From 0bb43975c81308afa4ee9d02a2f9c46fc1eae56c Mon Sep 17 00:00:00 2001
From: LTLA <infinite.monkeys.with.keyboards@gmail.com>
Date: Thu, 26 Sep 2024 08:40:44 -0700
Subject: [PATCH] Check for sortedness of the slabs to populate/reuse before
 re-sorting.

This makes the dense oracular extractor consistent with its sparse counterpart.
---
 include/tatami_hdf5/DenseMatrix.hpp | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/tatami_hdf5/DenseMatrix.hpp b/include/tatami_hdf5/DenseMatrix.hpp
index a779e2e..0654780 100644
--- a/include/tatami_hdf5/DenseMatrix.hpp
+++ b/include/tatami_hdf5/DenseMatrix.hpp
@@ -308,6 +308,16 @@ class OracularCoreNormal {
     size_t my_offset = 0;
 
 private:
+    template<class Function_>
+    static void sort_by_field(std::vector<std::pair<Index_, Slab*> >& indices, Function_ field) {
+        auto comp = [&field](const std::pair<Index_, Slab*>& l, const std::pair<Index_, Slab*>& r) -> bool {
+            return field(l) < field(r);
+        };
+        if (!std::is_sorted(indices.begin(), indices.end(), comp)) {
+            std::sort(indices.begin(), indices.end(), comp);
+        }
+    }
+
     template<typename Value_, class Unionize_>
     void fetch_raw([[maybe_unused]] Index_ i, Value_* buffer, size_t non_target_length, Unionize_ unionize) {
         auto info = my_cache.next(
@@ -323,9 +333,7 @@ class OracularCoreNormal {
             /* populate = */ [&](std::vector<std::pair<Index_, Slab*> >& chunks, std::vector<std::pair<Index_, Slab*> >& to_reuse) {
                 // Defragmenting the existing chunks. We sort by offset to make 
                 // sure that we're not clobbering in-use slabs during the copy().
-                std::sort(to_reuse.begin(), to_reuse.end(), [](const std::pair<Index_, Slab*>& left, const std::pair<Index_, Slab*>& right) -> bool { 
-                    return left.second->offset < right.second->offset; 
-                });
+                sort_by_field(to_reuse, [](const std::pair<Index_, Slab*>& x) -> size_t { return x.second->offset; });
 
                 auto dest = my_memory_pool.data();
                 size_t running_offset = 0;
@@ -343,7 +351,7 @@ class OracularCoreNormal {
                 // to populate the contiguous memory pool that we made available after
                 // defragmentation; then we just update the slab pointers to refer
                 // to the slices of memory corresponding to each slab.
-                std::sort(chunks.begin(), chunks.end());
+                sort_by_field(chunks, [](const std::pair<Index_, Slab*>& x) -> Index_ { return x.first; });
 
                 serialize([&]() -> void {
                     auto& components = *my_h5comp;