Adding upstream version 1.20.14.upstream/1.20.14upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

8 files changed, 1683 insertions, 0 deletions

diff --git a/src/go/token/example_test.go b/src/go/token/example_test.go
new file mode 100644
index 0000000..0011703
--- /dev/null
+++ b/src/go/token/example_test.go
@@ -0,0 +1,77 @@
+// Copyright 2018 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 token_test
+
+import (
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+)
+
+func Example_retrievePositionInfo() {
+	fset := token.NewFileSet()
+
+	const src = `package main
+
+import "fmt"
+
+import "go/token"
+
+//line :1:5
+type p = token.Pos
+
+const bad = token.NoPos
+
+//line fake.go:42:11
+func ok(pos p) bool {
+	return pos != bad
+}
+
+/*line :7:9*/func main() {
+	fmt.Println(ok(bad) == bad.IsValid())
+}
+`
+
+	f, err := parser.ParseFile(fset, "main.go", src, 0)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+
+	// Print the location and kind of each declaration in f.
+	for _, decl := range f.Decls {
+		// Get the filename, line, and column back via the file set.
+		// We get both the relative and absolute position.
+		// The relative position is relative to the last line directive.
+		// The absolute position is the exact position in the source.
+		pos := decl.Pos()
+		relPosition := fset.Position(pos)
+		absPosition := fset.PositionFor(pos, false)
+
+		// Either a FuncDecl or GenDecl, since we exit on error.
+		kind := "func"
+		if gen, ok := decl.(*ast.GenDecl); ok {
+			kind = gen.Tok.String()
+		}
+
+		// If the relative and absolute positions differ, show both.
+		fmtPosition := relPosition.String()
+		if relPosition != absPosition {
+			fmtPosition += "[" + absPosition.String() + "]"
+		}
+
+		fmt.Printf("%s: %s\n", fmtPosition, kind)
+	}
+
+	//Output:
+	//
+	// main.go:3:1: import
+	// main.go:5:1: import
+	// main.go:1:5[main.go:8:1]: type
+	// main.go:3:1[main.go:10:1]: const
+	// fake.go:42:11[main.go:13:1]: func
+	// fake.go:7:9[main.go:17:14]: func
+}
diff --git a/src/go/token/position.go b/src/go/token/position.go
new file mode 100644
index 0000000..cbc2ddb
--- /dev/null
+++ b/src/go/token/position.go
@@ -0,0 +1,556 @@
+// 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 token
+
+import (
+	"fmt"
+	"sort"
+	"sync"
+	"sync/atomic"
+)
+
+// -----------------------------------------------------------------------------
+// Positions
+
+// Position describes an arbitrary source position
+// including the file, line, and column location.
+// A Position is valid if the line number is > 0.
+type Position struct {
+	Filename string // filename, if any
+	Offset   int    // offset, starting at 0
+	Line     int    // line number, starting at 1
+	Column   int    // column number, starting at 1 (byte count)
+}
+
+// IsValid reports whether the position is valid.
+func (pos *Position) IsValid() bool { return pos.Line > 0 }
+
+// String returns a string in one of several forms:
+//
+//	file:line:column    valid position with file name
+//	file:line           valid position with file name but no column (column == 0)
+//	line:column         valid position without file name
+//	line                valid position without file name and no column (column == 0)
+//	file                invalid position with file name
+//	-                   invalid position without file name
+func (pos Position) String() string {
+	s := pos.Filename
+	if pos.IsValid() {
+		if s != "" {
+			s += ":"
+		}
+		s += fmt.Sprintf("%d", pos.Line)
+		if pos.Column != 0 {
+			s += fmt.Sprintf(":%d", pos.Column)
+		}
+	}
+	if s == "" {
+		s = "-"
+	}
+	return s
+}
+
+// Pos is a compact encoding of a source position within a file set.
+// It can be converted into a Position for a more convenient, but much
+// larger, representation.
+//
+// The Pos value for a given file is a number in the range [base, base+size],
+// where base and size are specified when a file is added to the file set.
+// The difference between a Pos value and the corresponding file base
+// corresponds to the byte offset of that position (represented by the Pos value)
+// from the beginning of the file. Thus, the file base offset is the Pos value
+// representing the first byte in the file.
+//
+// To create the Pos value for a specific source offset (measured in bytes),
+// first add the respective file to the current file set using FileSet.AddFile
+// and then call File.Pos(offset) for that file. Given a Pos value p
+// for a specific file set fset, the corresponding Position value is
+// obtained by calling fset.Position(p).
+//
+// Pos values can be compared directly with the usual comparison operators:
+// If two Pos values p and q are in the same file, comparing p and q is
+// equivalent to comparing the respective source file offsets. If p and q
+// are in different files, p < q is true if the file implied by p was added
+// to the respective file set before the file implied by q.
+type Pos int
+
+// The zero value for Pos is NoPos; there is no file and line information
+// associated with it, and NoPos.IsValid() is false. NoPos is always
+// smaller than any other Pos value. The corresponding Position value
+// for NoPos is the zero value for Position.
+const NoPos Pos = 0
+
+// IsValid reports whether the position is valid.
+func (p Pos) IsValid() bool {
+	return p != NoPos
+}
+
+// -----------------------------------------------------------------------------
+// File
+
+// A File is a handle for a file belonging to a FileSet.
+// A File has a name, size, and line offset table.
+type File struct {
+	name string // file name as provided to AddFile
+	base int    // Pos value range for this file is [base...base+size]
+	size int    // file size as provided to AddFile
+
+	// lines and infos are protected by mutex
+	mutex sync.Mutex
+	lines []int // lines contains the offset of the first character for each line (the first entry is always 0)
+	infos []lineInfo
+}
+
+// Name returns the file name of file f as registered with AddFile.
+func (f *File) Name() string {
+	return f.name
+}
+
+// Base returns the base offset of file f as registered with AddFile.
+func (f *File) Base() int {
+	return f.base
+}
+
+// Size returns the size of file f as registered with AddFile.
+func (f *File) Size() int {
+	return f.size
+}
+
+// LineCount returns the number of lines in file f.
+func (f *File) LineCount() int {
+	f.mutex.Lock()
+	n := len(f.lines)
+	f.mutex.Unlock()
+	return n
+}
+
+// AddLine adds the line offset for a new line.
+// The line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise the line offset is ignored.
+func (f *File) AddLine(offset int) {
+	f.mutex.Lock()
+	if i := len(f.lines); (i == 0 || f.lines[i-1] < offset) && offset < f.size {
+		f.lines = append(f.lines, offset)
+	}
+	f.mutex.Unlock()
+}
+
+// MergeLine merges a line with the following line. It is akin to replacing
+// the newline character at the end of the line with a space (to not change the
+// remaining offsets). To obtain the line number, consult e.g. Position.Line.
