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fix: clamp content differences to prevent FMA-induced numerical instability #1455

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Merged
barendgehrels merged 1 commit into from
Apr 21, 2026
Merged

fix: clamp content differences to prevent FMA-induced numerical instability #1455

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13 changes: 13 additions & 0 deletions include/boost/geometry/index/detail/algorithms/content.hpp
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Expand Up @@ -112,6 +112,19 @@ typename default_content_result<Indexable>::type content(Indexable const& b)
>::apply(b);
}

// Returns the content increase when expanding 'original' to 'expanded'.
// Precondition: 'original' is covered by 'expanded'. Under this precondition
// the result is mathematically non-negative, so clamping to 0 is valid and
// guards against floating-point contraction artifacts (e.g. GCC FMA fusion
// across function boundaries) that can make the difference negative or
// spuriously non-zero for equal boxes. See GitHub issue #1452.
template <typename Box>
typename default_content_result<Box>::type content_diff(Box const& expanded, Box const& original)
{
using content_type = typename default_content_result<Box>::type;
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return (std::max)(content_type(0), content(expanded) - content(original));
}

#if defined(BOOST_GCC)
#pragma GCC pop_options
#endif
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7 changes: 5 additions & 2 deletions include/boost/geometry/index/detail/rtree/linear/redistribute_elements.hpp
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Expand Up @@ -16,6 +16,7 @@
#ifndef BOOST_GEOMETRY_INDEX_DETAIL_RTREE_LINEAR_REDISTRIBUTE_ELEMENTS_HPP
#define BOOST_GEOMETRY_INDEX_DETAIL_RTREE_LINEAR_REDISTRIBUTE_ELEMENTS_HPP

#include <algorithm>
#include <type_traits>

#include <boost/core/ignore_unused.hpp>
Expand Down Expand Up @@ -432,8 +433,10 @@ struct redistribute_elements<MembersHolder, linear_tag>
content_type enlarged_content1 = index::detail::content(enlarged_box1);
content_type enlarged_content2 = index::detail::content(enlarged_box2);

content_type content_increase1 = enlarged_content1 - content1;
content_type content_increase2 = enlarged_content2 - content2;
content_type const content_increase1
= (std::max)(content_type(0), enlarged_content1 - content1);
content_type const content_increase2
= (std::max)(content_type(0), enlarged_content2 - content2);

// choose group which box content have to be enlarged least or has smaller content or has fewer elements
if ( content_increase1 < content_increase2 ||
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27 changes: 15 additions & 12 deletions include/boost/geometry/index/detail/rtree/quadratic/redistribute_elements.hpp
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Expand Up @@ -74,9 +74,10 @@ inline void pick_seeds(Elements const& elements,

bounded_indexable_view bounded_ind1(ind1, strategy);
bounded_indexable_view bounded_ind2(ind2, strategy);
content_type free_content = ( index::detail::content(enlarged_box)
- index::detail::content(bounded_ind1) )
- index::detail::content(bounded_ind2);
content_type free_content = (std::max)(content_type(0),
( index::detail::content(enlarged_box)
- index::detail::content(bounded_ind1) )
- index::detail::content(bounded_ind2));

if ( greatest_free_content < free_content )
{
Expand Down Expand Up @@ -273,7 +274,7 @@ struct redistribute_elements<MembersHolder, quadratic_tag>
typedef typename boost::iterator_value<It>::type element_type;
typedef typename rtree::element_indexable_type<element_type, translator_type>::type indexable_type;

content_type greatest_content_incrase_diff = 0;
content_type greatest_content_increase_diff = 0;
It out_it = first;
out_content_increase1 = 0;
out_content_increase2 = 0;
Expand All @@ -291,18 +292,20 @@ struct redistribute_elements<MembersHolder, quadratic_tag>
content_type enlarged_content1 = index::detail::content(enlarged_box1);
content_type enlarged_content2 = index::detail::content(enlarged_box2);

content_type content_incrase1 = (enlarged_content1 - content1);
content_type content_incrase2 = (enlarged_content2 - content2);
content_type const content_increase1
= (std::max)(content_type(0), enlarged_content1 - content1);
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content_type const content_increase2
= (std::max)(content_type(0), enlarged_content2 - content2);

content_type content_incrase_diff = content_incrase1 < content_incrase2 ?
content_incrase2 - content_incrase1 : content_incrase1 - content_incrase2;
content_type content_increase_diff = content_increase1 < content_increase2 ?
content_increase2 - content_increase1 : content_increase1 - content_increase2;

if ( greatest_content_incrase_diff < content_incrase_diff )
if ( greatest_content_increase_diff < content_increase_diff )
{
greatest_content_incrase_diff = content_incrase_diff;
greatest_content_increase_diff = content_increase_diff;
out_it = el_it;
out_content_increase1 = content_incrase1;
out_content_increase2 = content_incrase2;
out_content_increase1 = content_increase1;
out_content_increase2 = content_increase2;
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}
}

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4 changes: 2 additions & 2 deletions include/boost/geometry/index/detail/rtree/rstar/choose_next_node.hpp
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Expand Up @@ -117,7 +117,7 @@ class choose_next_node<MembersHolder, choose_by_overlap_diff_tag>

// areas difference
content_type content = index::detail::content(box_exp);
content_type content_diff = content - index::detail::content(ch_i.first);
content_type content_diff = index::detail::content_diff(box_exp, ch_i.first);

children_contents[i].set(i, content, content_diff);

Expand Down Expand Up @@ -246,7 +246,7 @@ class choose_next_node<MembersHolder, choose_by_overlap_diff_tag>

// areas difference
content_type content = index::detail::content(box_exp);
content_type content_diff = content - index::detail::content(ch_i.first);
content_type content_diff = index::detail::content_diff(box_exp, ch_i.first);

// update the result
if ( content_diff < smallest_content_diff ||
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2 changes: 1 addition & 1 deletion include/boost/geometry/index/detail/rtree/visitors/insert.hpp
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Expand Up @@ -89,7 +89,7 @@ class choose_next_node<MembersHolder, choose_by_content_diff_tag>

// areas difference
content_type content = index::detail::content(box_exp);
content_type content_diff = content - index::detail::content(ch_i.first);
content_type content_diff = index::detail::content_diff(box_exp, ch_i.first);

// update the result
if ( content_diff < smallest_content_diff ||
Expand Down
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