add utility to inspect gzip'ed payload

gziputil/util.go

@@ -0,0 +1,59 @@
+package gziputil
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+type Gzip struct {
+	magic []byte
+	cm    byte
+	flg   byte
+	mtime uint32
+	xfl   byte
+	os    byte
+	xlen  uint16
+	xflds []byte
+	crc32 uint32
+	isize uint32
+
+	rest    []byte
+	trailer []byte
+}
+
+func explainGzip(bs []byte) Gzip {
+	g := Gzip{
+		magic: bs[0:2],
+		cm:    bs[2],
+		flg:   bs[3],
+		mtime: binary.BigEndian.Uint32(bs[4:8]),
+		xfl:   bs[8],
+		os:    bs[9],
+	}
+
+	offset := 10
+	if g.xfl&2 == 1 {
+		g.xlen = binary.BigEndian.Uint16(bs[offset : offset+2])
+		offset += 2
+
+		g.xflds = bs[offset : offset+int(g.xlen)]
+		offset += int(g.xlen)
+	}
+
+	g.crc32 = binary.BigEndian.Uint32(bs[offset : offset+4])
+	offset += 4
+
+	g.isize = binary.BigEndian.Uint32(bs[offset : offset+4])
+	offset += 4
+
+	g.rest = bs[offset : len(bs)-8]
+	offset = len(bs) - 8
+
+	g.trailer = bs[offset:]
+
+	return g
+}
+
+func (g Gzip) String() string {
+	return fmt.Sprintf("magic=%04x cm=%02x flg=%02x mtime=%d xfl=%02x os=%02x xlen=%d crc32=%d isize=%d rest=%x trailer=%08x", g.magic, g.cm, g.flg, g.mtime, g.xfl, g.os, g.xlen, g.crc32, g.isize, g.rest, g.trailer)
+}