src/go/token/position.go - go.git - Git at Google

 // 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"
 	"strconv"
 	"sync"
 	"sync/atomic"
 )

 // If debug is set, invalid offset and position values cause a panic
 // (go.dev/issue/57490).
 const debug = false

 // -----------------------------------------------------------------------------
 // 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 += strconv.Itoa(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]
 }

 // Lines returns the effective line offset table of the form described by [File.SetLines].
 // Callers must not mutate the result.
 func (f *File) Lines() []int {
 	f.mutex.Lock()
 	lines := f.lines
 	f.mutex.Unlock()
 	return lines
 }

 // 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 [File.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 [File.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()
 }

 // fixOffset fixes an out-of-bounds offset such that 0 <= offset <= f.size.
 func (f *File) fixOffset(offset int) int {
 	switch {
 	case offset < 0:
 		if !debug {
 			return 0
 		}
 	case offset > f.size:
 		if !debug {
 			return f.size
 		}
 	default:
 		return offset
 	}

 	// only generate this code if needed
 	if debug {
 		panic(fmt.Sprintf("offset %d out of bounds [%d, %d] (position %d out of bounds [%d, %d])",
 			0 /* for symmetry */, offset, f.size,
 			f.base+offset, f.base, f.base+f.size))
 	}
 	return 0
 }

 // Pos returns the Pos value for the given file offset.
 //
 // If offset is negative, the result is the file's start
 // position; if the offset is too large, the result is
 // the file's end position (see also go.dev/issue/57490).
 //
 // The following invariant, though not true for Pos values
 // in general, holds for the result p:
 // f.Pos(f.Offset(p)) == p.
 func (f *File) Pos(offset int) Pos {
 	return Pos(f.base + f.fixOffset(offset))
 }

 // Offset returns the offset for the given file position p.
 //
 // If p is before the file's start position (or if p is NoPos),
 // the result is 0; if p is past the file's end position, the
 // the result is the file size (see also go.dev/issue/57490).
 //
 // The following invariant, though not true for offset values
 // in general, holds for the result offset:
 // f.Offset(f.Pos(offset)) == offset
 func (f *File) Offset(p Pos) int {
 	return f.fixOffset(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 := f.fixOffset(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 p is out of bounds, it is adjusted to match the File.Offset behavior.
 // 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 {
 		pos = f.position(p, adjusted)
 	}
 	return
 }

 // Position returns the Position value for the given file position p.
 // If p is out of bounds, it is adjusted to match the File.Offset behavior.
 // 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. [FileSet.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
 // [FileSet.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.Base], and
 // size must not be negative. As a special case, if a negative base is provided,
 // the current value of the [FileSet.Base] is used instead.
 //
 // Adding the file will set the file set's [FileSet.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
 }
	// 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"
	"strconv"
	"sync"
	"sync/atomic"
	)

	// If debug is set, invalid offset and position values cause a panic
	// (go.dev/issue/57490).
	const debug = false

	// -----------------------------------------------------------------------------
	// 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 += strconv.Itoa(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]
	}

	// Lines returns the effective line offset table of the form described by [File.SetLines].
	// Callers must not mutate the result.
	func (f *File) Lines() []int {
	f.mutex.Lock()
	lines := f.lines
	f.mutex.Unlock()
	return lines
	}

	// 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 [File.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 [File.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()
	}

	// fixOffset fixes an out-of-bounds offset such that 0 <= offset <= f.size.
	func (f *File) fixOffset(offset int) int {
	switch {
	case offset < 0:
	if !debug {
	return 0
	}
	case offset > f.size:
	if !debug {
	return f.size
	}
	default:
	return offset
	}

	// only generate this code if needed
	if debug {
	panic(fmt.Sprintf("offset %d out of bounds [%d, %d] (position %d out of bounds [%d, %d])",
	0 /* for symmetry */, offset, f.size,
	f.base+offset, f.base, f.base+f.size))
	}
	return 0
	}

	// Pos returns the Pos value for the given file offset.
	//
	// If offset is negative, the result is the file's start
	// position; if the offset is too large, the result is
	// the file's end position (see also go.dev/issue/57490).
	//
	// The following invariant, though not true for Pos values
	// in general, holds for the result p:
	// f.Pos(f.Offset(p)) == p.
	func (f *File) Pos(offset int) Pos {
	return Pos(f.base + f.fixOffset(offset))
	}

	// Offset returns the offset for the given file position p.
	//
	// If p is before the file's start position (or if p is NoPos),
	// the result is 0; if p is past the file's end position, the
	// the result is the file size (see also go.dev/issue/57490).
	//
	// The following invariant, though not true for offset values
	// in general, holds for the result offset:
	// f.Offset(f.Pos(offset)) == offset
	func (f *File) Offset(p Pos) int {
	return f.fixOffset(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 := f.fixOffset(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 p is out of bounds, it is adjusted to match the File.Offset behavior.
	// 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 {
	pos = f.position(p, adjusted)
	}
	return
	}

	// Position returns the Position value for the given file position p.
	// If p is out of bounds, it is adjusted to match the File.Offset behavior.
	// 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. [FileSet.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
	// [FileSet.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.Base], and
	// size must not be negative. As a special case, if a negative base is provided,
	// the current value of the [FileSet.Base] is used instead.
	//
	// Adding the file will set the file set's [FileSet.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
	}