github.com/anacrolix/torrent@v1.61.0/bep40.go (about)

     1  package torrent
     2  
     3  import (
     4  	"bytes"
     5  	"encoding/binary"
     6  	"errors"
     7  	"fmt"
     8  	"hash/crc32"
     9  	"net"
    10  )
    11  
    12  var table = crc32.MakeTable(crc32.Castagnoli)
    13  
    14  type peerPriority = uint32
    15  
    16  func sameSubnet(ones, bits int, a, b net.IP) bool {
    17  	mask := net.CIDRMask(ones, bits)
    18  	return a.Mask(mask).Equal(b.Mask(mask))
    19  }
    20  
    21  func ipv4Mask(a, b net.IP) net.IPMask {
    22  	if !sameSubnet(16, 32, a, b) {
    23  		return net.IPv4Mask(0xff, 0xff, 0x55, 0x55)
    24  	}
    25  	if !sameSubnet(24, 32, a, b) {
    26  		return net.IPv4Mask(0xff, 0xff, 0xff, 0x55)
    27  	}
    28  	return net.IPv4Mask(0xff, 0xff, 0xff, 0xff)
    29  }
    30  
    31  func mask(prefix, bytes int) net.IPMask {
    32  	ret := make(net.IPMask, bytes)
    33  	for i := range ret {
    34  		ret[i] = 0x55
    35  	}
    36  	for i := 0; i < prefix; i++ {
    37  		ret[i] = 0xff
    38  	}
    39  	return ret
    40  }
    41  
    42  func ipv6Mask(a, b net.IP) net.IPMask {
    43  	for i := 6; i <= 16; i++ {
    44  		if !sameSubnet(i*8, 128, a, b) {
    45  			return mask(i, 16)
    46  		}
    47  	}
    48  	panic(fmt.Sprintf("%s %s", a, b))
    49  }
    50  
    51  func bep40PriorityBytes(a, b IpPort) ([]byte, error) {
    52  	if a.IP.Equal(b.IP) {
    53  		var ret [4]byte
    54  		binary.BigEndian.PutUint16(ret[0:2], a.Port)
    55  		binary.BigEndian.PutUint16(ret[2:4], b.Port)
    56  		return ret[:], nil
    57  	}
    58  	if a4, b4 := a.IP.To4(), b.IP.To4(); a4 != nil && b4 != nil {
    59  		m := ipv4Mask(a.IP, b.IP)
    60  		return append(a4.Mask(m), b4.Mask(m)...), nil
    61  	}
    62  	if a6, b6 := a.IP.To16(), b.IP.To16(); a6 != nil && b6 != nil {
    63  		m := ipv6Mask(a.IP, b.IP)
    64  		return append(a6.Mask(m), b6.Mask(m)...), nil
    65  	}
    66  	return nil, errors.New("incomparable IPs")
    67  }
    68  
    69  func bep40Priority(a, b IpPort) (peerPriority, error) {
    70  	bs, err := bep40PriorityBytes(a, b)
    71  	if err != nil {
    72  		return 0, err
    73  	}
    74  	i := len(bs) / 2
    75  	_a, _b := bs[:i], bs[i:]
    76  	if bytes.Compare(_a, _b) > 0 {
    77  		bs = append(_b, _a...)
    78  	}
    79  	return crc32.Checksum(bs, table), nil
    80  }
    81  
    82  func bep40PriorityIgnoreError(a, b IpPort) peerPriority {
    83  	prio, _ := bep40Priority(a, b)
    84  	return prio
    85  }