+// MergeLine will panic if given an invalid line number.
+func (f *File) MergeLine(line int) {
+	if line < 1 {
+		panic(fmt.Sprintf("invalid line number %d (should be >= 1)", line))
+	}
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	if line >= len(f.lines) {
+		panic(fmt.Sprintf("invalid line number %d (should be < %d)", line, len(f.lines)))
+	}
+	// To merge the line numbered <line> with the line numbered <line+1>,
+	// we need to remove the entry in lines corresponding to the line
+	// numbered <line+1>. The entry in lines corresponding to the line
+	// numbered <line+1> is located at index <line>, since indices in lines
+	// are 0-based and line numbers are 1-based.
+	copy(f.lines[line:], f.lines[line+1:])
+	f.lines = f.lines[:len(f.lines)-1]
+}
+
+// SetLines sets the line offsets for a file and reports whether it succeeded.
+// The line offsets are the offsets of the first character of each line;
+// for instance for the content "ab\nc\n" the line offsets are {0, 3}.
+// An empty file has an empty line offset table.
+// Each line offset must be larger than the offset for the previous line
+// and smaller than the file size; otherwise SetLines fails and returns
+// false.
+// Callers must not mutate the provided slice after SetLines returns.
+func (f *File) SetLines(lines []int) bool {
+	// verify validity of lines table
+	size := f.size
+	for i, offset := range lines {
+		if i > 0 && offset <= lines[i-1] || size <= offset {
+			return false
+		}
+	}
+
+	// set lines table
+	f.mutex.Lock()
+	f.lines = lines
+	f.mutex.Unlock()
+	return true
+}
+
+// SetLinesForContent sets the line offsets for the given file content.
+// It ignores position-altering //line comments.
+func (f *File) SetLinesForContent(content []byte) {
+	var lines []int
+	line := 0
+	for offset, b := range content {
+		if line >= 0 {
+			lines = append(lines, line)
+		}
+		line = -1
+		if b == '\n' {
+			line = offset + 1
+		}
+	}
+
+	// set lines table
+	f.mutex.Lock()
+	f.lines = lines
+	f.mutex.Unlock()
+}
+
+// LineStart returns the Pos value of the start of the specified line.
+// It ignores any alternative positions set using AddLineColumnInfo.
+// LineStart panics if the 1-based line number is invalid.
+func (f *File) LineStart(line int) Pos {
+	if line < 1 {
+		panic(fmt.Sprintf("invalid line number %d (should be >= 1)", line))
+	}
+	f.mutex.Lock()
+	defer f.mutex.Unlock()
+	if line > len(f.lines) {
+		panic(fmt.Sprintf("invalid line number %d (should be < %d)", line, len(f.lines)))
+	}
+	return Pos(f.base + f.lines[line-1])
+}
+
+// A lineInfo object describes alternative file, line, and column
+// number information (such as provided via a //line directive)
+// for a given file offset.
+type lineInfo struct {
+	// fields are exported to make them accessible to gob
+	Offset       int
+	Filename     string
+	Line, Column int
+}
+
+// AddLineInfo is like AddLineColumnInfo with a column = 1 argument.
+// It is here for backward-compatibility for code prior to Go 1.11.
+func (f *File) AddLineInfo(offset int, filename string, line int) {
+	f.AddLineColumnInfo(offset, filename, line, 1)
+}
+
+// AddLineColumnInfo adds alternative file, line, and column number
+// information for a given file offset. The offset must be larger
+// than the offset for the previously added alternative line info
+// and smaller than the file size; otherwise the information is
+// ignored.
+//
+// AddLineColumnInfo is typically used to register alternative position
+// information for line directives such as //line filename:line:column.
+func (f *File) AddLineColumnInfo(offset int, filename string, line, column int) {
+	f.mutex.Lock()
+	if i := len(f.infos); (i == 0 || f.infos[i-1].Offset < offset) && offset < f.size {
+		f.infos = append(f.infos, lineInfo{offset, filename, line, column})
+	}
+	f.mutex.Unlock()
+}
+
+// Pos returns the Pos value for the given file offset;
+// the offset must be <= f.Size().
+// f.Pos(f.Offset(p)) == p.
+func (f *File) Pos(offset int) Pos {
+	if offset > f.size {
+		panic(fmt.Sprintf("invalid file offset %d (should be <= %d)", offset, f.size))
+	}
+	return Pos(f.base + offset)
+}
+
+// Offset returns the offset for the given file position p;
+// p must be a valid Pos value in that file.
+// f.Offset(f.Pos(offset)) == offset.
+func (f *File) Offset(p Pos) int {
+	if int(p) < f.base || int(p) > f.base+f.size {
+		panic(fmt.Sprintf("invalid Pos value %d (should be in [%d, %d])", p, f.base, f.base+f.size))
+	}
+	return int(p) - f.base
+}
+
+// Line returns the line number for the given file position p;
+// p must be a Pos value in that file or NoPos.
+func (f *File) Line(p Pos) int {
+	return f.Position(p).Line
+}
+
+func searchLineInfos(a []lineInfo, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].Offset > x }) - 1
+}
+
+// unpack returns the filename and line and column number for a file offset.
+// If adjusted is set, unpack will return the filename and line information
+// possibly adjusted by //line comments; otherwise those comments are ignored.
+func (f *File) unpack(offset int, adjusted bool) (filename string, line, column int) {
+	f.mutex.Lock()
+	filename = f.name
+	if i := searchInts(f.lines, offset); i >= 0 {
+		line, column = i+1, offset-f.lines[i]+1
+	}
+	if adjusted && len(f.infos) > 0 {
+		// few files have extra line infos
+		if i := searchLineInfos(f.infos, offset); i >= 0 {
+			alt := &f.infos[i]
+			filename = alt.Filename
+			if i := searchInts(f.lines, alt.Offset); i >= 0 {
+				// i+1 is the line at which the alternative position was recorded
+				d := line - (i + 1) // line distance from alternative position base
+				line = alt.Line + d
+				if alt.Column == 0 {
+					// alternative column is unknown => relative column is unknown
+					// (the current specification for line directives requires
+					// this to apply until the next PosBase/line directive,
+					// not just until the new newline)
+					column = 0
+				} else if d == 0 {
+					// the alternative position base is on the current line
+					// => column is relative to alternative column
+					column = alt.Column + (offset - alt.Offset)
+				}
+			}
+		}
+	}
+	// TODO(mvdan): move Unlock back under Lock with a defer statement once
+	// https://go.dev/issue/38471 is fixed to remove the performance penalty.
