check header again

main.go

@@ -68,12 +68,8 @@ func main() {
 		bail(1, "input error: %v", err)
 	}
 
-	buf := make([]byte, 8)
+	p := newParser(r)
-	if _, err := r.Read(buf); err != nil {
+	if _, err := p.checkHeader(); err != nil {
-		bail(1, "error reading header: %v", err)
+		bail(1, "parse error: %v", err)
 	}
-	if string(buf) != replay_header {
-		bail(1, "unexpected replay header: %s", string(buf))
-	}
-	fmt.Println(string(buf))
 }

parser.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"fmt"
+	"io"
+)
+
+type parser struct {
+	// the source of replay bytes. Must NOT be compressed.
+	source io.Reader
+}
+
+func newParser(r io.Reader) *parser {
+	return &parser{source: r}
+}
+
+// checks whether we have an acceptable header at the current reader position.
+func (p *parser) checkHeader() (bool, error) {
+	buf := make([]byte, 8)
+	if _, err := p.source.Read(buf); err != nil {
+		return false, fmt.Errorf("unable to read header bytes: %v", err)
+	}
+	return string(buf) == replay_header, nil
+}