first commit

.gitignore

@@ -0,0 +1,3 @@
+*.iml
+.idea
+.DS_Store

data/main.go

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+package main
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+type Data struct {
+	Value  uint32   // 4 bytes
+	Label  [10]byte // 10 bytes
+	Active bool     // 1 byte
+	// padded with 1 byte to make it align
+}
+
+func DataFromBytesUnsafe(b [16]byte) Data {
+	return *(*Data)(unsafe.Pointer(&b))
+}
+
+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])
+	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))
+}
+
+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 & 255)
+	copy(out[4:14], d.Label[:])
+	if d.Active {
+		out[14] = 1
+	}
+	return out
+}
+
+func main() {
+	d := Data{
+		Value:  8675309,
+		Active: true,
+	}
+	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}
+	fmt.Println(b)
+
+	b1 := BytesFromData(d)
+	b2 := BytesFromDataUnsafe(d)
+	if b1 != b2 {
+		panic(fmt.Sprintf("%v %v", b1, b2))
+	}
+	fmt.Printf("%+v\n", b1)
+	d1 := DataFromBytes(b1)
+	d2 := DataFromBytesUnsafe(b1)
+	if d1 != d2 {
+		panic(fmt.Sprintf("%v %v", d1, d2))
+	}
+	fmt.Println(d1)
+}

data/main_test.go

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+package main
+
+import (
+	"testing"
+)
+
+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 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)
+	}
+	if b1 != input {
+		t.Fatal(b1,input)
+	}
+	d1 := DataFromBytes(b1)
+	d2 := DataFromBytesUnsafe(b1)
+	if d1 != d2 {
+		t.Fatal(d1,d2)
+	}
+	if d1 != inputData {
+		t.Fatal(d1,inputData)
+	}
+}
+
+func BenchmarkBytesFromData(b *testing.B) {
+	for i := 0;i<b.N;i++ {
+		bh = BytesFromData(inputData)
+	}
+}
+
+func BenchmarkBytesFromDataUnsafe(b *testing.B) {
+	for i := 0;i<b.N;i++ {
+		bh = BytesFromDataUnsafe(inputData)
+	}
+}
+
+func BenchmarkDataFromBytes(b *testing.B) {
+	for i := 0;i<b.N;i++ {
+		dh = DataFromBytes(input)
+	}
+}
+
+func BenchmarkDataFromBytesUnsafe(b *testing.B) {
+	for i := 0;i<b.N;i++ {
+		dh = DataFromBytesUnsafe(input)
+	}
+}

go.mod

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+module unsafe_examples
+
+go 1.14

numbers/main.go

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+package main
+
+import (
+	"fmt"
+	"unsafe"
+)
+
+func main() {
+	var i64 int64 = 1_000_000
+	i64ptr := uintptr(unsafe.Pointer(&i64))
+	bytes := *(*[unsafe.Sizeof(i64)]byte)(unsafe.Pointer(i64ptr))
+	fmt.Println(bytes)
+	i32 := *(*int32)(unsafe.Pointer(i64ptr))
+	fmt.Println(i32)
+
+	i64ptr += 4
+	bytesShifted := *(*[unsafe.Sizeof(i64)]byte)(unsafe.Pointer(i64ptr))
+	fmt.Println(bytesShifted)
+
+	i32Shifted := *(*int32)(unsafe.Pointer(i64ptr))
+	fmt.Println(i32Shifted)
+
+	i64Shifted := *(*int64)(unsafe.Pointer(i64ptr))
+	fmt.Println(i64Shifted)
+}
+

string_modification1/main.go

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+package main
+
+import (
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+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.Println(string(bp))
+	}
+	sHdr.Len = sHdr.Len+10
+	fmt.Println(s)
+
+	bp := (*byte)(unsafe.Pointer(sHdr.Data+2))
+	*bp = *bp + 1
+	fmt.Println(s)
+}

string_modification2/main.go

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+package main
+
+import (
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+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")
+	bHdr := (*reflect.SliceHeader)(unsafe.Pointer(&b))
+	sHdr.Data = bHdr.Data
+	sHdr.Len = bHdr.Len
+	fmt.Println(s)
+
+	b[0] = 'x'
+	fmt.Println(s)
+}