package adminstore

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"fmt"
	"regexp"
	"strings"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgconn"
)

// randomHex returns n random lowercase hex characters for fallback ltree slugs.
func randomHex(n int) string {
	b := make([]byte, (n+1)/2)
	if _, err := rand.Read(b); err != nil {
		return "x"
	}
	return hex.EncodeToString(b)[:n]
}

// Category-management errors, mapped to client statuses by the handler.
var (
	// ErrDuplicatePath is returned when a category path already exists.
	ErrDuplicatePath = errors.New("duplicate category path")
	// ErrCategoryHasChildren blocks deleting a node that still has children.
	ErrCategoryHasChildren = errors.New("category has children")
	// ErrCategoryInUse blocks deleting a node still referenced by products.
	ErrCategoryInUse = errors.New("category in use")
	// ErrInvalidParent is returned for a missing parent or an illegal move
	// (onto itself or one of its own descendants).
	ErrInvalidParent = errors.New("invalid parent category")
)

// CategoryInput is the payload accepted when creating or editing a category.
// Slug is the ltree label (ASCII); when empty it is derived from NameEN, then
// from a random suffix, since ltree labels cannot contain CJK or spaces.
type CategoryInput struct {
	NameZH       string  `json:"name_zh"`
	NameEN       *string `json:"name_en"`
	Slug         *string `json:"slug"`
	ParentID     *string `json:"parent_id"`
	GPCBrickCode *string `json:"gpc_brick_code"`
}

var slugInvalid = regexp.MustCompile(`[^a-z0-9_]+`)

// slugify converts a string into a valid ltree label ([a-z0-9_]).
func slugify(s string) string {
	s = strings.ToLower(strings.TrimSpace(s))
	s = slugInvalid.ReplaceAllString(s, "_")
	s = strings.Trim(s, "_")
	for strings.Contains(s, "__") {
		s = strings.ReplaceAll(s, "__", "_")
	}
	return s
}

// resolveSlug picks an ltree label from the explicit slug, then NameEN, then a
// random fallback so a Chinese-only category still gets a valid path label.
func resolveSlug(in CategoryInput) string {
	if in.Slug != nil {
		if s := slugify(*in.Slug); s != "" {
			return s
		}
	}
	if in.NameEN != nil {
		if s := slugify(*in.NameEN); s != "" {
			return s
		}
	}
	return "cat_" + randomHex(6)
}

func trimPtr(p *string) *string {
	if p == nil {
		return nil
	}
	t := strings.TrimSpace(*p)
	if t == "" {
		return nil
	}
	return &t
}

// CreateCategory inserts a new category node. With no parent it becomes a root
// (level 0); otherwise its path is parentPath.slug and level is parentLevel+1.
func (s *Store) CreateCategory(ctx context.Context, actor string, in CategoryInput) (*Category, error) {
	name := strings.TrimSpace(in.NameZH)
	if name == "" {
		return nil, errors.New("name_zh required")
	}

	parentPath := ""
	parentLevel := -1
	kind := DefaultKind
	var parentID *string
	if pid := trimPtr(in.ParentID); pid != nil {
		var path string
		var level int
		err := s.pool.QueryRow(ctx, "SELECT path::text, level, archive_kind FROM category WHERE id = $1", *pid).Scan(&path, &level, &kind)
		if errors.Is(err, pgx.ErrNoRows) {
			return nil, ErrInvalidParent
		}
		if err != nil {
			return nil, err
		}
		parentPath, parentLevel, parentID = path, level, pid
	}

	slug := resolveSlug(in)
	path := slug
	if parentPath != "" {
		path = parentPath + "." + slug
	}
	level := parentLevel + 1

	var c Category
	err := s.pool.QueryRow(ctx, `
INSERT INTO category (name_zh, name_en, parent_id, path, gpc_brick_code, level, archive_kind)
VALUES ($1, $2, $3, $4::ltree, $5, $6, $7)
RETURNING id, name_zh, name_en, path::text, level, parent_id::text, gpc_brick_code, archive_kind, 0`,
		name, trimPtr(in.NameEN), parentID, path, trimPtr(in.GPCBrickCode), level, kind).
		Scan(&c.ID, &c.NameZH, &c.NameEN, &c.Path, &c.Level, &c.ParentID, &c.GPCBrickCode, &c.ArchiveKind, &c.ProductCount)
	if isUniqueViolation(err) {
		return nil, ErrDuplicatePath
	}
	if err != nil {
		return nil, err
	}

	_ = s.writeAudit(ctx, actor, "create", "category", &c.ID, []string{"name_zh", "path"}, nil, c)
	return &c, nil
}

