BLI: Generalize short algorithm for finding bounds

Finding the greatest and/or smallest element in an array is a common
need. This commit refactors the point cloud bounds code added in
6d7dbdbb44f379682 to a more general header in blenlib.
This will allow reusing the algorithm for curves without duplicating it.

Differential Revision: https://developer.blender.org/D14053

4 files changed, 162 insertions, 53 deletions

diff --git a/source/blender/blenkernel/intern/pointcloud.cc b/source/blender/blenkernel/intern/pointcloud.cc
index 2a4e0715293..3ee46fc4f15 100644
--- a/source/blender/blenkernel/intern/pointcloud.cc
+++ b/source/blender/blenkernel/intern/pointcloud.cc
@@ -11,6 +11,7 @@
 #include "DNA_object_types.h"
 #include "DNA_pointcloud_types.h"
 
+#include "BLI_bounds.hh"
 #include "BLI_index_range.hh"
 #include "BLI_listbase.h"
 #include "BLI_math_vec_types.hh"
@@ -254,68 +255,28 @@ PointCloud *BKE_pointcloud_new_nomain(const int totpoint)
   return pointcloud;
 }
 
-struct MinMaxResult {
-  float3 min;
-  float3 max;
-};
-
-static MinMaxResult min_max_no_radii(Span<float3> positions)
+static std::optional<blender::bounds::MinMaxResult<float3>> point_cloud_bounds(
+    const PointCloud &pointcloud)
 {
-  using namespace blender::math;
-
-  return blender::threading::parallel_reduce(
-      positions.index_range(),
-      1024,
-      MinMaxResult{float3(FLT_MAX), float3(-FLT_MAX)},
-      [&](IndexRange range, const MinMaxResult &init) {
-        MinMaxResult result = init;
-        for (const int i : range) {
-          min_max(positions[i], result.min, result.max);
-        }
-        return result;
-      },
-      [](const MinMaxResult &a, const MinMaxResult &b) {
-        return MinMaxResult{min(a.min, b.min), max(a.max, b.max)};
-      });
-}
-
-static MinMaxResult min_max_with_radii(Span<float3> positions, Span<float> radii)
-{
-  using namespace blender::math;
-
-  return blender::threading::parallel_reduce(
-      positions.index_range(),
-      1024,
-      MinMaxResult{float3(FLT_MAX), float3(-FLT_MAX)},
-      [&](IndexRange range, const MinMaxResult &init) {
-        MinMaxResult result = init;
-        for (const int i : range) {
-          result.min = min(positions[i] - radii[i], result.min);
-          result.max = max(positions[i] + radii[i], result.max);
-        }
-        return result;
-      },
-      [](const MinMaxResult &a, const MinMaxResult &b) {
-        return MinMaxResult{min(a.min, b.min), max(a.max, b.max)};
-      });
+  Span<float3> positions{reinterpret_cast<float3 *>(pointcloud.co), pointcloud.totpoint};
+  if (pointcloud.radius) {
+    Span<float> radii{pointcloud.radius, pointcloud.totpoint};
+    return blender::bounds::min_max_with_radii(positions, radii);
+  }
+  return blender::bounds::min_max(positions);
 }
 
 bool BKE_pointcloud_minmax(const PointCloud *pointcloud, float r_min[3], float r_max[3])
 {
-  using namespace blender::math;
+  using namespace blender;
 
-  if (!pointcloud->totpoint) {
+  const std::optional<bounds::MinMaxResult<float3>> min_max = point_cloud_bounds(*pointcloud);
+  if (!min_max) {
     return false;
   }
 
-  Span<float3> positions{reinterpret_cast<float3 *>(pointcloud->co), pointcloud->totpoint};
-  const MinMaxResult min_max = (pointcloud->radius) ?
-                                   min_max_with_radii(positions,
-                                                      {pointcloud->radius, pointcloud->totpoint}) :
-                                   min_max_no_radii(positions);
-
-  copy_v3_v3(r_min, min(min_max.min, float3(r_min)));
-  copy_v3_v3(r_max, max(min_max.max, float3(r_max)));
+  copy_v3_v3(r_min, math::min(min_max->min, float3(r_min)));
+  copy_v3_v3(r_max, math::max(min_max->max, float3(r_max)));
 
