vis: adding a pure golang Vis()

The current Vis() and Unvis() are using the C implementation from
MTREE(8).

But that means that cgo is used, which is not always desired.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

vis.go

@@ -1,26 +1,89 @@
 package mtree
 
-// #include "vis.h"
-// #include <stdlib.h>
-import "C"
-import (
-	"fmt"
-	"math"
-	"unsafe"
-)
+import "unicode"
 
 // Vis is a wrapper of the C implementation of the function vis, which encodes
-// a character with a particular format/style
-func Vis(src string) (string, error) {
-	// dst needs to be 4 times the length of str, must check appropriate size
-	if uint32(len(src)*4+1) >= math.MaxUint32/4 {
-		return "", fmt.Errorf("failed to encode: %q", src)
-	}
-	dst := string(make([]byte, 4*len(src)+1))
-	cDst, cSrc := C.CString(dst), C.CString(src)
-	defer C.free(unsafe.Pointer(cDst))
-	defer C.free(unsafe.Pointer(cSrc))
-	C.strvis(cDst, cSrc, C.VIS_WHITE|C.VIS_OCTAL|C.VIS_GLOB)
-
-	return C.GoString(cDst), nil
+// a character with a particular format/style.
+// For most use-cases use DefaultVisFlags.
+func Vis(src string, flags VisFlag) (string, error) {
+	return vis(src, flags)
+}
+
+// DefaultVisFlags are the typical flags used for encoding strings in mtree
+// manifests.
+var DefaultVisFlags = VisWhite | VisOctal | VisGlob
+
+// VisFlag sets the extent of charactures to be encoded
+type VisFlag int
+
+// flags for encoding
+const (
+	// to select alternate encoding format
+	VisOctal  VisFlag = 0x01 // use octal \ddd format
+	VisCstyle VisFlag = 0x02 // use \[nrft0..] where appropriate
+
+	// to alter set of characters encoded (default is to encode all non-graphic
+	// except space, tab, and newline).
+	VisSp    VisFlag = 0x04 // also encode space
+	VisTab   VisFlag = 0x08 // also encode tab
+	VisNl    VisFlag = 0x10 // also encode newline
+	VisWhite VisFlag = (VisSp | VisTab | VisNl)
+	VisSafe  VisFlag = 0x20 // only encode "unsafe" characters
+
+	// other
+	VisNoSlash   VisFlag = 0x40  // inhibit printing '\'
+	VisHttpstyle VisFlag = 0x80  // http-style escape % HEX HEX
+	VisGlob      VisFlag = 0x100 // encode glob(3) magics
+
+)
+
+// errors used in the tokenized decoding strings
+const (
+	// unvis return codes
+	unvisValid            unvisErr = 1  // character valid
+	unvisValidPush        unvisErr = 2  // character valid, push back passed char
+	unvisNochar           unvisErr = 3  // valid sequence, no character produced
+	unvisErrSynbad        unvisErr = -1 // unrecognized escape sequence
+	unvisErrUnrecoverable unvisErr = -2 // decoder in unknown state (unrecoverable)
+	unvisNone             unvisErr = 0
+
+	// unvisEnd means there are no more characters
+	unvisEnd VisFlag = 1 // no more characters
+)
+
+// unvisErr are the return conditions for Unvis
+type unvisErr int
+
+func (ue unvisErr) Error() string {
+	switch ue {
+	case unvisValid:
+		return "character valid"
+	case unvisValidPush:
+		return "character valid, push back passed char"
+	case unvisNochar:
+		return "valid sequence, no character produced"
+	case unvisErrSynbad:
+		return "unrecognized escape sequence"
+	case unvisErrUnrecoverable:
+		return "decoder in unknown state (unrecoverable)"
+	}
+	return "Unknown Error"
+}
+
+func ishex(r rune) bool {
+	lr := unicode.ToLower(r)
+	return (lr >= '0' && lr <= '9') || (lr >= 'a' && lr <= 'f')
+}
+
+func isoctal(r rune) bool {
+	return r <= '7' && r >= '0'
+}
+
+// the ctype isgraph is "any printable character except space"
+func isgraph(r rune) bool {
+	return unicode.IsPrint(r) && !unicode.IsSpace(r)
+}
+
+func isalnum(r rune) bool {
+	return unicode.IsNumber(r) || unicode.IsLetter(r)
 }