package post

import (
	"fmt"
	"io"
	"text/template"
)

// PreprocessFunctions are functions which can be used by posts themselves to
// interleave dynamic content into their bodies. Usually this is used for
// properly constructing URLs, but also for things like displaying images.
type PreprocessFunctions struct {

	// BlogURL returns the given string, rooted to the blog's base url (which
	// may or may not include path components itself).
	//
	// The given path should not have a leading slash.
	BlogURL func(path string) string

	// AssetURL returns the URL of the asset with the given ID.
	AssetURL func(id string) string

	// PostURL returns the URL of the post with the given ID.
	PostURL func(id string) string

	// StaticURL returns the URL of a file being served from the static
	// directory. The given path should _not_ include the prefixed 'static/'
	// path element.
	StaticURL func(path string) string

	// Image returns a string which should be inlined into the post body in
	// order to display an.
	//
	// The first argument to Image _must_ be the ID of an image asset. The
	// second argument _may_ be a description of the image which will be used as
	// alt text, or possibly displayed to the user with the image.
	Image func(args ...string) (string, error)
}

// PreprocessBody interprets the Post's Body as a text template which may use
// any of the functions found in PreprocessFunctions (all must be set). It
// executes the template and writes the result to the given writer.
func (p Post) PreprocessBody(into io.Writer, funcs PreprocessFunctions) error {

	tpl := template.New("")

	tpl.Funcs(template.FuncMap{
		"BlogURL":   funcs.BlogURL,
		"AssetURL":  funcs.AssetURL,
		"PostURL":   funcs.PostURL,
		"StaticURL": funcs.StaticURL,
		"Image":     funcs.Image,
	})

	tpl, err := tpl.Parse(p.Body)

	if err != nil {
		return fmt.Errorf("parsing post body as template: %w", err)
	}

	if err := tpl.Execute(into, nil); err != nil {
		return fmt.Errorf("executing post body as template: %w", err)
	}

	return nil
}