From cd080ddda2639017f980b7fab8b86104e1c89f1f Mon Sep 17 00:00:00 2001
From: V <vlad@dev.local>
Date: Sun, 31 Aug 2025 17:38:50 +0100
Subject: [PATCH] Sync

---
 tour_of_go/l5/3                               | 12 +++++
 tour_of_go/l5/empty-interface.go              | 18 +++++++
 tour_of_go/l5/error.go                        | 28 ++++++++++
 tour_of_go/l5/ex-reader/exercise-reader.go    | 16 ++++++
 .../l5/ex-reader/exercise-rot-reader.go       | 30 +++++++++++
 tour_of_go/l5/ex-reader/go.mod                |  5 ++
 tour_of_go/l5/ex-reader/go.sum                |  2 +
 tour_of_go/l5/exercise-errors.go              | 41 +++++++++++++++
 tour_of_go/l5/exercise-stringer.go            | 19 +++++++
 tour_of_go/l5/indirection.go                  | 30 +++++++++++
 tour_of_go/l5/interfaces-continued.go         | 51 +++++++++++++++++++
 tour_of_go/l5/interfaces.go                   | 40 +++++++++++++++
 tour_of_go/l5/methods-continued.go            | 20 ++++++++
 tour_of_go/l5/methods-pointers.go             | 25 +++++++++
 tour_of_go/l5/methods.go                      | 29 +++++++++++
 tour_of_go/l5/pointers-functions.go           | 25 +++++++++
 tour_of_go/l5/reader.go                       | 21 ++++++++
 tour_of_go/l5/stringer.go                     | 18 +++++++
 tour_of_go/l5/type-assertions.go              | 22 ++++++++
 tour_of_go/l5/type-switches.go                | 20 ++++++++
 20 files changed, 472 insertions(+)
 create mode 100644 tour_of_go/l5/3
 create mode 100644 tour_of_go/l5/empty-interface.go
 create mode 100644 tour_of_go/l5/error.go
 create mode 100644 tour_of_go/l5/ex-reader/exercise-reader.go
 create mode 100644 tour_of_go/l5/ex-reader/exercise-rot-reader.go
 create mode 100644 tour_of_go/l5/ex-reader/go.mod
 create mode 100644 tour_of_go/l5/ex-reader/go.sum
 create mode 100644 tour_of_go/l5/exercise-errors.go
 create mode 100644 tour_of_go/l5/exercise-stringer.go
 create mode 100644 tour_of_go/l5/indirection.go
 create mode 100644 tour_of_go/l5/interfaces-continued.go
 create mode 100644 tour_of_go/l5/interfaces.go
 create mode 100644 tour_of_go/l5/methods-continued.go
 create mode 100644 tour_of_go/l5/methods-pointers.go
 create mode 100644 tour_of_go/l5/methods.go
 create mode 100644 tour_of_go/l5/pointers-functions.go
 create mode 100644 tour_of_go/l5/reader.go
 create mode 100644 tour_of_go/l5/stringer.go
 create mode 100644 tour_of_go/l5/type-assertions.go
 create mode 100644 tour_of_go/l5/type-switches.go

diff --git a/tour_of_go/l5/3 b/tour_of_go/l5/3
new file mode 100644
index 0000000..3e392fb
--- /dev/null
+++ b/tour_of_go/l5/3
@@ -0,0 +1,12 @@
+package main
+
+import "fmt"
+
+func main() {
+	var i interface{} = "hello"
+
+	s := i.(string)
+	fmt.Println(s)
+
+	fmt.Printf("%v- %T\n", s, s)
+}
diff --git a/tour_of_go/l5/empty-interface.go b/tour_of_go/l5/empty-interface.go
new file mode 100644
index 0000000..349afd1
--- /dev/null
+++ b/tour_of_go/l5/empty-interface.go
@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+func describe(i interface{}) {
+	fmt.Printf("(%v, %T)\n", i, i)
+}
+
+func main() {
+	var i interface{}
+	describe(i)
+
+	i = 42
+	describe(i)
+
+	i = "hello"
+	describe(i)
+}
diff --git a/tour_of_go/l5/error.go b/tour_of_go/l5/error.go
new file mode 100644
index 0000000..2b3fde3
--- /dev/null
+++ b/tour_of_go/l5/error.go
