#!/bin/sh

die() { printf "%s: Error: %s\n" "$0" "$*" >&2; exit 1; }

[ "$(id -u)" = 0 ] || die 'you are not root'

shrink()
{
    local shrinkmegs=100 mountpoint="$1"
    while true; do
        while ! btrfs filesystem resize -${shrinkmegs}M "$mountpoint"/; do
            shrinkmegs=$((shrinkmegs - 10 ))
            if [ $shrinkmegs -lt 10 ]; then
                return
            fi
        done
    done
}

btrfs_truncate()
{
    local img="$1" bytes
#   548044800/1176715264 bytes used
    bytes=$(file "$img" | sed -ne 's?.*/\([0-9]*\) bytes used.*?\1?p')
    if [ "$bytes" ]; then
        truncate -s "$bytes" "$img"
    fi
}

main()
{
    if [ -d "$1" ]; then
        mountpoint=$1
        mountpoint -q "$mountpoint" || die "not a mountpoint: $1"
        shrink "$mountpoint"
    elif [ -f "$1" ]; then
        mountpoint="$1".mnt.tmp
        mkdir "$mountpoint"
        mount -t btrfs "$1" "$mountpoint"
        result=$?
        if [ $result = 0 ]; then
            shrink "$mountpoint"
            result=$?
            umount "$mountpoint"
        fi
        rmdir "$mountpoint"
        btrfs_truncate "$1"
        return $result
    else
        die "not a file or directory: $1"
    fi
}

main "$@"