Implement preprocessing of post bodies for gemini

1 files changed, 66 insertions, 0 deletions

diff --git a/src/post/preprocess.go b/src/post/preprocess.go
new file mode 100644
index 0000000..4d4be9d
--- /dev/null
+++ b/src/post/preprocess.go
@@ -0,0 +1,66 @@
+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
+}