push_swap

CODAM project Curriculum 2019, added to common core in March 2021.

❗ Important: The subject changed in May 2021. This project has been done according to the previous subject.

• Project type: Algorithm implementation.
• Objective: Sorting data on a stack with a limited set of instructions, using the lowest possible number of actions, through manipulating various types of algorithms and choosing the one most appropriate solution for an optimized data sorting.

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Requirements 📌

Write 2 programs in C:

• The first, named 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.

• The second one called push_swap which calculates and displays on the standard output the smallest progam using Push_swap instruction language that sorts integer arguments received.

Allowed functions

write, read, malloc, free, exit

Allowed movements

Swap : Do nothing if there is only one or no nodes.

• sa -> swap a - swap the first 2 nodes at the top of stack a.
• sb -> swap b - swap the first 2 nodes at the top of stack b.
• ss -> sa and sb at the same time.

Push

• 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.

Rotate : The first element becomes the last one.

• ra -> rotate a - shift up all nodes of stack a by 1.
• rb -> rotate b - shift up all nodes of stack b by 1.
• rr -> ra and rb at the same time.

Reverse : The last element becomes the first one.

• rra -> reverse rotate a - shift down all nodes of stack a by 1.
• rrb -> reverse rotate b - shift down all nodes of stack b by 1.
• rrr -> rra and rrb at the same time

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Strategy 💡

Solving checker

The sorting algorithm used for checker was merge-sort.

Solving push_swap

This project used circular doubly linked list to make rotate and reverse operations more efficient and, therefore, use fewer movements. An example of this is shown in the following image:

The behavior could be similar to a boat steering wheel: depending on the movement, it could be turned to the right or to the left to quickly access the head or tail.

For stack size less or equal to 3, a simpler sort method was used. For stack size greater than 3, it was used an insertion sort modified algorithm. The method consists in to move numbers from stack "a" to stack "b".

Before pushing a number on stack "b", it is checked in stack "a" which number costs fewer movements to be positionable at the head of stack "b" and ensuring to maintain the descending order in stack "b".

This process is done until stack "a" is almost empty, when just one or two numbers left in stack a depending on the numbers on stack "b". Then, the numbers were push back to stack "a".

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How to build 🔨

1. Clone the repository and go to folder cd push_swap
2. Type in the terminal make

Usage of checker and push_swap

1. They can be used separately using spaces and double quotes:

./checker 4 2 8 1 3

./checker "4 2 8 1 3"

./checker "4" "2" "8" "1" "3"

2. They can be used together:

A="5 6 8 7 1 9 10 4 3"; ./push_swap $A | ./checker $A

3. They can be used together with an extra flag "-v" to show the stacks in every sorting step:

A="5 6 8 7 1 9 10 4 3"; ./push_swap $A | ./checker -v $A

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Resources 📚

More info in the Wiki!