#!/usr/bin/env bash

# Title               : mac_to_ip6_ll_addr
# Last modified date  : 13.03.2023
# Author              : Martin Tonusoo
# Description         : Script finds the IPv6 link local address
#                       from the MAC address.
# Options             : MAC address.
# Notes               : Script expects the MAC address as a
#                       command line argument in the format
#                       returned by "ip link" command, e.g
#                       52:54:00:db:99:99.


mac_regex="^([0-9a-f]{2}:){5}([0-9a-f]{2})$"
if [[ ! "$1" =~ $mac_regex ]]; then
	printf "%s\n%s\n" "Usage:   ${0##*/} <MAC_addr>" \
		"Example: ${0##*/} 52:54:00:db:99:99" \
		>&2
	exit 1
fi

IFS=: read -r o1 o2 o3 o4 o5 o6 <<< "$1"

# Convert the four least significant bits of
# the first octet of the MAC address from hex
# to decimal.
o1_dec=$(( 16#"${o1:1:1}" ))

# Loop through the four least significant bits of
# the first octet of the MAC address by shifting
# one bit to right and checking if the bit is set
# or not on each cycle starting from the least
# significant bit and moving towards the most
# significant bit. This will convert the second
# hex character of the MAC address reading from
# left to binary.
for (( n="$o1_dec"; n>0; n >>= 1 )); do
	o1_bits="$(( n & 1 ))$o1_bits"
done

# Prepend leading zeros if needed.
o1_bits=$(printf "%04d" "$o1_bits")

# Flip the second-least-significant bit of the
# first octet of the MAC address and convert
# four bits back to hex digit.
flipped_bit=$(tr 01 10 <<< "${o1_bits:2:1}")
o1_bits="${o1_bits:0:2}$flipped_bit${o1_bits:3:1}"
o1_bits=$(printf "%x" $((2#"$o1_bits")))

# Finally, rebuild the first octet of the MAC address.
o1="${o1:0:1}$o1_bits"


# Build the hextets for the IPv6 link local address.
if [[ $o1 == 00 ]] && [[ $o2 == 00 ]]; then
	h5=""
else
	# Strip possible leading zeros.
	h5="$(printf "%x:" "0x$o1$o2")"
fi

if [[ $o3 == 00 ]]; then
	h6="ff:"
else
	h6="$(printf "%xff:" "0x$o3")"
fi

h7="fe$o4:"

h8="$(printf "%x" "0x$o5$o6")"

printf "fe80::%s%s%s%s\n" "$h5" "$h6" "$h7" "$h8"