internal/licenses/licenses.go - pkgsite - Git at Google

 // Copyright 2020 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 licenses detects licenses and determines whether they are redistributable.
 // The functions in this package do not return errors; instead, they log any problems
 // they encounter and fail closed by reporting that the module or package is not
 // redistributable.
 //
 // Example (modproxy):
 //   d := licenses.NewDetector(modulePath, version, zipReader, log.Infof)
 //   modRedist := d.ModuleIsRedistributable()
 //
 // Example (discovery):
 //   d := licenses.NewDetector(modulePath, version, zipReader, log.Infof)
 //   modRedist := d.ModuleIsRedistributable()
 //   lics := d.AllLicenses()
 //   pkgRedist, pkgMetas := d.PackageInfo(pkgSubdir)
 package licenses

 import (
 	"archive/zip"
 	"fmt"
 	"io"
 	"io/ioutil"
 	"path"
 	"path/filepath"
 	"sort"
 	"strings"

 	"github.com/google/licensecheck"
 	"golang.org/x/mod/module"
 	modzip "golang.org/x/mod/zip"
 )

 //go:generate rm -f exceptions.gen.go
 //go:generate go run gen_exceptions.go

 const (
 	// classifyThreshold is the minimum confidence percentage/threshold to
 	// classify a license
 	classifyThreshold = 90

 	// coverageThreshold is the minimum percentage of the file that must contain
 	// license text.
 	coverageThreshold = 75

 	// unknownLicenseType is for text in a license file that's not recognized.
 	unknownLicenseType = "UNKNOWN"
 )

 // maxLicenseSize is the maximum allowable size (in bytes) for a license file.
 // There are some license files larger than 1 million bytes: https://github.com/vmware/vic/LICENSE
 // and github.com/goharbor/harbor/LICENSE, for example.
 // var for testing
 var maxLicenseSize uint64 = modzip.MaxLICENSE

 // Metadata holds information extracted from a license file.
 type Metadata struct {
 	// Types is the set of license types, as determined by the licensecheck package.
 	Types []string
 	// FilePath is the '/'-separated path to the license file in the module zip,
 	// relative to the contents directory.
 	FilePath string
 	// The output of licensecheck.Cover.
 	Coverage licensecheck.Coverage
 }

 // A License is a classified license file path and its contents.
 type License struct {
 	*Metadata
 	Contents []byte
 }

 // RemoveNonRedistributableData methods removes the license contents
 // if the license is non-redistributable.
 func (l *License) RemoveNonRedistributableData() {
 	if !Redistributable(l.Types) {
 		l.Contents = nil
 	}
 }

 var (
 	FileNames = []string{
 		"COPYING",
 		"COPYING.md",
 		"COPYING.markdown",
 		"COPYING.txt",
 		"LICENCE",
 		"LICENCE.md",
 		"LICENCE.markdown",
 		"LICENCE.txt",
 		"LICENSE",
 		"LICENSE.md",
 		"LICENSE.markdown",
 		"LICENSE.txt",
 		"LICENSE-2.0.txt",
 		"LICENCE-2.0.txt",
 		"LICENSE-APACHE",
 		"LICENCE-APACHE",
 		"LICENSE-APACHE-2.0.txt",
 		"LICENCE-APACHE-2.0.txt",
 		"LICENSE-MIT",
 		"LICENCE-MIT",
 		"LICENSE.MIT",
 		"LICENCE.MIT",
 		"LICENSE.code",
 		"LICENCE.code",
 		"LICENSE.docs",
 		"LICENCE.docs",
 		"LICENSE.rst",
 		"LICENCE.rst",
 		"MIT-LICENSE",
 		"MIT-LICENCE",
 		"MIT-LICENSE.md",
 		"MIT-LICENCE.md",
 		"MIT-LICENSE.markdown",
 		"MIT-LICENCE.markdown",
 		"MIT-LICENSE.txt",
 		"MIT-LICENCE.txt",
 		"MIT_LICENSE",
 		"MIT_LICENCE",
 		"UNLICENSE",
 		"UNLICENCE",
 	}

