#!/usr/bin/perl -w

use strict;
use bignum;
use Time::HiRes qw (time);

if (@ARGV != 2) {
  print STDERR "usage: $0 device {100|1000}\n";
  exit 1;
}

my $devname = $ARGV[0];
my $speed = $ARGV[1] * 1e6;

$| = 1;
my ($lt, $t, $lrecv, $lxmit);
while (1) {
  open FH, "< /proc/net/dev" or die "can't open /proc/net/dev";
  $t = time;
  my $info = undef;
  while (<FH>) {
    chomp;
    if (s/^\s+$devname\s*:\s*//) {
      $info = $_;
      last;
    }
  }
  close FH;
  die "can't locate info for $devname" unless defined $info;

  my @info = split ' ', $info;
  my $recv = $info[0] + 0;
  my $xmit = $info[8] + 0;

  if (defined $lt) {
    my $dr = $recv - $lrecv;
    my $dx = $xmit - $lxmit;
    my $dt = $t - $lt;
    my $ur = 100 * (8 * $dr) / ($dt * $speed);
    my $xr = 100 * (8 * $dx) / ($dt * $speed);
    printf "r %5.1f x %5.1f (%8d %8d)\n", $ur, $xr, $dr, $dx;
  }

  $lt = $t;
  $lrecv = $recv;
  $lxmit = $xmit;
  sleep 1;
}