+	f.mutex.Unlock()
+	return
+}
+
+func (f *File) position(p Pos, adjusted bool) (pos Position) {
+	offset := int(p) - f.base
+	pos.Offset = offset
+	pos.Filename, pos.Line, pos.Column = f.unpack(offset, adjusted)
+	return
+}
+
+// PositionFor returns the Position value for the given file position p.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in f or NoPos.
+func (f *File) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if int(p) < f.base || int(p) > f.base+f.size {
+			panic(fmt.Sprintf("invalid Pos value %d (should be in [%d, %d])", p, f.base, f.base+f.size))
+		}
+		pos = f.position(p, adjusted)
+	}
+	return
+}
+
+// Position returns the Position value for the given file position p.
+// Calling f.Position(p) is equivalent to calling f.PositionFor(p, true).
+func (f *File) Position(p Pos) (pos Position) {
+	return f.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// FileSet
+
+// A FileSet represents a set of source files.
+// Methods of file sets are synchronized; multiple goroutines
+// may invoke them concurrently.
+//
+// The byte offsets for each file in a file set are mapped into
+// distinct (integer) intervals, one interval [base, base+size]
+// per file. Base represents the first byte in the file, and size
+// is the corresponding file size. A Pos value is a value in such
+// an interval. By determining the interval a Pos value belongs
+// to, the file, its file base, and thus the byte offset (position)
+// the Pos value is representing can be computed.
+//
+// When adding a new file, a file base must be provided. That can
+// be any integer value that is past the end of any interval of any
+// file already in the file set. For convenience, FileSet.Base provides
+// such a value, which is simply the end of the Pos interval of the most
+// recently added file, plus one. Unless there is a need to extend an
+// interval later, using the FileSet.Base should be used as argument
+// for FileSet.AddFile.
+//
+// A File may be removed from a FileSet when it is no longer needed.
+// This may reduce memory usage in a long-running application.
+type FileSet struct {
+	mutex sync.RWMutex         // protects the file set
+	base  int                  // base offset for the next file
+	files []*File              // list of files in the order added to the set
+	last  atomic.Pointer[File] // cache of last file looked up
+}
+
+// NewFileSet creates a new file set.
+func NewFileSet() *FileSet {
+	return &FileSet{
+		base: 1, // 0 == NoPos
+	}
+}
+
+// Base returns the minimum base offset that must be provided to
+// AddFile when adding the next file.
+func (s *FileSet) Base() int {
+	s.mutex.RLock()
+	b := s.base
+	s.mutex.RUnlock()
+	return b
+
+}
+
+// AddFile adds a new file with a given filename, base offset, and file size
+// to the file set s and returns the file. Multiple files may have the same
+// name. The base offset must not be smaller than the FileSet's Base(), and
+// size must not be negative. As a special case, if a negative base is provided,
+// the current value of the FileSet's Base() is used instead.
+//
+// Adding the file will set the file set's Base() value to base + size + 1
+// as the minimum base value for the next file. The following relationship
+// exists between a Pos value p for a given file offset offs:
+//
+//	int(p) = base + offs
+//
+// with offs in the range [0, size] and thus p in the range [base, base+size].
+// For convenience, File.Pos may be used to create file-specific position
+// values from a file offset.
+func (s *FileSet) AddFile(filename string, base, size int) *File {
+	// Allocate f outside the critical section.
+	f := &File{name: filename, size: size, lines: []int{0}}
+
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	if base < 0 {
+		base = s.base
+	}
+	if base < s.base {
+		panic(fmt.Sprintf("invalid base %d (should be >= %d)", base, s.base))
+	}
+	f.base = base
+	if size < 0 {
+		panic(fmt.Sprintf("invalid size %d (should be >= 0)", size))
+	}
+	// base >= s.base && size >= 0
+	base += size + 1 // +1 because EOF also has a position
+	if base < 0 {
+		panic("token.Pos offset overflow (> 2G of source code in file set)")
+	}
+	// add the file to the file set
+	s.base = base
+	s.files = append(s.files, f)
+	s.last.Store(f)
+	return f
+}
+
+// RemoveFile removes a file from the FileSet so that subsequent
+// queries for its Pos interval yield a negative result.
+// This reduces the memory usage of a long-lived FileSet that
+// encounters an unbounded stream of files.
+//
+// Removing a file that does not belong to the set has no effect.
+func (s *FileSet) RemoveFile(file *File) {
+	s.last.CompareAndSwap(file, nil) // clear last file cache
+
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+
+	if i := searchFiles(s.files, file.base); i >= 0 && s.files[i] == file {
+		last := &s.files[len(s.files)-1]
+		s.files = append(s.files[:i], s.files[i+1:]...)
+		*last = nil // don't prolong lifetime when popping last element
+	}
+}
+
+// Iterate calls f for the files in the file set in the order they were added
+// until f returns false.
+func (s *FileSet) Iterate(f func(*File) bool) {
+	for i := 0; ; i++ {
+		var file *File
+		s.mutex.RLock()
+		if i < len(s.files) {
+			file = s.files[i]
+		}
+		s.mutex.RUnlock()
+		if file == nil || !f(file) {
+			break
+		}
+	}
+}
+
+func searchFiles(a []*File, x int) int {
+	return sort.Search(len(a), func(i int) bool { return a[i].base > x }) - 1
+}
+
+func (s *FileSet) file(p Pos) *File {
+	// common case: p is in last file.
+	if f := s.last.Load(); f != nil && f.base <= int(p) && int(p) <= f.base+f.size {
+		return f
+	}
+
+	s.mutex.RLock()
+	defer s.mutex.RUnlock()
+
+	// p is not in last file - search all files
+	if i := searchFiles(s.files, int(p)); i >= 0 {
+		f := s.files[i]
+		// f.base <= int(p) by definition of searchFiles
+		if int(p) <= f.base+f.size {
+			// Update cache of last file. A race is ok,
+			// but an exclusive lock causes heavy contention.
+			s.last.Store(f)
+			return f
+		}
+	}
+	return nil
+}
+
+// File returns the file that contains the position p.
+// If no such file is found (for instance for p == NoPos),
+// the result is nil.
+func (s *FileSet) File(p Pos) (f *File) {
+	if p != NoPos {
+		f = s.file(p)
+	}
+	return
+}
+
+// PositionFor converts a Pos p in the fileset into a Position value.
+// If adjusted is set, the position may be adjusted by position-altering
+// //line comments; otherwise those comments are ignored.
+// p must be a Pos value in s or NoPos.
