backport: Optimize chained hyperslab selection. (#1032)

A common pattern for creating semi-unstructured selection is to use many
(small) RegularHyperSlab and chain them:

```
HyperSlab hyperslab;
for(auto slab : regular_hyper_slabs) {
  hyperslab |= slab;
}
```

This eventually triggers calling:
```
for(auto slab : regular_hyper_slabs) {
  auto [offset, stride, counts, blocks] = slab;
  H5Sselect_hyperslab(space_id, offset, stride, counts, block);
}
```

Measurements show that this has runtime that's quadratic in the number
of regular hyper slabs. This starts becoming prohibitive at 10k - 40k
slabs.

We noticed that `H5Scombine_select` does not suffer from the same
performance issue. This allows us to optimize (long) chain of `Op::Or`
using divide and conquer.

The current implementation only optimizes streaks of `Op::Or`.

include/highfive/bits/H5Slice_traits.hpp

@@ -172,20 +172,7 @@ class HyperSlab {
     }
 
     DataSpace apply(const DataSpace& space_) const {
-        auto space = space_.clone();
-        for (const auto& sel: selects) {
-            if (sel.op == Op::None) {
-                detail::h5s_select_none(space.getId());
-            } else {
-                detail::h5s_select_hyperslab(space.getId(),
-                                             convert(sel.op),
-                                             sel.offset.empty() ? nullptr : sel.offset.data(),
-                                             sel.stride.empty() ? nullptr : sel.stride.data(),
-                                             sel.count.empty() ? nullptr : sel.count.data(),
-                                             sel.block.empty() ? nullptr : sel.block.data());
-            }
-        }
-        return space;
+        return apply_impl(space_);
     }
 
   private:
@@ -239,6 +226,113 @@ class HyperSlab {
     };
 
     std::vector<Select_> selects;
+
+  protected:
+    DataSpace select_none(const DataSpace& outer_space) const {
+        auto space = outer_space.clone();
+        detail::h5s_select_none(space.getId());
+        return space;
+    }
+
+    void select_hyperslab(DataSpace& space, const Select_& sel) const {
+        detail::h5s_select_hyperslab(space.getId(),
+                                     convert(sel.op),
+                                     sel.offset.empty() ? nullptr : sel.offset.data(),
+                                     sel.stride.empty() ? nullptr : sel.stride.data(),
+                                     sel.count.empty() ? nullptr : sel.count.data(),
+                                     sel.block.empty() ? nullptr : sel.block.data());
+    }
+
+#if H5_VERSION_GE(1, 10, 6)
+    /// The length of a stream of `Op::Or` starting at `begin`.
+    size_t detect_streak(Select_ const* begin, Select_ const* end, Op op) const {
+        assert(op == Op::Or);
+        auto it = std::find_if(begin, end, [op](const Select_& sel) { return sel.op != op; });
+        return static_cast<size_t>(it - begin);
+    }
+
+    DataSpace combine_selections(const DataSpace& left_space,
+                                 Op op,
+                                 const DataSpace& right_space) const {
+        return detail::make_data_space(
+            H5Scombine_select(left_space.getId(), convert(op), right_space.getId()));
+    }
+
+    /// Reduce a sequence of `Op::Or` efficiently.
+    ///
+    /// The issue is that `H5Sselect_hyperslab` runs in time that linear of the
+    /// number of block in the existing selection. Therefore, a loop that adds
+    /// slab-by-slab has quadratic runtime in the number of slabs.
+    ///
+    /// Fortunately, `H5Scombine_select` doesn't suffer from the same problem.
+    /// However, it's only available in 1.10.6 and newer.
+    ///
+    /// The solution is to use divide-and-conquer to reduce (long) streaks of
+    /// `Op::Or` in what seems to be log-linear time.
+    DataSpace reduce_streak(const DataSpace& outer_space,
+                            Select_ const* begin,
+                            Select_ const* end,
+                            Op op) const {
+        assert(op == Op::Or);
+
+        if (begin == end) {
+            throw std::runtime_error("Broken logic in 'DataSpace::reduce_streak'.");
+        }
+
+        std::ptrdiff_t distance = end - begin;
+        if (distance == 1) {
+            auto space = select_none(outer_space);
+            select_hyperslab(space, *begin);
+            return space;
+        }
+
+        Select_ const* mid = begin + distance / 2;
+        auto right_space = reduce_streak(outer_space, begin, mid, op);
+        auto left_space = reduce_streak(outer_space, mid, end, op);
+
+        return combine_selections(left_space, op, right_space);
+    }
+
+    DataSpace apply_impl(const DataSpace& space_) const {
+        auto space = space_.clone();
+        auto n_selects = selects.size();
+        for (size_t i = 0; i < n_selects; ++i) {
+            auto begin = selects.data() + i;
+            auto end = selects.data() + n_selects;
+
+            auto n_ors = detect_streak(begin, end, Op::Or);
+
+            if (n_ors > 1) {
+                auto right_space = reduce_streak(space_, begin, begin + n_ors, Op::Or);
+                // Since HDF5 doesn't allow `combine_selections` with a None
+                // selection, we need to avoid the issue:
+                if (detail::h5s_get_select_type(space.getId()) == H5S_SEL_NONE) {
+                    space = right_space;
+                } else {
+                    space = combine_selections(space, Op::Or, right_space);
+                }
+                i += n_ors - 1;
+            } else if (selects[i].op == Op::None) {
+                detail::h5s_select_none(space.getId());
+            } else {
+                select_hyperslab(space, selects[i]);
+            }
+        }
+        return space;
+    }
+#else
+    DataSpace apply_impl(const DataSpace& space_) const {
+        auto space = space_.clone();
+        for (const auto& sel: selects) {
+            if (sel.op == Op::None) {
+                detail::h5s_select_none(space.getId());
+            } else {
+                select_hyperslab(space, sel);
+            }
+        }
+        return space;
+    }
+#endif
 };
 
