#!/usr/bin/env ruby

require 'date'

SNAPSHOTS_TO_RETAIN = 7
SNAPSHOT_FORMAT = "daily-%Y-%m-%d"

input = ARGV

def usage
  puts "Usage: "
  puts "    #{$PROGRAM_NAME} <pool>"

  exit -1
end

def run_command(*cmd)
  pipe = IO.popen(cmd)
  stdout = pipe.read
  puts stdout
  pipe.close

  return stdout, $?.success?
end

class Array
  def drop_tail(n)
    slice(0...(length - n))
  end
end

class Dataset
  def initialize(name)
    _, dataset_exists = run_command("zfs", "list", "-H", name)
    raise "No such pool: #{name}" unless dataset_exists

    @name = name
  end

  def has_snapshot(name)
    _, snapshot_exists = run_command("zfs", "list", "-H", "#{@name}@#{name}")
    return snapshot_exists
  end

  def create_snapshot_today
    current_snapshot_name = Date.today.strftime(SNAPSHOT_FORMAT)
    raise "Snapshot already created: #{@name}@#{current_snapshot_name}" if has_snapshot(current_snapshot_name)

    _, made_snapshot = run_command("zfs", "snapshot", "#{@name}@#{current_snapshot_name}")
    raise "Could not create new snapshot #{@name}@#{current_snapshot_name}" unless made_snapshot
  end

  def get_all_snapshots
    all_snapshots, could_list_all_snapshot = run_command("zfs", "list", "-t", "snapshot", "-H", @name)
    raise "Could not list snapshots for #{@name}" unless could_list_all_snapshot

    return all_snapshots.lines.map { |line| line.split("\t").first }.map { |snapshot_name| snapshot_name.split("@").last }
  end

  def clean_up_old_snapshots
    snapshots_to_delete = get_all_snapshots.drop_tail(SNAPSHOTS_TO_RETAIN)
    p snapshots_to_delete

    return if snapshots_to_delete.length == 0

    output, could_delete_old_snapshots = run_command("zfs", "destroy", "#{@name}@#{snapshots_to_delete.first}%#{snapshots_to_delete.last}")
    raise "Could not delete old snapshots for #{@name}" unless could_delete_old_snapshots
  end
end

usage unless input.length == 1
usage if input[0] == "-h" or input[0] == "--help"

dataset = Dataset.new input[0]
dataset.create_snapshot_today
dataset.clean_up_old_snapshots