/*
 * $COPYRIGHT$
 *
 * Additional copyrights may follow
 *
 * $HEADER$
 */

#include "ompi_config.h"
#include "coll_spacc.h"

#include "mpi.h"
#include "ompi/constants.h"
#include "opal/util/bit_ops.h"
#include "ompi/datatype/ompi_datatype.h"
#include "ompi/communicator/communicator.h"
#include "ompi/mca/coll/coll.h"
#include "ompi/mca/coll/base/coll_base_functions.h"
#include "ompi/mca/coll/base/coll_tags.h"
#include "ompi/mca/coll/base/coll_base_util.h"
#include "ompi/mca/pml/pml.h"
#include "ompi/op/op.h"

/*
 * mca_coll_spacc_exscan_intra_recursivedoubling
 *
 * Function:  Recursive doubling algorithm for exclusive scan.
 * Accepts:   Same as MPI_Exscan
 * Returns:   MPI_SUCCESS or error code
 *
 * Description:  Implements recursive doubling algorithm for MPI_Exscan.
 *               The algorithm preserves order of operations so it can
 *               be used both by commutative and non-commutative operations.
 *
 * Example for 5 processes and commutative operation MPI_SUM:
 * Process:  0                 1             2              3               4
 *  rbuf:    -                 -             -              -               -
 *  psend:  [0]               [1]           [2]            [3]             [4]
 *
 * Step 1:
 *  rbuf:    -                [0]            -             [2]              -
 *  psend:  [1+0]             [0+1]         [3+2]          [2+3]           [4]
 *
 * Step 2:
 *  rbuf:    -                [0]            [1+0]          [(0+1)+2]       -
 *  psend:  [(3+2)+(1+0)]     [(2+3)+(0+1)]  [(1+0)+(3+2)]  [(1+0)+(2+3)]  [4]
 *
 * Step 3:
 *  rbuf     -                [0]            [1+0]          [(0+1)+2]      [(3+2)+(1+0)]
 *  psend:  [4+((3+2)+(1+0))]                                              [((3+2)+(1+0))+4]
 *
 * Time complexity (worst case): \ceil(\log_2(p))(2\alpha + 2m\beta + 2m\gamma)
 * Memory requirements (per process): 2 * count * typesize = O(count)
 * Limitations: intra-communicators only
 */
int mca_coll_spacc_exscan_intra_recursivedoubling(
    const void *sbuf, void *rbuf, int count, struct ompi_datatype_t *dtype,
    struct ompi_op_t *op, struct ompi_communicator_t *comm,
    mca_coll_base_module_t *module)
{
    int err = MPI_SUCCESS;
    char *tmpsend_raw = NULL, *tmprecv_raw = NULL;
    int comm_size = ompi_comm_size(comm);
    int rank = ompi_comm_rank(comm);

    OPAL_OUTPUT((mca_coll_spacc_stream, "coll:spacc:exscan_intra_recursivedoubling: rank %d/%d",
                 rank, comm_size));
    if (count == 0)
        return MPI_SUCCESS;
    if (comm_size < 2)
        return MPI_SUCCESS;

    ptrdiff_t dsize, gap;
    dsize = opal_datatype_span(&dtype->super, count, &gap);
    tmpsend_raw = malloc(dsize);
    tmprecv_raw = malloc(dsize);
    if (NULL == tmpsend_raw || NULL == tmprecv_raw) {
        err = OMPI_ERR_OUT_OF_RESOURCE;
        goto cleanup_and_return;
    }
    char *psend = tmpsend_raw - gap;
    char *precv = tmprecv_raw - gap;
    if (sbuf != MPI_IN_PLACE) {
        err = ompi_datatype_copy_content_same_ddt(dtype, count, psend, sbuf);
        if (MPI_SUCCESS != err) { goto cleanup_and_return; }
    } else {
        err = ompi_datatype_copy_content_same_ddt(dtype, count, psend, rbuf);
        if (MPI_SUCCESS != err) { goto cleanup_and_return; }
    }
    int is_commute = ompi_op_is_commute(op);
    int is_first_block = 1;

    for (int mask = 1; mask < comm_size; mask <<= 1) {
        int remote = rank ^ mask;
        if (remote < comm_size) {
            err = ompi_coll_base_sendrecv(psend, count, dtype, remote,
                                          MCA_COLL_BASE_TAG_SCAN,
                                          precv, count, dtype, remote,
                                          MCA_COLL_BASE_TAG_SCAN, comm,
                                          MPI_STATUS_IGNORE, rank);
            if (MPI_SUCCESS != err) { goto cleanup_and_return; }

            if (rank > remote) {
                /* Assertion: rank > 0 and rbuf is valid */
                if (is_first_block) {
                    err = ompi_datatype_copy_content_same_ddt(dtype, count, rbuf, precv);
                    if (MPI_SUCCESS != err) { goto cleanup_and_return; }
                    is_first_block = 0;
                } else {
                    /* Accumulate prefix reduction: rbuf = precv <op> rbuf */
                    ompi_op_reduce(op, precv, rbuf, count, dtype);
                }
                /* Partial result: psend = precv <op> psend */
                ompi_op_reduce(op, precv, psend, count, dtype);
            } else {
                if (is_commute) {
                    /* psend = precv <op> psend */
                    ompi_op_reduce(op, precv, psend, count, dtype);
                } else {
                    /* precv = psend <op> precv */
                    ompi_op_reduce(op, psend, precv, count, dtype);
                    char *tmp = psend;
                    psend = precv;
                    precv = tmp;
                }
            }
        }
    }

cleanup_and_return:
    if (NULL != tmpsend_raw)
        free(tmpsend_raw);
    if (NULL != tmprecv_raw)
        free(tmprecv_raw);
    return err;
}