From 9d11c63b64d1f29f43cfd7397614baa24c72a151 Mon Sep 17 00:00:00 2001
From: Eric Pauley <eric@pauley.me>
Date: Sun, 29 Apr 2018 00:15:03 -0400
Subject: [PATCH] bytes, strings: improve EqualFold fast version for ASCII

The existing implementation only considers the special ASCII
case when the lower character is an upper case letter. This
means that most ASCII comparisons use unicode.SimpleFold even
when it is not necessary.

benchmark                old ns/op     new ns/op     delta
BenchmarkEqualFold-8     450           390           -13.33%

Change-Id: I735ca3c30fc0145c186d2a54f31fd39caab2c3fa
Reviewed-on: https://go-review.googlesource.com/110018
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
---
 src/bytes/bytes.go          |  6 +++---
 src/strings/strings.go      |  6 +++---
 src/strings/strings_test.go | 12 ++++++++++++
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/bytes/bytes.go b/src/bytes/bytes.go
index 32bf6ab30d..437a6e12df 100644
--- a/src/bytes/bytes.go
+++ b/src/bytes/bytes.go
@@ -805,9 +805,9 @@ func EqualFold(s, t []byte) bool {
 			tr, sr = sr, tr
 		}
 		// Fast check for ASCII.
-		if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
-			// ASCII, and sr is upper case.  tr must be lower case.
-			if tr == sr+'a'-'A' {
+		if tr < utf8.RuneSelf {
+			// ASCII only, sr/tr must be upper/lower case
+			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
 				continue
 			}
 			return false
diff --git a/src/strings/strings.go b/src/strings/strings.go
index b0a53fdefd..45345e0088 100644
--- a/src/strings/strings.go
+++ b/src/strings/strings.go
@@ -908,9 +908,9 @@ func EqualFold(s, t string) bool {
 			tr, sr = sr, tr
 		}
 		// Fast check for ASCII.
-		if tr < utf8.RuneSelf && 'A' <= sr && sr <= 'Z' {
-			// ASCII, and sr is upper case.  tr must be lower case.
-			if tr == sr+'a'-'A' {
+		if tr < utf8.RuneSelf {
+			// ASCII only, sr/tr must be upper/lower case
+			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
 				continue
 			}
 			return false
diff --git a/src/strings/strings_test.go b/src/strings/strings_test.go
index d8fcb62a87..876f06c674 100644
--- a/src/strings/strings_test.go
+++ b/src/strings/strings_test.go
@@ -1379,6 +1379,8 @@ var EqualFoldTests = []struct {
 	{"abcdefghijK", "abcdefghij\u212A", true},
 	{"abcdefghijkz", "abcdefghij\u212Ay", false},
 	{"abcdefghijKz", "abcdefghij\u212Ay", false},
+	{"1", "2", false},
+	{"utf-8", "US-ASCII", false},
 }
 
 func TestEqualFold(t *testing.T) {
@@ -1392,6 +1394,16 @@ func TestEqualFold(t *testing.T) {
 	}
 }
 
+func BenchmarkEqualFold(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tt := range EqualFoldTests {
+			if out := EqualFold(tt.s, tt.t); out != tt.out {
+				b.Fatal("wrong result")
+			}
+		}
+	}
+}
+
 var CountTests = []struct {
 	s, sep string
 	num    int