"""Diff engine — catalog × canonical products → apply plan + preview records (SD-0002 §6.5.2).

Classifies each canonical product against the storefront's current catalog as
add / update / unchanged / error. One walk produces two views of the same
computation: a typed apply *plan* carrying resolved native values (Decimal etc.)
for the confirm transaction, and JSON-ready preview records derived from that
walk (stored as draft JSONB, served verbatim to the SPA) — so what confirm
applies is exactly what preview showed (INV-11). A summary and a deterministic
fingerprint over the records detect catalog drift between preview and confirm.

Deliberately DB-free: the catalog snapshot dataclasses are defined here and the
repo layer builds them. Only fields present in the file's canonical fields{}
participate in a comparison — an absent column is untouched, never a change
(§6.5.1); a present-but-empty cell resolves to the field's CLEAR_DEFAULTS entry
— except a variant's position, whose default is the variant's 1-based file
order within its product ("defaults to file order"), resolved here at diff time.
Catalog variants/images absent from the file are likewise untouched (INV-10);
file variants match catalog variants by their option-value combination (INV-13).
"""
from __future__ import annotations

import hashlib
import json
from dataclasses import dataclass, field, replace
from decimal import Decimal

from . import hosted
from .models import CLEAR_DEFAULTS, CanonicalProduct, CanonicalVariant, RowError


@dataclass
class CatalogVariant:
    id: int
    options: tuple[str | None, str | None, str | None]
    position: int
    # sku, barcode, price (Decimal|None), cost, weight, weight_unit, volume,
    # volume_unit, tax_id_1, tax_id_2, inventory_tracker, inventory_qty,
    # variant_image (linked image source_url or None)
    fields: dict[str, object]


@dataclass
class CatalogImage:
    id: int
    source_url: str
    position: int
    alt_text: str | None
    status: str = "pending"


@dataclass
class CatalogProduct:
    id: int
    handle: str
    title: str
    option_names: tuple[str | None, str | None, str | None]
    # title, description_html, vendor, product_type, google_product_category,
    # tags (list[str]), status, published (bool)
    fields: dict[str, object]
    variants: list[CatalogVariant]
    images: list[CatalogImage]


# ---------------------------------------------------------------------------
# Apply plan — the typed twin of the preview records. confirm_draft executes
# these; the values are resolved natives (Decimal, bool, list), never the
# json-safe strings the records carry for display.
# ---------------------------------------------------------------------------


@dataclass
class VariantPlan:
    kind: str  # "add" | "update"
    canonical: CanonicalVariant
    catalog_id: int | None  # None for add
    # 1-based index within the product's file variants — the position default.
    file_order: int = 0
    # field -> resolved after-value (update only); adds resolve from canonical.fields
    changes: dict[str, object] = field(default_factory=dict)


@dataclass
class ImagePlan:
    kind: str  # "add" | "update"
    source_url: str
    position: int
    alt_text: str | None
    image_id: int | None  # None for add
    changes: dict[str, object] = field(default_factory=dict)  # subset of {"position","alt_text"}


@dataclass
class ProductPlan:
    kind: str  # "add" | "update" | "unchanged" | "error"
    canonical: CanonicalProduct
    catalog: CatalogProduct | None
    # field -> resolved after-value (update only; may include title/option*_name)
    product_changes: dict[str, object] = field(default_factory=dict)
    variant_plans: list[VariantPlan] = field(default_factory=list)
    image_plans: list[ImagePlan] = field(default_factory=list)


@dataclass(frozen=True)
class DiffResult:
    records: list[dict]
    summary: dict
    fingerprint: str
    plan: list[ProductPlan]


# Option-name presence markers in fields{} (see validate.py); the values are
# compared via the option_names attribute, not through the generic field loop.
_OPTION_NAME_FIELDS = ("option1_name", "option2_name", "option3_name")
_SUMMARY_KEY = {"add": "adds", "update": "updates", "unchanged": "unchanged", "error": "errors"}


