utter: add quoting strategy support

WIP:
* needs tests
* API needs more thought

common.go

@@ -153,6 +153,8 @@ func unsafeReflectValue(v reflect.Value) (rv reflect.Value) {
 // Some constants in the form of bytes to avoid string overhead.  This mirrors
 // the technique used in the fmt package.
 var (
+	backQuoteBytes        = []byte("`")
+	quoteBytes            = []byte(`"`)
 	plusBytes             = []byte("+")
 	iBytes                = []byte("i")
 	trueBytes             = []byte("true")

config.go

@@ -50,6 +50,9 @@ type ConfigState struct {
 	// a string slice or array. Zero specifies all entries on one line.
 	StringWidth int
 
+	// Quoting specifies the quoting strategy to use when printing strings.
+	Quoting Quoting
+
 	// BytesWidth specifies the number of byte columns to use when dumping a
 	// byte slice or array.
 	BytesWidth int
@@ -84,6 +87,26 @@ type ConfigState struct {
 	SortKeys bool
 }
 
+// Quoting describes string quoting strategies.
+type Quoting uint
+
+const (
+	// Quote strings with double quotes.
+	DoubleQuote Quoting = 0
+
+	// AvoidEscapes quotes strings using backquotes to avoid escape sequences
+	// if possible, otherwise double quotes are used.
+	AvoidEscapes Quoting = 1 << iota
+
+	// Backquote always quotes strings using backquotes if possible, otherwise
+	// double quotes are used.
+	Backquote
+
+	// Force is a modifier of AvoidEscapes and Backquote that adds additional
+	// double quote syntax to represent parts that cannot be backquoted.
+	Force
+)
+
 // Config is the active configuration of the top-level functions.
 // The configuration can be changed by modifying the contents of utter.Config.
 var Config = ConfigState{

dump.go

@@ -26,6 +26,8 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+	"unicode"
+	"unicode/utf8"
 )
 
 var (
@@ -463,7 +465,7 @@ func (d *dumpState) dump(v reflect.Value, wasPtr, static, canElideCompound bool,
 		d.dumpSlice(v, !interfaceContext)
 
 	case reflect.String:
-		d.w.Write([]byte(strconv.Quote(v.String())))
+		d.writeQuoted(v.String())
 
 	case reflect.Interface:
 		// The only time we should get here is for nil interfaces due to
@@ -585,6 +587,107 @@ func (d *dumpState) dump(v reflect.Value, wasPtr, static, canElideCompound bool,
 	}
 }
 
+// writeQuoted writes the string s quoted according to the quoting strategy.
+func (d *dumpState) writeQuoted(s string) {
+	switch d.cs.Quoting {
+	default:
+		fallthrough
+	case DoubleQuote:
+		d.w.Write([]byte(strconv.Quote(s)))
+
+	case AvoidEscapes:
+		if !needsEscape(s) || !canBackquoteString(s) {
+			d.w.Write([]byte(strconv.Quote(s)))
+			return
+		}
+		d.backQuote(s)
+
+	case AvoidEscapes | Force:
+		if !needsEscape(s) {
+			d.w.Write([]byte(strconv.Quote(s)))
+			return
+		}
+
+		fallthrough
+	case Backquote, Backquote | Force:
+		if canBackquoteString(s) {
+			d.backQuote(s)
+			return
+		}
+
+		var last int
+		inBackquote := true
+		for i, r := range s {
+			if canBackquote(r) != inBackquote {
+				if last != 0 {
+					d.w.Write(plusBytes)
+				}
+				if inBackquote {
+					d.backQuote(s[last:i])
+				} else {
+					d.w.Write([]byte(strconv.Quote(s[last:i])))
+				}
+				last = i
+				inBackquote = !inBackquote
+			}
+		}
+		if last != len(s) {
+			if last != 0 {
+				d.w.Write(plusBytes)
+			}
+			if !inBackquote {
+				d.w.Write([]byte(strconv.Quote(s[last:])))
+				return
+			}
+			d.backQuote(s[last:])
+		}
+	}
+}
+
+// backQuote writes s backquoted.
+func (d *dumpState) backQuote(s string) {
+	d.w.Write(backQuoteBytes)
+	d.w.Write([]byte(s))
+	d.w.Write(backQuoteBytes)
+}
+
+// needsEscape returns whether the string s needs any escape sequence to be
+// double quote printed.
+func needsEscape(s string) bool {
+	for _, r := range s {
+		if r == '"' || r == '\\' {
+			return true
+		}
+		if !strconv.IsPrint(r) && !strconv.IsGraphic(r) {
+			return true
+		}
+	}
+	return false
+}
+
+// canBackquoteString returns whether the string s can be represented
+// unchanged as a backquoted string without non-space control characters.
+func canBackquoteString(s string) bool {
+	for _, r := range s {
+		if !canBackquote(r) {
+			return false
+		}
+	}
+	return true
+}
+
+// canBackquote returns whether the rune r can be represented unchanged as a
+// backquoted string without non-space control characters.
+func canBackquote(r rune) bool {
+	if r == utf8.RuneError {
+		return false
+	}
+	if utf8.RuneLen(r) > 1 {
+		return r != '\ufeff'
+	}
+	return (unicode.IsSpace(r) || ' ' < r) && r != '`' && r != '\u007f'
+}
+
 // typeString returns the string representation of the reflect.Type with the local
 // package selector removed.
 func typeString(typ reflect.Type, local string) string {