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package hcl
import (
// RangeScanner is a helper that will scan over a buffer using a bufio.SplitFunc
// and visit a source range for each token matched.
// For example, this can be used with bufio.ScanLines to find the source range
// for each line in the file, skipping over the actual newline characters, which
// may be useful when printing source code snippets as part of diagnostic
// messages.
// The line and column information in the returned ranges is produced by
// counting newline characters and grapheme clusters respectively, which
// mimics the behavior we expect from a parser when producing ranges.
type RangeScanner struct {
filename string
b []byte
cb bufio.SplitFunc
pos Pos // position of next byte to process in b
cur Range // latest range
tok []byte // slice of b that is covered by cur
err error // error from last scan, if any
// NewRangeScanner creates a new RangeScanner for the given buffer, producing
// ranges for the given filename.
// Since ranges have grapheme-cluster granularity rather than byte granularity,
// the scanner will produce incorrect results if the given SplitFunc creates
// tokens between grapheme cluster boundaries. In particular, it is incorrect
// to use RangeScanner with bufio.ScanRunes because it will produce tokens
// around individual UTF-8 sequences, which will split any multi-sequence
// grapheme clusters.
func NewRangeScanner(b []byte, filename string, cb bufio.SplitFunc) *RangeScanner {
return NewRangeScannerFragment(b, filename, InitialPos, cb)
// NewRangeScannerFragment is like NewRangeScanner but the ranges it produces
// will be offset by the given starting position, which is appropriate for
// sub-slices of a file, whereas NewRangeScanner assumes it is scanning an
// entire file.
func NewRangeScannerFragment(b []byte, filename string, start Pos, cb bufio.SplitFunc) *RangeScanner {
return &RangeScanner{
filename: filename,
b: b,
cb: cb,
pos: start,
func (sc *RangeScanner) Scan() bool {
if sc.pos.Byte >= len(sc.b) || sc.err != nil {
// All done
return false
// Since we're operating on an in-memory buffer, we always pass the whole
// remainder of the buffer to our SplitFunc and set isEOF to let it know
// that it has the whole thing.
advance, token, err := sc.cb(sc.b[sc.pos.Byte:], true)
// Since we are setting isEOF to true this should never happen, but
// if it does we will just abort and assume the SplitFunc is misbehaving.
if advance == 0 && token == nil && err == nil {
return false
if err != nil {
sc.err = err
sc.cur = Range{
Filename: sc.filename,
Start: sc.pos,
End: sc.pos,
sc.tok = nil
return false
sc.tok = token
start := sc.pos
end := sc.pos
new := sc.pos
// adv is similar to token but it also includes any subsequent characters
// we're being asked to skip over by the SplitFunc.
// adv is a slice covering any additional bytes we are skipping over, based
// on what the SplitFunc told us to do with advance.
adv := sc.b[sc.pos.Byte : sc.pos.Byte+advance]
// We now need to scan over our token to count the grapheme clusters
// so we can correctly advance Column, and count the newlines so we
// can correctly advance Line.
advR := bytes.NewReader(adv)
gsc := bufio.NewScanner(advR)
advanced := 0
for gsc.Scan() {
gr := gsc.Bytes()
new.Byte += len(gr)
// We rely here on the fact that \r\n is considered a grapheme cluster
// and so we don't need to worry about miscounting additional lines
// on files with Windows-style line endings.
if len(gr) != 0 && (gr[0] == '\r' || gr[0] == '\n') {
new.Column = 1
if advanced < len(token) {
// If we've not yet found the end of our token then we'll
// also push our "end" marker along.
// (if advance > len(token) then we'll stop moving "end" early
// so that the caller only sees the range covered by token.)
end = new
advanced += len(gr)
sc.cur = Range{
Filename: sc.filename,
Start: start,
End: end,
sc.pos = new
return true
// Range returns a range that covers the latest token obtained after a call
// to Scan returns true.
func (sc *RangeScanner) Range() Range {
return sc.cur
// Bytes returns the slice of the input buffer that is covered by the range
// that would be returned by Range.
func (sc *RangeScanner) Bytes() []byte {
return sc.tok
// Err can be called after Scan returns false to determine if the latest read
// resulted in an error, and obtain that error if so.
func (sc *RangeScanner) Err() error {
return sc.err
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