def compute_diff(catalog: dict[str, CatalogProduct], products: list[CanonicalProduct]) -> DiffResult:
    records: list[dict] = []
    plan: list[ProductPlan] = []
    summary = {"adds": 0, "updates": 0, "unchanged": 0, "errors": 0}
    for product in products:
        current = catalog.get(product.handle)
        _resolve_hosted_images(product, current)
        if product.errors:
            product_plan = ProductPlan(kind="error", canonical=product, catalog=current)
            detail: dict = {"errors": [e.as_json() for e in product.errors]}
        elif current is None:
            product_plan = _add_plan(product)
            detail = _add_detail(product_plan)
        else:
            product_plan, detail = _update_plan(product, current)
        kind = product_plan.kind
        plan.append(product_plan)
        records.append(
            {
                "handle": product.handle,
                "title": product.title or (current.title if current else ""),
                "kind": kind,
                "variant_count": len(current.variants) if kind == "unchanged" else len(product.variants),
                "detail": detail,
            }
        )
        summary[_SUMMARY_KEY[kind]] += 1
    fingerprint = hashlib.sha256(
        json.dumps(records, sort_keys=True, separators=(",", ":")).encode()
    ).hexdigest()
    return DiffResult(records=records, summary=summary, fingerprint=fingerprint, plan=plan)


def _resolve_hosted_images(product: CanonicalProduct, current: CatalogProduct | None) -> None:
    """Recognize re-imported hosted image URLs (INV-12). A hosted URL is resolved
    to this product's existing image with that id — re-keyed onto that image's
    source_url so the by_src match treats it as the existing image (never an add,
    never a fetch). A hosted URL whose id is not one of this product's images
    (cross-storefront, deleted, or a brand-new product) is a row error."""
    current_by_id = {i.id: i for i in current.images} if current else {}
    resolved: list = []
    for image in product.images:
        hosted_id = hosted.parse_image_id(image.source_url)
        if hosted_id is None:
            resolved.append(image)
            continue
        match = current_by_id.get(hosted_id)
        if match is None:
            product.errors.append(
                RowError(image.line_number, "Image Src",
                         f"image '{image.source_url}' refers to an unknown hosted id")
            )
            resolved.append(image)
            continue
        resolved.append(replace(image, source_url=match.source_url))
    product.images = resolved


def _add_plan(product: CanonicalProduct) -> ProductPlan:
    return ProductPlan(
        kind="add",
        canonical=product,
        catalog=None,
        variant_plans=[
            VariantPlan(kind="add", canonical=v, catalog_id=None, file_order=order)
            for order, v in enumerate(product.variants, start=1)
        ],
        image_plans=[
            ImagePlan(kind="add", source_url=i.source_url, position=i.position,
                      alt_text=i.alt_text, image_id=None)
            for i in product.images
        ],
    )


def _add_detail(plan: ProductPlan) -> dict:
    # On add, absent fields fall back to their defaults for display where one
    # exists — the detail shows what will actually be set.
    set_fields = resolved_product_fields(plan.canonical)
    for field_name, default in CLEAR_DEFAULTS.items():
        set_fields.setdefault(field_name, default)
    return {
        "set": {f: _json_safe(v) for f, v in set_fields.items()},
        "option_names": list(plan.canonical.option_names),
        "variants": [
            {"options": list(vp.canonical.options),
             "set": _resolved_variant_fields(vp.canonical, vp.file_order)}
            for vp in plan.variant_plans
        ],
        "images": [
            {"src": ip.source_url, "position": ip.position, "alt_text": ip.alt_text}
            for ip in plan.image_plans
        ],
    }


def _update_plan(product: CanonicalProduct, current: CatalogProduct) -> tuple[ProductPlan, dict]:
    """One walk, two outputs: the resolved-value plan entries and the json-safe
    record detail entries are appended side by side, so they can never diverge."""
    product_changes: dict[str, object] = {}
    changes: list[dict] = []
    # Title is always file-present (required header column); "" means the block
    # already carries an error and never reaches here.
    if product.title and product.title != current.title:
        product_changes["title"] = product.title
        changes.append(_change("title", current.title, product.title))
    for field_name, resolved in resolved_product_fields(product).items():
        before = current.fields.get(field_name)
        if resolved != before:
            product_changes[field_name] = resolved
            changes.append(_change(field_name, before, resolved))
    for slot in (1, 2, 3):
        if f"option{slot}_name" not in product.fields:
            continue
        before, after = current.option_names[slot - 1], product.option_names[slot - 1]
        if after != before:
            product_changes[f"option{slot}_name"] = after
            changes.append(_change(f"option{slot}_name", before, after))

