format.go

     1  // Copyright 2010 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package image
     6  
     7  import (
     8  	"bufio"
     9  	"errors"
    10  	"io"
    11  	"sync"
    12  	"sync/atomic"
    13  )
    14  
    15  // ErrFormat indicates that decoding encountered an unknown format.
    16  var ErrFormat = errors.New("image: unknown format")
    17  
    18  // A format holds an image format's name, magic header and how to decode it.
    19  type format struct {
    20  	name, magic  string
    21  	decode       func(io.Reader) (Image, error)
    22  	decodeConfig func(io.Reader) (Config, error)
    23  }
    24  
    25  // Formats is the list of registered formats.
    26  var (
    27  	formatsMu     sync.Mutex
    28  	atomicFormats atomic.Value
    29  )
    30  
    31  // RegisterFormat registers an image format for use by [Decode].
    32  // Name is the name of the format, like "jpeg" or "png".
    33  // Magic is the magic prefix that identifies the format's encoding. The magic
    34  // string can contain "?" wildcards that each match any one byte.
    35  // [Decode] is the function that decodes the encoded image.
    36  // [DecodeConfig] is the function that decodes just its configuration.
    37  func RegisterFormat(name, magic string, decode func(io.Reader) (Image, error), decodeConfig func(io.Reader) (Config, error)) {
    38  	formatsMu.Lock()
    39  	formats, _ := atomicFormats.Load().([]format)
    40  	atomicFormats.Store(append(formats, format{name, magic, decode, decodeConfig}))
    41  	formatsMu.Unlock()
    42  }
    43  
    44  // A reader is an io.Reader that can also peek ahead.
    45  type reader interface {
    46  	io.Reader
    47  	Peek(int) ([]byte, error)
    48  }
    49  
    50  // asReader converts an io.Reader to a reader.
    51  func asReader(r io.Reader) reader {
    52  	if rr, ok := r.(reader); ok {
    53  		return rr
    54  	}
    55  	return bufio.NewReader(r)
    56  }
    57  
    58  // match reports whether magic matches b. Magic may contain "?" wildcards.
    59  func match(magic string, b []byte) bool {
    60  	if len(magic) != len(b) {
    61  		return false
    62  	}
    63  	for i, c := range b {
    64  		if magic[i] != c && magic[i] != '?' {
    65  			return false
    66  		}
    67  	}
    68  	return true
    69  }
    70  
    71  // sniff determines the format of r's data.
    72  func sniff(r reader) format {
    73  	formats, _ := atomicFormats.Load().([]format)
    74  	for _, f := range formats {
    75  		b, err := r.Peek(len(f.magic))
    76  		if err == nil && match(f.magic, b) {
    77  			return f
    78  		}
    79  	}
    80  	return format{}
    81  }
    82  
    83  // Decode decodes an image that has been encoded in a registered format.
    84  // The string returned is the format name used during format registration.
    85  // Format registration is typically done by an init function in the codec-
    86  // specific package.
    87  func Decode(r io.Reader) (Image, string, error) {
    88  	rr := asReader(r)
    89  	f := sniff(rr)
    90  	if f.decode == nil {
    91  		return nil, "", ErrFormat
    92  	}
    93  	m, err := f.decode(rr)
    94  	return m, f.name, err
    95  }
    96  
    97  // DecodeConfig decodes the color model and dimensions of an image that has
    98  // been encoded in a registered format. The string returned is the format name
    99  // used during format registration. Format registration is typically done by
   100  // an init function in the codec-specific package.
   101  func DecodeConfig(r io.Reader) (Config, string, error) {
   102  	rr := asReader(r)
   103  	f := sniff(rr)
   104  	if f.decodeConfig == nil {
   105  		return Config{}, "", ErrFormat
   106  	}
   107  	c, err := f.decodeConfig(rr)
   108  	return c, f.name, err
   109  }
   110
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