// UpdateCategory renames a node and/or moves it under a new parent. Moving
// rewrites the path of the node and every descendant via ltree, keeping level
// in sync. Moving a node onto itself or a descendant is rejected.
func (s *Store) UpdateCategory(ctx context.Context, id, actor string, in CategoryInput) (*Category, error) {
	name := strings.TrimSpace(in.NameZH)
	if name == "" {
		return nil, errors.New("name_zh required")
	}

	tx, err := s.pool.Begin(ctx)
	if err != nil {
		return nil, err
	}
	defer tx.Rollback(ctx)

	var oldPath string
	var oldLevel int
	var oldParent *string
	err = tx.QueryRow(ctx, "SELECT path::text, level, parent_id::text FROM category WHERE id = $1", id).
		Scan(&oldPath, &oldLevel, &oldParent)
	if errors.Is(err, pgx.ErrNoRows) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, err
	}

	if _, err := tx.Exec(ctx,
		"UPDATE category SET name_zh = $1, name_en = $2, gpc_brick_code = $3 WHERE id = $4",
		name, trimPtr(in.NameEN), trimPtr(in.GPCBrickCode), id); err != nil {
		return nil, err
	}

	newParent := trimPtr(in.ParentID)
	if !strEq(newParent, oldParent) {
		if err := s.moveCategoryTx(ctx, tx, id, oldPath, newParent); err != nil {
			return nil, err
		}
	}

	if err := tx.Commit(ctx); err != nil {
		return nil, err
	}

	out, err := s.getCategory(ctx, id)
	if err != nil {
		return nil, err
	}
	_ = s.writeAudit(ctx, actor, "update", "category", &id, []string{"name_zh", "name_en", "gpc_brick_code", "parent_id"}, nil, out)
	return out, nil
}

// moveCategoryTx re-parents a subtree. The node's slug (last path label) is
// preserved; only its ancestor prefix and level change.
func (s *Store) moveCategoryTx(ctx context.Context, tx pgx.Tx, id, oldPath string, newParent *string) error {
	slug := oldPath
	if i := strings.LastIndex(oldPath, "."); i >= 0 {
		slug = oldPath[i+1:]
	}

	newBase := slug
	if newParent != nil {
		var parentPath string
		err := tx.QueryRow(ctx, "SELECT path::text FROM category WHERE id = $1", *newParent).Scan(&parentPath)
		if errors.Is(err, pgx.ErrNoRows) {
			return ErrInvalidParent
		}
		if err != nil {
			return err
		}
		// Disallow moving a node under itself or one of its descendants.
		if parentPath == oldPath || strings.HasPrefix(parentPath, oldPath+".") {
			return ErrInvalidParent
		}
		newBase = parentPath + "." + slug
	}

	// Rewrite the node and all descendants in one statement; level tracks depth.
	_, err := tx.Exec(ctx, `
UPDATE category
SET path  = ($1::ltree || subpath(path, nlevel($2::ltree) - 1)),
    level = nlevel($1::ltree) + (nlevel(path) - nlevel($2::ltree)) - 1
WHERE path = $2::ltree OR path <@ $2::ltree`, newBase, oldPath)
	if isUniqueViolation(err) {
		return ErrDuplicatePath
	}
	if err != nil {
		return err
	}

	_, err = tx.Exec(ctx, "UPDATE category SET parent_id = $1 WHERE id = $2", newParent, id)
	return err
}

// DeleteCategory removes a leaf node not referenced by any product. Nodes with
// children or in-use nodes are rejected with a specific error.
func (s *Store) DeleteCategory(ctx context.Context, id, actor string) error {
	before, err := s.getCategory(ctx, id)
	if err != nil {
		return err
	}

	var children int
	if err := s.pool.QueryRow(ctx, "SELECT count(*) FROM category WHERE parent_id = $1", id).Scan(&children); err != nil {
		return err
	}
	if children > 0 {
		return ErrCategoryHasChildren
	}

	var products int
	if err := s.pool.QueryRow(ctx, "SELECT count(*) FROM product WHERE category_id = $1", id).Scan(&products); err != nil {
		return err
	}
	if products > 0 {
		return fmt.Errorf("%w: %d products", ErrCategoryInUse, products)
	}

	ct, err := s.pool.Exec(ctx, "DELETE FROM category WHERE id = $1", id)
	if err != nil {
		// A concurrent product assignment can still trip the FK.
		if isForeignKeyViolation(err) {
			return ErrCategoryInUse
		}
		return err
	}
	if ct.RowsAffected() == 0 {
		return ErrNotFound
	}

	_ = s.writeAudit(ctx, actor, "delete", "category", &id, []string{"path"}, before, nil)
	return nil
}

func (s *Store) getCategory(ctx context.Context, id string) (*Category, error) {
	var c Category
	err := s.pool.QueryRow(ctx, `
SELECT c.id, c.name_zh, c.name_en, c.path::text, c.level, c.parent_id::text,
       c.gpc_brick_code, c.archive_kind,
       (SELECT count(*) FROM product p WHERE p.category_id = c.id)
FROM category c WHERE c.id = $1`, id).
		Scan(&c.ID, &c.NameZH, &c.NameEN, &c.Path, &c.Level, &c.ParentID, &c.GPCBrickCode, &c.ArchiveKind, &c.ProductCount)
	if errors.Is(err, pgx.ErrNoRows) {
		return nil, ErrNotFound
	}
	if err != nil {
		return nil, err
	}
	return &c, nil
}

func isUniqueViolation(err error) bool {
	var pgErr *pgconn.PgError
	return errors.As(err, &pgErr) && pgErr.Code == "23505"
}

func isForeignKeyViolation(err error) bool {
	var pgErr *pgconn.PgError
	return errors.As(err, &pgErr) && pgErr.Code == "23503"
}