    variant_plans: list[VariantPlan] = []
    variant_entries: list[dict] = []
    by_options = {v.options: v for v in current.variants}
    for file_order, variant in enumerate(product.variants, start=1):
        match = by_options.get(variant.options)
        if match is None:
            variant_plans.append(
                VariantPlan(kind="add", canonical=variant, catalog_id=None, file_order=file_order)
            )
            variant_entries.append(
                {"options": list(variant.options), "kind": "add",
                 "set": _resolved_variant_fields(variant, file_order)}
            )
            continue
        variant_changes: dict[str, object] = {}
        variant_change_entries: list[dict] = []
        for f, resolved in resolved_variant_fields(variant, file_order).items():
            # position lives on the catalog variant as an attribute, not in
            # fields{} — compare it explicitly (as images do for theirs).
            before = match.position if f == "position" else match.fields.get(f)
            if resolved != before:
                variant_changes[f] = resolved
                variant_change_entries.append(_change(f, before, resolved))
        if variant_changes:
            variant_plans.append(
                VariantPlan(kind="update", canonical=variant, catalog_id=match.id,
                            file_order=file_order, changes=variant_changes)
            )
            variant_entries.append(
                {"options": list(variant.options), "kind": "update", "changes": variant_change_entries}
            )

    image_plans: list[ImagePlan] = []
    image_entries: list[dict] = []
    by_src = {i.source_url: i for i in current.images}
    for image in product.images:
        match = by_src.get(image.source_url)
        if match is None:
            image_plans.append(
                ImagePlan(kind="add", source_url=image.source_url, position=image.position,
                          alt_text=image.alt_text, image_id=None)
            )
            image_entries.append(
                {"src": image.source_url, "kind": "add", "position": image.position, "alt_text": image.alt_text}
            )
            continue
        image_changes: dict[str, object] = {}
        image_change_entries: list[dict] = []
        for f, before, after in (
            ("position", match.position, image.position),
            ("alt_text", match.alt_text, image.alt_text),
        ):
            if after != before:
                image_changes[f] = after
                image_change_entries.append(_change(f, before, after))
        if image_changes:
            image_plans.append(
                ImagePlan(kind="update", source_url=image.source_url, position=image.position,
                          alt_text=image.alt_text, image_id=match.id, changes=image_changes)
            )
            image_entries.append(
                {"src": image.source_url, "kind": "update", "changes": image_change_entries}
            )

    detail: dict = {}
    if changes:
        detail["changes"] = changes
    if variant_entries:
        detail["variants"] = variant_entries
    if image_entries:
        detail["images"] = image_entries
    kind = "update" if detail else "unchanged"
    return (
        ProductPlan(kind=kind, canonical=product, catalog=current, product_changes=product_changes,
                    variant_plans=variant_plans, image_plans=image_plans),
        detail,
    )


def resolved_fields(fields: dict[str, object]) -> dict[str, object]:
    """File-present fields with None (an explicit clear) resolved to the default."""
    return {f: (v if v is not None else CLEAR_DEFAULTS.get(f)) for f, v in fields.items()}


def resolved_product_fields(product: CanonicalProduct) -> dict[str, object]:
    """Resolved product-level fields, minus the option-name presence markers."""
    return {
        f: v for f, v in resolved_fields(product.fields).items() if f not in _OPTION_NAME_FIELDS
    }


def resolved_variant_fields(variant: CanonicalVariant, file_order: int) -> dict[str, object]:
    """File-present variant fields with clears resolved. A cleared position has
    no CLEAR_DEFAULTS entry — it resets to the variant's 1-based file order
    within its product ("defaults to file order"), never to NULL."""
    resolved = resolved_fields(variant.fields)
    if "position" in resolved and resolved["position"] is None:
        resolved["position"] = file_order
    return resolved


def _resolved_variant_fields(variant: CanonicalVariant, file_order: int) -> dict[str, object]:
    return {f: _json_safe(v) for f, v in resolved_variant_fields(variant, file_order).items()}


def _change(field_name: str, before: object, after: object) -> dict:
    return {"field": field_name, "before": _json_safe(before), "after": _json_safe(after)}


def _json_safe(value: object) -> object:
    if isinstance(value, Decimal):
        return str(value)
    if isinstance(value, (tuple, list)):
        return [_json_safe(v) for v in value]
    return value