// Copyright 2010 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package image
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import (
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"bufio"
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"errors"
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"io"
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"sync"
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"sync/atomic"
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)
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// ErrFormat indicates that decoding encountered an unknown format.
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var ErrFormat = errors.New("image: unknown format")
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// A format holds an image format's name, magic header and how to decode it.
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type format struct {
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name, magic string
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decode func(io.Reader) (Image, error)
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decodeConfig func(io.Reader) (Config, error)
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}
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// Formats is the list of registered formats.
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var (
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formatsMu sync.Mutex
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atomicFormats atomic.Value
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)
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// RegisterFormat registers an image format for use by Decode.
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// Name is the name of the format, like "jpeg" or "png".
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// Magic is the magic prefix that identifies the format's encoding. The magic
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// string can contain "?" wildcards that each match any one byte.
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// Decode is the function that decodes the encoded image.
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// DecodeConfig is the function that decodes just its configuration.
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func RegisterFormat(name, magic string, decode func(io.Reader) (Image, error), decodeConfig func(io.Reader) (Config, error)) {
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formatsMu.Lock()
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formats, _ := atomicFormats.Load().([]format)
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atomicFormats.Store(append(formats, format{name, magic, decode, decodeConfig}))
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formatsMu.Unlock()
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}
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// A reader is an io.Reader that can also peek ahead.
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type reader interface {
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io.Reader
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Peek(int) ([]byte, error)
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}
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// asReader converts an io.Reader to a reader.
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func asReader(r io.Reader) reader {
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if rr, ok := r.(reader); ok {
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return rr
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}
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return bufio.NewReader(r)
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}
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// Match reports whether magic matches b. Magic may contain "?" wildcards.
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func match(magic string, b []byte) bool {
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if len(magic) != len(b) {
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return false
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}
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for i, c := range b {
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if magic[i] != c && magic[i] != '?' {
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return false
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}
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}
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return true
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}
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// Sniff determines the format of r's data.
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func sniff(r reader) format {
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formats, _ := atomicFormats.Load().([]format)
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for _, f := range formats {
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b, err := r.Peek(len(f.magic))
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if err == nil && match(f.magic, b) {
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return f
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}
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}
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return format{}
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}
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// Decode decodes an image that has been encoded in a registered format.
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// The string returned is the format name used during format registration.
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// Format registration is typically done by an init function in the codec-
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// specific package.
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func Decode(r io.Reader) (Image, string, error) {
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rr := asReader(r)
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f := sniff(rr)
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if f.decode == nil {
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return nil, "", ErrFormat
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}
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m, err := f.decode(rr)
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return m, f.name, err
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}
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// DecodeConfig decodes the color model and dimensions of an image that has
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// been encoded in a registered format. The string returned is the format name
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// used during format registration. Format registration is typically done by
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// an init function in the codec-specific package.
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func DecodeConfig(r io.Reader) (Config, string, error) {
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rr := asReader(r)
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f := sniff(rr)
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if f.decodeConfig == nil {
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return Config{}, "", ErrFormat
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}
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c, err := f.decodeConfig(rr)
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return c, f.name, err
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}
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