package main

import (
	"bytes"
	"compress/gzip"
	"encoding/binary"
	"errors"
	"io"
)

// Reject truncated, oversized or compressed-bomb frames before slicing payloads.
func decodeVolc(data []byte) (typ, flags byte, code uint32, payload []byte, err error) {
	invalid := errors.New("invalid ASR frame")
	if len(data) < 8 || data[0]>>4 != 1 {
		return 0, 0, 0, nil, invalid
	}
	typ, flags = data[1]>>4, data[1]&15
	off := int(data[0]&15) * 4
	if off < 4 || off > len(data) || (typ != msgFullServer && typ != msgError) {
		return 0, 0, 0, nil, invalid
	}
	if flags&1 != 0 {
		off += 4
	} // Bit 1 means last packet; it does not imply a sequence field.
	if off > len(data) {
		return 0, 0, 0, nil, invalid
	}
	if typ == msgError {
		if off+4 > len(data) {
			return 0, 0, 0, nil, invalid
		}
		code = binary.BigEndian.Uint32(data[off : off+4])
		off += 4
	}
	if off+4 > len(data) {
		return 0, 0, 0, nil, invalid
	}
	size := uint64(binary.BigEndian.Uint32(data[off : off+4]))
	off += 4
	if size > 1<<20 || size != uint64(len(data)-off) {
		return 0, 0, 0, nil, invalid
	}
	payload = data[off:]
	switch data[2] & 15 {
	case compNone:
	case compGzip:
		reader, e := gzip.NewReader(bytes.NewReader(payload))
		if e != nil {
			return 0, 0, 0, nil, invalid
		}
		payload, e = io.ReadAll(io.LimitReader(reader, (1<<20)+1))
		closeErr := reader.Close()
		if e != nil || closeErr != nil || len(payload) > 1<<20 {
			return 0, 0, 0, nil, invalid
		}
	default:
		return 0, 0, 0, nil, invalid
	}
	return typ, flags, code, payload, nil
}