#!/usr/bin/env bash
netstat_parse_linux() {
    sed -r -n 's/tcp.*\.[0-9]+:([0-9]+).*/\1/p' <<< "$1" | sort -u
}

netstat_parse_macos() {
    sed -r -n 's/.*([0-9]+\.[0-9]+\.[0-9]+\.[0-9]+\.([0-9]+)).*/\2/p;s/.*\*\.([0-9]+).*/\1/p' <<< "$1" | sort -u
}

netstat_parse() {
    if [[ $(uname -s) == Darwin ]]; then
        netstat_parse_macos "$1"
    else
        netstat_parse_linux "$1"
    fi
}

netstat_args="-a -t -n"
if [[ $(uname -s) == Darwin ]]; then
    netstat_args="-p tcp -a -n"
fi

# Return a listing of all TCP ports in use
get_ports_tcp() {
    local data=$(netstat $netstat_args)
    local ports=$(netstat_parse "$data")

    while read port; do
        if ! [[ $port =~ ^[0-9]+$ ]]; then
            continue
        fi
        echo "$port"
    done <<< "$ports"
}

# Determine whether a port is in use
# Return
#  0 if yes
# >0 if no
get_port_status() {
    local port="$1"
    grep "$port" <<< $(get_ports_tcp) &>/dev/null
    # returns exit code from grep
}

usage() {
    printf \
"usage: %s [-h] [range_low] [range_high]

Positional arguments:
    range_low    (default: $PORT_MIN)
    range_high   (default: $PORT_MAX)

Arguments:
    -h    show this usage statement
" $(basename $0)
}

# main()
PORT_MIN=1024
PORT_MAX=65535

# Parse arguments
argv=($*)
argc="${#argv[@]}"
args=()

i=0
while [[ $i < $argc ]]; do
    key="${argv[$i]}"
    if [[ $key =~ ^- ]]; then
        key="${key#-*}"
        case "$key" in
            h)
                usage
                exit 0
                ;;
            *)
                echo "error: unknown argument" >&2
                usage
                exit 1
                ;;
        esac
    fi
    args+=("$key")
    (( i++ ))
done

available=-1
range_low=${args[0]:-$PORT_MIN}
range_high=${args[1]:-$PORT_MAX}

# Check input boundaries
if (( range_low > range_high )); then
    echo "Invalid port range: $range_low > $range_high" >&2
    usage
    exit 1
elif (( range_high < 0 )) || (( range_low < 0 )); then
    echo "Range value must be a positive integer" >&2
    usage
    exit 1
elif (( range_high > PORT_MAX )) || (( range_low > PORT_MAX )); then
    echo "Range value must be less than $PORT_MAX" >&2
    usage
    exit 1
fi

# Print the next available port or -1 if unable
ports=($(seq $range_low $range_high))
for port in "${ports[@]}"; do
    if ! $(get_port_status $port); then
        available=$port
        break
    fi
done

echo $available