package main

import (
	"bytes"
	"encoding/binary"
	"encoding/hex"
	"net"
	"time"
)

type TrackerMeta struct {
	Seeders   int
	Leechers  int
	Completed int
}

func udpScrape(host string, infoHashes [][]byte) map[string]TrackerMeta {
	out := map[string]TrackerMeta{}
	if len(infoHashes) == 0 {
		return out
	}
	addr, err := net.ResolveUDPAddr("udp", host)
	if err != nil {
		return out
	}
	conn, err := net.DialUDP("udp", nil, addr)
	if err != nil {
		return out
	}
	defer conn.Close()
	_ = conn.SetDeadline(time.Now().Add(6 * time.Second))
	transID := uint32(time.Now().UnixNano() & 0x7fffffff)
	var req bytes.Buffer
	_ = binary.Write(&req, binary.BigEndian, uint64(0x41727101980))
	_ = binary.Write(&req, binary.BigEndian, uint32(0))
	_ = binary.Write(&req, binary.BigEndian, transID)
	if _, err := conn.Write(req.Bytes()); err != nil {
		return out
	}
	resp := make([]byte, 16)
	n, err := conn.Read(resp)
	if err != nil || n < 16 || binary.BigEndian.Uint32(resp[4:8]) != transID {
		return out
	}
	connID := resp[8:16]
	if len(infoHashes) > 70 {
		infoHashes = infoHashes[:70]
	}
	var sc bytes.Buffer
	sc.Write(connID)
	_ = binary.Write(&sc, binary.BigEndian, uint32(2))
	_ = binary.Write(&sc, binary.BigEndian, transID+1)
	for _, h := range infoHashes {
		if len(h) == 20 {
			sc.Write(h)
		}
	}
	if _, err := conn.Write(sc.Bytes()); err != nil {
		return out
	}
	buf := make([]byte, 8+12*74)
	_ = conn.SetDeadline(time.Now().Add(6 * time.Second))
	n, err = conn.Read(buf)
	if err != nil || n < 8 {
		return out
	}
	off := 8
	for i, h := range infoHashes {
		if off+12 > n {
			break
		}
		_ = i
		seeders := int(binary.BigEndian.Uint32(buf[off : off+4]))
		done := int(binary.BigEndian.Uint32(buf[off+4 : off+8]))
		leech := int(binary.BigEndian.Uint32(buf[off+8 : off+12]))
		out[hex.EncodeToString(h)] = TrackerMeta{Seeders: seeders, Leechers: leech, Completed: done}
		off += 12
	}
	return out
}