encoding/json: simplify some pieces of the encoder

Some WriteByte('\\') calls can be deduplicated.

fillField is used in two occasions, but it is unnecessary when adding
fields to the "next" stack, as those aren't used for the final encoding.
Inline the func with its only remaining call.

Finally, unindent a default-if block.

The performance of the encoder is unaffected:

name           old time/op    new time/op    delta
CodeEncoder-4    6.65ms ± 1%    6.65ms ± 0%   ~     (p=0.662 n=6+5)

Change-Id: Ie55baeab89abad9b9f13e9f6ca886a670c30dba9
Reviewed-on: https://go-review.googlesource.com/122461
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/json/encode.go

@@ -839,10 +839,8 @@ func isValidTag(s string) bool {
 			// Backslash and quote chars are reserved, but
 			// otherwise any punctuation chars are allowed
 			// in a tag name.
-		default:
-			if !unicode.IsLetter(c) && !unicode.IsDigit(c) {
-				return false
-			}
+		case !unicode.IsLetter(c) && !unicode.IsDigit(c):
+			return false
 		}
 	}
 	return true
@@ -897,18 +895,15 @@ func (e *encodeState) string(s string, escapeHTML bool) {
 			if start < i {
 				e.WriteString(s[start:i])
 			}
+			e.WriteByte('\\')
 			switch b {
 			case '\\', '"':
-				e.WriteByte('\\')
 				e.WriteByte(b)
 			case '\n':
-				e.WriteByte('\\')
 				e.WriteByte('n')
 			case '\r':
-				e.WriteByte('\\')
 				e.WriteByte('r')
 			case '\t':
-				e.WriteByte('\\')
 				e.WriteByte('t')
 			default:
 				// This encodes bytes < 0x20 except for \t, \n and \r.
@@ -916,7 +911,7 @@ func (e *encodeState) string(s string, escapeHTML bool) {
 				// because they can lead to security holes when
 				// user-controlled strings are rendered into JSON
 				// and served to some browsers.
-				e.WriteString(`\u00`)
+				e.WriteString(`u00`)
 				e.WriteByte(hex[b>>4])
 				e.WriteByte(hex[b&0xF])
 			}
@@ -972,18 +967,15 @@ func (e *encodeState) stringBytes(s []byte, escapeHTML bool) {
 			if start < i {
 				e.Write(s[start:i])
 			}
+			e.WriteByte('\\')
 			switch b {
 			case '\\', '"':
-				e.WriteByte('\\')
 				e.WriteByte(b)
 			case '\n':
-				e.WriteByte('\\')
 				e.WriteByte('n')
 			case '\r':
-				e.WriteByte('\\')
 				e.WriteByte('r')
 			case '\t':
-				e.WriteByte('\\')
 				e.WriteByte('t')
 			default:
 				// This encodes bytes < 0x20 except for \t, \n and \r.
@@ -991,7 +983,7 @@ func (e *encodeState) stringBytes(s []byte, escapeHTML bool) {
 				// because they can lead to security holes when
 				// user-controlled strings are rendered into JSON
 				// and served to some browsers.
-				e.WriteString(`\u00`)
+				e.WriteString(`u00`)
 				e.WriteByte(hex[b>>4])
 				e.WriteByte(hex[b&0xF])
 			}
@@ -1052,12 +1044,6 @@ type field struct {
 	encoder encoderFunc
 }
 
-func fillField(f field) field {
-	f.nameBytes = []byte(f.name)
-	f.equalFold = foldFunc(f.nameBytes)
-	return f
-}
-
 // byIndex sorts field by index sequence.
 type byIndex []field
 
@@ -1164,14 +1150,16 @@ func typeFields(t reflect.Type) []field {
 					if name == "" {
 						name = sf.Name
 					}
-					field := fillField(field{
+					field := field{
 						name:      name,
 						tag:       tagged,
 						index:     index,
 						typ:       ft,
 						omitEmpty: opts.Contains("omitempty"),
 						quoted:    quoted,
-					})
+					}
+					field.nameBytes = []byte(field.name)
+					field.equalFold = foldFunc(field.nameBytes)
 
 					// Build nameEscHTML and nameNonEsc ahead of time.
 					nameEscBuf.Reset()
@@ -1195,7 +1183,7 @@ func typeFields(t reflect.Type) []field {
 				// Record new anonymous struct to explore in next round.
 				nextCount[ft]++
 				if nextCount[ft] == 1 {
-					next = append(next, fillField(field{name: ft.Name(), index: index, typ: ft}))
+					next = append(next, field{name: ft.Name(), index: index, typ: ft})
 				}
 			}
 		}