 	// redistributableLicenseTypes is the list of license types, as reported by
 	// licensecheck, that allow redistribution.
 	redistributableLicenseTypes = map[string]bool{
 		// Licenses acceptable by OSI.
 		"AGPL-3.0":             true,
 		"Apache-2.0":           true,
 		"Artistic-2.0":         true,
 		"BlueOak-1.0":          true,
 		"BSD-0-Clause":         true,
 		"BSD-2-Clause":         true,
 		"BSD-2-Clause-FreeBSD": true,
 		"BSD-3-Clause":         true,
 		"BSL-1.0":              true,
 		"CC-BY-3.0":            true,
 		"CC-BY-4.0":            true,
 		"CC-BY-SA-3.0":         true,
 		"CC-BY-SA-4.0":         true,
 		"CC0-1.0":              true,
 		"EPL-1.0":              true,
 		"EPL-2.0":              true,
 		"EUPL-1.2":             true,
 		"GPL2":                 true,
 		"GPL3":                 true,
 		"ISC":                  true,
 		"JSON":                 true,
 		"LGPL-2.1":             true,
 		"LGPL-3.0":             true,
 		"MIT":                  true,
 		"MIT-0":                true,
 		"MPL-2.0":              true,
 		"NIST":                 true,
 		"NCSA":                 true,
 		"OpenSSL":              true,
 		"OSL-3.0":              true,
 		"Unlicense":            true,
 		"Zlib":                 true,
 	}

 	// These aren't technically licenses, but they are recognized by
 	// licensecheck and safe to ignore.
 	ignorableLicenseTypes = map[string]bool{
 		"CC-Notice":          true,
 		"GooglePatentClause": true,
 	}
 )

 // spdxIdentifierOverrides maps a licensecheck license type to the corresponding
 // SPDX short identifier, if they differ.
 var spdxIdentifierOverrides = map[string]string{
 	"BlueOak-1.0":  "BlueOak-1.0.0",
 	"BSD-0-Clause": "0BSD",
 	"GPL2":         "GPL-2.0",
 	"GPL3":         "GPL-3.0",
 	"NIST":         "NIST-PD",
 }

 // nonOSILicenses lists licenses that are not approved by OSI.
 var nonOSILicenses = map[string]bool{
 	"BlueOak-1.0":          true,
 	"BSD-2-Clause-FreeBSD": true,
 	"CC-BY-3.0":            true,
 	"CC-BY-4.0":            true,
 	"CC-BY-SA-3.0":         true,
 	"CC-BY-SA-4.0":         true,
 	"CC0-1.0":              true,
 	"JSON":                 true,
 	"NIST":                 true,
 	"OpenSSL":              true,
 }

 // fileNamesLowercase has all the entries of FileNames, downcased and made a set
 // for fast case-insensitive matching.
 var fileNamesLowercase = map[string]bool{}

 func init() {
 	for _, f := range FileNames {
 		fileNamesLowercase[strings.ToLower(f)] = true
 	}
 }

 // AcceptedLicenseInfo describes a license that is accepted by the discovery site.
 type AcceptedLicenseInfo struct {
 	Name string
 	URL  string
 }

 // AcceptedLicenses returns a sorted slice of license types that are accepted as
 // redistributable. Its result is intended to be displayed to users.
 func AcceptedLicenses() []AcceptedLicenseInfo {
 	var lics []AcceptedLicenseInfo
 	for l := range redistributableLicenseTypes {
 		identifier := spdxIdentifierOverrides[l]
 		if identifier == "" {
 			identifier = l
 		}
 		var link string
 		if nonOSILicenses[l] {
 			link = fmt.Sprintf("https://spdx.org/licenses/%s.html", identifier)
 		} else {
 			link = fmt.Sprintf("https://opensource.org/licenses/%s", identifier)
 		}
 		lics = append(lics, AcceptedLicenseInfo{identifier, link})
 	}
 	sort.Slice(lics, func(i, j int) bool { return lics[i].Name < lics[j].Name })
 	return lics
 }

 var checker *licensecheck.Checker = licensecheck.New(licensecheck.BuiltinLicenses())

 // A Detector detects licenses in a module and its packages.
 type Detector struct {
 	modulePath     string
 	version        string
 	zr             *zip.Reader
 	logf           func(string, ...interface{})
 	moduleRedist   bool
 	moduleLicenses []*License // licenses at module root directory, or list from exceptions
 	allLicenses    []*License
 	licsByDir      map[string][]*License // from directory to list of licenses
 }

 // NewDetector returns a Detector for the given module and version.
 // zr should be the zip file for that module and version.
 // logf is for logging; if nil, no logging is done.
 func NewDetector(modulePath, version string, zr *zip.Reader, logf func(string, ...interface{})) *Detector {
 	if logf == nil {
 		logf = func(string, ...interface{}) {}
 	}
 	d := &Detector{
 		modulePath: modulePath,
 		version:    version,
 		zr:         zr,
 		logf:       logf,
 	}
 	d.computeModuleInfo()
 	return d
 }

 // ModuleIsRedistributable reports whether the given module is redistributable.
 func (d *Detector) ModuleIsRedistributable() bool {
 	return d.moduleRedist
 }

