add: png detector example

interfaces/15-png-detector/main.go

@@ -0,0 +1,53 @@
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+// For more tutorials  : https://learngoprogramming.com
+// In-person training  : https://www.linkedin.com/in/inancgumus/
+// Follow me on twitter: https://twitter.com/inancgumus
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+)
+
+// you can download the rosie.unknown image in the link:
+// https://inancgumus.github.com/x/rosie.unknown
+
+// then feed the file to the standard input of this program:
+// go run . < rosie.unknown
+
+func main() {
+	const pngSign = "\x89PNG\r\n\x1a\n"
+	const pngSignLen = 8
+
+	// create an in-memory buffer (bytes.Buffer is an io.Reader and an io.Writer).
+	memory := bytes.Buffer{}
+
+	// get it from the web server
+	resp, err := http.Get("https://inancgumus.github.com/x/rosie.unknown")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer resp.Body.Close()
+
+	// stream from the standard input to the in-memory buffer in 32KB data chunks.
+	// os.Stdin.Read(...) -> memory.Write(...)
+	if _, err := io.Copy(&memory, resp.Body); err != nil {
+		log.Fatal(err)
+	}
+
+	// get the accumulated bytes from the in-memory buffer.
+	buf := memory.Bytes()
+
+	// print the first eight bytes.
+	fmt.Printf("% x\n", buf[:pngSignLen])
+
+	// compare it with the png signature.
+	fmt.Printf("% x\n", pngSign)
+}