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)
}