scikit-learn · ogrisel · Mar 14, 2023 · Jul 29, 2022 · Jul 31, 2022 · Aug 1, 2022
diff --git a/.gitignore b/.gitignore
@@ -90,6 +90,7 @@ sklearn/metrics/_dist_metrics.pyx
 sklearn/metrics/_dist_metrics.pxd
 sklearn/metrics/_pairwise_distances_reduction/_argkmin.pxd
 sklearn/metrics/_pairwise_distances_reduction/_argkmin.pyx
+sklearn/metrics/_pairwise_distances_reduction/_argkmin_classmode.pyx
 sklearn/metrics/_pairwise_distances_reduction/_base.pxd
 sklearn/metrics/_pairwise_distances_reduction/_base.pyx
 sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.pxd

diff --git a/doc/whats_new/v1.3.rst b/doc/whats_new/v1.3.rst
@@ -292,6 +292,11 @@ Changelog
   dissimilarity is not a metric and cannot be supported by the BallTree.
   :pr:`25417` by :user:`Guillaume Lemaitre <glemaitre>`.
 
+- |Enhancement| The performance of :meth:`neighbors.KNeighborsClassifier.predict`
+  and of :meth:`neighbors.KNeighborsClassifier.predict_proba` has been improved
+  when `n_neighbors` is large and `algorithm="brute"` with non Euclidean metrics.
+  :pr:`24076` by :user:`Meekail Zain <micky774>`, :user:`Julien Jerphanion <jjerphan>`.
+
 :mod:`sklearn.neural_network`
 .............................
 

diff --git a/setup.cfg b/setup.cfg
@@ -90,6 +90,7 @@ ignore =
     sklearn/metrics/_dist_metrics.pxd
     sklearn/metrics/_pairwise_distances_reduction/_argkmin.pxd
     sklearn/metrics/_pairwise_distances_reduction/_argkmin.pyx
+    sklearn/metrics/_pairwise_distances_reduction/_argkmin_classmode.pyx
     sklearn/metrics/_pairwise_distances_reduction/_base.pxd
     sklearn/metrics/_pairwise_distances_reduction/_base.pyx
     sklearn/metrics/_pairwise_distances_reduction/_datasets_pair.pxd

diff --git a/setup.py b/setup.py
@@ -277,6 +277,12 @@ def check_package_status(package, min_version):
             "include_np": True,
             "extra_compile_args": ["-std=c++11"],
         },
+        {
+            "sources": ["_argkmin_classmode.pyx.tp"],
+            "language": "c++",
+            "include_np": True,
+            "extra_compile_args": ["-std=c++11"],
+        },
         {
             "sources": ["_radius_neighbors.pyx.tp", "_radius_neighbors.pxd.tp"],
             "language": "c++",

diff --git a/sklearn/metrics/_pairwise_distances_reduction/__init__.py b/sklearn/metrics/_pairwise_distances_reduction/__init__.py
@@ -90,12 +90,14 @@
     BaseDistancesReductionDispatcher,
     ArgKmin,
     RadiusNeighbors,
+    ArgKminClassMode,
     sqeuclidean_row_norms,
 )
 
 __all__ = [
     "BaseDistancesReductionDispatcher",
     "ArgKmin",
     "RadiusNeighbors",
+    "ArgKminClassMode",
     "sqeuclidean_row_norms",
 ]
diff --git a/sklearn/metrics/_pairwise_distances_reduction/_argkmin.pyx.tp b/sklearn/metrics/_pairwise_distances_reduction/_argkmin.pyx.tp
@@ -178,7 +178,6 @@ cdef class ArgKmin{{name_suffix}}(BaseDistancesReduction{{name_suffix}}):
         self.heaps_r_distances_chunks[thread_num] = &self.argkmin_distances[X_start, 0]
         self.heaps_indices_chunks[thread_num] = &self.argkmin_indices[X_start, 0]
 