+func (s *FileSet) PositionFor(p Pos, adjusted bool) (pos Position) {
+	if p != NoPos {
+		if f := s.file(p); f != nil {
+			return f.position(p, adjusted)
+		}
+	}
+	return
+}
+
+// Position converts a Pos p in the fileset into a Position value.
+// Calling s.Position(p) is equivalent to calling s.PositionFor(p, true).
+func (s *FileSet) Position(p Pos) (pos Position) {
+	return s.PositionFor(p, true)
+}
+
+// -----------------------------------------------------------------------------
+// Helper functions
+
+func searchInts(a []int, x int) int {
+	// This function body is a manually inlined version of:
+	//
+	//   return sort.Search(len(a), func(i int) bool { return a[i] > x }) - 1
+	//
+	// With better compiler optimizations, this may not be needed in the
+	// future, but at the moment this change improves the go/printer
+	// benchmark performance by ~30%. This has a direct impact on the
+	// speed of gofmt and thus seems worthwhile (2011-04-29).
+	// TODO(gri): Remove this when compilers have caught up.
+	i, j := 0, len(a)
+	for i < j {
+		h := int(uint(i+j) >> 1) // avoid overflow when computing h
+		// i ≤ h < j
+		if a[h] <= x {
+			i = h + 1
+		} else {
+			j = h
+		}
+	}
+	return i - 1
+}
diff --git a/src/go/token/position_bench_test.go b/src/go/token/position_bench_test.go
new file mode 100644
index 0000000..41be728
--- /dev/null
+++ b/src/go/token/position_bench_test.go
@@ -0,0 +1,24 @@
+// Copyright 2021 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 token
+
+import (
+	"testing"
+)
+
+func BenchmarkSearchInts(b *testing.B) {
+	data := make([]int, 10000)
+	for i := 0; i < 10000; i++ {
+		data[i] = i
+	}
+	const x = 8
+	if r := searchInts(data, x); r != x {
+		b.Errorf("got index = %d; want %d", r, x)
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		searchInts(data, x)
+	}
+}
diff --git a/src/go/token/position_test.go b/src/go/token/position_test.go
new file mode 100644
index 0000000..65cb242
--- /dev/null
+++ b/src/go/token/position_test.go
@@ -0,0 +1,477 @@
+// 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 token
+
+import (
+	"fmt"
+	"math/rand"
+	"reflect"
+	"sync"
+	"testing"
+)
+
+func checkPos(t *testing.T, msg string, got, want Position) {
+	if got.Filename != want.Filename {
+		t.Errorf("%s: got filename = %q; want %q", msg, got.Filename, want.Filename)
+	}
+	if got.Offset != want.Offset {
+		t.Errorf("%s: got offset = %d; want %d", msg, got.Offset, want.Offset)
+	}
+	if got.Line != want.Line {
+		t.Errorf("%s: got line = %d; want %d", msg, got.Line, want.Line)
+	}
+	if got.Column != want.Column {
+		t.Errorf("%s: got column = %d; want %d", msg, got.Column, want.Column)
+	}
+}
+
+func TestNoPos(t *testing.T) {
+	if NoPos.IsValid() {
+		t.Errorf("NoPos should not be valid")
+	}
+	var fset *FileSet
+	checkPos(t, "nil NoPos", fset.Position(NoPos), Position{})
+	fset = NewFileSet()
+	checkPos(t, "fset NoPos", fset.Position(NoPos), Position{})
+}
+
+var tests = []struct {
+	filename string
+	source   []byte // may be nil
+	size     int
+	lines    []int
+}{
+	{"a", []byte{}, 0, []int{}},
+	{"b", []byte("01234"), 5, []int{0}},
+	{"c", []byte("\n\n\n\n\n\n\n\n\n"), 9, []int{0, 1, 2, 3, 4, 5, 6, 7, 8}},
+	{"d", nil, 100, []int{0, 5, 10, 20, 30, 70, 71, 72, 80, 85, 90, 99}},
+	{"e", nil, 777, []int{0, 80, 100, 120, 130, 180, 267, 455, 500, 567, 620}},
+	{"f", []byte("package p\n\nimport \"fmt\""), 23, []int{0, 10, 11}},
+	{"g", []byte("package p\n\nimport \"fmt\"\n"), 24, []int{0, 10, 11}},
+	{"h", []byte("package p\n\nimport \"fmt\"\n "), 25, []int{0, 10, 11, 24}},
+}
+
+func linecol(lines []int, offs int) (int, int) {
+	prevLineOffs := 0
+	for line, lineOffs := range lines {
+		if offs < lineOffs {
+			return line, offs - prevLineOffs + 1
+		}
+		prevLineOffs = lineOffs
+	}
+	return len(lines), offs - prevLineOffs + 1
+}
+
+func verifyPositions(t *testing.T, fset *FileSet, f *File, lines []int) {
+	for offs := 0; offs < f.Size(); offs++ {
+		p := f.Pos(offs)
+		offs2 := f.Offset(p)
+		if offs2 != offs {
+			t.Errorf("%s, Offset: got offset %d; want %d", f.Name(), offs2, offs)
+		}
+		line, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{f.Name(), offs, line, col})
+		checkPos(t, msg, fset.Position(p), Position{f.Name(), offs, line, col})
+	}
+}
+
+func makeTestSource(size int, lines []int) []byte {
+	src := make([]byte, size)
+	for _, offs := range lines {
+		if offs > 0 {
+			src[offs-1] = '\n'
+		}
+	}
+	return src
+}
+
+func TestPositions(t *testing.T) {
+	const delta = 7 // a non-zero base offset increment
+	fset := NewFileSet()
+	for _, test := range tests {
+		// verify consistency of test case
+		if test.source != nil && len(test.source) != test.size {
+			t.Errorf("%s: inconsistent test case: got file size %d; want %d", test.filename, len(test.source), test.size)
+		}
+
+		// add file and verify name and size
+		f := fset.AddFile(test.filename, fset.Base()+delta, test.size)
+		if f.Name() != test.filename {
+			t.Errorf("got filename %q; want %q", f.Name(), test.filename)
+		}
+		if f.Size() != test.size {
+			t.Errorf("%s: got file size %d; want %d", f.Name(), f.Size(), test.size)
+		}
+		if fset.File(f.Pos(0)) != f {
+			t.Errorf("%s: f.Pos(0) was not found in f", f.Name())
+		}
+
+		// add lines individually and verify all positions
+		for i, offset := range test.lines {
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			// adding the same offset again should be ignored
+			f.AddLine(offset)
+			if f.LineCount() != i+1 {
+				t.Errorf("%s, AddLine: got unchanged line count %d; want %d", f.Name(), f.LineCount(), i+1)
+			}
+			verifyPositions(t, fset, f, test.lines[0:i+1])
+		}
+
+		// add lines with SetLines and verify all positions
+		if ok := f.SetLines(test.lines); !ok {
+			t.Errorf("%s: SetLines failed", f.Name())
+		}
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLines: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+
+		// add lines with SetLinesForContent and verify all positions
+		src := test.source
+		if src == nil {
+			// no test source available - create one from scratch
+			src = makeTestSource(test.size, test.lines)
+		}
+		f.SetLinesForContent(src)
+		if f.LineCount() != len(test.lines) {
+			t.Errorf("%s, SetLinesForContent: got line count %d; want %d", f.Name(), f.LineCount(), len(test.lines))
+		}
+		verifyPositions(t, fset, f, test.lines)
+	}
+}
+
+func TestLineInfo(t *testing.T) {
+	fset := NewFileSet()
+	f := fset.AddFile("foo", fset.Base(), 500)
+	lines := []int{0, 42, 77, 100, 210, 220, 277, 300, 333, 401}
+	// add lines individually and provide alternative line information
+	for _, offs := range lines {
+		f.AddLine(offs)
+		f.AddLineInfo(offs, "bar", 42)
+	}
+	// verify positions for all offsets
+	for offs := 0; offs <= f.Size(); offs++ {
+		p := f.Pos(offs)
+		_, col := linecol(lines, offs)
+		msg := fmt.Sprintf("%s (offs = %d, p = %d)", f.Name(), offs, p)
+		checkPos(t, msg, f.Position(f.Pos(offs)), Position{"bar", offs, 42, col})
+		checkPos(t, msg, fset.Position(p), Position{"bar", offs, 42, col})
+	}
+}
+
+func TestFiles(t *testing.T) {
+	fset := NewFileSet()
+	for i, test := range tests {
+		base := fset.Base()
+		if i%2 == 1 {
+			// Setting a negative base is equivalent to
+			// fset.Base(), so test some of each.