@@ -0,0 +1,28 @@
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+type MyError struct {
+	When time.Time
+	What string
+}
+
+func (e *MyError) Error() string {
+	return fmt.Sprintf("at %v, %s", e.When, e.What)
+}
+
+func run() error {
+	return &MyError {
+		time.Now(),
+		"it didn't work",
+	}
+}
+
+func main() {
+	if err := run(); err != nil {
+		fmt.Println(err)
+	}
+}
diff --git a/tour_of_go/l5/ex-reader/exercise-reader.go b/tour_of_go/l5/ex-reader/exercise-reader.go
new file mode 100644
index 0000000..e269f44
--- /dev/null
+++ b/tour_of_go/l5/ex-reader/exercise-reader.go
@@ -0,0 +1,16 @@
+package main
+
+import "golang.org/x/tour/reader"
+
+type MyReader struct{}
+
+func (r MyReader) Read(b []byte) (int, error) {
+	for i := range(b) {
+		b[i] = 'A'
+	}
+	return len(b), nil
+}
+
+func main() {
+	reader.Validate(MyReader{})
+}
diff --git a/tour_of_go/l5/ex-reader/exercise-rot-reader.go b/tour_of_go/l5/ex-reader/exercise-rot-reader.go
new file mode 100644
index 0000000..37f6b5f
--- /dev/null
+++ b/tour_of_go/l5/ex-reader/exercise-rot-reader.go
@@ -0,0 +1,30 @@
+package main
+
+import (
+	"io"
+	"os"
+	"strings"
+)
+
+type rot13Reader struct {
+	r io.Reader
+}
+
+func (r rot13Reader) Read(b []byte) (int, error) {
+	n, err := r.r.Read(b)
+	for i := 0; i < n; i++ {
+		switch {
+		case 'A' <= b[i] && 'Z' >= b[i]:
+			b[i] = 'A' + (b[i]-'A'+13)%26
+		case 'a' <= b[i] && 'z' >= b[i]:
+			b[i] = 'a' + (b[i]-'a'+13)%26
+		}
+	}
+	return n, err
+}
+
+func main() {
+	s := strings.NewReader("Lbh penpxrq gur pbqr!")
+	r := rot13Reader{s}
+	io.Copy(os.Stdout, &r)
+}
diff --git a/tour_of_go/l5/ex-reader/go.mod b/tour_of_go/l5/ex-reader/go.mod
new file mode 100644
index 0000000..feaf2d2
--- /dev/null
+++ b/tour_of_go/l5/ex-reader/go.mod
@@ -0,0 +1,5 @@
+module exercies-reader.go
+
+go 1.24.5
+
+require golang.org/x/tour v0.1.0
diff --git a/tour_of_go/l5/ex-reader/go.sum b/tour_of_go/l5/ex-reader/go.sum
new file mode 100644
index 0000000..cde48ea
--- /dev/null
+++ b/tour_of_go/l5/ex-reader/go.sum
@@ -0,0 +1,2 @@
+golang.org/x/tour v0.1.0 h1:OWzbINRoGf1wwBhKdFDpYwM88NM0d1SL/Nj6PagS6YE=
+golang.org/x/tour v0.1.0/go.mod h1:DUZC6G8mR1AXgXy73r8qt/G5RsefKIlSj6jBMc8b9Wc=
diff --git a/tour_of_go/l5/exercise-errors.go b/tour_of_go/l5/exercise-errors.go
new file mode 100644
index 0000000..a1fcdd1
--- /dev/null
+++ b/tour_of_go/l5/exercise-errors.go
@@ -0,0 +1,41 @@
+package main
+
+import (
+	"fmt"
+)
+
+type ErrNegativeSqrt float64
+
+func (e ErrNegativeSqrt) Error() string {
+	return fmt.Sprintf("cannot Sqrt negative number: %v", float64(e))
+}
+
+func Sqrt(x float64) (float64, error) {
+	if x < 0 {
+		return 0, ErrNegativeSqrt(x)
+	}	
+	precision := 0.00001
+	old_z := 0.0
+	z := x
+	iter := 0
+	for (z - old_z) > precision {
+		iter += 1
+		old_z := z
+		z -= (z*z - x) / (2*z)
+		if old_z - z < 0 {
+			if ((old_z - z) * -1) < precision {
+				fmt.Println("Required precision has been reached after", iter, "iterations! Square root of", x , "is", z)
+				return z, nil
+			}
+		} else if (old_z - z) < precision {
+			fmt.Println("Required precision has been reached after", iter, "iterations! Square root of", x , "is", z)
+			return z, nil
+		}
+	}
+	return z, nil
+}
+
+func main() {
+	fmt.Println(Sqrt(2))
+	fmt.Println(Sqrt(-2))
+}
