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// 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 span
import (
// Range represents a source code range in token.Pos form.
// It also carries the FileSet that produced the positions, so that it is
// self contained.
type Range struct {
FileSet *token.FileSet
Start token.Pos
End token.Pos
Converter Converter
type FileConverter struct {
file *token.File
// TokenConverter is a Converter backed by a token file set and file.
// It uses the file set methods to work out the conversions, which
// makes it fast and does not require the file contents.
type TokenConverter struct {
fset *token.FileSet
// NewRange creates a new Range from a FileSet and two positions.
// To represent a point pass a 0 as the end pos.
func NewRange(fset *token.FileSet, start, end token.Pos) Range {
return Range{
FileSet: fset,
Start: start,
End: end,
// NewTokenConverter returns an implementation of Converter backed by a
// token.File.
func NewTokenConverter(fset *token.FileSet, f *token.File) *TokenConverter {
return &TokenConverter{fset: fset, FileConverter: FileConverter{file: f}}
// NewContentConverter returns an implementation of Converter for the
// given file content.
func NewContentConverter(filename string, content []byte) *TokenConverter {
fset := token.NewFileSet()
f := fset.AddFile(filename, -1, len(content))
return NewTokenConverter(fset, f)
// IsPoint returns true if the range represents a single point.
func (r Range) IsPoint() bool {
return r.Start == r.End
// Span converts a Range to a Span that represents the Range.
// It will fill in all the members of the Span, calculating the line and column
// information.
func (r Range) Span() (Span, error) {
if !r.Start.IsValid() {
return Span{}, fmt.Errorf("start pos is not valid")
f := r.FileSet.File(r.Start)
if f == nil {
return Span{}, fmt.Errorf("file not found in FileSet")
return FileSpan(f, r.Converter, r.Start, r.End)
// FileSpan returns a span within tok, using converter to translate between
// offsets and positions.
func FileSpan(tok *token.File, converter Converter, start, end token.Pos) (Span, error) {
var s Span
var err error
var startFilename string
startFilename, s.v.Start.Line, s.v.Start.Column, err = position(tok, start)
if err != nil {
return Span{}, err
s.v.URI = URIFromPath(startFilename)
if end.IsValid() {
var endFilename string
endFilename, s.v.End.Line, s.v.End.Column, err = position(tok, end)
if err != nil {
return Span{}, err
// In the presence of line directives, a single File can have sections from
// multiple file names.
if endFilename != startFilename {
return Span{}, fmt.Errorf("span begins in file %q but ends in %q", startFilename, endFilename)
if converter != nil {
return s.WithOffset(converter)
if startFilename != tok.Name() {
return Span{}, fmt.Errorf("must supply Converter for file %q containing lines from %q", tok.Name(), startFilename)
return s.WithOffset(&FileConverter{tok})
func position(f *token.File, pos token.Pos) (string, int, int, error) {
off, err := offset(f, pos)
if err != nil {
return "", 0, 0, err
return positionFromOffset(f, off)
func positionFromOffset(f *token.File, offset int) (string, int, int, error) {
if offset > f.Size() {
return "", 0, 0, fmt.Errorf("offset %v is past the end of the file %v", offset, f.Size())
pos := f.Pos(offset)
p := f.Position(pos)
// TODO(golang/go#41029): Consider returning line, column instead of line+1, 1 if
// the file's last character is not a newline.
if offset == f.Size() {
return p.Filename, p.Line + 1, 1, nil
return p.Filename, p.Line, p.Column, nil
// offset is a copy of the Offset function in go/token, but with the adjustment
// that it does not panic on invalid positions.
func offset(f *token.File, pos token.Pos) (int, error) {
if int(pos) < f.Base() || int(pos) > f.Base()+f.Size() {
return 0, fmt.Errorf("invalid pos")
return int(pos) - f.Base(), nil
// Range converts a Span to a Range that represents the Span for the supplied
// File.
func (s Span) Range(converter *TokenConverter) (Range, error) {
s, err := s.WithOffset(converter)
if err != nil {
return Range{}, err
// go/token will panic if the offset is larger than the file's size,
// so check here to avoid panicking.
if s.Start().Offset() > converter.file.Size() {
return Range{}, fmt.Errorf("start offset %v is past the end of the file %v", s.Start(), converter.file.Size())
if s.End().Offset() > converter.file.Size() {
return Range{}, fmt.Errorf("end offset %v is past the end of the file %v", s.End(), converter.file.Size())
return Range{
FileSet: converter.fset,
Start: converter.file.Pos(s.Start().Offset()),
End: converter.file.Pos(s.End().Offset()),
Converter: converter,
}, nil
func (l *FileConverter) ToPosition(offset int) (int, int, error) {
_, line, col, err := positionFromOffset(l.file, offset)
return line, col, err
func (l *FileConverter) ToOffset(line, col int) (int, error) {
if line < 0 {
return -1, fmt.Errorf("line is not valid")
lineMax := l.file.LineCount() + 1
if line > lineMax {
return -1, fmt.Errorf("line is beyond end of file %v", lineMax)
} else if line == lineMax {
if col > 1 {
return -1, fmt.Errorf("column is beyond end of file")
// at the end of the file, allowing for a trailing eol
return l.file.Size(), nil
pos := lineStart(l.file, line)
if !pos.IsValid() {
return -1, fmt.Errorf("line is not in file")
// we assume that column is in bytes here, and that the first byte of a
// line is at column 1
pos += token.Pos(col - 1)
return offset(l.file, pos)