 // ModuleLicenses returns the licenses that apply to the module.
 func (d *Detector) ModuleLicenses() []*License {
 	return d.moduleLicenses
 }

 // AllLicenses returns all the licenses detected in the entire module, including
 // package licenses.
 func (d *Detector) AllLicenses() []*License {
 	if d.allLicenses == nil {
 		d.computeAllLicenseInfo()
 	}
 	return d.allLicenses
 }

 // PackageInfo reports whether the package at dir, a directory relative to the
 // module root, is redistributable. It also returns all the licenses that apply
 // to the package.
 func (d *Detector) PackageInfo(dir string) (isRedistributable bool, lics []*License) {
 	cleanDir := filepath.ToSlash(filepath.Clean(dir))
 	if path.IsAbs(cleanDir) || strings.HasPrefix(cleanDir, "..") {
 		return false, nil
 	}
 	if d.allLicenses == nil {
 		d.computeAllLicenseInfo()
 	}
 	// Collect all the license metadata for directories dir and above, excluding the root.
 	for prefix, plics := range d.licsByDir {
 		// append a slash so that prefix a/b does not match a/bc/d
 		if strings.HasPrefix(cleanDir+"/", prefix+"/") {
 			lics = append(lics, plics...)
 		}
 	}
 	// A package is redistributable if its module is, and if other licenses on
 	// the path to the root are redistributable. Note that this is not the same
 	// as asking if the module licenses plus the package licenses are
 	// redistributable. A module that is granted an exception (see DetectFiles)
 	// may have licenses that are non-redistributable.
 	ltypes := types(lics)
 	isRedistributable = d.ModuleIsRedistributable() && (len(ltypes) == 0 || Redistributable(ltypes))
 	// A package's licenses include the ones we've already computed, as well
 	// as the module licenses.
 	return isRedistributable, append(lics, d.moduleLicenses...)
 }

 // computeModuleInfo determines values for the moduleRedist and moduleLicenses fields of d.
 func (d *Detector) computeModuleInfo() {
 	// Check that all licenses in the contents directory are redistributable.
 	d.moduleLicenses = d.detectFiles(d.Files(RootFiles))
 	d.moduleRedist = Redistributable(types(d.moduleLicenses))
 }

 // computeAllLicenseInfo collects all the detected licenses in the zip and
 // stores them in the allLicenses field of d. It also maps detected licenses to
 // their directories, to optimize Detector.PackageInfo.
 func (d *Detector) computeAllLicenseInfo() {
 	d.allLicenses = []*License{}
 	d.allLicenses = append(d.allLicenses, d.moduleLicenses...)
 	nonRootLicenses := d.detectFiles(d.Files(NonRootFiles))
 	d.allLicenses = append(d.allLicenses, nonRootLicenses...)
 	d.licsByDir = map[string][]*License{}
 	for _, l := range nonRootLicenses {
 		prefix := path.Dir(l.FilePath)
 		d.licsByDir[prefix] = append(d.licsByDir[prefix], l)
 	}
 }

 // WhichFiles describes which files from the zip should be returned by Detector.Files.
 type WhichFiles int

 const (
 	// Only files from the root (contents) directory.
 	RootFiles WhichFiles = iota
 	// Only files that are not in the root directory.
 	NonRootFiles
 	// All files; the union of root and non-root.
 	AllFiles
 )

 // Files returns a list of license files from the zip. The which argument
 // determines the location of the files considered.
 func (d *Detector) Files(which WhichFiles) []*zip.File {
 	cdir := contentsDir(d.modulePath, d.version)
 	prefix := pathPrefix(cdir)
 	var files []*zip.File
 	for _, f := range d.zr.File {
 		if !fileNamesLowercase[strings.ToLower(path.Base(f.Name))] {
 			continue
 		}
 		if !strings.HasPrefix(f.Name, prefix) {
 			d.logf("potential license file %q found outside of the expected path %q", f.Name, cdir)
 			continue
 		}
 		// Skip files we should ignore.
 		if ignoreFiles[d.modulePath+" "+strings.TrimPrefix(f.Name, prefix)] {
 			continue
 		}
 		if which == RootFiles && path.Dir(f.Name) != cdir {
 			// Skip f since it's not at root.
 			continue
 		}
 		if which == NonRootFiles && path.Dir(f.Name) == cdir {
 			// Skip f since it is at root.
 			continue
 		}
 		if isVendoredFile(f.Name) {
 			// Skip if f is in the vendor directory.
 			continue
 		}
 		if err := module.CheckFilePath(f.Name); err != nil {
 			// Skip if the file path is bad.
 			d.logf("module.CheckFilePath(%q): %v", f.Name, err)
 			continue
 		}
 		files = append(files, f)
 	}
 	return files
 }