diff --git a/tour_of_go/l5/exercise-stringer.go b/tour_of_go/l5/exercise-stringer.go
new file mode 100644
index 0000000..69b43fb
--- /dev/null
+++ b/tour_of_go/l5/exercise-stringer.go
@@ -0,0 +1,19 @@
+package main
+
+import "fmt"
+
+type IPAddr [4]byte
+
+func (i IPAddr) String() string {
+	return fmt.Sprintf("%d.%d.%d.%d", i[0], i[1], i[2], i[3])
+}
+
+func main() {
+	hosts := map[string]IPAddr{
+		"loopback": {127, 0, 0, 1},
+		"googleDNS": {8, 8, 8, 8},
+	}
+	for name, ip := range hosts {
+		fmt.Printf("%v: %v\n", name, ip)
+	}
+}
diff --git a/tour_of_go/l5/indirection.go b/tour_of_go/l5/indirection.go
new file mode 100644
index 0000000..ba13bd1
--- /dev/null
+++ b/tour_of_go/l5/indirection.go
@@ -0,0 +1,30 @@
+package main
+
+import "fmt"
+
+type Vertex struct {
+	X, Y float64
+}
+
+func (v *Vertex) Scale(f float64) {
+	v.X = v.X * f
+	v.Y = v.Y * f
+}
+
+func ScaleFunc(v *Vertex, f float64) {
+	v.X = v.X * f
+	v.Y = v.Y * f
+}
+
+func main() {
+	v := Vertex{3, 4}
+	v.Scale(2)
+	ScaleFunc(&v, 10)
+
+	p := &Vertex{4, 3}
+	p.Scale(3)
+	ScaleFunc(p, 8)
+
+	fmt.Println(v, p)
+}
+
diff --git a/tour_of_go/l5/interfaces-continued.go b/tour_of_go/l5/interfaces-continued.go
new file mode 100644
index 0000000..5db9893
--- /dev/null
+++ b/tour_of_go/l5/interfaces-continued.go
@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type I interface {
+	M()
+}
+
+type T struct {
+	S string
+}
+
+func (t *T) M() {
+	if t == nil {
+		fmt.Println("<nil>")
+		return
+	}
+	fmt.Println(t.S)
+}
+
+type F float64
+
+func (f F) M() {
+	fmt.Println(f)
+}
+func describe(i I) {
+	fmt.Printf("(%v, %T)\n", i, i)
+}
+
+func main() {
+	var i I 
+	// These lines will cause a run-time error due to nil pointer dereferencing
+	// describe(i)
+	// i.M()
+
+	var t *T
+	i = t
+	describe(i)
+	i.M()
+
+	i = &T{"hello"}
+	describe(i)
+	i.M()
+
+	i = F(math.Pi)
+	describe(i)
+	i.M()
+}
diff --git a/tour_of_go/l5/interfaces.go b/tour_of_go/l5/interfaces.go
new file mode 100644
index 0000000..8be66f4
--- /dev/null
+++ b/tour_of_go/l5/interfaces.go
@@ -0,0 +1,40 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type MyFloat float64
+
+type Vertex struct {
+	X, Y float64
+}
+
+type Abser interface {
+	Abs() float64
+}
+
+func (f MyFloat) Abs() float64 {
+	if f < 0 {
+		return float64(-f)
+	}
+	return float64(f)
+}
+
+func (v *Vertex) Abs() float64 {
+	return math.Sqrt(v.X*v.X + v.Y*v.Y)
+}
+
+func main() {
+	var a Abser
+	f := MyFloat(-math.Sqrt2)
+	v := Vertex{3, 4}
+
+	a = f
+	a = &v
+
+	// a = v
+
+	fmt.Println(a.Abs())
+}
diff --git a/tour_of_go/l5/methods-continued.go b/tour_of_go/l5/methods-continued.go
new file mode 100644
index 0000000..397be47
--- /dev/null
+++ b/tour_of_go/l5/methods-continued.go
@@ -0,0 +1,20 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type MyFloat float64
+
+func (f MyFloat) Abs() float64 {
+	if f < 0 {
+		return float64(-f)
+	}
+	return float64(f)
+}
+
+func main() {
+	f := MyFloat(-math.Sqrt2)
+	fmt.Println(f.Abs())
+}
diff --git a/tour_of_go/l5/methods-pointers.go b/tour_of_go/l5/methods-pointers.go
new file mode 100644
index 0000000..7807acd
--- /dev/null
+++ b/tour_of_go/l5/methods-pointers.go
@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type Vertex struct {
+	X, Y float64
+}
+
+func (v Vertex) Abs() float64 {
+	return math.Sqrt(v.X*v.X + v.Y*v.Y)
+}
+
