image: introduce Decode and RegisterFormat.

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

diff --git a/src/pkg/image/Makefile b/src/pkg/image/Makefile
index 9c886f9f9fac5043940ec028c7fe5944932c0651..e26deeac67a108e629340d5e5f453d9ea55bf0ea 100644 (file)
--- a/src/pkg/image/Makefile
+++ b/src/pkg/image/Makefile
@@ -7,6 +7,7 @@ include ../../Make.$(GOARCH)
 TARG=image
 GOFILES=\
        color.go\
+       format.go\
        image.go\
        names.go\
 

diff --git a/src/pkg/image/format.go b/src/pkg/image/format.go
new file mode 100644 (file)
index 0000000..b20f02e
--- /dev/null
+++ b/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
+}

diff --git a/src/pkg/image/jpeg/reader.go b/src/pkg/image/jpeg/reader.go
index ec036ef4d66fa563163a1a2967fc65359fe26ed9..55cc89aa311794f2018767722f2c2f9243325109 100644 (file)
--- a/src/pkg/image/jpeg/reader.go
+++ b/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)
+}

diff --git a/src/pkg/image/png/reader.go b/src/pkg/image/png/reader.go
index fddb70423a73333293612578af430be6633add20..b23aa7071a7f5eca1f9e264cd3d046fe5ddccda7 100644 (file)
--- a/src/pkg/image/png/reader.go
+++ b/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)
+}