 // isVendoredFile reports if the given file is in a proper subdirectory nested
 // under a 'vendor' directory, to allow for Go packages named 'vendor'.
 //
 // e.g. isVendoredFile("vendor/LICENSE") == false, and
 //      isVendoredFile("vendor/foo/LICENSE") == true
 func isVendoredFile(name string) bool {
 	var vendorOffset int
 	if strings.HasPrefix(name, "vendor/") {
 		vendorOffset = len("vendor/")
 	} else if i := strings.Index(name, "/vendor/"); i >= 0 {
 		vendorOffset = i + len("/vendor/")
 	} else {
 		// no vendor directory
 		return false
 	}
 	// check if the file is in a proper subdirectory of vendor
 	return strings.Contains(name[vendorOffset:], "/")
 }

 // detectFiles runs DetectFile on each of the given files.
 // If a file cannot be read, the error is logged and a license
 // of type unknown is added.
 func (d *Detector) detectFiles(files []*zip.File) []*License {
 	prefix := pathPrefix(contentsDir(d.modulePath, d.version))
 	var licenses []*License
 	for _, f := range files {
 		bytes, err := readZipFile(f)
 		if err != nil {
 			d.logf("reading zip file %s: %v", f.Name, err)
 			licenses = append(licenses, &License{
 				Metadata: &Metadata{
 					Types:    []string{unknownLicenseType},
 					FilePath: strings.TrimPrefix(f.Name, prefix),
 				},
 			})
 			continue
 		}
 		types, cov := DetectFile(bytes, f.Name, d.logf)
 		licenses = append(licenses, &License{
 			Metadata: &Metadata{
 				Types:    types,
 				FilePath: strings.TrimPrefix(f.Name, prefix),
 				Coverage: cov,
 			},
 			Contents: bytes,
 		})
 	}
 	return licenses
 }

 // DetectFile return the set of license types for the given file contents. It
 // also returns the licensecheck coverage information. The filename is used
 // solely for logging.
 func DetectFile(contents []byte, filename string, logf func(string, ...interface{})) ([]string, licensecheck.Coverage) {
 	if logf == nil {
 		logf = func(string, ...interface{}) {}
 	}
 	if types := exceptionFileTypes(contents); types != nil {
 		logf("%s is an exception", filename)
 		return types, licensecheck.Coverage{}
 	}
 	cov, ok := checker.Cover(contents, licensecheck.Options{})
 	if !ok {
 		logf("%s checker.Cover failed, skipping", filename)
 		return []string{unknownLicenseType}, licensecheck.Coverage{}
 	}
 	if cov.Percent < float64(coverageThreshold) {
 		logf("%s license coverage too low (%+v), skipping", filename, cov)
 		return []string{unknownLicenseType}, cov
 	}
 	types := make(map[string]bool)
 	for _, m := range cov.Match {
 		if m.Percent >= classifyThreshold {
 			types[canonicalizeName(m.Name)] = true
 		}
 	}
 	if len(types) == 0 {
 		logf("%s failed to classify license (%+v), skipping", filename, cov)
 		return []string{unknownLicenseType}, cov
 	}
 	return setToSortedSlice(types), cov
 }

 // Redistributable reports whether the set of license types establishes that a
 // module or package is redistributable.
 func Redistributable(licenseTypes []string) bool {
 	if len(licenseTypes) == 0 {
 		return false
 	}
 	for _, t := range licenseTypes {
 		if !redistributableLicenseTypes[t] && !ignorableLicenseTypes[t] {
 			return false
 		}
 	}
 	return true
 }

 var canonicalNames = map[string]string{
 	"AGPL-Header":         "AGPL-3.0",
 	"GPL-Header":          "GPL2",
 	"GPL-NotLater-Header": "GPL3",
 	"LGPL-Header":         "LGPL-2.1",
 }

 // canonicalizeName puts a license name in a standard form.
 func canonicalizeName(name string) string {
 	if c := canonicalNames[name]; c != "" {
 		return c
 	}
 	name = strings.TrimSuffix(name, "-Short")
 	return strings.TrimSuffix(name, "-Header")
 }

 func types(lics []*License) []string {
 	var types []string
 	for _, l := range lics {
 		types = append(types, l.Types...)
 	}
 	return types
 }

 func setToSortedSlice(m map[string]bool) []string {
 	var s []string
 	for e := range m {
 		s = append(s, e)
 	}
 	sort.Strings(s)
 	return s
 }

 func readZipFile(f *zip.File) ([]byte, error) {
 	if f.UncompressedSize64 > maxLicenseSize {
 		return nil, fmt.Errorf("file size %d exceeds max license size %d", f.UncompressedSize64, maxLicenseSize)
 	}
 	rc, err := f.Open()
 	if err != nil {
 		return nil, err
 	}
 	defer rc.Close()
 	return ioutil.ReadAll(io.LimitReader(rc, int64(maxLicenseSize)))
 }

 func contentsDir(modulePath, version string) string {
 	return modulePath + "@" + version
 }

