paleolimbot commented on code in PR #45459:
URL: https://github.com/apache/arrow/pull/45459#discussion_r1975823035


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cpp/src/parquet/geometry_util_internal.h:
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@@ -0,0 +1,184 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+#pragma once
+
+#include <algorithm>
+#include <limits>
+#include <sstream>
+#include <string>
+#include <unordered_set>
+
+#include "parquet/platform.h"
+
+namespace parquet::geometry {
+
+/// \brief Infinity, used to define bounds of empty bounding boxes
+constexpr double kInf = std::numeric_limits<double>::infinity();
+
+/// \brief Valid combinations of dimensions allowed by ISO well-known binary
+enum class Dimensions { XY = 0, XYZ = 1, XYM = 2, XYZM = 3, MIN = 0, MAX = 3 };
+
+/// \brief The supported set of geometry types allowed by ISO well-known binary
+enum class GeometryType {
+  POINT = 1,
+  LINESTRING = 2,
+  POLYGON = 3,
+  MULTIPOINT = 4,
+  MULTILINESTRING = 5,
+  MULTIPOLYGON = 6,
+  GEOMETRYCOLLECTION = 7,
+  MIN = 1,
+  MAX = 7
+};
+
+/// \brief A collection of intervals representing the encountered ranges of 
values
+/// in each dimension.
+struct BoundingBox {
+  using XY = std::array<double, 2>;
+  using XYZ = std::array<double, 3>;
+  using XYM = std::array<double, 3>;
+  using XYZM = std::array<double, 4>;
+
+  BoundingBox(const XYZM& mins, const XYZM& maxes) : min(mins), max(maxes) {}
+  BoundingBox() : min{kInf, kInf, kInf, kInf}, max{-kInf, -kInf, -kInf, -kInf} 
{}
+
+  BoundingBox(const BoundingBox& other) = default;
+  BoundingBox& operator=(const BoundingBox&) = default;
+
+  /// \brief Update the X and Y bounds to ensure these bounds contain coord
+  void UpdateXY(const XY& coord) { UpdateInternal(coord); }
+
+  /// \brief Update the X, Y, and Z bounds to ensure these bounds contain coord
+  void UpdateXYZ(const XYZ& coord) { UpdateInternal(coord); }
+
+  /// \brief Update the X, Y, and M bounds to ensure these bounds contain coord
+  void UpdateXYM(const XYM& coord) {
+    min[0] = std::min(min[0], coord[0]);
+    min[1] = std::min(min[1], coord[1]);
+    min[3] = std::min(min[3], coord[2]);
+    max[0] = std::max(max[0], coord[0]);
+    max[1] = std::max(max[1], coord[1]);
+    max[3] = std::max(max[3], coord[2]);
+  }
+
+  /// \brief Update the X, Y, Z, and M bounds to ensure these bounds contain 
coord
+  void UpdateXYZM(const XYZM& coord) { UpdateInternal(coord); }
+
+  /// \brief Reset these bounds to an empty state such that they contain no 
coordinates
+  void Reset() {
+    for (int i = 0; i < 4; i++) {
+      min[i] = kInf;
+      max[i] = -kInf;
+    }
+  }
+
+  /// \brief Update these bounds such they also contain other
+  void Merge(const BoundingBox& other) {
+    for (int i = 0; i < 4; i++) {
+      min[i] = std::min(min[i], other.min[i]);
+      max[i] = std::max(max[i], other.max[i]);
+    }
+  }
+
+  std::string ToString() const {
+    std::stringstream ss;
+    ss << "BoundingBox [" << min[0] << " => " << max[0];
+    for (int i = 1; i < 4; i++) {
+      ss << ", " << min[i] << " => " << max[i];
+    }
+
+    ss << "]";
+
+    return ss.str();
+  }
+
+  XYZM min;
+  XYZM max;
+
+ private:
+  // This works for XY, XYZ, and XYZM
+  template <typename Coord>
+  void UpdateInternal(Coord coord) {
+    static_assert(coord.size() <= 4);
+
+    for (size_t i = 0; i < coord.size(); i++) {
+      min[i] = std::min(min[i], coord[i]);
+      max[i] = std::max(max[i], coord[i]);
+    }
+  }
+};
+
+inline bool operator==(const BoundingBox& lhs, const BoundingBox& rhs) {
+  return lhs.min == rhs.min && lhs.max == rhs.max;
+}
+
+class WKBBuffer;
+
+/// \brief Accumulate a BoundingBox and geometry types based on zero or more 
well-known
+/// binary blobs
+class PARQUET_EXPORT WKBGeometryBounder {
+ public:
+  WKBGeometryBounder() = default;
+  WKBGeometryBounder(const WKBGeometryBounder&) = default;
+
+  /// \brief Accumulate the bounds of a serialized well-known binary geometry
+  ///
+  /// Returns SerializationError for any parse errors encountered. Bounds for
+  /// any encountered coordinates are accumulated and the geometry type of
+  /// the geometry is added to the internal geometry type list.
+  ///
+  /// Note that this method is NOT appropriate for bounding a GEOGRAPHY,
+  /// whose bounds are not a function purely of the vertices. Geography 
bounding
+  /// is not yet implemented.
+  ::arrow::Status ReadGeometry(const uint8_t* data, int64_t size);
+
+  /// \brief Accumulate the bounds of a previously-calculated BoundingBox
+  void ReadBox(const BoundingBox& box) { box_.Merge(box); }
+
+  /// \brief Accumulate a previously-calculated list of geometry types
+  void ReadGeometryTypes(const std::vector<int32_t>& geospatial_types) {
+    geospatial_types_.insert(geospatial_types.begin(), geospatial_types.end());
+  }
+
+  /// \brief Retrieve the accumulated bounds
+  const BoundingBox& Bounds() const { return box_; }
+
+  /// \brief Retrieve the accumulated geometry types
+  std::vector<int32_t> GeometryTypes() const {
+    std::vector<int32_t> out(geospatial_types_.begin(), 
geospatial_types_.end());
+    std::sort(out.begin(), out.end());

Review Comment:
   I think I successfully moved all non-trivial things to 
`geometry_util_internal.cc`!
   
   The typical size of the output here is one or two elements (e.g., usually 
people are writing 100% points to a file, even though the WKB could contain any 
type of geometry). I quite like this as canonically sorted whenever possible 
(for comparison, either programatically or visually) but it's not strictly 
necessary.



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