package be.ac.vub.ir.mpi;

import mpi.MPI;

class MPIWavefront {
  public static void main(String[] args) 
  {
    /*
     * this solution assigns a row of the matrix to each process
     */

    int[] matrix;         // matrix stored at process 0
    int[] row;            // each process keeps its row
    int[] receiveBuffer;  // to receive a value from ``row - 1''
    int[] sendBuffer;     // to send a value to ``row + 1''

    MPI.Init(args);

    int rank = MPI.COMM_WORLD.Rank();
    int size = MPI.COMM_WORLD.Size();
    

    // initialize data structures
    receiveBuffer = new int[1];
    sendBuffer = new int[1];
    row  = new int[size];
    if (rank == 0) {
      matrix = new int[size * size];
      for (int i = 0; i < size * size; ++i) {
        matrix[i] = 0;
      }
      matrix[0] = 1;
      receiveBuffer[0] = 0; // process 0 never receives!!!
    }
    else {
      matrix = null;
    }

    // distribute the rows of the matrix to the appropriate processes
    MPI.COMM_WORLD.Scatter(matrix, rank * size, size, MPI.INT, row, 0, size, MPI.INT, 0);
    for (int col = 0; col < size; ++col) {
    	
    	if (rank > 0) {
    		MPI.COMM_WORLD.Recv(receiveBuffer, 0, 1, MPI.INT, rank - 1, 0);
    	}
    	int leftValue = col == 0 ? 0 : row[col - 1];
    	row[col] = row[col] + leftValue + receiveBuffer[0];
    	sendBuffer[0] = row[col];
    	if (rank + 1 < size) {
    		MPI.COMM_WORLD.Send(sendBuffer, 0, 1, MPI.INT, rank + 1, 0);
    	}    	
    }
    // retrieve the rows from the processes into the matrix
    MPI.COMM_WORLD.Gather(row, 0, size, MPI.INT, matrix, rank * size, size, MPI.INT, 0);

    // display the result
    if (rank == 0) {
      for (int y = 0; y < size; ++y) {
        for (int x = 0; x < size; ++x) {
          System.out.print(matrix[size * y + x] + " ");
        }
        System.out.println();
      }
    }

    MPI.Finalize();
  }
}