 // pathPrefix appends a "/" to its argument if the argument is non-empty.
 func pathPrefix(s string) string {
 	if s != "" {
 		return s + "/"
 	}
 	return ""
 }
	// Copyright 2020 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 licenses detects licenses and determines whether they are redistributable.
	// The functions in this package do not return errors; instead, they log any problems
	// they encounter and fail closed by reporting that the module or package is not
	// redistributable.
	//
	// Example (modproxy):
	// d := licenses.NewDetector(modulePath, version, zipReader, log.Infof)
	// modRedist := d.ModuleIsRedistributable()
	//
	// Example (discovery):
	// d := licenses.NewDetector(modulePath, version, zipReader, log.Infof)
	// modRedist := d.ModuleIsRedistributable()
	// lics := d.AllLicenses()
	// pkgRedist, pkgMetas := d.PackageInfo(pkgSubdir)
	package licenses

	import (
	"archive/zip"
	"fmt"
	"io"
	"io/ioutil"
	"path"
	"path/filepath"
	"sort"
	"strings"

	"github.com/google/licensecheck"
	"golang.org/x/mod/module"
	modzip "golang.org/x/mod/zip"
	)

	//go:generate rm -f exceptions.gen.go
	//go:generate go run gen_exceptions.go

	const (
	// classifyThreshold is the minimum confidence percentage/threshold to
	// classify a license
	classifyThreshold = 90

	// coverageThreshold is the minimum percentage of the file that must contain
	// license text.
	coverageThreshold = 75

	// unknownLicenseType is for text in a license file that's not recognized.
	unknownLicenseType = "UNKNOWN"
	)

	// maxLicenseSize is the maximum allowable size (in bytes) for a license file.
	// There are some license files larger than 1 million bytes: https://github.com/vmware/vic/LICENSE
	// and github.com/goharbor/harbor/LICENSE, for example.
	// var for testing
	var maxLicenseSize uint64 = modzip.MaxLICENSE

	// Metadata holds information extracted from a license file.
	type Metadata struct {
	// Types is the set of license types, as determined by the licensecheck package.
	Types []string
	// FilePath is the '/'-separated path to the license file in the module zip,
	// relative to the contents directory.
	FilePath string
	// The output of licensecheck.Cover.
	Coverage licensecheck.Coverage
	}

	// A License is a classified license file path and its contents.
	type License struct {
	*Metadata
	Contents []byte
	}

	// RemoveNonRedistributableData methods removes the license contents
	// if the license is non-redistributable.
	func (l *License) RemoveNonRedistributableData() {
	if !Redistributable(l.Types) {
	l.Contents = nil
	}
	}

	var (
	FileNames = []string{
	"COPYING",
	"COPYING.md",
	"COPYING.markdown",
	"COPYING.txt",
	"LICENCE",
	"LICENCE.md",
	"LICENCE.markdown",
	"LICENCE.txt",
	"LICENSE",
	"LICENSE.md",
	"LICENSE.markdown",
	"LICENSE.txt",
	"LICENSE-2.0.txt",
	"LICENCE-2.0.txt",
	"LICENSE-APACHE",
	"LICENCE-APACHE",
	"LICENSE-APACHE-2.0.txt",
	"LICENCE-APACHE-2.0.txt",
	"LICENSE-MIT",
	"LICENCE-MIT",
	"LICENSE.MIT",
	"LICENCE.MIT",
	"LICENSE.code",
	"LICENCE.code",
	"LICENSE.docs",
	"LICENCE.docs",
	"LICENSE.rst",
	"LICENCE.rst",
	"MIT-LICENSE",
	"MIT-LICENCE",
	"MIT-LICENSE.md",
	"MIT-LICENCE.md",
	"MIT-LICENSE.markdown",
	"MIT-LICENCE.markdown",
	"MIT-LICENSE.txt",
	"MIT-LICENCE.txt",
	"MIT_LICENSE",
	"MIT_LICENCE",
	"UNLICENSE",
	"UNLICENCE",
	}