+func (v *Vertex) Scale(f float64) {
+	v.X = v.X * f
+	v.Y = v.Y * f
+}
+
+func main() {
+	v := Vertex{3, 4}
+	v.Scale(10)
+	fmt.Println(v.Abs())
+}
diff --git a/tour_of_go/l5/methods.go b/tour_of_go/l5/methods.go
new file mode 100644
index 0000000..b53bc81
--- /dev/null
+++ b/tour_of_go/l5/methods.go
@@ -0,0 +1,29 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type Vertex struct {
+	X, Y float64
+}
+
+// This is a method - a function that implements a receiver (the part between "func" and "Abs()")
+
+func (v Vertex) Abs() float64 {
+	return math.Sqrt(v.X*v.X + v.Y*v.Y)
+}
+
+// The method above has the same functionality as the function below
+
+func Abs(v Vertex) float64 {
+	return math.Sqrt(v.X*v.X + v.Y*v.Y)
+}
+
+func main() {
+	v := Vertex{3,4}
+	fmt.Println(v.Abs())
+	w := Vertex{3,4}
+	fmt.Println(Abs(w))
+}
diff --git a/tour_of_go/l5/pointers-functions.go b/tour_of_go/l5/pointers-functions.go
new file mode 100644
index 0000000..bc0e612
--- /dev/null
+++ b/tour_of_go/l5/pointers-functions.go
@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"math"
+)
+
+type Vertex struct {
+	X, Y float64
+}
+
+func Abs(v Vertex) float64 {
+	return math.Sqrt(v.X*v.X + v.Y*v.Y)
+}
+
+func Scale(v Vertex, f float64) {
+	v.X = v.X * f
+	v.Y = v.Y * f
+}
+
+func main() {
+	v := Vertex{3, 4}
+	Scale(v, 10)
+	fmt.Println(Abs(v))
+}
diff --git a/tour_of_go/l5/reader.go b/tour_of_go/l5/reader.go
new file mode 100644
index 0000000..f22a1f2
--- /dev/null
+++ b/tour_of_go/l5/reader.go
@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"strings"
+)
+
+func main() {
+	r := strings.NewReader("Hello, Reader!")
+
+	b := make([]byte, 8)
+	for {
+		n, err := r.Read(b)
+		fmt.Printf("n = %v err = %v b = %v\n", n, err, b)
+		fmt.Printf("b[:n] = %q\n", b[:n])
+		if err == io.EOF {
+			break
+		}
+	}
+}
diff --git a/tour_of_go/l5/stringer.go b/tour_of_go/l5/stringer.go
new file mode 100644
index 0000000..357dcac
--- /dev/null
+++ b/tour_of_go/l5/stringer.go
@@ -0,0 +1,18 @@
+package main
+
+import "fmt"
+
+type Person struct {
+	Name string
+	Age	 int
+}
+
+func (p Person) String() string {
+	return fmt.Sprintf("%v (%v years)\n", p.Name, p.Age)
+}
+
+func main() {
+	a := Person{"Arthur The King", 51}
+	b := Person{"Darth Vader", 49}
+	fmt.Println(a, b)
+}
diff --git a/tour_of_go/l5/type-assertions.go b/tour_of_go/l5/type-assertions.go
new file mode 100644
index 0000000..b10a261
--- /dev/null
+++ b/tour_of_go/l5/type-assertions.go
@@ -0,0 +1,22 @@
+package main
+
+import "fmt"
+
+func main() {
+	var i interface{} = "hello"
+
+	s := i.(string)
+	fmt.Println(s)
+
+	s, ok := i.(string)
+	fmt.Println(s, ok)
+
+	f, ok := i.(float64)
+	fmt.Println(f, ok)
+
+	// This will cause a panic
+	// f = i.(float64)
+	// fmt.Println(f)
+
+	fmt.Printf("Type info: %v- %T\n", s, s)
+}
diff --git a/tour_of_go/l5/type-switches.go b/tour_of_go/l5/type-switches.go
new file mode 100644
index 0000000..a92b76f
--- /dev/null
+++ b/tour_of_go/l5/type-switches.go
@@ -0,0 +1,20 @@
+package main
+
+import "fmt"
+
+func do(i interface{}) {
+	switch v := i.(type) {
+	case int:
+		fmt.Printf("Twice %v is %v\n", v, v*2)
+	case string:
+		fmt.Printf("%q is %v bytes long\n", v, len(v))
+	default:
+		fmt.Printf("I don't know about this type %T!\n", v)
+	}
+}
+
+func main() {
+	do(21)
+	do("Golang")
+	do(true)
+}