# Authors: The scikit-learn developers
# SPDX-License-Identifier: BSD-3-Clause

from cython cimport floating
from cython.parallel cimport prange
from libc.math cimport fabs

from ..utils._typedefs cimport intp_t

from ..utils._openmp_helpers import _openmp_effective_n_threads


def _chi2_kernel_fast(floating[:, :] X,
                      floating[:, :] Y,
                      floating[:, :] result):
    cdef intp_t i, j, k
    cdef intp_t n_samples_X = X.shape[0]
    cdef intp_t n_samples_Y = Y.shape[0]
    cdef intp_t n_features = X.shape[1]
    cdef double res, nom, denom

    with nogil:
        for i in range(n_samples_X):
            for j in range(n_samples_Y):
                res = 0
                for k in range(n_features):
                    denom = (X[i, k] - Y[j, k])
                    nom = (X[i, k] + Y[j, k])
                    if nom != 0:
                        res += denom * denom / nom
                result[i, j] = -res


def _sparse_manhattan(
    const floating[::1] X_data,
    const int[:] X_indices,
    const int[:] X_indptr,
    const floating[::1] Y_data,
    const int[:] Y_indices,
    const int[:] Y_indptr,
    double[:, ::1] D,
):
    """Pairwise L1 distances for CSR matrices.

    Usage:
    >>> D = np.zeros(X.shape[0], Y.shape[0])
    >>> _sparse_manhattan(X.data, X.indices, X.indptr,
    ...                   Y.data, Y.indices, Y.indptr,
    ...                   D)
    """
    cdef intp_t px, py, i, j, ix, iy
    cdef double d = 0.0

    cdef int m = D.shape[0]
    cdef int n = D.shape[1]

    cdef int X_indptr_end = 0
    cdef int Y_indptr_end = 0

    cdef int num_threads = _openmp_effective_n_threads()

    # We scan the matrices row by row.
    # Given row px in X and row py in Y, we find the positions (i and j
    # respectively), in .indices where the indices for the two rows start.
    # If the indices (ix and iy) are the same, the corresponding data values
    # are processed and the cursors i and j are advanced.
    # If not, the lowest index is considered. Its associated data value is
    # processed and its cursor is advanced.
    # We proceed like this until one of the cursors hits the end for its row.
    # Then we process all remaining data values in the other row.

    # Below the avoidance of inplace operators is intentional.
    # When prange is used, the inplace operator has a special meaning, i.e. it
    # signals a "reduction"

    for px in prange(m, nogil=True, num_threads=num_threads):
        X_indptr_end = X_indptr[px + 1]
        for py in range(n):
            Y_indptr_end = Y_indptr[py + 1]
            i = X_indptr[px]
            j = Y_indptr[py]
            d = 0.0
            while i < X_indptr_end and j < Y_indptr_end:
                ix = X_indices[i]
                iy = Y_indices[j]

                if ix == iy:
                    d = d + fabs(X_data[i] - Y_data[j])
                    i = i + 1
                    j = j + 1
                elif ix < iy:
                    d = d + fabs(X_data[i])
                    i = i + 1
                else:
                    d = d + fabs(Y_data[j])
                    j = j + 1

            if i == X_indptr_end:
                while j < Y_indptr_end:
                    d = d + fabs(Y_data[j])
                    j = j + 1
            else:
                while i < X_indptr_end:
                    d = d + fabs(X_data[i])
                    i = i + 1

            D[px, py] = d