	// redistributableLicenseTypes is the list of license types, as reported by
	// licensecheck, that allow redistribution.
	redistributableLicenseTypes = map[string]bool{
	// Licenses acceptable by OSI.
	"AGPL-3.0": true,
	"Apache-2.0": true,
	"Artistic-2.0": true,
	"BlueOak-1.0": true,
	"BSD-0-Clause": true,
	"BSD-2-Clause": true,
	"BSD-2-Clause-FreeBSD": true,
	"BSD-3-Clause": true,
	"BSL-1.0": true,
	"CC-BY-3.0": true,
	"CC-BY-4.0": true,
	"CC-BY-SA-3.0": true,
	"CC-BY-SA-4.0": true,
	"CC0-1.0": true,
	"EPL-1.0": true,
	"EPL-2.0": true,
	"EUPL-1.2": true,
	"GPL2": true,
	"GPL3": true,
	"ISC": true,
	"JSON": true,
	"LGPL-2.1": true,
	"LGPL-3.0": true,
	"MIT": true,
	"MIT-0": true,
	"MPL-2.0": true,
	"NIST": true,
	"NCSA": true,
	"OpenSSL": true,
	"OSL-3.0": true,
	"Unlicense": true,
	"Zlib": true,
	}

	// These aren't technically licenses, but they are recognized by
	// licensecheck and safe to ignore.
	ignorableLicenseTypes = map[string]bool{
	"CC-Notice": true,
	"GooglePatentClause": true,
	}
	)

	// spdxIdentifierOverrides maps a licensecheck license type to the corresponding
	// SPDX short identifier, if they differ.
	var spdxIdentifierOverrides = map[string]string{
	"BlueOak-1.0": "BlueOak-1.0.0",
	"BSD-0-Clause": "0BSD",
	"GPL2": "GPL-2.0",
	"GPL3": "GPL-3.0",
	"NIST": "NIST-PD",
	}

	// nonOSILicenses lists licenses that are not approved by OSI.
	var nonOSILicenses = map[string]bool{
	"BlueOak-1.0": true,
	"BSD-2-Clause-FreeBSD": true,
	"CC-BY-3.0": true,
	"CC-BY-4.0": true,
	"CC-BY-SA-3.0": true,
	"CC-BY-SA-4.0": true,
	"CC0-1.0": true,
	"JSON": true,
	"NIST": true,
	"OpenSSL": true,
	}

	// fileNamesLowercase has all the entries of FileNames, downcased and made a set
	// for fast case-insensitive matching.
	var fileNamesLowercase = map[string]bool{}

	func init() {
	for _, f := range FileNames {
	fileNamesLowercase[strings.ToLower(f)] = true
	}
	}

	// AcceptedLicenseInfo describes a license that is accepted by the discovery site.
	type AcceptedLicenseInfo struct {
	Name string
	URL string
	}

	// AcceptedLicenses returns a sorted slice of license types that are accepted as
	// redistributable. Its result is intended to be displayed to users.
	func AcceptedLicenses() []AcceptedLicenseInfo {
	var lics []AcceptedLicenseInfo
	for l := range redistributableLicenseTypes {
	identifier := spdxIdentifierOverrides[l]
	if identifier == "" {
	identifier = l
	}
	var link string
	if nonOSILicenses[l] {
	link = fmt.Sprintf("https://spdx.org/licenses/%s.html", identifier)
	} else {
	link = fmt.Sprintf("https://opensource.org/licenses/%s", identifier)
	}
	lics = append(lics, AcceptedLicenseInfo{identifier, link})
	}
	sort.Slice(lics, func(i, j int) bool { return lics[i].Name < lics[j].Name })
	return lics
	}

	var checker *licensecheck.Checker = licensecheck.New(licensecheck.BuiltinLicenses())

	// A Detector detects licenses in a module and its packages.
	type Detector struct {
	modulePath string
	version string
	zr *zip.Reader
	logf func(string, ...interface{})
	moduleRedist bool
	moduleLicenses []*License // licenses at module root directory, or list from exceptions
	allLicenses []*License
	licsByDir map[string][]*License // from directory to list of licenses
	}

	// NewDetector returns a Detector for the given module and version.
	// zr should be the zip file for that module and version.
	// logf is for logging; if nil, no logging is done.
	func NewDetector(modulePath, version string, zr zip.Reader, logf func(string, ...interface{})) Detector {
	if logf == nil {
	logf = func(string, ...interface{}) {}
	}
	d := &Detector{
	modulePath: modulePath,
	version: version,
	zr: zr,
	logf: logf,
	}
	d.computeModuleInfo()
	return d
	}

	// ModuleIsRedistributable reports whether the given module is redistributable.
	func (d *Detector) ModuleIsRedistributable() bool {
	return d.moduleRedist
	}

	// ModuleLicenses returns the licenses that apply to the module.
	func (d Detector) ModuleLicenses() []License {
	return d.moduleLicenses
	}

