lib/encoder: add string rendering and parsing for encodings

lib/encoder/encoder.go

@@ -16,6 +16,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"sort"
 	"strconv"
 	"strings"
 	"unicode/utf8"
@@ -32,6 +33,7 @@ const (
 )
 
 // NB keep the tests in fstests/fstests/fstests.go FsEncoding up to date with this
+// NB keep the aliases up to date below also
 
 // Possible flags for the MultiEncoder
 const (
@@ -99,6 +101,114 @@ type Encoder interface {
 // character classes
 type MultiEncoder uint
 
+// Aliases maps encodings to names and vice versa
+var (
+	encodingToName = map[MultiEncoder]string{}
+	nameToEncoding = map[string]MultiEncoder{}
+)
+
+// alias adds an alias for MultiEncoder.String() and MultiEncoder.Set()
+func alias(name string, mask MultiEncoder) {
+	nameToEncoding[name] = mask
+	// don't overwrite existing reverse translations
+	if _, ok := encodingToName[mask]; !ok {
+		encodingToName[mask] = name
+	}
+}
+
+func init() {
+	alias("None", EncodeZero)
+	alias("Slash", EncodeSlash)
+	alias("LtGt", EncodeLtGt)
+	alias("DoubleQuote", EncodeDoubleQuote)
+	alias("SingleQuote", EncodeSingleQuote)
+	alias("BackQuote", EncodeBackQuote)
+	alias("Dollar", EncodeDollar)
+	alias("Colon", EncodeColon)
+	alias("Question", EncodeQuestion)
+	alias("Asterisk", EncodeAsterisk)
+	alias("Pipe", EncodePipe)
+	alias("Hash", EncodeHash)
+	alias("Percent", EncodePercent)
+	alias("BackSlash", EncodeBackSlash)
+	alias("CrLf", EncodeCrLf)
+	alias("Del", EncodeDel)
+	alias("Ctl", EncodeCtl)
+	alias("LeftSpace", EncodeLeftSpace)
+	alias("LeftPeriod", EncodeLeftPeriod)
+	alias("LeftTilde", EncodeLeftTilde)
+	alias("LeftCrLfHtVt", EncodeLeftCrLfHtVt)
+	alias("RightSpace", EncodeRightSpace)
+	alias("RightPeriod", EncodeRightPeriod)
+	alias("RightCrLfHtVt", EncodeRightCrLfHtVt)
+	alias("InvalidUtf8", EncodeInvalidUtf8)
+	alias("Dot", EncodeDot)
+}
+
+// validStrings returns all the valid MultiEncoder strings
+func validStrings() string {
+	var out []string
+	for k := range nameToEncoding {
+		out = append(out, k)
+	}
+	sort.Strings(out)
+	return strings.Join(out, ", ")
+}
+
+// String converts the MultiEncoder into text
+func (mask MultiEncoder) String() string {
+	// See if there is an exact translation - if so return that
+	if name, ok := encodingToName[mask]; ok {
+		return name
+	}
+	var out []string
+	// Otherwise decompose bit by bit
+	for bit := MultiEncoder(1); bit != 0; bit *= 2 {
+		if (mask & bit) != 0 {
+			if name, ok := encodingToName[bit]; ok {
+				out = append(out, name)
+			} else {
+				out = append(out, fmt.Sprintf("0x%X", uint(bit)))
+			}
+		}
+	}
+	return strings.Join(out, ",")
+}
+
+// Set converts a string into a MultiEncoder
+func (mask *MultiEncoder) Set(in string) error {
+	var out MultiEncoder
+	parts := strings.Split(in, ",")
+	for _, part := range parts {
+		part = strings.TrimSpace(part)
+		if bits, ok := nameToEncoding[part]; ok {
+			out |= bits
+		} else {
+			i, err := strconv.ParseInt(part, 0, 64)
+			if err != nil {
+				return fmt.Errorf("bad encoding %q: possible values are: %s", part, validStrings())
+			}
+			out |= MultiEncoder(i)
+		}
+	}
+	*mask = out
+	return nil
+}
+
+// Type returns a textual type of the MultiEncoder to satsify the pflag.Value interface
+func (mask MultiEncoder) Type() string {
+	return "Encoding"
+}
+
+// Scan implements the fmt.Scanner interface
+func (mask *MultiEncoder) Scan(s fmt.ScanState, ch rune) error {
+	token, err := s.Token(true, nil)
+	if err != nil {
+		return err
+	}
+	return mask.Set(string(token))
+}
+
 // Encode takes a raw name and substitutes any reserved characters and
 // patterns in it
 func (mask MultiEncoder) Encode(in string) string {