add: custom io reader ifaces

interfaces/17-write-an-io-reader/.gitignore

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+rosie.unknown
+rosie.png
+_other.go
+toc

interfaces/17-write-an-io-reader/main.go

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+// 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 (
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+)
+
+func main() {
+	resp, err := http.Get("https://inancgumus.github.com/x/rosie.unknown")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return
+	}
+	defer resp.Body.Close()
+
+	file, err := os.Create("rosie.png")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return
+	}
+	defer file.Close()
+
+	// io.Copy reads from pngReader first
+	// pngReader reads from the resp.Body
+	n, err := io.Copy(file, pngReader(resp.Body))
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		return
+	}
+	fmt.Printf("%d bytes transferred.\n", n)
+}

interfaces/17-write-an-io-reader/reader.go

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+// 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 (
+	"errors"
+	"io"
+)
+
+// create a reader for detecting the png signatures.
+func pngReader(r io.Reader) io.Reader {
+	return &signatureReader{r: r, sign: []byte("\x89PNG\r\n\x1a\n")}
+}
+
+// reader reads from `r` if the stream starts with PNG signature.
+// otherwise it stops and returns with an error.
+type signatureReader struct {
+	r    io.Reader // reads from the response.Body (or from any reader)
+	sign []byte    // stream should start with this initial signature
+}
+
+// Read implements the io.Reader interface.
+func (sr *signatureReader) Read(b []byte) (n int, err error) {
+	n, err = sr.r.Read(b)
+
+	l := len(sr.sign)
+
+	// simply return if the signature has already been detected.
+	if l == 0 {
+		return
+	}
+	// limit the compared bytes at most to the buffer length.
+	if lb := len(b); l > lb {
+		l = lb
+	}
+	if string(b[:l]) != string(sr.sign[:l]) {
+		err = errors.New("not png")
+	}
+	// the next read will compare the rest of the signature after `l`
+	// `sr.sign` will be zero after one or several reads later.
+	sr.sign = sr.sign[l:]
+	return
+}

interfaces/17-write-an-io-reader/reader_test.go

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+package main
+
+import (
+	"io"
+	"strings"
+	"testing"
+)
+
+func Test_PNG_Reader_Correct_Signature(t *testing.T) {
+	const input = "\x89PNG\r\n\x1a\nHELLO"
+
+	out, err := readSignature(input)
+	if err != nil {
+		t.Fatalf("got: %q; want: <nil> err", err)
+	}
+	if out != input {
+		t.Fatalf("invalid output, got: '%x'; want: '%x'", out, input)
+	}
+}
+
+func Test_PNG_Reader_Incorrect_Signature(t *testing.T) {
+	_, err := readSignature("\x89INCORRECT")
+	if err == nil {
+		t.Fatal("got: nil; want: !nil err")
+	}
+}
+
+func readSignature(in string) (string, error) {
+	r := strings.NewReader(in)
+	w := strings.Builder{}
+	_, err := io.Copy(&w, pngReader(r))
+	return w.String(), err
+}