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Source file src/mime/type.go

  // 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 mime implements parts of the MIME spec.
  package mime
  
  import (
  	"fmt"
  	"strings"
  	"sync"
  )
  
  var (
  	mimeLock       sync.RWMutex      // guards following 3 maps
  	mimeTypes      map[string]string // ".Z" => "application/x-compress"
  	mimeTypesLower map[string]string // ".z" => "application/x-compress"
  
  	// extensions maps from MIME type to list of lowercase file
  	// extensions: "image/jpeg" => [".jpg", ".jpeg"]
  	extensions map[string][]string
  )
  
  // setMimeTypes is used by initMime's non-test path, and by tests.
  // The two maps must not be the same, or nil.
  func setMimeTypes(lowerExt, mixExt map[string]string) {
  	if lowerExt == nil || mixExt == nil {
  		panic("nil map")
  	}
  	mimeTypesLower = lowerExt
  	mimeTypes = mixExt
  	extensions = invert(lowerExt)
  }
  
  var builtinTypesLower = map[string]string{
  	".css":  "text/css; charset=utf-8",
  	".gif":  "image/gif",
  	".htm":  "text/html; charset=utf-8",
  	".html": "text/html; charset=utf-8",
  	".jpg":  "image/jpeg",
  	".js":   "application/x-javascript",
  	".pdf":  "application/pdf",
  	".png":  "image/png",
  	".svg":  "image/svg+xml",
  	".xml":  "text/xml; charset=utf-8",
  }
  
  func clone(m map[string]string) map[string]string {
  	m2 := make(map[string]string, len(m))
  	for k, v := range m {
  		m2[k] = v
  		if strings.ToLower(k) != k {
  			panic("keys in builtinTypesLower must be lowercase")
  		}
  	}
  	return m2
  }
  
  func invert(m map[string]string) map[string][]string {
  	m2 := make(map[string][]string, len(m))
  	for k, v := range m {
  		justType, _, err := ParseMediaType(v)
  		if err != nil {
  			panic(err)
  		}
  		m2[justType] = append(m2[justType], k)
  	}
  	return m2
  }
  
  var once sync.Once // guards initMime
  
  var testInitMime, osInitMime func()
  
  func initMime() {
  	if fn := testInitMime; fn != nil {
  		fn()
  	} else {
  		setMimeTypes(builtinTypesLower, clone(builtinTypesLower))
  		osInitMime()
  	}
  }
  
  // TypeByExtension returns the MIME type associated with the file extension ext.
  // The extension ext should begin with a leading dot, as in ".html".
  // When ext has no associated type, TypeByExtension returns "".
  //
  // Extensions are looked up first case-sensitively, then case-insensitively.
  //
  // The built-in table is small but on unix it is augmented by the local
  // system's mime.types file(s) if available under one or more of these
  // names:
  //
  //   /etc/mime.types
  //   /etc/apache2/mime.types
  //   /etc/apache/mime.types
  //
  // On Windows, MIME types are extracted from the registry.
  //
  // Text types have the charset parameter set to "utf-8" by default.
  func TypeByExtension(ext string) string {
  	once.Do(initMime)
  	mimeLock.RLock()
  	defer mimeLock.RUnlock()
  
  	// Case-sensitive lookup.
  	if v := mimeTypes[ext]; v != "" {
  		return v
  	}
  
  	// Case-insensitive lookup.
  	// Optimistically assume a short ASCII extension and be
  	// allocation-free in that case.
  	var buf [10]byte
  	lower := buf[:0]
  	const utf8RuneSelf = 0x80 // from utf8 package, but not importing it.
  	for i := 0; i < len(ext); i++ {
  		c := ext[i]
  		if c >= utf8RuneSelf {
  			// Slow path.
  			return mimeTypesLower[strings.ToLower(ext)]
  		}
  		if 'A' <= c && c <= 'Z' {
  			lower = append(lower, c+('a'-'A'))
  		} else {
  			lower = append(lower, c)
  		}
  	}
  	// The conversion from []byte to string doesn't allocate in
  	// a map lookup.
  	return mimeTypesLower[string(lower)]
  }
  
  // ExtensionsByType returns the extensions known to be associated with the MIME
  // type typ. The returned extensions will each begin with a leading dot, as in
  // ".html". When typ has no associated extensions, ExtensionsByType returns an
  // nil slice.
  func ExtensionsByType(typ string) ([]string, error) {
  	justType, _, err := ParseMediaType(typ)
  	if err != nil {
  		return nil, err
  	}
  
  	once.Do(initMime)
  	mimeLock.RLock()
  	defer mimeLock.RUnlock()
  	s, ok := extensions[justType]
  	if !ok {
  		return nil, nil
  	}
  	return append([]string{}, s...), nil
  }
  
  // AddExtensionType sets the MIME type associated with
  // the extension ext to typ. The extension should begin with
  // a leading dot, as in ".html".
  func AddExtensionType(ext, typ string) error {
  	if !strings.HasPrefix(ext, ".") {
  		return fmt.Errorf("mime: extension %q missing leading dot", ext)
  	}
  	once.Do(initMime)
  	return setExtensionType(ext, typ)
  }
  
  func setExtensionType(extension, mimeType string) error {
  	justType, param, err := ParseMediaType(mimeType)
  	if err != nil {
  		return err
  	}
  	if strings.HasPrefix(mimeType, "text/") && param["charset"] == "" {
  		param["charset"] = "utf-8"
  		mimeType = FormatMediaType(mimeType, param)
  	}
  	extLower := strings.ToLower(extension)
  
  	mimeLock.Lock()
  	defer mimeLock.Unlock()
  	mimeTypes[extension] = mimeType
  	mimeTypesLower[extLower] = mimeType
  	for _, v := range extensions[justType] {
  		if v == extLower {
  			return nil
  		}
  	}
  	extensions[justType] = append(extensions[justType], extLower)
  	return nil
  }
  

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