reader.go

package lexer

import "io"

// reader represents a buffered rune reader used by the scanner.
// It provides a fixed-length circular buffer that can be unread.
type reader struct {
	r   io.RuneScanner
	i   int // buffer index
	n   int // buffer char count
	pos Pos // last read rune position
	buf [3]struct {
		ch  rune
		pos Pos
	}
	eof bool // true if reader has ever seen eof.
}

// ReadRune reads the next rune from the reader.
// This is a wrapper function to implement the io.RuneReader interface.
// Note that this function does not return size.
func (r *reader) ReadRune() (ch rune, size int, err error) {
	ch, _ = r.read()
	if ch == eof {
		err = io.EOF
	}
	return
}

// UnreadRune pushes the previously read rune back onto the buffer.
// This is a wrapper function to implement the io.RuneScanner interface.
func (r *reader) UnreadRune() error {
	r.unread()
	return nil
}

// read reads the next rune from the reader.
func (r *reader) read() (ch rune, pos Pos) {
	// If we have unread characters then read them off the buffer first.
	if r.n > 0 {
		r.n--
		return r.curr()
	}

	// Read next rune from underlying reader.
	// Any error (including io.EOF) should return as EOF.
	ch, _, err := r.r.ReadRune()
	if err != nil {
		ch = eof
	} else if ch == '\r' {
		if ch, _, err := r.r.ReadRune(); err != nil {
			// nop
		} else if ch != '\n' {
			_ = r.r.UnreadRune()
		}
		ch = '\n'
	}

	// Save character and position to the buffer.
	r.i = (r.i + 1) % len(r.buf)
	buf := &r.buf[r.i]
	buf.ch, buf.pos = ch, r.pos

	// Update position.
	// Only count EOF once.
	if ch == '\n' {
		r.pos.Line++
		r.pos.Char = 0
	} else if !r.eof {
		r.pos.Char++
	}

	// Mark the reader as EOF.
	// This is used so we don't double count EOF characters.
	if ch == eof {
		r.eof = true
	}

	return r.curr()
}

// unread pushes the previously read rune back onto the buffer.
func (r *reader) unread() {
	r.n++
}

// curr returns the last read character and position.
func (r *reader) curr() (ch rune, pos Pos) {
	i := (r.i - r.n + len(r.buf)) % len(r.buf)
	buf := &r.buf[i]
	return buf.ch, buf.pos
}