add: ifaces testing

interfaces/18-testing/reader.go

@@ -0,0 +1,48 @@
+// 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"
+)
+
+// 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)
+	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.signature)
+	if l == 0 {
+		return
+	}
+	if lb := len(b); lb < l {
+		l = lb
+	}
+	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)
+	}
+	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"),
+	}
+}