+			base = -1
+		}
+		fset.AddFile(test.filename, base, test.size)
+		j := 0
+		fset.Iterate(func(f *File) bool {
+			if f.Name() != tests[j].filename {
+				t.Errorf("got filename = %s; want %s", f.Name(), tests[j].filename)
+			}
+			j++
+			return true
+		})
+		if j != i+1 {
+			t.Errorf("got %d files; want %d", j, i+1)
+		}
+	}
+}
+
+// FileSet.File should return nil if Pos is past the end of the FileSet.
+func TestFileSetPastEnd(t *testing.T) {
+	fset := NewFileSet()
+	for _, test := range tests {
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	if f := fset.File(Pos(fset.Base())); f != nil {
+		t.Errorf("got %v, want nil", f)
+	}
+}
+
+func TestFileSetCacheUnlikely(t *testing.T) {
+	fset := NewFileSet()
+	offsets := make(map[string]int)
+	for _, test := range tests {
+		offsets[test.filename] = fset.Base()
+		fset.AddFile(test.filename, fset.Base(), test.size)
+	}
+	for file, pos := range offsets {
+		f := fset.File(Pos(pos))
+		if f.Name() != file {
+			t.Errorf("got %q at position %d, want %q", f.Name(), pos, file)
+		}
+	}
+}
+
+// issue 4345. Test that concurrent use of FileSet.Pos does not trigger a
+// race in the FileSet position cache.
+func TestFileSetRace(t *testing.T) {
+	fset := NewFileSet()
+	for i := 0; i < 100; i++ {
+		fset.AddFile(fmt.Sprintf("file-%d", i), fset.Base(), 1031)
+	}
+	max := int32(fset.Base())
+	var stop sync.WaitGroup
+	r := rand.New(rand.NewSource(7))
+	for i := 0; i < 2; i++ {
+		r := rand.New(rand.NewSource(r.Int63()))
+		stop.Add(1)
+		go func() {
+			for i := 0; i < 1000; i++ {
+				fset.Position(Pos(r.Int31n(max)))
+			}
+			stop.Done()
+		}()
+	}
+	stop.Wait()
+}
+
+// issue 16548. Test that concurrent use of File.AddLine and FileSet.PositionFor
+// does not trigger a race in the FileSet position cache.
+func TestFileSetRace2(t *testing.T) {
+	const N = 1e3
+	var (
+		fset = NewFileSet()
+		file = fset.AddFile("", -1, N)
+		ch   = make(chan int, 2)
+	)
+
+	go func() {
+		for i := 0; i < N; i++ {
+			file.AddLine(i)
+		}
+		ch <- 1
+	}()
+
+	go func() {
+		pos := file.Pos(0)
+		for i := 0; i < N; i++ {
+			fset.PositionFor(pos, false)
+		}
+		ch <- 1
+	}()
+
+	<-ch
+	<-ch
+}
+
+func TestPositionFor(t *testing.T) {
+	src := []byte(`
+foo
+b
+ar
+//line :100
+foobar
+//line bar:3
+done
+`)
+
+	const filename = "foo"
+	fset := NewFileSet()
+	f := fset.AddFile(filename, fset.Base(), len(src))
+	f.SetLinesForContent(src)
+
+	// verify position info
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "1. PositionFor unadjusted", got1, want)
+		checkPos(t, "1. PositionFor adjusted", got2, want)
+		checkPos(t, "1. Position", got3, want)
+	}
+
+	// manually add //line info on lines l1, l2
+	const l1, l2 = 5, 7
+	f.AddLineInfo(f.lines[l1-1], "", 100)
+	f.AddLineInfo(f.lines[l2-1], "bar", 3)
+
+	// unadjusted position info must remain unchanged
+	for i, offs := range f.lines {
+		got1 := f.PositionFor(f.Pos(offs), false)
+		want := Position{filename, offs, i + 1, 1}
+		checkPos(t, "2. PositionFor unadjusted", got1, want)
+	}
+
+	// adjusted position info should have changed
+	for i, offs := range f.lines {
+		got2 := f.PositionFor(f.Pos(offs), true)
+		got3 := f.Position(f.Pos(offs))
+		want := Position{filename, offs, i + 1, 1}
+		// manually compute wanted filename and line
+		line := want.Line
+		if i+1 >= l1 {
+			want.Filename = ""
+			want.Line = line - l1 + 100
+		}
+		if i+1 >= l2 {
+			want.Filename = "bar"
+			want.Line = line - l2 + 3
+		}
+		checkPos(t, "3. PositionFor adjusted", got2, want)
+		checkPos(t, "3. Position", got3, want)
+	}
+}
+
+func TestLineStart(t *testing.T) {
+	const src = "one\ntwo\nthree\n"
+	fset := NewFileSet()
+	f := fset.AddFile("input", -1, len(src))
+	f.SetLinesForContent([]byte(src))
+
+	for line := 1; line <= 3; line++ {
+		pos := f.LineStart(line)
+		position := fset.Position(pos)
+		if position.Line != line || position.Column != 1 {
+			t.Errorf("LineStart(%d) returned wrong pos %d: %s", line, pos, position)
+		}
+	}
+}
+
+func TestRemoveFile(t *testing.T) {
+	contentA := []byte("this\nis\nfileA")
+	contentB := []byte("this\nis\nfileB")
+	fset := NewFileSet()
+	a := fset.AddFile("fileA", -1, len(contentA))
+	a.SetLinesForContent(contentA)
+	b := fset.AddFile("fileB", -1, len(contentB))
+	b.SetLinesForContent(contentB)
+
+	checkPos := func(pos Pos, want string) {
+		if got := fset.Position(pos).String(); got != want {
+			t.Errorf("Position(%d) = %s, want %s", pos, got, want)
+		}
+	}
+	checkNumFiles := func(want int) {
+		got := 0
+		fset.Iterate(func(*File) bool { got++; return true })
+		if got != want {
+			t.Errorf("Iterate called %d times, want %d", got, want)
+		}
+	}
+
+	apos3 := a.Pos(3)
+	bpos3 := b.Pos(3)
+	checkPos(apos3, "fileA:1:4")
+	checkPos(bpos3, "fileB:1:4")
+	checkNumFiles(2)
+
+	// After removal, queries on fileA fail.
