# L-9 MCS 572 Mon 16 Sep 2024 : fan_out_integers.jl

"""
This program fans out an array of 80 integers over 8 processors,
so it must be executed with

mpiexecjl -n 8 julia fan_out_integers.jl

To synchronize the printing, MPI_Barrier is used.
"""

using MPI
MPI.Init()

COMM = MPI.COMM_WORLD
# size of the problem
SIZE = 80               

"""
    function parity_offset(n::Int, s::Int)
 
returns the offset of node with label n
for data of size s based on parity of n.
"""
function parity_offset(n::Int, s::Int)
    offset = 0
    dim = n
    wsz = div(s, 2)
    while dim > 0
        d = dim % 2
        if d > 0
            offset += wsz
        end
        dim = div(dim, 2)
        wsz = div(wsz, 2)
    end
    return offset
end

"""
    function main(verbose=True)

fans out 80 integers to 8 processors.
"""
function main(verbose::Bool=true)
    myid = MPI.Comm_rank(COMM)
    p = MPI.Comm_size(COMM)
    # manager initializes, workers allocate space
    if myid == 0
        data = [i for i=1:SIZE]
    else
        data = zeros(SIZE)
    end
    d = 1
    s = SIZE
    b = 0
    for i=0:2
        s = div(s, 2)
        if verbose
            MPI.Barrier(COMM)
            if myid == 0
                println("stage ", i, " d = ", d)
            end
            MPI.Barrier(COMM)
        end
        for j=0:d-1
            b = parity_offset(myid, SIZE) + 1
            if myid == j
                if verbose
                    MPI.Barrier(COMM)
                    println(j, " sends ", s, " integers to ", j+d,
                            " at ", b+s, " start ", data[b+s],
                            " to ", data[b+2*s-1])
                    MPI.Barrier(COMM)
                end
                slice = data[b+s: b+2*s]
                MPI.send(slice, COMM; dest=j+d, tag=11)
            elseif myid == j+d
                data[b: b+s] = MPI.recv(COMM; source=j, tag=11)
                if verbose
                    MPI.Barrier(COMM)
                    println(j+d, " received ", s, " integers from ", j,
                            " at ", b, " start ", data[b],
                            " to ", data[b+s-1])
                    MPI.Barrier(COMM)
                end
            end
        end
        d = 2*d
    end
    if verbose
        MPI.Barrier(COMM)
        println(myid, " has ", div(SIZE, p), " integers starting at ", b,
                " with ", data[b], data[b+1], " to ", data[b+div(SIZE, p)-1])
        MPI.Barrier(COMM)
    end
end

main()