

    rgba = _normalize_rgba(row.hex_color)
    if rgba is None:
        return None
    return rgba, row.extra_colors, row.effect_type, cross_material


def _readable_validation_error(exc: ValidationError) -> str:
    """Flatten a Pydantic ValidationError into one short, user-facing line."""
    parts = []
    for err in exc.errors():
        loc = ".".join(str(p) for p in err.get("loc", ())) or "value"
        parts.append(f"{loc}: {err.get('msg', 'invalid')}")
    return "; ".join(parts)


def _empty_preview(warnings: list[str]) -> ImportPreview:
    """A preview with no rows — used for the early-exit cases (bad/empty file)."""
    return ImportPreview(
        columns=CSV_COLUMNS,
        total=0,
        valid_count=0,
        error_count=0,
        skipped_count=0,
        rows=[],
        warnings=warnings,
    )


async def parse_and_validate(raw_bytes: bytes, db: AsyncSession) -> ImportPreview:
    """Parse a CSV blob, validate + colour-resolve each row. Never writes.

    Decodes UTF-8 (BOM tolerant), reads with DictReader against the fixed
    schema, and classifies each row as valid / error / skipped. Valid rows
    carry a SpoolCreate-shaped `spool` dict ready to persist.
    """
    warnings: list[str] = []

    try:
        text = raw_bytes.decode("utf-8-sig")
    except UnicodeDecodeError:
        return _empty_preview(["File is not valid UTF-8 text."])

    reader = csv.reader(io.StringIO(text))
    try:
        header = next(reader)
    except StopIteration:
        return _empty_preview(["CSV is empty."])

    norm_header = [_normalize_header(h) for h in header]
    known = set(CSV_COLUMNS)
    unknown = [h for h in norm_header if h and h not in known]
    if unknown:
        warnings.append(f"Ignoring unknown columns: {', '.join(unknown)}")
    # Map canonical field name → column index in this file (first occurrence).
    col_index: dict[str, int] = {}
    for idx, h in enumerate(norm_header):
        if h in known and h not in col_index:
            col_index[h] = idx

    if "material" not in col_index:
        return _empty_preview(warnings + ["Required column 'material' is missing from the header."])

    # Pull the catalog and the existing-spool keys once; per-row colour
    # resolution and the duplicate soft-warn both match in memory rather than
    # issuing a SELECT per row.
    catalog = await _load_color_catalog(db)
    existing_keys = await _load_existing_spool_keys(db)

    def cell(row: list[str], field: str) -> str:
        idx = col_index.get(field)
        if idx is None or idx >= len(row):
            return ""
        # Strip whitespace, then undo any export-side formula-injection quoting
        # so export → import round-trips without accumulating a leading quote.
        return _desanitize_cell(row[idx].strip())

    rows: list[ImportRowResult] = []
    valid = error = skipped = 0

    for row_number, raw_row in enumerate(reader, start=1):
        # Fully blank row (no non-empty cell) → skip silently.
        if not any(c.strip() for c in raw_row):
            rows.append(ImportRowResult(row_number=row_number, status="skipped", reason="Empty row"))
            skipped += 1
            continue

        material = cell(raw_row, "material")
        brand = cell(raw_row, "brand") or None
        color_name = cell(raw_row, "color_name") or None

        if not material:
            rows.append(
                ImportRowResult(
                    row_number=row_number,
                    status="error",
                    reason="material is required",
                    brand=brand,
                    color_name=color_name,
                )
            )
            error += 1
            continue

        data: dict = {"material": material}
        if brand:
            data["brand"] = brand
        if color_name:
            data["color_name"] = color_name

        row_error: str | None = None

        # Plain text passthrough columns.
        for field in ("subtype", "effect_type", "extra_colors", "note", "storage_location", "category"):
            value = cell(raw_row, field)
            if value:
                data[field] = value

        # Numeric columns: parse only if present, else leave to schema defaults.
        for field in _INT_COLUMNS:
            value = cell(raw_row, field)
            if value:
                try:
                    data[field] = int(value)
                except ValueError:
                    row_error = f"{field} must be a whole number (got '{value}')"
                    break
        if row_error is None:
            for field in _FLOAT_COLUMNS:
                value = cell(raw_row, field)
                if value:
                    try:
                        data[field] = float(value)
                    except ValueError:
                        row_error = f"{field} must be a number (got '{value}')"
                        break

        # Bounds check: weight_used must be within [0, label_weight]. The schema
        # accepts any float, so a negative or over-full value would otherwise be
        # imported silently. label_weight falls back to the schema default when
        # the CSV omits it.
        if row_error is None and "weight_used" in data:
            used = data["weight_used"]
            label = data.get("label_weight", _DEFAULT_LABEL_WEIGHT)
            if used < 0:
                row_error = f"weight_used cannot be negative (got {used})"
            elif used > label:
                row_error = f"weight_used ({used}) exceeds label_weight ({label})"

