Stacked Dozen

This is one of the new possible maps for Sokoban Level 2 in Hack'EM. It has 11 boulders and 10 holes, leaving 1 spare boulder. In the starting position, all the boulders are stacked on top of each other.

---------         ------  0 boulder
|.......|     ----|....-- ^ hole
|.......|    --..--.....| > starting position/downstairs
|...<...|  ---......-.|.| < upstairs
|.......|  |..........|.| + door
|.......|  |..0..-......|
|+------------.....--...|
|.^^^^^^^^^^^...|>......|
-------------------------

Setup

Start by moving moving the boulder pile right to create a new boulder.

---------         ------  0 r
|.......|     ----|....--
|.......|    --..--.....|
|...<...|  ---......-.|.|
|.......|  |..........|.|
|.......|  |..0..-......|
|+------------.....--...|
|.^^^^^^^^^^^...|>......|
-------------------------

The solution expects that each step will create a new boulder by following the step above.

---------         ------  1 urrr rrrd r
|.......|     ----|....-- 1 urrr rrru ul
|.......|    --..--.....| 1 urrr rrru
|...<...|  ---......-.|.| 1 urrr rdrr
|.......|  |..........|.| 1 urrr rd
|.......|  |..01.-......| 1 uurr rr
|+------------.....--...|
|.^^^^^^^^^^^...|>......|
-------------------------

---------         ------  a ld
|.......|     ----|....-- b dd
|.......|    --..--.0...| 1 uurr rr
|...<...|  ---.....b-0|.| 1 uurr r
|.......|  |..........|.| 1 uur
|.......|  |..01.-.a.00.| 1 r
|+------------.....--...|
|.^^^^^^^^^^^...|>......|
-------------------------

---------         ------  1 r
|.......|     ----|....-- 1 d
|.......|    --..--.0...|
|...<...|  ---..0.00-0|.|
|.......|  |..........|.|
|.......|  |..1..-.0.00.|
|+------------....0--...|
|.^^^^^^^^^^^...|>......|
-------------------------

Strategy

First move the boulders from the stack to the position below. Then the boulders can be cleared as follows:

---------         ------   B u
|.......|     ----|....--  A l
|.......|    --..--.G...|  C dll
|...<...|  ---..D.EF-H|.|  A rddl ll
|.......|  |..........|.|  B lddd llll
|.......|  |..AB.-.J.KL.|  D dlld ddll lll
|+------------C...I--...|
|.^^^^^^^^^^^...|>......|
-------------------------

---------         ------   F u
|.......|     ----|....--  E llld ddld llll ll
|.......|    --..--.G...|  I uull lldd dlll llll
|...<...|  ---....EF-H|.|  J lull lldd dlll llll l
|.......|  |..........|.|  F ddll llld ddll llll lll
|.......|  |.....-.J.KL.|  G lddl llll dddl llll llll l
|+------------....I--...|
|.^^^^^^^.......|>......|
-------------------------

---------         ------   H ulld dlll lldd dlll llll llll
|.......|     ----|....--  K d
|.......|    --..--.....|  L llll ulll lddd llll llll llll
|...<...|  ---......-H|.|
|.......|  |..........|.|
|.......|  |.....-...KL.|
|+------------.....--...|
|.^^............|>......|
-------------------------