Question

This is a interview Question which was asked and wanted to find an efficient solution.

Problem

Consider a intersection of four roads as shown in the diagram. Each road is defined to have a direction. How would you solve the problem so as to improve the traffic condition and avoid deadlocks.

1. the intersection is divided into four quadrants [shown in yellow].
2. Cars enter the intersection at random from the four direction [0,1,2,3]
3. At the intersection each car can make a single move. the three possible moves are
   1. Go left
   2. Go straight
   3. Go right

e.g :

if a car enters from direction 2 and wants to take a left turn. it should pass through
quandrant 2 , quandrant 1 and finally quandrant 0

Semi complete solution

Each quadrant marked in yellow has a semaphore associated with it.
What I imagined was a 2 phase protocol wherein each car would

1. get list of quadrants it will pass through
2. lock each of the quadrants starting from the last quadrant
   3. In the above example the car from direction 2 would lock quadrant 0, quadrant 1 and then quadrant 2.
3. Move through the intersection
4. Release the locks acquired.
   3. The locks are released in the same order. quadrant 0, quandrant 1 and the quandrant 2

However the above solution is less than optimal as it results in a deadlock.

My Question is

1. What other /better way can this be achieved?
2. Can I implement this using a semaphore in C?
   2. If not what synchronization method should I be looking at?

Updates

1. I want a solution which allows multiple cars can enter the intersection and still avoid deadlock as well as collision.Having a single lock on the entire intersection would be less than optimized.

Answer 1

I was able to implement a solution in C using the following logic (The actual implementation used semaphores for each of the quadrant).

The solution I mentioned in the question was partially correct however I was experiencing deadlock due to the matter in which locks were made.
I was able to solve the issue by assigning priority to the locks.
The locks are gained by locking highest priority lock first.
Following is a pseudo-code algorithm used by each of the car:

Assumptions:

1. Each quadrant has a lock associated with it.
2. Each lock has an associated priority. The priorities are as follows

Priority(Lock_0) < Priority(Lock_1) < Priority(Lock_2) < Priority(Lock_3)
*where Lock_x is the lock for Quadrant x

Algorithm

1. Each car gets list of quadrants it will pass through

2. Lock each of the quadrants starting from the quadrant with highest priority.

   e.g: In the above example the car from direction 2 taking a left turn would lock quadrant 2, quadrant 0 and then quadrant 1.

3. Move through the intersection

4. Release the locks acquired.

   e.g: In the above example the locks are released in the same order. quadrant 2, quandrant 1 and the quandrant 0