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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:

def reconstruct_queue(process)
    
    temp = []
    temp2 = Array.new(process.length, Array.new(process[0].length, 0))
    for k in (0...process.length)
      temp.push([process[k][0], process[k][1]])
    end
    # First sort priorities by priority and then by the k value.
    # priority in descending order and k value in ascending order.
    temp=temp.sort_by {|e| [-e[0], e[1]]}
    for i in (0...temp.length)
        temp2.insert(temp[i][1], temp[i])
    end
    # Place the result back in original 2d array
    for l in (0... process.length())
        process[l][0] = temp2[l][0]
        process[l][1] = temp2[l][1]
    end
    return process
end
# Driver Code
p1 =  [[7,0], [4,4], [7,1], [5,0], [6,1], [5,2]]
sol = reconstruct_queue(p1)
print(sol)
Queue reconstruction by priority
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