-    @final
     cdef void _parallel_on_X_prange_iter_finalize(
         self,
         ITYPE_t thread_num,

diff --git a/sklearn/metrics/_pairwise_distances_reduction/_argkmin_classmode.pyx.tp b/sklearn/metrics/_pairwise_distances_reduction/_argkmin_classmode.pyx.tp
@@ -0,0 +1,208 @@
+{{py:
+
+implementation_specific_values = [
+    # Values are the following ones:
+    #
+    #       name_suffix, INPUT_DTYPE_t, INPUT_DTYPE
+    #
+    # We also use the float64 dtype and C-type names as defined in
+    # `sklearn.utils._typedefs` to maintain consistency.
+    #
+    ('64', 'DTYPE_t', 'DTYPE'),
+    ('32', 'cnp.float32_t', 'np.float32')
+]
+
+}}
+
+from cython cimport floating, integral
+from cython.parallel cimport parallel, prange
+from libcpp.map cimport map as cpp_map, pair as cpp_pair
+from libc.stdlib cimport free
+
+cimport numpy as cnp
+
+cnp.import_array()
+
+from ...utils._typedefs cimport ITYPE_t, DTYPE_t
+from ...utils._typedefs import ITYPE, DTYPE
+import numpy as np
+from scipy.sparse import issparse
+from sklearn.utils.fixes import threadpool_limits
+
+cpdef enum WeightingStrategy:
+    uniform = 0
+    # TODO: Implement the following options, most likely in
+    # `weighted_histogram_mode`
+    distance = 1
+    callable = 2
+
+{{for name_suffix, INPUT_DTYPE_t, INPUT_DTYPE in implementation_specific_values}}
+from ._argkmin cimport ArgKmin{{name_suffix}}
+from ._datasets_pair cimport DatasetsPair{{name_suffix}}
+
+cdef class ArgKminClassMode{{name_suffix}}(ArgKmin{{name_suffix}}):
+    """
+    {{name_suffix}}bit implementation of ArgKminClassMode.
+    """
+    cdef:
+        const ITYPE_t[:] class_membership,
+        const ITYPE_t[:] unique_labels
+        DTYPE_t[:, :] class_scores
+        cpp_map[ITYPE_t, ITYPE_t] labels_to_index
+        WeightingStrategy weight_type
+
+    @classmethod
+    def compute(
+        cls,
+        X,
+        Y,
+        ITYPE_t k,
+        weights,
+        class_membership,
+        unique_labels,
+        str metric="euclidean",
+        chunk_size=None,
+        dict metric_kwargs=None,
+        str strategy=None,
+    ):
+        """Compute the argkmin reduction with class_membership.
+
+        This classmethod is responsible for introspecting the arguments
+        values to dispatch to the most appropriate implementation of
+        :class:`ArgKminClassMode{{name_suffix}}`.
+
+        This allows decoupling the API entirely from the implementation details
+        whilst maintaining RAII: all temporarily allocated datastructures necessary
+        for the concrete implementation are therefore freed when this classmethod
+        returns.
+
+        No instance _must_ directly be created outside of this class method.
+        """
+        # Use a generic implementation that handles most scipy
+        # metrics by computing the distances between 2 vectors at a time.
+        pda = ArgKminClassMode{{name_suffix}}(
+            datasets_pair=DatasetsPair{{name_suffix}}.get_for(X, Y, metric, metric_kwargs),
+            k=k,
+            chunk_size=chunk_size,
+            strategy=strategy,
+            weights=weights,
+            class_membership=class_membership,
+            unique_labels=unique_labels,
+        )
+
+        # Limit the number of threads in second level of nested parallelism for BLAS
+        # to avoid threads over-subscription (in GEMM for instance).
+        with threadpool_limits(limits=1, user_api="blas"):
+            if pda.execute_in_parallel_on_Y:
+                pda._parallel_on_Y()
+            else:
+                pda._parallel_on_X()
+
+        return pda._finalize_results()
+
+    def __init__(
+        self,
+        DatasetsPair{{name_suffix}} datasets_pair,
+        const ITYPE_t[:] class_membership,
