feat: add embed.go example

This commit adds the embed.go example, which focuses on demonstrating
how to embed an svg within another. The example adds a title and a red
border around the svg file provided via the -f argument.

Closes ajstarks#57.

README.markdown

@@ -149,6 +149,7 @@ A video describing how to use the package can be seen on YouTube at <http://www.
 * bulletgraph:	Bullet Graphs (via Stephen Few)
 * colortab: Display SVG named colors with RGB values
 * compx:  Component diagrams
+* embed:  Embedding svgs within another
 * flower:	Random "flowers"
 * fontcompare:	Compare two fonts
 * f50:		Get 50 photos from Flickr based on a query

embed/embed.go

@@ -0,0 +1,100 @@
+// embed -- including the contents of an SVG within your own
+// +build !appengine
+
+package main
+
+import (
+	"encoding/xml"
+	"flag"
+	"fmt"
+	"io"
+	"os"
+
+	svg "github.com/ajstarks/svgo"
+)
+
+var (
+	canvas   = svg.New(os.Stdout)
+	filename string
+)
+
+var (
+	titleHeight = 64
+	borderSize  = 4
+	borderfmt   = "fill:transparent;stroke:red;stroke-width:%d"
+	titlefmt    = `text-anchor:middle;width:%d;font-size:32px`
+	groupfmt    = `transform="translate(%d, %d)"`
+)
+
+func embed() error {
+	// load and parse svg
+	f, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	s, err := parseSVG(f)
+	if err != nil {
+		return err
+	}
+	width := s.Width + 2*borderSize
+	height := s.Height + titleHeight + 2*borderSize
+	// create svg
+	canvas.Start(width, height)
+	placeHeader(width, height, f.Name())
+	// embed svg
+	placeSVG(s)
+	return nil
+}
+
+// SVG contains the parsed attributes and xml from the given file.
+type SVG struct {
+	// Width and Height are attributes of the <svg> tag
+	Width  int `xml:"width,attr"`
+	Height int `xml:"height,attr"`
+	// Doc is all all of the contents within the <svg> tags, specified by the
+	// `innerxml` struct tag
+	Doc []byte `xml:",innerxml"`
+}
+
+func parseSVG(src io.Reader) (SVG, error) {
+	var s SVG
+	data, err := io.ReadAll(src)
+	if err != nil {
+		return SVG{}, err
+	}
+	if xml.Unmarshal(data, &s); err != nil {
+		return SVG{}, err
+	}
+	return s, nil
+}
+
+func placeHeader(width, height int, name string) {
+	// add border
+	canvas.Rect(0, 0, width, height, fmt.Sprintf(borderfmt, borderSize))
+	// add title from file name
+	canvas.Text(width/2, titleHeight*3/4, name, fmt.Sprintf(titlefmt, width))
+}
+
+func placeSVG(s SVG) {
+	// create clip path of svg size
+	canvas.Group(`clip-path="url(#embed)"`, fmt.Sprintf(groupfmt, borderSize, titleHeight+borderSize))
+	canvas.ClipPath(`id="embed"`)
+	canvas.Rect(0, 0, s.Width, s.Height)
+	canvas.ClipEnd()
+	// append embedded svg
+	canvas.Writer.Write(s.Doc)
+	canvas.Gend()
+	canvas.End()
+}
+
+func init() {
+	flag.StringVar(&filename, "f", "embed.svg", "file name")
+	flag.Parse()
+}
+
+func main() {
+	if err := embed(); err != nil {
+		fmt.Fprintf(os.Stdout, "error: %v\n", err)
+		os.Exit(1)
+	}
+}