	// AllLicenses returns all the licenses detected in the entire module, including
	// package licenses.
	func (d Detector) AllLicenses() []License {
	if d.allLicenses == nil {
	d.computeAllLicenseInfo()
	}
	return d.allLicenses
	}

	// PackageInfo reports whether the package at dir, a directory relative to the
	// module root, is redistributable. It also returns all the licenses that apply
	// to the package.
	func (d Detector) PackageInfo(dir string) (isRedistributable bool, lics []License) {
	cleanDir := filepath.ToSlash(filepath.Clean(dir))
	if path.IsAbs(cleanDir) \|\| strings.HasPrefix(cleanDir, "..") {
	return false, nil
	}
	if d.allLicenses == nil {
	d.computeAllLicenseInfo()
	}
	// Collect all the license metadata for directories dir and above, excluding the root.
	for prefix, plics := range d.licsByDir {
	// append a slash so that prefix a/b does not match a/bc/d
	if strings.HasPrefix(cleanDir+"/", prefix+"/") {
	lics = append(lics, plics...)
	}
	}
	// A package is redistributable if its module is, and if other licenses on
	// the path to the root are redistributable. Note that this is not the same
	// as asking if the module licenses plus the package licenses are
	// redistributable. A module that is granted an exception (see DetectFiles)
	// may have licenses that are non-redistributable.
	ltypes := types(lics)
	isRedistributable = d.ModuleIsRedistributable() && (len(ltypes) == 0 \|\| Redistributable(ltypes))
	// A package's licenses include the ones we've already computed, as well
	// as the module licenses.
	return isRedistributable, append(lics, d.moduleLicenses...)
	}

	// computeModuleInfo determines values for the moduleRedist and moduleLicenses fields of d.
	func (d *Detector) computeModuleInfo() {
	// Check that all licenses in the contents directory are redistributable.
	d.moduleLicenses = d.detectFiles(d.Files(RootFiles))
	d.moduleRedist = Redistributable(types(d.moduleLicenses))
	}

	// computeAllLicenseInfo collects all the detected licenses in the zip and
	// stores them in the allLicenses field of d. It also maps detected licenses to
	// their directories, to optimize Detector.PackageInfo.
	func (d *Detector) computeAllLicenseInfo() {
	d.allLicenses = []*License{}
	d.allLicenses = append(d.allLicenses, d.moduleLicenses...)
	nonRootLicenses := d.detectFiles(d.Files(NonRootFiles))
	d.allLicenses = append(d.allLicenses, nonRootLicenses...)
	d.licsByDir = map[string][]*License{}
	for _, l := range nonRootLicenses {
	prefix := path.Dir(l.FilePath)
	d.licsByDir[prefix] = append(d.licsByDir[prefix], l)
	}
	}

	// WhichFiles describes which files from the zip should be returned by Detector.Files.
	type WhichFiles int

	const (
	// Only files from the root (contents) directory.
	RootFiles WhichFiles = iota
	// Only files that are not in the root directory.
	NonRootFiles
	// All files; the union of root and non-root.
	AllFiles
	)

	// Files returns a list of license files from the zip. The which argument
	// determines the location of the files considered.
	func (d Detector) Files(which WhichFiles) []zip.File {
	cdir := contentsDir(d.modulePath, d.version)
	prefix := pathPrefix(cdir)
	var files []*zip.File
	for _, f := range d.zr.File {
	if !fileNamesLowercase[strings.ToLower(path.Base(f.Name))] {
	continue
	}
	if !strings.HasPrefix(f.Name, prefix) {
	d.logf("potential license file %q found outside of the expected path %q", f.Name, cdir)
	continue
	}
	// Skip files we should ignore.
	if ignoreFiles[d.modulePath+" "+strings.TrimPrefix(f.Name, prefix)] {
	continue
	}
	if which == RootFiles && path.Dir(f.Name) != cdir {
	// Skip f since it's not at root.
	continue
	}
	if which == NonRootFiles && path.Dir(f.Name) == cdir {
	// Skip f since it is at root.
	continue
	}
	if isVendoredFile(f.Name) {
	// Skip if f is in the vendor directory.
	continue
	}
	if err := module.CheckFilePath(f.Name); err != nil {
	// Skip if the file path is bad.
	d.logf("module.CheckFilePath(%q): %v", f.Name, err)
	continue
	}
	files = append(files, f)
	}
	return files
	}

