package storage

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"strings"
	"text/template"
	"time"

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

// titleTemplateData is the data model exposed to a classification template's
// (or the tenant-wide default's) title Go text/template. Fields are kept simple
// (plain strings / times) so patterns stay readable, e.g.
//
//	{{.Correspondent}} {{.DocumentType}} {{dateFormat "02.01.2006" .Belegdatum}}
//
// Belegdatum and UploadDate are passed as time.Time (zero value when unknown);
// use the dateFormat template func to render them, which yields "" for a zero
// time instead of Go's "0001-01-01..." default.
type titleTemplateData struct {
	Correspondent string
	DocumentType  string
	Belegdatum    time.Time
	UploadDate    time.Time
	Tags          string
	OCRTitle      string
}

// titleTemplateFuncs provides the custom template functions available inside a
// title template. dateFormat takes a Go reference layout ("02.01.2006") and a
// time.Time, returning "" for a zero time so an unknown Belegdatum does not
// leak a placeholder date into the title.
var titleTemplateFuncs = template.FuncMap{
	"dateFormat": func(layout string, t time.Time) string {
		if t.IsZero() {
			return ""
		}
		return t.Format(layout)
	},
}

// ValidateTitleTemplate parses (but does not execute) a title template pattern
// so the API layer can reject a syntactically invalid pattern up front. An
// empty pattern is valid (means "no template title"). Exported for the
// settings / template CRUD handlers.
func ValidateTitleTemplate(pattern string) error {
	if strings.TrimSpace(pattern) == "" {
		return nil
	}
	_, err := template.New("title").Option("missingkey=zero").Funcs(titleTemplateFuncs).Parse(pattern)
	if err != nil {
		return fmt.Errorf("invalid title template: %w", err)
	}
	return nil
}

// renderTitleTemplate parses and executes a title template against data. The
// result is whitespace-trimmed. missingkey=zero guards against crashes when a
// pattern references a field that does not exist. A parse/execute error or an
// empty result is signalled to the caller so it can fall back (never an empty
// title).
func renderTitleTemplate(pattern string, data titleTemplateData) (string, error) {
	tmpl, err := template.New("title").Option("missingkey=zero").Funcs(titleTemplateFuncs).Parse(pattern)
	if err != nil {
		return "", fmt.Errorf("storage: parse title template: %w", err)
	}
	var buf bytes.Buffer
	if err := tmpl.Execute(&buf, data); err != nil {
		return "", fmt.Errorf("storage: execute title template: %w", err)
	}
	return strings.TrimSpace(buf.String()), nil
}

// tenantDefaultTitleTemplate reads the tenant-wide fallback title template
// straight from the tenants table (same DB pool). Returns "" when unset
// (NULL) so callers can treat "no default" uniformly.
func (s *Store) tenantDefaultTitleTemplate(ctx context.Context, tenantID int64) (string, error) {
	var v *string
	err := s.db.QueryRow(ctx, `SELECT default_title_template FROM tenants WHERE id = $1`, tenantID).Scan(&v)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return "", nil
		}
		return "", fmt.Errorf("storage: read tenant default_title_template: %w", err)
	}
	if v == nil {
		return "", nil
	}
	return strings.TrimSpace(*v), nil
}

// taxonomyNameByID resolves a taxonomy entity's display name (tenant-scoped).
// table must be a fixed internal literal ("correspondents" / "document_types"),
// never user input. Returns "" (not an error) when the row does not exist so a
// dangling reference cannot break title generation.
func (s *Store) taxonomyNameByID(ctx context.Context, table string, id, tenantID int64) (string, error) {
	var name string
	err := s.db.QueryRow(ctx, fmt.Sprintf(`SELECT name FROM %s WHERE id = $1 AND tenant_id = $2`, table), id, tenantID).Scan(&name)
	if err != nil {
		if errors.Is(err, pgx.ErrNoRows) {
			return "", nil
		}
		return "", fmt.Errorf("storage: lookup %s name: %w", table, err)
	}
	return name, nil
}

// applyTemplateTitle derives and persists a document title from a classification
// template's title_template (or, if that is empty, the tenant-wide
// default_title_template). Rules (see feature spec):
//
//   - Never touches a manually renamed document (title_manually_set = true).
//   - Chooses the template's own pattern first, then the tenant default; if both
//     are empty, does nothing (existing title kept).
//   - On an empty or errored render result, falls back to keeping the existing
//     title — never sets an empty string.
//   - Persists via UpdateDocumentTitleAuto so title_manually_set stays false
//     (a later correction + re-apply must still work).
//
// Called at the end of ApplyTemplate, so it runs identically for the manual
// endpoint and the workflow trigger (both funnel through ApplyTemplate).
func (s *Store) applyTemplateTitle(ctx context.Context, documentID, tenantID int64, tmpl *ClassificationTemplate) error {
	doc, err := s.GetDocument(ctx, documentID, tenantID)
	if err != nil {
		return err
	}
	if doc.TitleManuallySet {
		return nil
	}

	// Resolve the effective pattern: template-own first, then tenant default.
	pattern := ""
	if tmpl.TitleTemplate != nil {
		pattern = strings.TrimSpace(*tmpl.TitleTemplate)
	}
	if pattern == "" {
		def, err := s.tenantDefaultTitleTemplate(ctx, tenantID)
		if err != nil {
			return err
		}
		pattern = def
	}
	if pattern == "" {
		return nil // no template title configured at either level
	}

	// Build the render data. Name lookups are best-effort (missing rows -> "").
	data := titleTemplateData{
		OCRTitle:   doc.Title,
		UploadDate: doc.CreatedAt,
	}
	if doc.DocumentDate != nil {
		data.Belegdatum = *doc.DocumentDate
	}
	if doc.CorrespondentID != nil {
		name, err := s.taxonomyNameByID(ctx, "correspondents", *doc.CorrespondentID, tenantID)
		if err != nil {
			return err
		}
		data.Correspondent = name
	}
	// Document type: prefer the template's target type (what is being applied),
	// falling back to the document's current type.
	docTypeID := tmpl.DocTypeID
	if docTypeID == nil {
		docTypeID = doc.DocTypeID
	}
	if docTypeID != nil {
		name, err := s.taxonomyNameByID(ctx, "document_types", *docTypeID, tenantID)
		if err != nil {
			return err
		}
		data.DocumentType = name
	}
	tags, err := s.ListDocumentTags(ctx, documentID, tenantID)
	if err != nil {
		return err
	}
	names := make([]string, 0, len(tags))
	for _, t := range tags {
		names = append(names, t.Name)
	}
	data.Tags = strings.Join(names, ", ")

	rendered, err := renderTitleTemplate(pattern, data)
	if err != nil || rendered == "" {
		// Fallback: keep the existing (OCR-derived) title, never blank it.
		return nil //nolint:nilerr // intentional: a bad template must not fail the apply
	}
	if rendered == doc.Title {
		return nil
	}
	return s.UpdateDocumentTitleAuto(ctx, documentID, tenantID, rendered)
}