#!/bin/sh
default_sink_name()
{
        pactl stat | sed -ne 's/^Default Sink: //p'
}

die()
{
        printf 'Error: %s\n' "$1" >&2
        exit 1
}

sink=$(default_sink_name) || die "could not determine default sink"

set_volume() # uses $sink
{
        pactl set-sink-volume "$sink" "$1"%
}

show_volume_line() # uses $sink
{
        local want_next=
        pactl list sinks | while read line; do
                case "$line" in
                        "Name: $sink") want_next=1 ;;
                        'Name: '*) want_next= ;;
                        'Volume: '*)
                                [ "$want_next" ] || continue
                                echo "$line"
                                break
                        ;;
                esac
        done
}

show_volume_number() # uses $sink
{
        set -- $(show_volume_line)
        local total=0 num=0 p
        for p; do
                case "$p" in
                        *%)
                                total=$((total + ${p%%%}))
                                num=$((num + 1))
                                ;;
                esac
        done
        if [ "$num" -gt 0 ]; then
                echo $(( total / num ))
        fi
}

case "$1" in
        [0-9]*)       set_volume "$1" ;;
        [-+*/][0-9]*) set_volume $(( $(show_volume_number) $1 )) ;;
        '')           show_volume_number ;;
        *)            die 'usage' ;;
esac