+	fset.RemoveFile(a)
+	checkPos(apos3, "-")
+	checkPos(bpos3, "fileB:1:4")
+	checkNumFiles(1)
+
+	// idempotent / no effect
+	fset.RemoveFile(a)
+	checkPos(apos3, "-")
+	checkPos(bpos3, "fileB:1:4")
+	checkNumFiles(1)
+}
+
+func TestFileAddLineColumnInfo(t *testing.T) {
+	const (
+		filename = "test.go"
+		filesize = 100
+	)
+
+	tests := []struct {
+		name  string
+		infos []lineInfo
+		want  []lineInfo
+	}{
+		{
+			name: "normal",
+			infos: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+				{Offset: 50, Filename: filename, Line: 3, Column: 1},
+				{Offset: 80, Filename: filename, Line: 4, Column: 2},
+			},
+			want: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+				{Offset: 50, Filename: filename, Line: 3, Column: 1},
+				{Offset: 80, Filename: filename, Line: 4, Column: 2},
+			},
+		},
+		{
+			name: "offset1 == file size",
+			infos: []lineInfo{
+				{Offset: filesize, Filename: filename, Line: 2, Column: 1},
+			},
+			want: nil,
+		},
+		{
+			name: "offset1 > file size",
+			infos: []lineInfo{
+				{Offset: filesize + 1, Filename: filename, Line: 2, Column: 1},
+			},
+			want: nil,
+		},
+		{
+			name: "offset2 == file size",
+			infos: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+				{Offset: filesize, Filename: filename, Line: 3, Column: 1},
+			},
+			want: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+			},
+		},
+		{
+			name: "offset2 > file size",
+			infos: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+				{Offset: filesize + 1, Filename: filename, Line: 3, Column: 1},
+			},
+			want: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+			},
+		},
+		{
+			name: "offset2 == offset1",
+			infos: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+				{Offset: 10, Filename: filename, Line: 3, Column: 1},
+			},
+			want: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+			},
+		},
+		{
+			name: "offset2 < offset1",
+			infos: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+				{Offset: 9, Filename: filename, Line: 3, Column: 1},
+			},
+			want: []lineInfo{
+				{Offset: 10, Filename: filename, Line: 2, Column: 1},
+			},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			fs := NewFileSet()
+			f := fs.AddFile(filename, -1, filesize)
+			for _, info := range test.infos {
+				f.AddLineColumnInfo(info.Offset, info.Filename, info.Line, info.Column)
+			}
+			if !reflect.DeepEqual(f.infos, test.want) {
+				t.Errorf("\ngot %+v, \nwant %+v", f.infos, test.want)
+			}
+		})
+	}
+}
diff --git a/src/go/token/serialize.go b/src/go/token/serialize.go
new file mode 100644
index 0000000..04a48d9
--- /dev/null
+++ b/src/go/token/serialize.go
@@ -0,0 +1,70 @@
+// Copyright 2011 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 token
+
+type serializedFile struct {
+	// fields correspond 1:1 to fields with same (lower-case) name in File
+	Name  string
+	Base  int
+	Size  int
+	Lines []int
+	Infos []lineInfo
+}
+
+type serializedFileSet struct {
+	Base  int
+	Files []serializedFile
+}
+
+// Read calls decode to deserialize a file set into s; s must not be nil.
+func (s *FileSet) Read(decode func(any) error) error {
+	var ss serializedFileSet
+	if err := decode(&ss); err != nil {
+		return err
+	}
+
+	s.mutex.Lock()
+	s.base = ss.Base
+	files := make([]*File, len(ss.Files))
+	for i := 0; i < len(ss.Files); i++ {
+		f := &ss.Files[i]
+		files[i] = &File{
+			name:  f.Name,
+			base:  f.Base,
+			size:  f.Size,
+			lines: f.Lines,
+			infos: f.Infos,
+		}
+	}
+	s.files = files
+	s.last.Store(nil)
+	s.mutex.Unlock()
+
+	return nil
+}
+
+// Write calls encode to serialize the file set s.
+func (s *FileSet) Write(encode func(any) error) error {
+	var ss serializedFileSet
+
+	s.mutex.Lock()
+	ss.Base = s.base
+	files := make([]serializedFile, len(s.files))
+	for i, f := range s.files {
+		f.mutex.Lock()
+		files[i] = serializedFile{
+			Name:  f.name,
+			Base:  f.base,
+			Size:  f.size,
+			Lines: append([]int(nil), f.lines...),
+			Infos: append([]lineInfo(nil), f.infos...),
+		}
+		f.mutex.Unlock()
+	}
+	ss.Files = files
+	s.mutex.Unlock()
+
+	return encode(ss)
+}
diff --git a/src/go/token/serialize_test.go b/src/go/token/serialize_test.go
new file mode 100644
index 0000000..8d97995
--- /dev/null
+++ b/src/go/token/serialize_test.go
@@ -0,0 +1,105 @@
+// Copyright 2011 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 token
+
+import (
+	"bytes"
+	"encoding/gob"
+	"fmt"
+	"testing"
+)
+
+// equal returns nil if p and q describe the same file set;
+// otherwise it returns an error describing the discrepancy.
