perf: replace char comparison with array lookup (#16)

* perf: replace char comparison with array lookup

isNotEscapedSingleChar is a hotsport that was taking significant cpu time
during encoding for string-heavy structs.
Replacing char comparison with a array lookup yields a significant performance
improvement:

                 │   old.txt   │               new.txt
                 │   sec/op    │   sec/op     vs base
String/string-20   35.72n ± 3%   18.61n ± 2%  -47.89% (p=0.000 n=10)

Both variant are zero allocations.

* refactor: remove unused var

* build: add missing license headers

tables.go

@@ -0,0 +1,136 @@
+// Copyright 2018 Elasticsearch BV
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package fastjson provides a library for fast JSON encoding,
+// optimised for static code generation.
+//
+// Fastjson functions and interfaces are structured such that
+// all encoding appends to a buffer, enabling buffer reuse
+// without forcing specific mechanisms such as sync.Pool. This
+// enables zero-allocation encoding without incurring any
+// concurrency overhead in certain applications.
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fastjson
+
+import "unicode/utf8"
+
+// htmlSafeSet holds the value true if the ASCII character with the given
+// array position can be safely represented inside a JSON string, embedded
+// inside of HTML <script> tags, without any additional escaping.
+//
+// All values are true except for the ASCII control characters (0-31), the
+// double quote ("), the backslash character ("\"), HTML opening and closing
+// tags ("<" and ">"), and the ampersand ("&").
+var htmlSafeSet = [utf8.RuneSelf]bool{
+	' ':      true,
+	'!':      true,
+	'"':      false,
+	'#':      true,
+	'$':      true,
+	'%':      true,
+	'&':      false,
+	'\'':     true,
+	'(':      true,
+	')':      true,
+	'*':      true,
+	'+':      true,
+	',':      true,
+	'-':      true,
+	'.':      true,
+	'/':      true,
+	'0':      true,
+	'1':      true,
+	'2':      true,
+	'3':      true,
+	'4':      true,
+	'5':      true,
+	'6':      true,
+	'7':      true,
+	'8':      true,
+	'9':      true,
+	':':      true,
+	';':      true,
+	'<':      false,
+	'=':      true,
+	'>':      false,
+	'?':      true,
+	'@':      true,
+	'A':      true,
+	'B':      true,
+	'C':      true,
+	'D':      true,
+	'E':      true,
+	'F':      true,
+	'G':      true,
+	'H':      true,
+	'I':      true,
+	'J':      true,
+	'K':      true,
+	'L':      true,
+	'M':      true,
+	'N':      true,
+	'O':      true,
+	'P':      true,
+	'Q':      true,
+	'R':      true,
+	'S':      true,
+	'T':      true,
+	'U':      true,
+	'V':      true,
+	'W':      true,
+	'X':      true,
+	'Y':      true,
+	'Z':      true,
+	'[':      true,
+	'\\':     false,
+	']':      true,
+	'^':      true,
+	'_':      true,
+	'`':      true,
+	'a':      true,
+	'b':      true,
+	'c':      true,
+	'd':      true,
+	'e':      true,
+	'f':      true,
+	'g':      true,
+	'h':      true,
+	'i':      true,
+	'j':      true,
+	'k':      true,
+	'l':      true,
+	'm':      true,
+	'n':      true,
+	'o':      true,
+	'p':      true,
+	'q':      true,
+	'r':      true,
+	's':      true,
+	't':      true,
+	'u':      true,
+	'v':      true,
+	'w':      true,
+	'x':      true,
+	'y':      true,
+	'z':      true,
+	'{':      true,
+	'|':      true,
+	'}':      true,
+	'~':      true,
+	'\u007f': true,
+}

writer.go

@@ -109,29 +109,20 @@ func (w *Writer) String(s string) {
 
 const chars = "0123456789abcdef"
 
-func isNotEscapedSingleChar(c byte, escapeHTML bool) bool {
-	// Note: might make sense to use a table if there are more chars to escape. With 4 chars
-	// it benchmarks the same.
-	if escapeHTML {
-		return c != '<' && c != '>' && c != '&' && c != '\\' && c != '"' && c >= 0x20 && c < utf8.RuneSelf
-	}
-	return c != '\\' && c != '"' && c >= 0x20 && c < utf8.RuneSelf
-}
-
 // StringContents is the same as String, but without the surrounding quotes.
 func (w *Writer) StringContents(s string) {
 	// Portions of the string that contain no escapes are appended as byte slices.
 
 	p := 0 // last non-escape symbol
 
 	for i := 0; i < len(s); {
-		c := s[i]
 
-		if isNotEscapedSingleChar(c, true) {
-			// single-width character, no escaping is required
-			i++
-			continue
-		} else if c < utf8.RuneSelf {
+		if c := s[i]; c < utf8.RuneSelf {
+			if htmlSafeSet[c] {
+				i++
+				continue
+			}
+
 			// single-with character, need to escape
 			w.RawString(s[p:i])
 			switch c {

writer_test.go

@@ -53,3 +53,15 @@ func TestWriterStringEscapes(t *testing.T) {
 	w.StringContents(string([]byte{255}))
 	assertEncoded(t, &w, `\ufffd`)
 }
+
+func BenchmarkString(b *testing.B) {
+	w := Writer{
+		buf: make([]byte, 0, 1024),
+	}
+	b.Run("string", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			w.Reset()
+			w.String("foobarstring-go-fastjson")
+		}
+	})
+}