move decompressor to internal

internal/decompressor.go

@@ -0,0 +1,46 @@
+// Copyright 2014 The Cayley Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package internal
+
+import (
+	"bufio"
+	"bytes"
+	"compress/bzip2"
+	"compress/gzip"
+	"io"
+)
+
+const (
+	gzipMagic  = "\x1f\x8b"
+	b2zipMagic = "BZh"
+)
+
+// Decompressor detects the file type of an io.Reader between
+// bzip, gzip, or raw quad file.
+func Decompressor(r io.Reader) (io.Reader, error) {
+	br := bufio.NewReader(r)
+	buf, err := br.Peek(3)
+	if err != nil {
+		return nil, err
+	}
+	switch {
+	case bytes.Compare(buf[:2], []byte(gzipMagic)) == 0:
+		return gzip.NewReader(br)
+	case bytes.Compare(buf[:3], []byte(b2zipMagic)) == 0:
+		return bzip2.NewReader(br), nil
+	default:
+		return br, nil
+	}
+}