+func equal(p, q *FileSet) error {
+	if p == q {
+		// avoid deadlock if p == q
+		return nil
+	}
+
+	// not strictly needed for the test
+	p.mutex.Lock()
+	q.mutex.Lock()
+	defer q.mutex.Unlock()
+	defer p.mutex.Unlock()
+
+	if p.base != q.base {
+		return fmt.Errorf("different bases: %d != %d", p.base, q.base)
+	}
+
+	if len(p.files) != len(q.files) {
+		return fmt.Errorf("different number of files: %d != %d", len(p.files), len(q.files))
+	}
+
+	for i, f := range p.files {
+		g := q.files[i]
+		if f.name != g.name {
+			return fmt.Errorf("different filenames: %q != %q", f.name, g.name)
+		}
+		if f.base != g.base {
+			return fmt.Errorf("different base for %q: %d != %d", f.name, f.base, g.base)
+		}
+		if f.size != g.size {
+			return fmt.Errorf("different size for %q: %d != %d", f.name, f.size, g.size)
+		}
+		for j, l := range f.lines {
+			m := g.lines[j]
+			if l != m {
+				return fmt.Errorf("different offsets for %q", f.name)
+			}
+		}
+		for j, l := range f.infos {
+			m := g.infos[j]
+			if l.Offset != m.Offset || l.Filename != m.Filename || l.Line != m.Line {
+				return fmt.Errorf("different infos for %q", f.name)
+			}
+		}
+	}
+
+	// we don't care about .last - it's just a cache
+	return nil
+}
+
+func checkSerialize(t *testing.T, p *FileSet) {
+	var buf bytes.Buffer
+	encode := func(x any) error {
+		return gob.NewEncoder(&buf).Encode(x)
+	}
+	if err := p.Write(encode); err != nil {
+		t.Errorf("writing fileset failed: %s", err)
+		return
+	}
+	q := NewFileSet()
+	decode := func(x any) error {
+		return gob.NewDecoder(&buf).Decode(x)
+	}
+	if err := q.Read(decode); err != nil {
+		t.Errorf("reading fileset failed: %s", err)
+		return
+	}
+	if err := equal(p, q); err != nil {
+		t.Errorf("filesets not identical: %s", err)
+	}
+}
+
+func TestSerialization(t *testing.T) {
+	p := NewFileSet()
+	checkSerialize(t, p)
+	// add some files
+	for i := 0; i < 10; i++ {
+		f := p.AddFile(fmt.Sprintf("file%d", i), p.Base()+i, i*100)
+		checkSerialize(t, p)
+		// add some lines and alternative file infos
+		line := 1000
+		for offs := 0; offs < f.Size(); offs += 40 + i {
+			f.AddLine(offs)
+			if offs%7 == 0 {
+				f.AddLineInfo(offs, fmt.Sprintf("file%d", offs), line)
+				line += 33
+			}
+		}
+		checkSerialize(t, p)
+	}
+}
diff --git a/src/go/token/token.go b/src/go/token/token.go
new file mode 100644
index 0000000..3ae10d8
--- /dev/null
+++ b/src/go/token/token.go
@@ -0,0 +1,341 @@
+// Copyright 2009 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 token defines constants representing the lexical tokens of the Go
+// programming language and basic operations on tokens (printing, predicates).
+package token
+
+import (
+	"strconv"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Token is the set of lexical tokens of the Go programming language.
+type Token int
+
+// The list of tokens.
+const (
+	// Special tokens
+	ILLEGAL Token = iota
+	EOF
+	COMMENT
+
+	literal_beg
+	// Identifiers and basic type literals
+	// (these tokens stand for classes of literals)
+	IDENT  // main
+	INT    // 12345
+	FLOAT  // 123.45
+	IMAG   // 123.45i
+	CHAR   // 'a'
+	STRING // "abc"
+	literal_end
+
+	operator_beg
+	// Operators and delimiters
+	ADD // +
+	SUB // -
+	MUL // *
+	QUO // /
+	REM // %
+
+	AND     // &
+	OR      // |
+	XOR     // ^
+	SHL     // <<
+	SHR     // >>
+	AND_NOT // &^
+
+	ADD_ASSIGN // +=
+	SUB_ASSIGN // -=
+	MUL_ASSIGN // *=
+	QUO_ASSIGN // /=
+	REM_ASSIGN // %=
+
+	AND_ASSIGN     // &=
+	OR_ASSIGN      // |=
+	XOR_ASSIGN     // ^=
+	SHL_ASSIGN     // <<=
+	SHR_ASSIGN     // >>=
+	AND_NOT_ASSIGN // &^=
+
+	LAND  // &&
+	LOR   // ||
+	ARROW // <-
+	INC   // ++
+	DEC   // --
+
+	EQL    // ==
+	LSS    // <
+	GTR    // >
+	ASSIGN // =
+	NOT    // !
+
+	NEQ      // !=
+	LEQ      // <=
+	GEQ      // >=
+	DEFINE   // :=
+	ELLIPSIS // ...
+
+	LPAREN // (
+	LBRACK // [
+	LBRACE // {
+	COMMA  // ,
+	PERIOD // .
