add: ifaces testing

interfaces/18-testing/reader_test.go

@@ -0,0 +1,34 @@
+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))
+	_, err := io.CopyBuffer(&w, pngReader(r), make([]byte, 1))
+	return w.String(), err
+}