move: interfaces error handling and advanced funcs

error-handling/02-png-detector/main.go

@@ -0,0 +1,146 @@
+// 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"
+)
+
+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",
+	}
+
+	valids := detect(files)
+
+	fmt.Printf("Correct Files:\n")
+	for _, valid := range valids {
+		fmt.Printf(" + %s\n", valid)
+	}
+}
+
+func detect(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) (valids []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) {
+			valids = append(valids, 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
+}