	// isVendoredFile reports if the given file is in a proper subdirectory nested
	// under a 'vendor' directory, to allow for Go packages named 'vendor'.
	//
	// e.g. isVendoredFile("vendor/LICENSE") == false, and
	// isVendoredFile("vendor/foo/LICENSE") == true
	func isVendoredFile(name string) bool {
	var vendorOffset int
	if strings.HasPrefix(name, "vendor/") {
	vendorOffset = len("vendor/")
	} else if i := strings.Index(name, "/vendor/"); i >= 0 {
	vendorOffset = i + len("/vendor/")
	} else {
	// no vendor directory
	return false
	}
	// check if the file is in a proper subdirectory of vendor
	return strings.Contains(name[vendorOffset:], "/")
	}

	// detectFiles runs DetectFile on each of the given files.
	// If a file cannot be read, the error is logged and a license
	// of type unknown is added.
	func (d Detector) detectFiles(files []zip.File) []*License {
	prefix := pathPrefix(contentsDir(d.modulePath, d.version))
	var licenses []*License
	for _, f := range files {
	bytes, err := readZipFile(f)
	if err != nil {
	d.logf("reading zip file %s: %v", f.Name, err)
	licenses = append(licenses, &License{
	Metadata: &Metadata{
	Types: []string{unknownLicenseType},
	FilePath: strings.TrimPrefix(f.Name, prefix),
	},
	})
	continue
	}
	types, cov := DetectFile(bytes, f.Name, d.logf)
	licenses = append(licenses, &License{
	Metadata: &Metadata{
	Types: types,
	FilePath: strings.TrimPrefix(f.Name, prefix),
	Coverage: cov,
	},
	Contents: bytes,
	})
	}
	return licenses
	}

	// DetectFile return the set of license types for the given file contents. It
	// also returns the licensecheck coverage information. The filename is used
	// solely for logging.
	func DetectFile(contents []byte, filename string, logf func(string, ...interface{})) ([]string, licensecheck.Coverage) {
	if logf == nil {
	logf = func(string, ...interface{}) {}
	}
	if types := exceptionFileTypes(contents); types != nil {
	logf("%s is an exception", filename)
	return types, licensecheck.Coverage{}
	}
	cov, ok := checker.Cover(contents, licensecheck.Options{})
	if !ok {
	logf("%s checker.Cover failed, skipping", filename)
	return []string{unknownLicenseType}, licensecheck.Coverage{}
	}
	if cov.Percent < float64(coverageThreshold) {
	logf("%s license coverage too low (%+v), skipping", filename, cov)
	return []string{unknownLicenseType}, cov
	}
	types := make(map[string]bool)
	for _, m := range cov.Match {
	if m.Percent >= classifyThreshold {
	types[canonicalizeName(m.Name)] = true
	}
	}
	if len(types) == 0 {
	logf("%s failed to classify license (%+v), skipping", filename, cov)
	return []string{unknownLicenseType}, cov
	}
	return setToSortedSlice(types), cov
	}

	// Redistributable reports whether the set of license types establishes that a
	// module or package is redistributable.
	func Redistributable(licenseTypes []string) bool {
	if len(licenseTypes) == 0 {
	return false
	}
	for _, t := range licenseTypes {
	if !redistributableLicenseTypes[t] && !ignorableLicenseTypes[t] {
	return false
	}
	}
	return true
	}

	var canonicalNames = map[string]string{
	"AGPL-Header": "AGPL-3.0",
	"GPL-Header": "GPL2",
	"GPL-NotLater-Header": "GPL3",
	"LGPL-Header": "LGPL-2.1",
	}

	// canonicalizeName puts a license name in a standard form.
	func canonicalizeName(name string) string {
	if c := canonicalNames[name]; c != "" {
	return c
	}
	name = strings.TrimSuffix(name, "-Short")
	return strings.TrimSuffix(name, "-Header")
	}

	func types(lics []*License) []string {
	var types []string
	for _, l := range lics {
	types = append(types, l.Types...)
	}
	return types
	}

	func setToSortedSlice(m map[string]bool) []string {
	var s []string
	for e := range m {
	s = append(s, e)
	}
	sort.Strings(s)
	return s
	}

	func readZipFile(f *zip.File) ([]byte, error) {
	if f.UncompressedSize64 > maxLicenseSize {
	return nil, fmt.Errorf("file size %d exceeds max license size %d", f.UncompressedSize64, maxLicenseSize)
	}
	rc, err := f.Open()
	if err != nil {
	return nil, err
	}
	defer rc.Close()
	return ioutil.ReadAll(io.LimitReader(rc, int64(maxLicenseSize)))
	}

	func contentsDir(modulePath, version string) string {
	return modulePath + "@" + version
	}

	// pathPrefix appends a "/" to its argument if the argument is non-empty.
	func pathPrefix(s string) string {
	if s != "" {
	return s + "/"
	}
	return ""
	}