--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/cmd/sievemgr/internal/config/scanner.go	Tue Nov 03 23:44:45 2020 +0100
@@ -0,0 +1,244 @@
+// Copyright (C) 2020 Guido Berhoerster <guido+sievemgr@berhoerster.name>
+//
+// Permission is hereby granted, free of charge, to any person obtaining
+// a copy of this software and associated documentation files (the
+// "Software"), to deal in the Software without restriction, including
+// without limitation the rights to use, copy, modify, merge, publish,
+// distribute, sublicense, and/or sell copies of the Software, and to
+// permit persons to whom the Software is furnished to do so, subject to
+// the following conditions:
+//
+// The above copyright notice and this permission notice shall be included
+// in all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+// MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+// IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+// CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+// TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
+// SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+package config
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"strings"
+	"unicode"
+)
+
+type token int
+
+const (
+	tokenEOF token = -(iota + 1)
+	tokenIllegal
+	tokenIdent
+	tokenNumber
+	tokenString
+)
+
+func (tok token) String() string {
+	switch tok {
+	case tokenIllegal:
+		return "illegal"
+	case tokenEOF:
+		return "EOF"
+	case tokenIdent:
+		return "identifier"
+	case tokenNumber:
+		return "number"
+	case tokenString:
+		return "string"
+	default:
+		return "unknown"
+	}
+}
+
+func isWhitespaceRune(r rune) bool {
+	return r == ' ' || r == '\t' || r == '\r' || r == '\n'
+}
+
+func isIdentRune(r rune) bool {
+	return r >= 'A' && r <= 'Z' || r >= 'a' && r <= 'z'
+}
+
+func isNumberRune(r rune) bool {
+	return r >= '0' && r <= '9'
+}
+
+type scanner struct {
+	br       *bufio.Reader
+	line     int  // line number for error messages
+	r        rune // last read rune
+	rdSize   int  // last read size
+	rdOffset int  // offset from beginning of file
+}
+
+func newScanner(br *bufio.Reader) *scanner {
+	return &scanner{line: 1, br: br}
+}
+
+func (s *scanner) read() error {
+	var size int
+again:
+	r, size, err := s.br.ReadRune()
+	if err != nil {
+		return err
+	}
+
+	// skip over BOM at the beginning of the file
+	if r == bom && s.rdOffset == 0 {
+		s.rdOffset += size
+		goto again
+	}
+
+	if s.r == '\n' {
+		s.line++
+	}
+
+	s.r = r
+	s.rdOffset += size
+	s.rdSize = size
+
+	if r == unicode.ReplacementChar && size == 1 {
+		return fmt.Errorf("illegal UTF-8 sequence")
+	} else if r == 0 {
+		return fmt.Errorf("illegal nul byte")
+	}
+	return nil
+}
+
+func (s *scanner) unread() {
+	if s.br.UnreadRune() != nil {
+		return
+	}
+
+	if b, _ := s.br.Peek(1); b[0] == '\n' {
+		// moved back to the previous line
+		s.line--
+	}
+
+	s.r = 0
+	s.rdOffset -= s.rdSize
+	s.rdSize = 0
+}
+
+func (s *scanner) skipWhitespace() error {
+	for {
+		if err := s.read(); err == io.EOF {
+			break
+		} else if err != nil {
+			return err
+		} else if !isWhitespaceRune(s.r) {
+			s.unread()
+			break
+		}
+	}
+	return nil
+}
+
+func (s *scanner) scanIdent() (token, string, error) {
+	var sb strings.Builder
+	for {
+		if err := s.read(); err == io.EOF {
+			break
+		} else if err != nil {
+			return tokenIllegal, "", err
+		}
+		if isIdentRune(s.r) {
+			sb.WriteRune(s.r)
+		} else if isWhitespaceRune(s.r) {
+			s.unread()
+			break
+		} else {
+			return tokenIllegal, "", fmt.Errorf("illegal rune in identifier: %q", s.r)
+		}
+	}
+	if sb.Len() == 0 {
+		return tokenIllegal, "", fmt.Errorf("expected identifier, got %q", s.r)
+	}
+	return tokenIdent, sb.String(), nil
+}
+
+func (s *scanner) scanNumber() (token, string, error) {
+	var sb strings.Builder
+	for {
+		if err := s.read(); err == io.EOF {
+			break
+		} else if err != nil {
+			return tokenIllegal, "", err
+		}
+		if isNumberRune(s.r) {
+			sb.WriteRune(s.r)
+		} else if isWhitespaceRune(s.r) {
+			s.unread()
+			break
+		} else {
+			return tokenIllegal, "", fmt.Errorf("illegal rune in number: %q", s.r)
+		}
+	}
+	if sb.Len() == 0 {
+		return tokenIllegal, "", fmt.Errorf("expected number, got %q", s.r)
+	}
+	return tokenNumber, sb.String(), nil
+}
+
+func (s *scanner) scanString() (token, string, error) {
+	if err := s.read(); err == io.EOF {
+		return tokenIllegal, "", fmt.Errorf("unexpected EOF")
+	} else if err != nil {
+		return tokenIllegal, "", err
+	}
+
+	if s.r != '"' {
+		return tokenIllegal, "",
+			fmt.Errorf("expected '\"', got %q", s.r)
+	}
+
+	var sb strings.Builder
+	var inEscape bool
+	for {
+		if err := s.read(); err == io.EOF {
+			return tokenIllegal, "", fmt.Errorf("unterminated string")
+		} else if err != nil {
+			return tokenIllegal, "", err
+		}
+
+		if s.r == '\\' && !inEscape {
+			inEscape = true
+		} else if s.r == '"' && !inEscape {
+			break
+		} else {
+			sb.WriteRune(s.r)
+			inEscape = false
+		}
+	}
+
+	return tokenString, sb.String(), nil
+}
+
+func (s *scanner) scan() (token, string, error) {
+	if err := s.skipWhitespace(); err != nil {
+		return tokenIllegal, "", err
+	}
+
+	if err := s.read(); err == io.EOF {
+		return tokenEOF, "", nil
+	} else if err != nil {
+		return tokenIllegal, "", err
+	}
+	r := s.r
+	s.unread()
+
+	switch {
+	case isIdentRune(r):
+		return s.scanIdent()
+	case isNumberRune(r):
+		return s.scanNumber()
+	case r == '"':
+		return s.scanString()
+	}
+	return tokenIllegal, string(r), nil
+}