implement metadata writing

Signed-off-by: Christine Dodrill <me@christine.website>

gempub.go

@@ -22,10 +22,82 @@ type Metadata struct {
 	PublishDate  time.Time // YYYY-MM-DD eg. 2006-01-02
 	RevisionDate time.Time // YYYY-MM-DD eg. 2006-01-02
 	Copyright    string    // copyright of the book
-	License      string    // license of the book	Version      string a // human readable only, not meant to be parsed
+	License      string    // license of the book
+	Version      string    // human readable only, not meant to be parsed
 	Cover        string    // a JPG or PNG image which can be anywhere in the directory structure
 }
 
+func (m Metadata) Write(w io.Writer) error {
+	if err := m.Valid(); err != nil {
+		return err
+	}
+
+	if _, err := fmt.Fprintln(w, "title: ", m.Title); err != nil {
+		return err
+	}
+	if _, err := fmt.Fprintln(w, "gpubVersion: ", m.GpubVersion); err != nil {
+		return err
+	}
+	if m.Index != "" {
+		if _, err := fmt.Fprintln(w, "index: ", m.Index); err != nil {
+			return err
+		}
+	}
+	if m.Author != "" {
+		if _, err := fmt.Fprintln(w, "author: ", m.Author); err != nil {
+			return err
+		}
+	}
+	if m.Language != "" {
+		if _, err := fmt.Fprintln(w, "language: ", m.Language); err != nil {
+			return err
+		}
+	}
+	if m.Charset != "" {
+		if _, err := fmt.Fprintln(w, "charset: ", m.Charset); err != nil {
+			return err
+		}
+	}
+	if m.Description != "" {
+		if _, err := fmt.Fprintln(w, "description: ", m.Description); err != nil {
+			return err
+		}
+	}
+	if m.Copyright != "" {
+		if _, err := fmt.Fprintln(w, "copyright: ", m.Copyright); err != nil {
+			return err
+		}
+	}
+	if m.Version != "" {
+		if _, err := fmt.Fprintln(w, "version: ", m.Version); err != nil {
+			return err
+		}
+	}
+	if m.Cover != "" {
+		if _, err := fmt.Fprintln(w, "cover: ", m.Cover); err != nil {
+			return err
+		}
+	}
+
+	if !m.Published.IsZero() {
+		if _, err := fmt.Fprintln(w, "published: ", m.Published.Format("2006-01-02")); err != nil {
+			return err
+		}
+	}
+	if !m.PublishDate.IsZero() {
+		if _, err := fmt.Fprintln(w, "publishDate: ", m.PublishDate.Format("2006-01-02")); err != nil {
+			return err
+		}
+	}
+	if !m.RevisionDate.IsZero() {
+		if _, err := fmt.Fprintln(w, "revisionDate: ", m.RevisionDate.Format("2006-01-02")); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
 var (
 	ErrNoTitle = errors.New("gempub: no title in document")
 )