add: custom io reader ifaces

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

@@ -0,0 +1,49 @@
+// 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
+}