From b3e93b704076820f2ca6c778f8725da7c587cfdf Mon Sep 17 00:00:00 2001
From: Masaaki Goshima <goccy54@gmail.com>
Date: Sat, 6 Feb 2021 01:09:24 +0900
Subject: [PATCH] Enable Boundary Check Elimination by pointer arithmetic

---
 decode_context.go |  6 ++++++
 decode_int.go     |  7 ++++---
 decode_string.go  |  5 +++--
 decode_struct.go  | 30 ++++++++++++++++--------------
 4 files changed, 29 insertions(+), 19 deletions(-)

diff --git a/decode_context.go b/decode_context.go
index 23e0d3f..de02a37 100644
--- a/decode_context.go
+++ b/decode_context.go
@@ -1,5 +1,7 @@
 package json
 
+import "unsafe"
+
 var (
 	isWhiteSpace = [256]bool{}
 )
@@ -11,6 +13,10 @@ func init() {
 	isWhiteSpace['\r'] = true
 }
 
+func char(ptr unsafe.Pointer, offset int64) byte {
+	return *(*byte)(unsafe.Pointer(uintptr(ptr) + uintptr(offset)))
+}
+
 func skipWhiteSpace(buf []byte, cursor int64) int64 {
 LOOP:
 	if isWhiteSpace[buf[cursor]] {
diff --git a/decode_int.go b/decode_int.go
index fb0d3c7..b091812 100644
--- a/decode_int.go
+++ b/decode_int.go
@@ -130,8 +130,9 @@ ERROR:
 }
 
 func (d *intDecoder) decodeByte(buf []byte, cursor int64) ([]byte, int64, error) {
+	b := (*sliceHeader)(unsafe.Pointer(&buf)).data
 	for {
-		switch buf[cursor] {
+		switch char(b, cursor) {
 		case ' ', '\n', '\t', '\r':
 			cursor++
 			continue
@@ -139,14 +140,14 @@ func (d *intDecoder) decodeByte(buf []byte, cursor int64) ([]byte, int64, error)
 			start := cursor
 			cursor++
 		LOOP:
-			if numTable[buf[cursor]] {
+			if numTable[char(b, cursor)] {
 				cursor++
 				goto LOOP
 			}
 			num := buf[start:cursor]
 			return num, cursor, nil
 		default:
-			return nil, 0, d.typeError([]byte{buf[cursor]}, cursor)
+			return nil, 0, d.typeError([]byte{char(b, cursor)}, cursor)
 		}
 	}
 }
diff --git a/decode_string.go b/decode_string.go
index a90f911..66412c9 100644
--- a/decode_string.go
+++ b/decode_string.go
@@ -249,11 +249,12 @@ func (d *stringDecoder) decodeByte(buf []byte, cursor int64) ([]byte, int64, err
 		case '"':
 			cursor++
 			start := cursor
+			b := (*sliceHeader)(unsafe.Pointer(&buf)).data
 			for {
-				switch buf[cursor] {
+				switch char(b, cursor) {
 				case '\\':
 					cursor++
-					switch buf[cursor] {
+					switch char(b, cursor) {
 					case '"':
 						buf[cursor] = '"'
 						buf = append(buf[:cursor-1], buf[cursor:]...)
diff --git a/decode_struct.go b/decode_struct.go
index 5b12aec..414c673 100644
--- a/decode_struct.go
+++ b/decode_struct.go
@@ -120,13 +120,14 @@ func decodeKeyByBitmapInt8(d *structDecoder, buf []byte, cursor int64) (int64, *
 		field  *structFieldSet
 		curBit int8 = math.MaxInt8
 	)
+	b := (*sliceHeader)(unsafe.Pointer(&buf)).data
 	for {
-		switch buf[cursor] {
+		switch char(b, cursor) {
 		case ' ', '\n', '\t', '\r':
 			cursor++
 		case '"':
 			cursor++
-			c := buf[cursor]
+			c := char(b, cursor)
 			switch c {
 			case '"':
 				cursor++
@@ -138,7 +139,7 @@ func decodeKeyByBitmapInt8(d *structDecoder, buf []byte, cursor int64) (int64, *
 			bitmap := d.keyBitmapInt8
 			keyBitmapLen := len(bitmap)
 			for {
-				c := buf[cursor]
+				c := char(b, cursor)
 				switch c {
 				case '"':
 					x := uint64(curBit & -curBit)
@@ -152,13 +153,13 @@ func decodeKeyByBitmapInt8(d *structDecoder, buf []byte, cursor int64) (int64, *
 					if keyIdx >= keyBitmapLen {
 						for {
 							cursor++
-							switch buf[cursor] {
+							switch char(b, cursor) {
 							case '"':
 								cursor++
 								return cursor, field, nil
 							case '\\':
 								cursor++
-								if buf[cursor] == nul {
+								if char(b, cursor) == nul {
 									return 0, nil, errUnexpectedEndOfJSON("string", cursor)
 								}
 							case nul:
@@ -170,13 +171,13 @@ func decodeKeyByBitmapInt8(d *structDecoder, buf []byte, cursor int64) (int64, *
 					if curBit == 0 {
 						for {
 							cursor++
-							switch buf[cursor] {
+							switch char(b, cursor) {
 							case '"':
 								cursor++
 								return cursor, field, nil
 							case '\\':
 								cursor++
-								if buf[cursor] == nul {
+								if char(b, cursor) == nul {
 									return 0, nil, errUnexpectedEndOfJSON("string", cursor)
 								}
 							case nul:
@@ -199,13 +200,14 @@ func decodeKeyByBitmapInt16(d *structDecoder, buf []byte, cursor int64) (int64,
 		field  *structFieldSet
 		curBit int16 = math.MaxInt16
 	)
+	b := (*sliceHeader)(unsafe.Pointer(&buf)).data
 	for {
-		switch buf[cursor] {
+		switch char(b, cursor) {
 		case ' ', '\n', '\t', '\r':
 			cursor++
 		case '"':
 			cursor++
-			c := buf[cursor]
+			c := char(b, cursor)
 			switch c {
 			case '"':
 				cursor++
@@ -217,7 +219,7 @@ func decodeKeyByBitmapInt16(d *structDecoder, buf []byte, cursor int64) (int64,
 			bitmap := d.keyBitmapInt16
 			keyBitmapLen := len(bitmap)
 			for {
-				c := buf[cursor]
+				c := char(b, cursor)
 				switch c {
 				case '"':
 					x := uint64(curBit & -curBit)
@@ -231,13 +233,13 @@ func decodeKeyByBitmapInt16(d *structDecoder, buf []byte, cursor int64) (int64,
 					if keyIdx >= keyBitmapLen {
 						for {
 							cursor++
-							switch buf[cursor] {
+							switch char(b, cursor) {
 							case '"':
 								cursor++
 								return cursor, field, nil
 							case '\\':
 								cursor++
-								if buf[cursor] == nul {
+								if char(b, cursor) == nul {
 									return 0, nil, errUnexpectedEndOfJSON("string", cursor)
 								}
 							case nul:
@@ -249,13 +251,13 @@ func decodeKeyByBitmapInt16(d *structDecoder, buf []byte, cursor int64) (int64,
 					if curBit == 0 {
 						for {
 							cursor++
-							switch buf[cursor] {
+							switch char(b, cursor) {
 							case '"':
 								cursor++
 								return cursor, field, nil
 							case '\\':
 								cursor++
-								if buf[cursor] == nul {
+								if char(b, cursor) == nul {
 									return 0, nil, errUnexpectedEndOfJSON("string", cursor)
 								}
 							case nul: