image: introduce Decode and RegisterFormat.

R=r, rsc
CC=golang-dev
https://golang.org/cl/1849054

src/pkg/image/Makefile

@@ -7,6 +7,7 @@ include ../../Make.$(GOARCH)
 TARG=image
 GOFILES=\
 	color.go\
+	format.go\
 	image.go\
 	names.go\
 

src/pkg/image/format.go

@@ -0,0 +1,73 @@
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package image
+
+import (
+	"bufio"
+	"io"
+	"os"
+)
+
+// An UnknownFormatErr indicates that decoding encountered an unknown format.
+var UnknownFormatErr = os.NewError("image: unknown format")
+
+// A format holds an image format's name, magic header and how to decode it.
+type format struct {
+	name, magic string
+	decode      func(io.Reader) (Image, os.Error)
+}
+
+// Formats is the list of registered formats.
+var formats []format
+
+// RegisterFormat registers an image format for use by Decode.
+// Name is the name of the format, like "jpeg" or "png".
+// Magic is the magic prefix that identifies the format's encoding.
+// Decode is the function that decodes the encoded image.
+func RegisterFormat(name, magic string, decode func(io.Reader) (Image, os.Error)) {
+	n := len(formats)
+	if n == cap(formats) {
+		x := make([]format, n+1, 2*n+4)
+		copy(x, formats)
+		formats = x
+	} else {
+		formats = formats[0 : n+1]
+	}
+	formats[n] = format{name, magic, decode}
+}
+
+// A reader is an io.Reader that can also peek ahead.
+type reader interface {
+	io.Reader
+	Peek(int) ([]byte, os.Error)
+}
+
+// AsReader converts an io.Reader to a reader.
+func asReader(r io.Reader) reader {
+	if rr, ok := r.(reader); ok {
+		return rr
+	}
+	return bufio.NewReader(r)
+}
+
+// Decode decodes an image that has been encoded in a registered format.
+// Format registration is typically done by the init method of the codec-
+// specific package.
+func Decode(r io.Reader) (m Image, formatName string, err os.Error) {
+	var f format
+	rr := asReader(r)
+	for _, g := range formats {
+		s, err := rr.Peek(len(g.magic))
+		if err == nil && string(s) == g.magic {
+			f = g
+			break
+		}
+	}
+	if f.decode == nil {
+		return nil, "", UnknownFormatErr
+	}
+	m, err = f.decode(rr)
+	return m, f.name, err
+}

src/pkg/image/jpeg/reader.go

@@ -432,3 +432,7 @@ func Decode(r io.Reader) (image.Image, os.Error) {
 	}
 	return d.image, nil
 }
+
+func init() {
+	image.RegisterFormat("jpeg", "\xff\xd8", Decode)
+}

src/pkg/image/png/reader.go

@@ -464,3 +464,7 @@ func Decode(r io.Reader) (image.Image, os.Error) {
 	}
 	return d.image, nil
 }
+
+func init() {
+	image.RegisterFormat("png", pngHeader, Decode)
+}