// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package templates is a file-backed store of admin post templates
// persisted as a [[templates]] array in TOML.
package templates

import (
	"errors"
	"fmt"
	"log/slog"
	"os"
	"slices"
	"sync"

	"sourcedock.dev/petrbalvin/interpres/v2"
	"sourcedock.dev/petrbalvin/volumen/internal/tomlfile"
)

// PostTemplate pre-fills the admin new-post form. Title, slug and tags
// have their own fields because the form gives them dedicated controls;
// Fields carries the remaining editor values to pre-fill, keyed by form
// field name (doi, orcid, series, series_order, author, lang, excerpt,
// cover, and so on), as strings.
type PostTemplate struct {
	Name  string
	Tags  []string
	Body  string
	Title string
	Slug  string

	Fields map[string]string
}

// Store manages post templates in a TOML file, with an mtime snapshot
// cache so that an unchanged file is not re-parsed on every read.
type Store struct {
	path string

	mu        sync.Mutex
	cached    []PostTemplate
	snapshot  fileSnapshot
	haveCache bool

	lock sync.Mutex
}

type fileSnapshot struct {
	present bool
	mtime   int64
	size    int64
}

// New opens the template store at path.
func New(path string) *Store {
	return &Store{path: path}
}

// All returns every template with a name.
func (s *Store) All() []PostTemplate {
	s.mu.Lock()
	defer s.mu.Unlock()
	templates, err := s.loadLocked()
	if err != nil {
		slog.Error("templates: cannot read the templates file", "path", s.path, "error", err)
		return nil
	}
	return slices.Clone(templates)
}

// loadLocked returns the cached templates, rebuilding them when the
// file changed. A missing file is not an error; an unreadable or
// unparsable one is. The caller must hold s.mu.
func (s *Store) loadLocked() ([]PostTemplate, error) {
	snapshot := s.buildSnapshot()
	if s.haveCache && snapshot == s.snapshot {
		return s.cached, nil
	}
	templates, err := s.readFile()
	if err != nil {
		return nil, err
	}
	s.snapshot = snapshot
	s.cached = templates
	s.haveCache = true
	return templates, nil
}

func (s *Store) buildSnapshot() fileSnapshot {
	info, err := os.Stat(s.path)
	if err != nil {
		return fileSnapshot{}
	}
	return fileSnapshot{present: true, mtime: info.ModTime().UnixNano(), size: info.Size()}
}

func (s *Store) readFile() ([]PostTemplate, error) {
	raw, err := os.ReadFile(s.path)
	if err != nil {
		if errors.Is(err, os.ErrNotExist) {
			return nil, nil
		}
		return nil, fmt.Errorf("read %s: %w", s.path, err)
	}
	data, err := interpres.ParseMap(raw)
	if err != nil {
		return nil, fmt.Errorf("parse %s: %w", s.path, err)
	}
	var out []PostTemplate
	for _, entry := range tomlfile.Tables(data["templates"]) {
		name := tomlfile.String(entry["name"])
		if name == "" {
			continue
		}
		out = append(out, PostTemplate{
			Name:   name,
			Body:   tomlfile.String(entry["body"]),
			Title:  tomlfile.String(entry["title"]),
			Slug:   tomlfile.String(entry["slug"]),
			Tags:   tomlfile.Strings(entry["tags"]),
			Fields: stringTable(entry["fields"]),
		})
	}
	return out, nil
}

// stringTable reads one table of scalar values into a string map;
// numbers and dates arrive already formatted by interpres.
func stringTable(v any) map[string]string {
	table, ok := v.(map[string]any)
	if !ok || len(table) == 0 {
		return nil
	}
	out := make(map[string]string, len(table))
	for key, value := range table {
		if value == nil {
			continue
		}
		out[key] = fmt.Sprintf("%v", value)
	}
	return out
}

// Errors a template change can be refused with.
var (
	// ErrNameTaken is returned when a template of that name exists.
	ErrNameTaken = errors.New("a template of that name already exists")
	// ErrEmptyName is returned when the name is blank.
	ErrEmptyName = errors.New("the template name must not be empty")
	// ErrNoSuchTemplate is returned when nothing matches the name.
	ErrNoSuchTemplate = errors.New("no such template")
)

// Add stores a template, or reports why it could not. The name is
// required and unique; the rest is stored as given.
func (s *Store) Add(tpl PostTemplate) (*PostTemplate, error) {
	if tpl.Name == "" {
		return nil, ErrEmptyName
	}
	s.lock.Lock()
	defer s.lock.Unlock()
	existing, err := s.reload()
	if err != nil {
		slog.Error("templates: refusing to add, the templates file is unreadable",
			"path", s.path, "error", err)
		return nil, err
	}
	for _, other := range existing {
		if other.Name == tpl.Name {
			return nil, ErrNameTaken
		}
	}
	created := tpl
	if err := s.persist(append(existing, created)); err != nil {
		return nil, err
	}
	return &created, nil
}

// Delete removes a template by name, or reports why it could not.
func (s *Store) Delete(name string) error {
	s.lock.Lock()
	defer s.lock.Unlock()
	templates, err := s.reload()
	if err != nil {
		slog.Error("templates: refusing to delete, the templates file is unreadable",
			"path", s.path, "error", err)
		return err
	}
	remaining := make([]PostTemplate, 0, len(templates))
	for _, tpl := range templates {
		if tpl.Name != name {
			remaining = append(remaining, tpl)
		}
	}
	if len(remaining) == len(templates) {
		return ErrNoSuchTemplate
	}
	return s.persist(remaining)
}

// reload re-reads the file, refusing to carry on when it cannot be
// parsed: writing back a list derived from an unreadable file would
// destroy the templates it contains. The caller must hold s.lock.
func (s *Store) reload() ([]PostTemplate, error) {
	s.Invalidate()
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.loadLocked()
}

// Health reports why the templates file cannot be read, or nil when it
// is fine or absent.
func (s *Store) Health() error {
	s.mu.Lock()
	defer s.mu.Unlock()
	_, err := s.loadLocked()
	return err
}

// Invalidate drops the cache so the next read re-reads the file.
func (s *Store) Invalidate() {
	s.mu.Lock()
	defer s.mu.Unlock()
	s.cached = nil
	s.snapshot = fileSnapshot{}
	s.haveCache = false
}

func (s *Store) persist(templates []PostTemplate) error {
	entries := make([]map[string]any, 0, len(templates))
	for _, tpl := range templates {
		entry := map[string]any{"name": tpl.Name}
		if tpl.Title != "" {
			entry["title"] = tpl.Title
		}
		if tpl.Slug != "" {
			entry["slug"] = tpl.Slug
		}
		if len(tpl.Tags) > 0 {
			tags := make([]string, len(tpl.Tags))
			for i, tag := range tpl.Tags {
				tags[i] = tag
			}
			entry["tags"] = tags
		}
		if tpl.Body != "" {
			entry["body"] = tpl.Body
		}
		if len(tpl.Fields) > 0 {
			fields := make(map[string]any, len(tpl.Fields))
			for key, value := range tpl.Fields {
				fields[key] = value
			}
			entry["fields"] = fields
		}
		entries = append(entries, entry)
	}
	if err := tomlfile.Write(s.path, "templates", entries); err != nil {
		slog.Error("templates: cannot persist", "path", s.path, "error", err)
		return err
	}
	s.Invalidate()
	return nil
}