update examples after feedback

learning-go-book · Aug 5, 2020 · 8be37cb · 8be37cb
1 parent 18a987a
commit 8be37cb
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Showing 5 changed files with 51 additions and 47 deletions.
diff --git a/data/main.go b/data/main.go
@@ -1,7 +1,9 @@
 package main
 
 import (
+	"encoding/binary"
 	"fmt"
+	"math/bits"
 	"unsafe"
 )
 
@@ -12,28 +14,41 @@ type Data struct {
 	// padded with 1 byte to make it align
 }
 
+var isLE bool
+
+func init() {
+	var x uint16 = 0xFF00
+	xb := *(*[2]byte)(unsafe.Pointer(&x))
+	isLE = (xb[0] == 0x00)
+}
+
 func DataFromBytesUnsafe(b [16]byte) Data {
-	return *(*Data)(unsafe.Pointer(&b))
+	data := *(*Data)(unsafe.Pointer(&b))
+	if isLE {
+		data.Value = bits.ReverseBytes32(data.Value)
+	}
+	return data
 }
 
 func DataFromBytes(b [16]byte) Data {
 	d := Data{}
-	d.Value = uint32(b[3])<<24 + uint32(b[2])<<16 + uint32(b[1])<<8 + uint32(b[0])
+	d.Value = binary.BigEndian.Uint32(b[:4])
 	copy(d.Label[:], b[4:14])
 	d.Active = b[14] != 0
 	return d
 }
 
 func BytesFromDataUnsafe(d Data) [16]byte {
-	return *(*[16]byte)(unsafe.Pointer(&d))
+	if isLE {
+		d.Value = bits.ReverseBytes32(d.Value)
+	}
+	b := *(*[16]byte)(unsafe.Pointer(&d))
+	return b
 }
 
 func BytesFromData(d Data) [16]byte {
 	out := [16]byte{}
-	out[3] = byte(d.Value >> 24)
-	out[2] = byte(d.Value >> 16)
-	out[1] = byte(d.Value >> 8)
-	out[0] = byte(d.Value)
+	binary.BigEndian.PutUint32(out[:4], d.Value)
 	copy(out[4:14], d.Label[:])
 	if d.Active {
 		out[14] = 1
@@ -49,7 +64,7 @@ func main() {
 	copy(d.Label[:], "Phone")
 	fmt.Println(d, unsafe.Alignof(d), unsafe.Alignof(d.Value), unsafe.Alignof(d.Label), unsafe.Alignof(d.Active))
 
-	b := [16]byte{237, 95, 132, 0, 80, 104, 111, 110, 101, 0, 0, 0, 0, 0, 1, 0}
+	b := [16]byte{0, 132, 95, 237, 80, 104, 111, 110, 101, 0, 0, 0, 0, 0, 1, 0}
 	fmt.Println(b)
 
 	b1 := BytesFromData(d)

diff --git a/data/main_test.go b/data/main_test.go
@@ -7,53 +7,53 @@ import (
 var bh [16]byte
 var dh Data
 
-var input = [16]byte{237, 95, 132, 0, 80, 104, 111, 110, 101, 0, 0, 0, 0, 0, 1, 0}
+var input = [16]byte{0, 132, 95, 237, 80, 104, 111, 110, 101, 0, 0, 0, 0, 0, 1, 0}
 
-var inputData = Data {
-	Value:8675309,
-	Label: [10]byte{80, 104, 111, 110, 101, 0, 0, 0, 0, 0},
+var inputData = Data{
+	Value:  8675309,
+	Label:  [10]byte{80, 104, 111, 110, 101, 0, 0, 0, 0, 0},
 	Active: true,
 }
 
 func TestIdentical(t *testing.T) {
 	b1 := BytesFromData(inputData)
 	b2 := BytesFromDataUnsafe(inputData)
 	if b1 != b2 {
-		t.Fatal(b1,b2)
+		t.Fatal(b1, b2)
 	}
 	if b1 != input {
-		t.Fatal(b1,input)
+		t.Fatal(b1, input)
 	}
 	d1 := DataFromBytes(b1)
 	d2 := DataFromBytesUnsafe(b1)
 	if d1 != d2 {
-		t.Fatal(d1,d2)
+		t.Fatal(d1, d2)
 	}
 	if d1 != inputData {
-		t.Fatal(d1,inputData)
+		t.Fatal(d1, inputData)
 	}
 }
 
 func BenchmarkBytesFromData(b *testing.B) {
-	for i := 0;i<b.N;i++ {
+	for i := 0; i < b.N; i++ {
 		bh = BytesFromData(inputData)
 	}
 }
 
