From 7eb7f8f5a75fcff520b5b93a7831fa7044d86887 Mon Sep 17 00:00:00 2001
From: Michael Schurter <michael.schurter@gmail.com>
Date: Sat, 3 Jun 2017 13:36:54 -0700
Subject: [PATCH] encoding/json: reduce allocations by Decoder for \uXXXX
MIME-Version: 1.0
Content-Type: text/plain; charset=utf8
Content-Transfer-Encoding: 8bit

Manually convert hex escape sequence to rune instead of calling
strconv.ParseUint.

This inlines the unhex func from docs (and many other packages).

name              old time/op    new time/op    delta
UnicodeDecoder-4     468ns ± 1%     402ns ± 1%  -14.26%  (p=0.000
n=10+10)

name              old speed      new speed      delta
UnicodeDecoder-4  29.9MB/s ± 1%  34.8MB/s ± 1%  +16.59%  (p=0.000
n=10+10)

name              old alloc/op   new alloc/op   delta
UnicodeDecoder-4     44.0B ± 0%     36.0B ± 0%  -18.18%  (p=0.000
n=10+10)

name              old allocs/op  new allocs/op  delta
UnicodeDecoder-4      4.00 ± 0%      2.00 ± 0%  -50.00%  (p=0.000
n=10+10)

Fixes #20567

Change-Id: If350978d5bb98ff517485752184d02249f5d1f3a
Reviewed-on: https://go-review.googlesource.com/44738
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
---
 src/encoding/json/bench_test.go | 15 +++++++++++++++
 src/encoding/json/decode.go     | 18 ++++++++++++++----
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/src/encoding/json/bench_test.go b/src/encoding/json/bench_test.go
index 85d7ae043b..42439eb705 100644
--- a/src/encoding/json/bench_test.go
+++ b/src/encoding/json/bench_test.go
@@ -133,6 +133,21 @@ func BenchmarkCodeDecoder(b *testing.B) {
 	b.SetBytes(int64(len(codeJSON)))
 }
 
+func BenchmarkUnicodeDecoder(b *testing.B) {
+	j := []byte(`"\uD83D\uDE01"`)
+	b.SetBytes(int64(len(j)))
+	r := bytes.NewReader(j)
+	dec := NewDecoder(r)
+	var out string
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		if err := dec.Decode(&out); err != nil {
+			b.Fatal("Decode:", err)
+		}
+		r.Seek(0, 0)
+	}
+}
+
 func BenchmarkDecoderStream(b *testing.B) {
 	b.StopTimer()
 	var buf bytes.Buffer
diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index 70179e60ac..4f98916105 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -1148,11 +1148,21 @@ func getu4(s []byte) rune {
 	if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
 		return -1
 	}
-	r, err := strconv.ParseUint(string(s[2:6]), 16, 64)
-	if err != nil {
-		return -1
+	var r rune
+	for _, c := range s[2:6] {
+		switch {
+		case '0' <= c && c <= '9':
+			c = c - '0'
+		case 'a' <= c && c <= 'f':
+			c = c - 'a' + 10
+		case 'A' <= c && c <= 'F':
+			c = c - 'A' + 10
+		default:
+			return -1
+		}
+		r = r*16 + rune(c)
 	}
-	return rune(r)
+	return r
 }
 
 // unquote converts a quoted JSON string literal s into an actual string t.
-- 
2.51.0