#! /usr/bin/env bash

# Generate a locally administered MAC address (LAA) number <num> based on the
# specified universally administered address <mac> (UAA, for example, an
# address corresponding to the host's physical Ethernet interface).
#
# Specifically, the resulting address is formed by combining the
# LAA-conforming first octet with the subsequent five octets from <mac>:
#
# x[26ae]:xx:xx:xx:xx:xx
#
# The first octet is derived from <num> as follows:
#
#  0-15 : 02-f2
# 16-31 : 06-f6
# 32-47 : 0a-fa
# 48-63 : 0e-fe
#
# For example, <num> can correspond to the interface number, such as tap0, for
# which the resulting MAC address will be used.
#
usage="usage: $0 <mac> <num>"

owd="$(pwd)"
trap "{ cd '$owd'; exit 1; }" ERR
set -o errtrace # Trap in functions.

function info () { echo "$*" 1>&2; }
function error () { info "$*"; exit 1; }

if [ -z "$1" ]; then
  error "$usage"
fi

o='[0-9a-fA-F]'
mac="$(sed -nr -e "s/^$o$o:($o$o:$o$o:$o$o:$o$o:$o$o)$/\1/p" <<<"$1")"

if [ -z "$mac" ]; then
  error "invalid MAC address '$1'"
fi

if [ -z "$2" ]; then
  error "$usage"
fi

num="$2"

if (( num < 0 || num > 63 )); then
  error "number '$num' is out of 0-63 range"
fi

if   (( num < 16 )); then
  printf "%x2:%s\n" $(( num      )) "$mac"
elif (( num < 32 )); then
  printf "%x6:%s\n" $(( num - 16 )) "$mac"
elif (( num < 48 )); then
  printf "%xa:%s\n" $(( num - 32 )) "$mac"
else
  printf "%xe:%s\n" $(( num - 48 )) "$mac"
fi