font/opentype: add Collection type and methods

The Collection type wraps sfnt.Collection and allows lazy-loading
of fonts from SFNT collections.

Signed-off-by: Greg Pomerantz <gmp.gio@wow.st>

font/opentype/opentype.go

@@ -5,6 +5,8 @@
 package opentype
 
 import (
+	"io"
+
 	"unicode"
 	"unicode/utf8"
 
@@ -23,6 +25,11 @@ type Font struct {
 	buf  sfnt.Buffer
 }
 
+// Collection is a collection of one or more fonts.
+type Collection struct {
+	coll *sfnt.Collection
+}
+
 type opentype struct {
 	Font    *sfnt.Font
 	Hinting font.Hinting
@@ -38,6 +45,46 @@ func Parse(src []byte) (*Font, error) {
 	return &Font{font: fnt}, nil
 }
 
+// ParseCollection parses an SFNT font collection, such as TTC or OTC data,
+// from a []byte data source.
+//
+// If passed data for a single font, a TTF or OTF instead of a TTC or OTC,
+// it will return a collection containing 1 font.
+func ParseCollection(src []byte) (*Collection, error) {
+	c, err := sfnt.ParseCollection(src)
+	if err != nil {
+		return nil, err
+	}
+	return &Collection{c}, nil
+}
+
+// ParseCollectionReaderAt parses an SFNT collection, such as TTC or OTC data,
+// from an io.ReaderAt data source.
+//
+// If passed data for a single font, a TTF or OTF instead of a TTC or OTC, it
+// will return a collection containing 1 font.
+func ParseCollectionReaderAt(src io.ReaderAt) (*Collection, error) {
+	c, err := sfnt.ParseCollectionReaderAt(src)
+	if err != nil {
+		return nil, err
+	}
+	return &Collection{c}, nil
+}
+
+// NumFonts returns the number of fonts in the collection.
+func (c *Collection) NumFonts() int {
+	return c.coll.NumFonts()
+}
+
+// Font returns the i'th font in the collection.
+func (c *Collection) Font(i int) (*Font, error) {
+	fnt, err := c.coll.Font(i)
+	if err != nil {
+		return nil, err
+	}
+	return &Font{font: fnt}, nil
+}
+
 func (f *Font) Layout(ppem fixed.Int26_6, str string, opts text.LayoutOptions) *text.Layout {
 	return layoutText(&f.buf, ppem, str, &opentype{Font: f.font, Hinting: font.HintingFull}, opts)
 }