 func BenchmarkBytesFromDataUnsafe(b *testing.B) {
-	for i := 0;i<b.N;i++ {
+	for i := 0; i < b.N; i++ {
 		bh = BytesFromDataUnsafe(inputData)
 	}
 }
 
 func BenchmarkDataFromBytes(b *testing.B) {
-	for i := 0;i<b.N;i++ {
+	for i := 0; i < b.N; i++ {
 		dh = DataFromBytes(input)
 	}
 }
 
 func BenchmarkDataFromBytesUnsafe(b *testing.B) {
-	for i := 0;i<b.N;i++ {
+	for i := 0; i < b.N; i++ {
 		dh = DataFromBytesUnsafe(input)
 	}
-}
+}
diff --git a/numbers/main.go b/numbers/main.go
@@ -7,20 +7,19 @@ import (
 
 func main() {
 	var i64 int64 = 1_000_000
-	i64ptr := uintptr(unsafe.Pointer(&i64))
-	bytes := *(*[unsafe.Sizeof(i64)]byte)(unsafe.Pointer(i64ptr))
+	i64ptr := unsafe.Pointer(&i64)
+	bytes := *(*[unsafe.Sizeof(i64)]byte)(i64ptr)
 	fmt.Println(bytes)
-	i32 := *(*int32)(unsafe.Pointer(i64ptr))
+	i32 := *(*int32)(i64ptr)
 	fmt.Println(i32)
 
-	i64ptr += 4
-	bytesShifted := *(*[unsafe.Sizeof(i64)]byte)(unsafe.Pointer(i64ptr))
+	i64ptrShifted := unsafe.Pointer(uintptr(unsafe.Pointer(&i64)) + 4)
+	bytesShifted := *(*[unsafe.Sizeof(i64)]byte)(i64ptrShifted)
 	fmt.Println(bytesShifted)
 
-	i32Shifted := *(*int32)(unsafe.Pointer(i64ptr))
+	i32Shifted := *(*int32)(unsafe.Pointer(&bytesShifted))
 	fmt.Println(i32Shifted)
 
-	i64Shifted := *(*int64)(unsafe.Pointer(i64ptr))
+	i64Shifted := *(*int64)(unsafe.Pointer(&bytesShifted))
 	fmt.Println(i64Shifted)
 }
-
diff --git a/string_modification1/main.go b/string_modification1/main.go
@@ -8,17 +8,18 @@ import (
 
 func main() {
 	s := "hello"
+	sHdrData := unsafe.Pointer((*reflect.StringHeader)(unsafe.Pointer(&s)).Data)
 	sHdr := (*reflect.StringHeader)(unsafe.Pointer(&s))
 	fmt.Println(sHdr.Len) // prints 5
 
-	for i :=0;i< sHdr.Len; i++ {
-		bp := *(*byte)(unsafe.Pointer(sHdr.Data+uintptr(i)))
-		fmt.Println(string(bp))
+	for i := 0; i < sHdr.Len; i++ {
+		bp := *(*byte)(unsafe.Pointer(uintptr(sHdrData) + uintptr(i)))
+		fmt.Print(string(bp))
 	}
-	sHdr.Len = sHdr.Len+10
+	fmt.Println()
+	sHdr.Len = sHdr.Len + 10
 	fmt.Println(s)
-
-	bp := (*byte)(unsafe.Pointer(sHdr.Data+2))
+	bp := (*byte)(unsafe.Pointer(uintptr(sHdrData) + 2))
 	*bp = *bp + 1
 	fmt.Println(s)
 }
diff --git a/string_modification2/main.go b/string_modification2/main.go
@@ -8,19 +8,8 @@ import (
 
 func main() {
 	s := "hello"
-	sHdr := (*reflect.StringHeader)(unsafe.Pointer(&s))
-	fmt.Println(sHdr.Len) // prints 5
-
-	for i :=0;i< sHdr.Len; i++ {
-		bp := *(*byte)(unsafe.Pointer(sHdr.Data+uintptr(i)))
-		fmt.Print(string(bp))
-	}
-	fmt.Println()
-
-	sHdr.Len = sHdr.Len+10
-	fmt.Println(s)
-
 	b := []byte("goodbye")
+	sHdr := (*reflect.StringHeader)(unsafe.Pointer(&s))
 	bHdr := (*reflect.SliceHeader)(unsafe.Pointer(&b))
 	sHdr.Data = bHdr.Data
 	sHdr.Len = bHdr.Len