   return true;
 }
diff --git a/source/blender/blenlib/BLI_bounds.hh b/source/blender/blenlib/BLI_bounds.hh
new file mode 100644
index 00000000000..ce440bef704
--- /dev/null
+++ b/source/blender/blenlib/BLI_bounds.hh
@@ -0,0 +1,89 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#pragma once
+
+/** \file
+ * \ingroup bli
+ *
+ * Generic algorithms for finding the largest and smallest elements in a span.
+ */
+
+#include <optional>
+
+#include "BLI_math_vector.hh"
+#include "BLI_task.hh"
+
+namespace blender::bounds {
+
+template<typename T> struct MinMaxResult {
+  T min;
+  T max;
+};
+
+/**
+ * Find the smallest and largest values element-wise in the span.
+ */
+template<typename T> static std::optional<MinMaxResult<T>> min_max(Span<T> values)
+{
+  if (values.is_empty()) {
+    return std::nullopt;
+  }
+  return threading::parallel_reduce(
+      values.index_range(),
+      1024,
+      MinMaxResult<T>(),
+      [&](IndexRange range, const MinMaxResult<T> &init) {
+        MinMaxResult<T> result = init;
+        for (const int i : range) {
+          math::min_max(values[i], result.min, result.max);
+        }
+        return result;
+      },
+      [](const MinMaxResult<T> &a, const MinMaxResult<T> &b) {
+        return MinMaxResult<T>{math::min(a.min, b.min), math::max(a.max, b.max)};
+      });
+}
+
+/**
+ * Find the smallest and largest values element-wise in the span, adding the radius to each element
+ * first. The template type T is expected to have an addition operator implemented with RadiusT.
+ */
+template<typename T, typename RadiusT>
+static std::optional<MinMaxResult<T>> min_max_with_radii(Span<T> values, Span<RadiusT> radii)
+{
+  BLI_assert(values.size() == radii.size());
+  if (values.is_empty()) {
+    return std::nullopt;
+  }
+  return threading::parallel_reduce(
+      values.index_range(),
+      1024,
+      MinMaxResult<T>(),
+      [&](IndexRange range, const MinMaxResult<T> &init) {
+        MinMaxResult<T> result = init;
+        for (const int i : range) {
+          result.min = math::min(values[i] - radii[i], result.min);
+          result.max = math::max(values[i] + radii[i], result.max);
+        }
+        return result;
+      },
+      [](const MinMaxResult<T> &a, const MinMaxResult<T> &b) {
+        return MinMaxResult<T>{math::min(a.min, b.min), math::max(a.max, b.max)};
+      });
+}
+
+}  // namespace blender::bounds
diff --git a/source/blender/blenlib/CMakeLists.txt b/source/blender/blenlib/CMakeLists.txt
index e67d673eb73..e1b6e218ff5 100644
--- a/source/blender/blenlib/CMakeLists.txt
+++ b/source/blender/blenlib/CMakeLists.txt
@@ -162,6 +162,7 @@ set(SRC
   BLI_bitmap_draw_2d.h
   BLI_blenlib.h
   BLI_boxpack_2d.h
+  BLI_bounds.hh
   BLI_buffer.h
   BLI_color.hh
   BLI_compiler_attrs.h
@@ -395,6 +396,7 @@ if(WITH_GTESTS)
     tests/BLI_array_store_test.cc
     tests/BLI_array_test.cc
     tests/BLI_array_utils_test.cc
+    tests/BLI_bounds_test.cc
     tests/BLI_color_test.cc
     tests/BLI_delaunay_2d_test.cc
     tests/BLI_disjoint_set_test.cc
diff --git a/source/blender/blenlib/tests/BLI_bounds_test.cc b/source/blender/blenlib/tests/BLI_bounds_test.cc
new file mode 100644
index 00000000000..fc3affd97de
--- /dev/null
+++ b/source/blender/blenlib/tests/BLI_bounds_test.cc
@@ -0,0 +1,57 @@
+/* Apache License, Version 2.0 */
+
+#include "testing/testing.h"
+
+#include "BLI_math_base.hh"
+
+#include "BLI_array.hh"
+#include "BLI_bounds.hh"
+
+namespace blender::tests {
+
+TEST(bounds, Empty)
+{
+  Span<float2> empty_span{};
+  EXPECT_TRUE(empty_span.is_empty());
+  auto result = bounds::min_max(empty_span);
+  EXPECT_EQ(result, std::nullopt);
+}
+
+TEST(bounds, MinMax)
+{
+  Array<float2> data = {float2(0, 1), float2(3, -1), float2(0, -2), float2(-1, 1)};
+  auto result = bounds::min_max(data.as_span());
+  EXPECT_EQ(result->min, float2(-1, -2));
+  EXPECT_EQ(result->max, float2(3, 1));
+}
+
+TEST(bounds, MinMaxFloat)
+{
+  Array<float> data = {1.0f, 3.0f, 0.0f, -1.0f};
+  auto result = bounds::min_max(data.as_span());
+  EXPECT_EQ(result->min, -1.0f);
+  EXPECT_EQ(result->max, 3.0f);
+}
+
+TEST(bounds, MinMaxRadii)
+{
+  Array<int2> data = {int2(0, 1), int2(3, -1), int2(0, -2), int2(-1, 1)};
+  Array<int> radii = {5, 1, 1, 4};
+  auto result = bounds::min_max_with_radii(data.as_span(), radii.as_span());
+  EXPECT_EQ(result->min, int2(-5, -4));
+  EXPECT_EQ(result->max, int2(5, 6));
+}
+
+TEST(bounds, Large)
+{
+  Array<int2> data(10000);
+  for (const int64_t i : data.index_range()) {
+    data[i] = int2(i, i);
+  }
+
+  auto result = bounds::min_max(data.as_span());
+  EXPECT_EQ(result->min, int2(0, 0));
+  EXPECT_EQ(result->max, int2(9999, 9999));
+}
+
+}  // namespace blender::tests