add: deferred funcs and png parser

27-functions-advanced/07-deferred-funcs/main.go

@@ -0,0 +1,68 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+func main() {
+	// single()
+	// stacked()
+	findTheMeaning()
+}
+
+func findTheMeaningNoDefer() {
+	start := time.Now()
+	fmt.Printf("%s starts...\n", "findTheMeaning")
+
+	// do some heavy calculation...
+	time.Sleep(time.Second * 2)
+
+	fmt.Printf("%s took %v\n", "findTheMeaning", time.Since(start))
+}
+
+func findTheMeaning() {
+	defer measure("findTheMeaning")()
+
+	// do some heavy calculation
+	time.Sleep(time.Second * 2)
+}
+
+func measure(name string) func() {
+	start := time.Now()
+	fmt.Printf("%s starts...\n", name)
+
+	return func() {
+		fmt.Printf("%s took %v\n", name, time.Since(start))
+	}
+}
+
+func stacked() {
+	for count := 1; count <= 5; count++ {
+		defer fmt.Println(count)
+	}
+
+	fmt.Println("the stacked func returns")
+}
+
+func single() {
+	var count int
+
+	// defer func() {
+	// 	fmt.Println(count)
+	// }()
+	defer fmt.Println(count)
+
+	count++
+	// fmt.Println(count)
+
+	// the defer runs here
+	// fmt.Println(count)
+}

27-functions-advanced/08-panic-recover-todo/TODO.md

@@ -1 +0,0 @@
-# TODO

27-functions-advanced/08-png-detector/main.go

@@ -0,0 +1,165 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+)
+
+/*
+	// Most other languages do this:
+	try {
+		// open a file
+		// throws an exception
+	} catch (ExceptionType name) {
+		// handle the error
+	} finally {
+		// close the file
+	}
+
+	// Go way:
+	file, err := // open the file
+	if err != nil {
+		// handle the error
+	}
+	// close the file
+*/
+
+func main() {
+	files := []string{
+		"pngs/cups-jpg.png",
+		"pngs/forest-jpg.png",
+		"pngs/golden.png",
+		"pngs/work.png",
+		"pngs/shakespeare-text.png",
+		"pngs/empty.png",
+	}
+
+	pngs := detectPNG(files)
+
+	fmt.Printf("Correct PNG Files:\n")
+	for _, png := range pngs {
+		fmt.Printf(" + %s\n", png)
+	}
+}
+
+func detectPNG(filenames []string) (pngs []string) {
+	const pngHeader = "\x89PNG\r\n\x1a\n"
+
+	buf := make([]byte, len(pngHeader))
+
+	for _, filename := range filenames {
+		if read(filename, buf) != nil {
+			continue
+		}
+
+		if bytes.Equal([]byte(pngHeader), buf) {
+			pngs = append(pngs, filename)
+		}
+	}
+	return
+}
+
+// read reads len(buf) bytes to buf from a file
+func read(filename string, buf []byte) error {
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	fi, err := file.Stat()
+	if err != nil {
+		return err
+	}
+
+	if fi.Size() <= int64(len(buf)) {
+		return fmt.Errorf("file size < len(buf)")
+	}
+
+	_, err = io.ReadFull(file, buf)
+	return err
+}
+
+func detectPNGUnsafeAndVerbose(filenames []string) (pngs []string) {
+	const pngHeader = "\x89PNG\r\n\x1a\n"
+
+	buf := make([]byte, len(pngHeader))
+
+	for _, filename := range filenames {
+		file, err := os.Open(filename)
+		if err != nil {
+			continue
+		}
+
+		fi, err := file.Stat()
+		if err != nil {
+			file.Close()
+			continue
+		}
+
+		if fi.Size() <= int64(len(pngHeader)) {
+			file.Close()
+			continue
+		}
+
+		_, err = io.ReadFull(file, buf)
+		file.Close()
+		if err != nil {
+			continue
+		}
+
+		if bytes.Equal([]byte(pngHeader), buf) {
+			pngs = append(pngs, filename)
+		}
+	}
+	return
+}
+
+// defers don't run when the loop block ends
+func detectPNGWrong(filenames []string) (pngs []string) {
+	const pngHeader = "\x89PNG\r\n\x1a\n"
+
+	buf := make([]byte, len(pngHeader))
+
+	for _, filename := range filenames {
+		fmt.Printf("processing: %s\n", filename)
+
+		file, err := os.Open(filename)
+		if err != nil {
+			continue
+		}
+
+		defer func() {
+			fmt.Printf("closes    : %s\n", filename)
+			file.Close()
+		}()
+
+		fi, err := file.Stat()
+		if err != nil {
+			continue
+		}
+
+		if fi.Size() <= int64(len(pngHeader)) {
+			continue
+		}
+
+		_, err = io.ReadFull(file, buf)
+		if err != nil {
+			continue
+		}
+
+		if bytes.Equal([]byte(pngHeader), buf) {
+			pngs = append(pngs, filename)
+		}
+	}
+	return
+}

27-functions-advanced/08-png-detector/pngs/shakespeare-text.png

@@ -0,0 +1,12 @@
+come night come romeo come thou day in night
+for thou wilt lie upon the wings of night
+whiter than new snow on a raven's back
+come gentle night come loving black-browed night
+give me my romeo and when he shall die
+take him and cut him out in little stars
+and he will make the face of heaven so fine
+that all the world will be in love with night
+and pay no worship to the garish sun
+oh i have bought the mansion of love
+but not possessed it and though i am sold
+not yet enjoyed