 template <typename Derivate>

include/highfive/bits/h5s_wrapper.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <H5Ipublic.h>
 #include <H5Spublic.h>
 namespace HighFive {
 namespace detail {
@@ -110,6 +111,26 @@ inline H5S_class_t h5s_get_simple_extent_type(hid_t space_id) {
     return cls;
 }
 
+inline H5S_sel_type h5s_get_select_type(hid_t space_id) {
+    H5S_sel_type type = H5Sget_select_type(space_id);
+    if (type < 0) {
+        HDF5ErrMapper::ToException<DataSpaceException>("Unable to get type of selection.");
+    }
+
+    return type;
+}
+
+#if H5_VERSION_GE(1, 10, 6)
+inline hid_t h5s_combine_select(hid_t space1_id, H5S_seloper_t op, hid_t space2_id) {
+    auto space_id = H5Scombine_select(space1_id, op, space2_id);
+    if (space_id == H5I_INVALID_HID) {
+        HDF5ErrMapper::ToException<DataSpaceException>("Unable to combine two selections.");
+    }
+
+    return space_id;
+}
+#endif
+
 
 }  // namespace detail
 }  // namespace HighFive

tests/unit/test_high_five_selection.cpp

@@ -25,6 +25,7 @@
 
 #include <highfive/highfive.hpp>
 #include "tests_high_five.hpp"
+#include "data_generator.hpp"
 
 using namespace HighFive;
 using Catch::Matchers::Equals;
@@ -534,3 +535,59 @@ void irregularHyperSlabSelectionWriteTest() {
 TEMPLATE_LIST_TEST_CASE("irregularHyperSlabSelectionWrite", "[template]", std::tuple<int>) {
     irregularHyperSlabSelectionWriteTest<TestType>();
 }
+
+TEST_CASE("select_multiple_ors", "[hyperslab]") {
+    size_t n = 100, m = 20;
+    size_t nsel = 30;
+    auto x = testing::DataGenerator<std::vector<std::vector<int>>>::create({n, m});
+
+    auto file = File("select_multiple_ors.h5", File::Truncate);
+    auto dset = file.createDataSet("x", x);
+
+    std::vector<std::array<size_t, 2>> indices;
+    auto hyperslab = HyperSlab();
+    for (size_t i = 0; i < nsel; ++i) {
+        std::vector<size_t> offsets{i, i % 10};
+        std::vector<size_t> counts{1, 3};
+        hyperslab |= RegularHyperSlab(offsets, counts);
+
+        for (size_t k = 0; k < counts[1]; ++k) {
+            indices.push_back({offsets[0], offsets[1] + k});
+        }
+    }
+
+    SECTION("Pure Or Chain") {
+        auto selected = dset.select(hyperslab).read<std::vector<int>>();
+        REQUIRE(selected.size() == indices.size());
+        for (size_t k = 0; k < selected.size(); ++k) {
+            size_t i = indices[k][0];
+            size_t j = indices[k][1];
+            REQUIRE(selected[k] == x[i][j]);
+        }
+    }
+
+    SECTION("Or Chain And Slab") {
+        std::vector<size_t> offsets{5, 2};
+        std::vector<size_t> counts{85, 12};
+
+        std::vector<std::array<size_t, 2>> selected_indices;
+        for (const auto ij: indices) {
+            std::array<size_t, 2> ij_max = {offsets[0] + counts[0], offsets[1] + counts[1]};
+
+            if (offsets[0] <= ij[0] && ij[0] < ij_max[0] && offsets[1] <= ij[1] &&
+                ij[1] < ij_max[1]) {
+                selected_indices.push_back(ij);
+            }
+        }
+
+        hyperslab &= RegularHyperSlab(offsets, counts);
+
+        auto selected = dset.select(hyperslab).read<std::vector<int>>();
+        REQUIRE(selected.size() == selected_indices.size());
+        for (size_t k = 0; k < selected.size(); ++k) {
+            size_t i = selected_indices[k][0];
+            size_t j = selected_indices[k][1];
+            REQUIRE(selected[k] == x[i][j]);
+        }
+    }
+}