refactor: ifaces write an io reader

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
+	r         io.Reader // reads from the response.Body (or from any reader)
+	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
 }