refactor: ifaces write an io reader

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

@@ -16,6 +16,7 @@ import (
 )
 
 func main() {
+	// resp, err := http.Get("https://inancgumus.github.com/x/rosie.jpg")
 	resp, err := http.Get("https://inancgumus.github.com/x/rosie.unknown")
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
@@ -30,8 +31,6 @@ func main() {
 	}
 	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)

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

@@ -9,41 +9,48 @@
 package main
 
 import (
-	"errors"
+	"bytes"
+	"fmt"
 	"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.
+// reader reads from `r` if the stream starts with a given 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
+	signature []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.
+	l := len(sr.signature)
 	if l == 0 {
+		// simply return if the signature has already been detected.
 		return
 	}
-	// limit the compared bytes at most to the buffer length.
-	if lb := len(b); l > lb {
+	// 1) buffer   : **** -> b[:3]            -> ***
+	//    signature: ***  -> sr.signature[:3] -> ***
+	// 2) buffer   : **   -> b[:2]            -> **
+	//    signature: **** -> sr.signature[:2] -> **
+	if lb := len(b); lb < l {
 		l = lb
 	}
-	if string(b[:l]) != string(sr.sign[:l]) {
-		err = errors.New("not png")
+	if got, want := b[:l], sr.signature[:l]; !bytes.Equal(got, want) {
+		err = fmt.Errorf("signature doesn't match, got: % x, want: % x", got, want)
+	}
+	// Assuming the `len(b)` is 4.
+	// 1st Read(): pr.signature[0:4] -> first part
+	// 2nd Read(): pr.signature[0:4] -> second part
+	sr.signature = sr.signature[l:]
+	return n, err
+}
+
+// create a reader for detecting only the png signatures.
+func pngReader(r io.Reader) io.Reader {
+	return &signatureReader{
+		r:         r,
+		signature: []byte("\x89PNG\r\n\x1a\n"),
 	}
-	// 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

@@ -1,33 +0,0 @@
-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
-}