+
+	RPAREN    // )
+	RBRACK    // ]
+	RBRACE    // }
+	SEMICOLON // ;
+	COLON     // :
+	operator_end
+
+	keyword_beg
+	// Keywords
+	BREAK
+	CASE
+	CHAN
+	CONST
+	CONTINUE
+
+	DEFAULT
+	DEFER
+	ELSE
+	FALLTHROUGH
+	FOR
+
+	FUNC
+	GO
+	GOTO
+	IF
+	IMPORT
+
+	INTERFACE
+	MAP
+	PACKAGE
+	RANGE
+	RETURN
+
+	SELECT
+	STRUCT
+	SWITCH
+	TYPE
+	VAR
+	keyword_end
+
+	additional_beg
+	// additional tokens, handled in an ad-hoc manner
+	TILDE
+	additional_end
+)
+
+var tokens = [...]string{
+	ILLEGAL: "ILLEGAL",
+
+	EOF:     "EOF",
+	COMMENT: "COMMENT",
+
+	IDENT:  "IDENT",
+	INT:    "INT",
+	FLOAT:  "FLOAT",
+	IMAG:   "IMAG",
+	CHAR:   "CHAR",
+	STRING: "STRING",
+
+	ADD: "+",
+	SUB: "-",
+	MUL: "*",
+	QUO: "/",
+	REM: "%",
+
+	AND:     "&",
+	OR:      "|",
+	XOR:     "^",
+	SHL:     "<<",
+	SHR:     ">>",
+	AND_NOT: "&^",
+
+	ADD_ASSIGN: "+=",
+	SUB_ASSIGN: "-=",
+	MUL_ASSIGN: "*=",
+	QUO_ASSIGN: "/=",
+	REM_ASSIGN: "%=",
+
+	AND_ASSIGN:     "&=",
+	OR_ASSIGN:      "|=",
+	XOR_ASSIGN:     "^=",
+	SHL_ASSIGN:     "<<=",
+	SHR_ASSIGN:     ">>=",
+	AND_NOT_ASSIGN: "&^=",
+
+	LAND:  "&&",
+	LOR:   "||",
+	ARROW: "<-",
+	INC:   "++",
+	DEC:   "--",
+
+	EQL:    "==",
+	LSS:    "<",
+	GTR:    ">",
+	ASSIGN: "=",
+	NOT:    "!",
+
+	NEQ:      "!=",
+	LEQ:      "<=",
+	GEQ:      ">=",
+	DEFINE:   ":=",
+	ELLIPSIS: "...",
+
+	LPAREN: "(",
+	LBRACK: "[",
+	LBRACE: "{",
+	COMMA:  ",",
+	PERIOD: ".",
+
+	RPAREN:    ")",
+	RBRACK:    "]",
+	RBRACE:    "}",
+	SEMICOLON: ";",
+	COLON:     ":",
+
+	BREAK:    "break",
+	CASE:     "case",
+	CHAN:     "chan",
+	CONST:    "const",
+	CONTINUE: "continue",
+
+	DEFAULT:     "default",
+	DEFER:       "defer",
+	ELSE:        "else",
+	FALLTHROUGH: "fallthrough",
+	FOR:         "for",
+
+	FUNC:   "func",
+	GO:     "go",
+	GOTO:   "goto",
+	IF:     "if",
+	IMPORT: "import",
+
+	INTERFACE: "interface",
+	MAP:       "map",
+	PACKAGE:   "package",
+	RANGE:     "range",
+	RETURN:    "return",
+
+	SELECT: "select",
+	STRUCT: "struct",
+	SWITCH: "switch",
+	TYPE:   "type",
+	VAR:    "var",
+
+	TILDE: "~",
+}
+
+// String returns the string corresponding to the token tok.
+// For operators, delimiters, and keywords the string is the actual
+// token character sequence (e.g., for the token ADD, the string is
+// "+"). For all other tokens the string corresponds to the token
+// constant name (e.g. for the token IDENT, the string is "IDENT").
+func (tok Token) String() string {
+	s := ""
+	if 0 <= tok && tok < Token(len(tokens)) {
+		s = tokens[tok]
+	}
+	if s == "" {
+		s = "token(" + strconv.Itoa(int(tok)) + ")"
+	}
+	return s
+}
+
+// A set of constants for precedence-based expression parsing.
+// Non-operators have lowest precedence, followed by operators
+// starting with precedence 1 up to unary operators. The highest
+// precedence serves as "catch-all" precedence for selector,
+// indexing, and other operator and delimiter tokens.
+const (
+	LowestPrec  = 0 // non-operators
+	UnaryPrec   = 6
+	HighestPrec = 7
+)
+
+// Precedence returns the operator precedence of the binary
+// operator op. If op is not a binary operator, the result
+// is LowestPrecedence.
+func (op Token) Precedence() int {
+	switch op {
+	case LOR:
+		return 1
+	case LAND:
+		return 2
+	case EQL, NEQ, LSS, LEQ, GTR, GEQ:
+		return 3
+	case ADD, SUB, OR, XOR:
+		return 4
+	case MUL, QUO, REM, SHL, SHR, AND, AND_NOT:
+		return 5
+	}
+	return LowestPrec
+}
+
+var keywords map[string]Token
+
+func init() {
+	keywords = make(map[string]Token, keyword_end-(keyword_beg+1))
+	for i := keyword_beg + 1; i < keyword_end; i++ {
+		keywords[tokens[i]] = i
+	}
+}
+
+// Lookup maps an identifier to its keyword token or IDENT (if not a keyword).
+func Lookup(ident string) Token {
+	if tok, is_keyword := keywords[ident]; is_keyword {
+		return tok
+	}
+	return IDENT
+}
+
+// Predicates
+
+// IsLiteral returns true for tokens corresponding to identifiers
+// and basic type literals; it returns false otherwise.
+func (tok Token) IsLiteral() bool { return literal_beg < tok && tok < literal_end }
+
+// IsOperator returns true for tokens corresponding to operators and
+// delimiters; it returns false otherwise.
+func (tok Token) IsOperator() bool {
+	return (operator_beg < tok && tok < operator_end) || tok == TILDE
+}
+
+// IsKeyword returns true for tokens corresponding to keywords;
+// it returns false otherwise.
+func (tok Token) IsKeyword() bool { return keyword_beg < tok && tok < keyword_end }
+
+// IsExported reports whether name starts with an upper-case letter.
+func IsExported(name string) bool {
+	ch, _ := utf8.DecodeRuneInString(name)
+	return unicode.IsUpper(ch)
+}
+
+// IsKeyword reports whether name is a Go keyword, such as "func" or "return".
+func IsKeyword(name string) bool {
+	// TODO: opt: use a perfect hash function instead of a global map.
+	_, ok := keywords[name]
+	return ok
+}
+
+// IsIdentifier reports whether name is a Go identifier, that is, a non-empty
+// string made up of letters, digits, and underscores, where the first character
+// is not a digit. Keywords are not identifiers.
+func IsIdentifier(name string) bool {
+	if name == "" || IsKeyword(name) {
+		return false
+	}
+	for i, c := range name {
+		if !unicode.IsLetter(c) && c != '_' && (i == 0 || !unicode.IsDigit(c)) {
+			return false
+		}
+	}
+	return true
+}
diff --git a/src/go/token/token_test.go b/src/go/token/token_test.go
new file mode 100644
index 0000000..eff38cc
--- /dev/null
+++ b/src/go/token/token_test.go
@@ -0,0 +1,33 @@
+// Copyright 2019 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 token
+
+import "testing"
+
+func TestIsIdentifier(t *testing.T) {
+	tests := []struct {
+		name string
+		in   string
+		want bool
+	}{
+		{"Empty", "", false},
+		{"Space", " ", false},
+		{"SpaceSuffix", "foo ", false},
+		{"Number", "123", false},
+		{"Keyword", "func", false},
+
+		{"LettersASCII", "foo", true},
+		{"MixedASCII", "_bar123", true},
+		{"UppercaseKeyword", "Func", true},
+		{"LettersUnicode", "fóö", true},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			if got := IsIdentifier(test.in); got != test.want {
+				t.Fatalf("IsIdentifier(%q) = %t, want %v", test.in, got, test.want)
+			}
+		})
+	}
+}