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Feature #15: Queue Reconstruction by Priority

Feature #15: Queue Reconstruction by Priority

Implement the "Queue Reconstruction by Priority" feature for our "Operating System" project.

We'll cover the following...

Description

A process queue contains the process priority. It also contains the number of processes ahead of a process in the queue that has a priority not less than its own. Suppose that the OS crashed and now we only have an array of processes, with each process at a random position in the array. In this feature, we’ll reconstruct the process queue from the information available to us.

Each element in the 2D array consists of a process’s priority and the number of processes with a higher or equal priority that are ahead of it in the queue. An entry [pi, ki] represents that a process with priority pi has ki other processes, with a priority of at least pi, ahead of it in the queue.

Our task is to reconstruct and return the process queue.

Let’s look at a few examples of this:

Solution

A process with a lower priority does not affect the placement of k processes with a higher priority. So, we will first insert the processes with a higher priority, into the output array. We will start by sorting the input array in descending order of process priority, and then in ascending order of the k-value. We will:

  • Sort the processes by priority, in a descending order.
  • Sort the processes with the same priority in ascending order of k.

We will pick elements from the sorted array, starting at index 0. If the element picked is [pi, ki], it will be inserted at index k in the output array. The following slides demonstrate this procedure:

Let’s take a look at an example of this:

internal object Solution {
    fun reconstructQueue(process:ArrayList<ArrayList<Int>>):List<List<Int>> {
        var temp = ArrayList<Pair<Int, Int>>();
        var temp2 = ArrayList<Pair<Int, Int>>(process.size);
        for (k in process.indices)
        {
            temp.add(Pair<Int,Int>(process[k][0], process[k][1]));
        }
        // First sort processes by priority and then by the k value.
        // priority in descending order and k value in ascending order.
        temp.sortWith(compareByDescending<Pair<Int,Int>> { it.first }.thenBy { it.second })
        for ( i in 0 until temp.size){
            temp2.add(temp[i].second, temp[i]);
        }
        // Place the result back in original 2d array
        for ( l in process.indices){
            process[l][0] = temp2[l].first;
            process[l][1] = temp2[l].second;
        }
        return process.toList();
    }
}
fun main(){
    var p  =  arrayListOf(
        arrayListOf(7, 0),
        arrayListOf(4, 4),
        arrayListOf(7, 1),
        arrayListOf(5, 0),
        arrayListOf(6, 1),
        arrayListOf(5, 2)
    )
    println(Solution.reconstructQueue(p));
    
}
Queue reconstruction by priority
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