package template

import (
	"fmt"
	"html/template"
	"io"
	"io/fs"
	"os"
	"path"
	"strings"
)

type Service struct {
	baseDir string
	cache   map[string]*template.Template
}

func createFuncMap() template.FuncMap {
	funcMap := template.FuncMap{
		"dict": func(pairs ...any) (map[string]any, error) {
			if len(pairs)%2 != 0 {
				return nil, fmt.Errorf("dict requires an even number of arguments")
			}
			m := make(map[string]any)
			for i := 0; i < len(pairs); i += 2 {
				key, ok := pairs[i].(string)
				if !ok {
					return nil, fmt.Errorf("dict keys must be strings")
				}
				m[key] = pairs[i+1]
			}
			return m, nil
		},
		"default": func(fallback string, value any) any {
			if value == nil {
				return fallback
			}
			return value
		},
	}

	return funcMap
}

func NewService(uiDir string) (*Service, error) {
	cache := make(map[string]*template.Template)

	uiFS := os.DirFS(uiDir)

	funcMap := createFuncMap()

	err := fs.WalkDir(uiFS, "pages", func(filePath string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if d.IsDir() {
			return nil
		}
		if !strings.HasSuffix(filePath, ".html") {
			return nil
		}

		// Key by the relative path under pages/, e.g. "auth/signin.html".
		relPath := strings.TrimPrefix(filePath, "pages/")

		tmpl, err := template.New(path.Base(filePath)).Funcs(funcMap).ParseFS(uiFS, filePath, "layouts/*.html")
		if err != nil {
			return fmt.Errorf("can't parse filesystem: %w", err)
		}
		cache[relPath] = tmpl

		return nil
	})
	if err != nil {
		return nil, fmt.Errorf("can't walk 'pages' in '%s': %w", uiDir, err)
	}

	return &Service{
		baseDir: uiDir,
		cache:   cache,
	}, nil
}

func (e *Service) Render(w io.Writer, pageName string, data any) error {
	tmpl, ok := e.cache[pageName]
	if !ok {
		return fmt.Errorf("no such template: %s", pageName)
	}
	return tmpl.Execute(w, data)
}