#!/usr/bin/env bash

set -o errexit
set -o pipefail
set -o nounset

readonly SCRIPT_NAME="${0##*/}"

check_deps() {
    command -v "curl" >/dev/null 2>&1 || die "curl: command not found"
}

usage() {
    cat <<EOF
Usage: ${SCRIPT_NAME} [--family auto|4|6] [-h|--help]

Query an external service using curl and print:
  1) public IP address
  2) reverse DNS (PTR record)
  3) curl version

Options:
  --family auto|4|6   select IP protocol version (default: auto)
  -h, --help          show help and exit
EOF
}

shell_quote() {
    local -r string="$1"
    printf "'%s'" "${string//'/'\\''}"
}

die() {
    local -r message="$1"
    printf "%s: %b\n" "${SCRIPT_NAME}" "${message}" >&2
    exit 1
}

invalid_option() {
    local -r option="$1"
    die "invalid option $(shell_quote "${option}")\nTry '${SCRIPT_NAME} --help' for usage."
}

parse_args() {
    family="auto"
    while (( $# > 0 )); do
        case "$1" in
            -h|--help) usage; exit 0 ;;
            --family)
                ( (( $# < 2 )) || [[ -z "$2" ]] ) && die "--family requires an argument (auto, 4 or 6)"
                family="$2"
                shift 2
                ;;
            --family=*)
                family="${1#*=}"
                [[ -z "${family}" ]] && die "--family requires an argument (auto, 4 or 6)"
                shift
                ;;
            *) invalid_option "$1" ;;
        esac
    done
    validate_args
}

validate_args() {
    case "${family}" in
        auto|4|6) ;;
        *) die "invalid value for family: '${family}' (expected auto, 4 or 6)"
    esac
}

show_ip() {
    local -a curl_args=()
    [[ ${family} == "4" ]] && curl_args=( "-4" )
    [[ ${family} == "6" ]] && curl_args=( "-6" )
    curl "${curl_args[@]}" "zx2c4.com/ip"
}

main() {
    check_deps
    parse_args "$@"
    show_ip
}

main "$@"