update: directory structure

advfuncs/08-png-detector-with-panic/main.go

@@ -0,0 +1,111 @@
+// 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"
+)
+
+/*
+	// ~~~ THE CLASSIC WAY ~~~
+	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
+
+	// really really exceptional problem occurs
+	// mostly due to a programmer
+
+	// panic("PANIC PANIC")
+*/
+
+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",
+	}
+
+	list("png", files)
+
+	// fmt.Println("catch me if you can!")
+}
+
+func list(format string, files []string) {
+	valids := detect(format, files)
+
+	fmt.Printf("Correct Files:\n")
+	for _, valid := range valids {
+		fmt.Printf(" + %s\n", valid)
+	}
+}
+
+func detect(format string, filenames []string) (valids []string) {
+	header := headerOf(format)
+
+	buf := make([]byte, len(header))
+
+	for _, filename := range filenames {
+		if read(filename, buf) != nil {
+			continue
+		}
+
+		if bytes.Equal([]byte(header), buf) {
+			valids = append(valids, filename)
+		}
+	}
+	return
+}
+
+func headerOf(format string) string {
+	switch format {
+	case "png":
+		return "\x89PNG\r\n\x1a\n"
+	case "jpg":
+		return "\xff\xd8\xff"
+	}
+	panic("unknown format: " + format)
+}
+
+// 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
+}

advfuncs/08-png-detector-with-panic/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