Push_Swap

Final Score 105/100

Challenge

Sort a random list of integers using the smallest number of moves, 2 stacks and a limited set of operations.

You start with two empty stacks: a and b. You are given a random list of integers via command line arguments.

Only these moves are allowed:

• 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 last element becomes the first one.
• rrb : reverse rotate b - shift down all elements of stack b by 1. The last element becomes the first one.
• rrr : rra and rrb at the same time.

At the end, stack b must empty empty and all integers must be in stack a, sorted in ascending order.

The Project

Create two programs: checker and push_swap.

The checker program reads a random list of integers from the stdin, stores them, and checks to see if they are sorted.

The push_swap program calculates the moves to sort the integers – pushing, popping, swapping and rotating them between stack a and stack b – and displays those directions on the stdout.

You can pipe push_swap into checker, and checker will verify that push_swap's instructions were successful.

Both programs must mandatorily parse input for errors, including empty strings, no parameters, non-numeric parameters, duplicates, and invalid/non-existent instructions.

Push_Swap must conform to the 42 Norm.
Using normal libc functions is strictly forbidden. Students are however, allowed to use: write, read, malloc, free, exit. It must not have any memory leaks. Errors must be handled carefully.
In no way can it quit in an unexpected manner (segmentation fault, bus error, double free, etc).

Approach

I stored all integers parsed into the stack in a doubly-circular linked list. This permitted me to access both the top and bottom of each stack (a and b) in the fewest number of moves, giving me the most efficient access to sort through all integers.

./push_swap writes recommended moves to the stdout, which ./checker then reads off the stdin and parses. I used a jump table to parse the moves and launch the corresponding function. This was much more efficient than an if tree, and triggered an error message for invalid input.

To try this out, launch ./checker. To push an integer to stack b, type pb and hit ‘enter’. To see if a combination of moves has sorted the stack, type control D to finish, and the ./checker will display “OK” for sorted or “KO” for unsorted.

The algorithms in ./push_swap to sort the stack are relatively straight-forward. I had 3 different algorithms: one for 5 numbers or less, one for 100 numbers or less, and one for 500 numbers or less.

For 100 < numbers, I find the median and push everything below the median into stack b. Then I identify each the largest and smallest integer in stack b, and determine which is most efficient to rotate up/down and push back to stack a (along with the specific moves to make that happen). Then I execute those moves.

In this way, integers are pushed back to stack a already sorted. I then repeat the process for everything above the median.

For 500 < numbers, I executed the same process but divided stack a by quarters instead of median.

Usage

Run make.

The checker program is used as follows:

  ./checker 5 2 3 1 4

  ./checker "-50 -400 -20 -1 -100"

  ./checker "-22" "35" "40" "-15" "75"

The push_swap program is used in the same way

  ./push_swap 5 2 3 1 4

You can run the two together using:

  ARG=`ruby -e "puts (0..100).to_a.shuffle.join(' ')"`; ./push_swap $ARG | ./checker -v $ARG

Note: the -v (debug) flag shows the stack status after each operation.

To see push_swap in action, run make and then the following script:

python3 pyviz.py `ruby -e "puts (1..20).to_a.shuffle.join(' ')"`