+        const ITYPE_t[:] unique_labels,
+        chunk_size=None,
+        strategy=None,
+        ITYPE_t k=1,
+        weights=None,
+    ):
+        super().__init__(
+            datasets_pair=datasets_pair,
+            chunk_size=chunk_size,
+            strategy=strategy,
+            k=k,
+        )
+
+        if weights == "uniform":
+            self.weight_type = WeightingStrategy.uniform
+        elif weights == "distance":
+            self.weight_type = WeightingStrategy.distance
+        else:
+            self.weight_type = WeightingStrategy.callable
+        self.class_membership = class_membership
+
+        self.unique_labels = unique_labels
+
+        cdef ITYPE_t idx, neighbor_class_idx
+        # Map from set of unique labels to their indices in `class_scores`
+        # Buffer used in building a histogram for one-pass weighted mode
+        self.class_scores = np.zeros(
+            (self.n_samples_X, unique_labels.shape[0]), dtype=DTYPE,
+        )
+
+    def _finalize_results(self):
+        probabilities = np.asarray(self.class_scores)
+        probabilities /= probabilities.sum(axis=1, keepdims=True)
+        return probabilities
+
+    cdef inline void weighted_histogram_mode(
+        self,
+        ITYPE_t sample_index,
+        ITYPE_t* indices,
+        DTYPE_t* distances,
+   ) noexcept nogil:
+        cdef:
+            ITYPE_t neighbor_idx, neighbor_class_idx, label_index, multi_output_index
+            DTYPE_t score_incr = 1
+            # TODO: Implement other WeightingStrategy values
+            bint use_distance_weighting = (
+                self.weight_type == WeightingStrategy.distance
+            )
+
+        # Iterate through the sample k-nearest neighbours
+        for neighbor_rank in range(self.k):
+            # Absolute indice of the neighbor_rank-th Nearest Neighbors
+            # in range [0, n_samples_Y)
+            # TODO: inspect if it worth permuting this condition
+            # and the for-loop above for improved branching.
+            if use_distance_weighting:
+                score_incr = 1 / distances[neighbor_rank]
+            neighbor_idx = indices[neighbor_rank]
+            neighbor_class_idx = self.class_membership[neighbor_idx]
+            self.class_scores[sample_index][neighbor_class_idx] += score_incr
+        return
+
+    cdef void _parallel_on_X_prange_iter_finalize(
+        self,
+        ITYPE_t thread_num,
+        ITYPE_t X_start,
+        ITYPE_t X_end,
+    ) noexcept nogil:
+        cdef:
+            ITYPE_t idx, sample_index
+        for idx in range(X_end - X_start):
+            # One-pass top-one weighted mode
+            # Compute the absolute index in [0, n_samples_X)
+            sample_index = X_start + idx
+            self.weighted_histogram_mode(
+                sample_index,
+                &self.heaps_indices_chunks[thread_num][idx * self.k],
+                &self.heaps_r_distances_chunks[thread_num][idx * self.k],
+            )
+        return
+
+    cdef void _parallel_on_Y_finalize(
+        self,
+    ) noexcept nogil:
+        cdef:
+            ITYPE_t sample_index, thread_num
+
+        with nogil, parallel(num_threads=self.chunks_n_threads):
+            # Deallocating temporary datastructures
+            for thread_num in prange(self.chunks_n_threads, schedule='static'):
+                free(self.heaps_r_distances_chunks[thread_num])
+                free(self.heaps_indices_chunks[thread_num])
+
+            for sample_index in prange(self.n_samples_X, schedule='static'):
+                self.weighted_histogram_mode(
+                    sample_index,
+                    &self.argkmin_indices[sample_index][0],
+                    &self.argkmin_distances[sample_index][0],
+                )
+        return
+
+{{endfor}}