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Push_Swap

The main goal is to sort data on a stack, with a limited set of instructions, using the lowest possible number of actions. To succeed you’ll have to manipulate various types of algorithms and choose the one (of many) most appropriate solution for an optimized data sorting.

Game rules

• The game is composed of 2 stacks named a and b.
  
• To start with:
  ◦ a contains a random number of either positive or negative numbers without any duplicates.
  ◦ b is empty
  
• The goal is to sort in ascending order numbers into stack a.
  
• To do this you have the following operations at your disposal:
  

Command	Description
sa	swap a - swap the first 2 elements at the top of stack a. Do nothing if there is only one or no elements).
sb	swap b - swap the first 2 elements at the top of stack b. Do nothing if there is only one or no elements).
ss	sa and sb at the same time.
pa	push a - take the first element at the top of b and put it at the top of a. Do nothing if b is empty.
pb	push b - take the first element at the top of a and put it at the top of b. Do nothing if a is empty.
ra	rotate a - shift up all elements of stack a by 1. The first element becomes the last one.
rb	rotate b - shift up all elements of stack b by 1. The first element becomes the last one.
rr	ra and rb at the same time.
rra	reverse rotate a - shift down all elements of stack a by 1. The flast element becomes the first one.
rrb	reverse rotate b - shift down all elements of stack b by 1. The flast element becomes the first one.
rrr	rra and rrb at the same time.

Sorting example

-------------------------------------------------------------------------------------------------------
Init a and b:
2
1
3
6
8
5
_ _
a b
-------------------------------------------------------------------------------------------------------
Exec sa:
1
2
3
6
8
5
_ _
a b
-------------------------------------------------------------------------------------------------------
Exec pb pb pb:
6 3
5 2
8 1
_ _
a b
-------------------------------------------------------------------------------------------------------
Exec ra rb (equiv. to rr):
5 2
8 1
6 3
_ _
a b
-------------------------------------------------------------------------------------------------------
Exec rra rrb (equiv. to rrr):
6 3
5 2
8 1
_ _
a b
-------------------------------------------------------------------------------------------------------
Exec sa:
5 3
6 2
8 1
_ _
a b
-------------------------------------------------------------------------------------------------------
Exec pa pa pa:
1
2
3
5
6
8
_ _
a b 
-------------------------------------------------------------------------------------------------------

This example sort integers from a in 12 instructions.

The checker

• You have to write a program named checker, which will get as an argument the stack a formatted as a list of integers. The first argument should be at the top of the stack (be careful about the order). If no argument is given checker stops and displays nothing.
• Checker will then wait and read instructions on the standard input, each instruction will be followed by ’\n’. Once all the instructions have been read, checker will execute them on the stack received as an argument.
• If after executing those instructions, stack a is actually sorted and b is empty, then checker must display "OK" followed by a ’\n’ on the standard output. In every other case, checker must display "KO" followed by a ’\n’ on the standard output.
• In case of error, you must display Error followed by a ’\n’ on the standard error. Errors include for example: some arguments are not integers, some arguments are bigger than an integer, there are duplicates, an instruction don’t exist and/or is incorrectly formatted.

Checker example

$>./checker 3 2 1 0
rra
pb
sa
rra
pa
OK
$>./checker 3 2 1 0
sa
rra
pb
KO
$>./checker 3 2 one 0
Error

The “push_swap” program

• You have to write a program named push_swap which will receive as an argument the stack a formatted as a list of integers. The first argument should be at the top of the stack (be careful about the order).
• The program must display the smallest list of instructions possible to sort the stack a, the smallest number being at the top.
• Instructions must be separaed by a ’\n’ and nothing else.
• The goal is to sort the stack with the minimum possible number of operations. During defence we’ll compare the number of instructions your program found with a maximum number of operation tolerated. If your program either displays a list too big or if the list isn’t sorted properly, you’ll get no points.
• In case of error, you must display Error followed by a ’\n’ on the standard error. Errors include for example: some arguments aren’t integers, some arguments are bigger than an integer, and/or there are duplicates.

Pushswap Example

$>./push_swap 2 1 3 6 5 8
sa
pb
pb
pb
sa
pa
pa
pa
$>./push_swap 0 one 2 3
Error
$>

Execution example

$>ARG="4 67 3 87 23"; ./push_swap $ARG | wc -l
6
$>ARG="4 67 3 87 23"; ./push_swap $ARG | ./checker $ARG
OK
$>

More about School 42 you can find here: https://en.wikipedia.org/wiki/42_(school)