        # `last_used` is an ORM-only timestamp (not on SpoolCreate); parse it
        # here and apply it to the validated dict after the SpoolCreate gate.
        last_used: datetime | None = None
        if row_error is None:
            last_used_cell = cell(raw_row, "last_used")
            if last_used_cell:
                last_used = _parse_datetime(last_used_cell)
                if last_used is None:
                    row_error = f"last_used must be an ISO date/time (got '{last_used_cell}')"

        resolved_color = False
        cross_material_color = False
        if row_error is None:
            # Colour precedence: explicit rgba wins; else resolve brand+name
            # from the catalog; else leave blank.
            rgba_cell = cell(raw_row, "rgba")
            if rgba_cell:
                normalized = _normalize_rgba(rgba_cell)
                if normalized is None:
                    row_error = f"rgba must be 6- or 8-char hex (got '{rgba_cell}')"
                else:
                    data["rgba"] = normalized
            else:
                resolved = _resolve_color(catalog, brand, color_name, material)
                if resolved is not None:
                    rgba_val, extra_val, effect_val, cross_material_color = resolved
                    data["rgba"] = rgba_val
                    # CSV-supplied extra_colors/effect_type take precedence over
                    # the catalog's; only fill from catalog when absent.
                    if extra_val and "extra_colors" not in data:
                        data["extra_colors"] = extra_val
                    if effect_val and "effect_type" not in data:
                        data["effect_type"] = effect_val
                    resolved_color = True

        if row_error is not None:
            rows.append(
                ImportRowResult(
                    row_number=row_number,
                    status="error",
                    reason=row_error,
                    material=material,
                    brand=brand,
                    color_name=color_name,
                )
            )
            error += 1
            continue

        # Final gate: SpoolCreate runs the same validators the form uses
        # (rgba pattern, extra_colors/effect_type normalisation, bounds).
        try:
            spool = SpoolCreate(**data)
        except ValidationError as exc:
            rows.append(
                ImportRowResult(
                    row_number=row_number,
                    status="error",
                    reason=_readable_validation_error(exc),
                    material=material,
                    brand=brand,
                    color_name=color_name,
                )
            )
            error += 1
            continue

        spool_data = spool.model_dump()
        if last_used is not None:
            # last_used isn't a SpoolCreate field; graft it onto the persisted
            # dict so the ORM object carries it.
            spool_data["last_used"] = last_used

        rows.append(
            ImportRowResult(
                row_number=row_number,
                status="valid",
                material=material,
                brand=brand,
                color_name=color_name,
                rgba=spool.rgba,
                resolved_color=resolved_color,
                cross_material_color=cross_material_color,
                duplicate_of_existing=_spool_key(material, brand, color_name) in existing_keys,
                spool=spool_data,
            )
        )
        valid += 1

    return ImportPreview(
        columns=CSV_COLUMNS,
        total=valid + error + skipped,
        valid_count=valid,
        error_count=error,
        skipped_count=skipped,
        rows=rows,
        warnings=warnings,
    )


def serialize(spools: list[Spool]) -> bytes:
    """Render spools to CSV bytes using the fixed schema (export side).

    rgba is written without a leading `#`, matching the import-side
    normalisation, so export → import round-trips without transformation.
    `remaining` is derived (label_weight - weight_used) and `last_used` is
    written as ISO-8601; empty/None fields become empty cells.
    """
    output = io.StringIO()
    writer = csv.writer(output)
    writer.writerow(CSV_COLUMNS)
    for spool in spools:
        writer.writerow([_sanitize_cell(_cell_value(spool, col)) for col in CSV_COLUMNS])
    return output.getvalue().encode("utf-8")


def _sanitize_cell(value: str) -> str:
    """Neutralise spreadsheet formula injection.

    A free-text field (note, color_name) starting with =, +, -, @, tab, or CR
    is evaluated as a formula by Excel/Sheets/LibreOffice when the CSV is
    opened. Prefixing with a single quote forces it to render as literal text.
    `_desanitize_cell` is the exact inverse, applied on import.
    """
    if value and value[0] in _FORMULA_INJECTION_PREFIXES:
        return "'" + value
    return value


def _desanitize_cell(value: str) -> str:
    """Undo `_sanitize_cell` on import so the round-trip is lossless.

    Export prefixes formula-looking cells with a single quote; strip exactly
    that quote back off when the next character is one of the guarded prefixes,
    so `'=SUM(A1)` reads back as `=SUM(A1)` and the value doesn't accumulate a
    leading quote on every export→import cycle. A quote followed by anything
    else is left untouched — only the prefix `_sanitize_cell` could have added
    is removed.
    """
    if len(value) >= 2 and value[0] == "'" and value[1] in _FORMULA_INJECTION_PREFIXES:
        return value[1:]
    return value


def _cell_value(spool: Spool, col: str) -> str:
    """Render one spool field for export. Handles the derived `remaining`
    column and ISO-formats `last_used`; everything else is str() of the value."""
    if col == "remaining":
        # Derived for display: label_weight - weight_used, clamped at 0.
        return str(max(0, round((spool.label_weight or 0) - (spool.weight_used or 0))))
    value = getattr(spool, col, None)
    if value is None:
        return ""
    if isinstance(value, datetime):
        return value.isoformat()
    # Whole-number floats (weight_used, cost_per_kg) export as ints — "300",
    # not "300.0" — for a cleaner, human-friendly CSV. import re-parses fine.
    if isinstance(value, float) and value.is_integer():
        return str(int(value))
    return str(value)