public/subjects/push-swap

push-swap

Objectives

Push-swap is a very simple and highly effective algorithm. You have at your disposal a list of int values, two stacks (a and b) and a set of instructions.

You will have to write 2 programs:

• push-swap, which calculates and displays on the standard output the smallest program using push-swap instruction language that sorts integer arguments received.
• checker, which takes integer arguments and reads instructions on the standard output. Once read, checker executes them and displays OK if integers are sorted. Otherwise, it will display KO.

As said before, you will have two stacks at your disposal. Your goal is to sort the stack a, that will contain the int values received, in ascending order, using both stacks and a set of instructions.

These are the instructions that you can use to sort the stack :

• pa push the top first element of stack b to stack a
• pb push the top first element of stack a to stack b
• sa swap first 2 elements of stack a
• sb swap first 2 elements of stack b
• ss execute sa and sb
• ra rotate stack a (shift up all elements of stack a by 1, the first element becomes the last one)
• rb rotate stack b
• rr execute ra and rb
• rra reverse rotate a (shift down all elements of stack a by 1, the last element becomes the first one)
• rrb reverse rotate b
• rrr execute rra and rrb

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
8 2
5 1
= =
a b
---------------------------------------
Exec rb :
6 2
8 1
5 3
= =
a b
---------------------------------------
Exec rra and rrb (equivalent to rrr):
5 3
6 2
8 1
= =
a b
---------------------------------------
Exec pa pa pa:
1
2
3
5
6
8
= =
a b

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 integer should be at the top of the stack.
• The program must display the smallest list of instructions possible to sort the stack a, with the smallest number being at the top.
• Instructions must be separated by a \n and nothing else.
• The goal is to sort the stack with the minimum possible number of operations.
• In case of error, you must display Error followed by a \n on the standard error. Errors are understood as: some arguments are not integers and/or there are duplicates.
• If there are no arguments, the program does not display anything (0 instructions).

$ go run . "2 1 3 6 5 8"
pb
pb
ra
sa
rrr
pa
pa
$ go run . "0 one 2 3"
Error
$ go run .
$

The checker program

• 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 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, there are duplicates, an instruction do not exist and/or is incorrectly formatted.
• In case of there are no arguments the program displays nothing.

$ go run ./checker "3 2 1 0"
sa
rra
pb
KO
$ echo -e "rra\npb\nsa\n" | go run ./checker "3 2 one 0"
Error
$ echo -e "rra\npb\nsa\nrra\npa"
rra
pb
sa
rra
pa
$ echo -e "rra\npb\nsa\nrra\npa" | go run ./checker "3 2 1 0"
OK
$ go run ./checker
$

Instructions

• Your project must be written in Go.
• The code must respect the good practices.
• It is recommended that the code should present a test file.
• The first executable file must be named checker and the second push-swap.
• You have to be able to handle the errors.

Allowed packages

• Only the standard Go packages are allowed

Usage

$ ARG="4 67 3 87 23"; ./push-swap "$ARG" | wc -l
6
$ ARG="4 67 3 87 23"; ./push-swap "$ARG" | go run ./checker "$ARG"
OK
$

If the program checker displays KO, it means that your push-swap came up with a list of instructions that does not sort the list.

This project will help you learn about :

• the use of basic algorithms
• the use of sorting algorithms
• the use of stacks