From 23b646c7f79f3b0e852ae4e5bd7952ca43135412 Mon Sep 17 00:00:00 2001
From: Erik Faulhaber <44124897+efaulhaber@users.noreply.github.com>
Date: Wed, 26 Jun 2024 16:12:12 +0200
Subject: [PATCH] Use serial update by default on one thread

---
 src/nhs_grid.jl             |  7 ++++++-
 test/neighborhood_search.jl | 14 ++++++++++----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/src/nhs_grid.jl b/src/nhs_grid.jl
index 783cf75..04149ff 100644
--- a/src/nhs_grid.jl
+++ b/src/nhs_grid.jl
@@ -77,7 +77,12 @@ struct GridNeighborhoodSearch{NDIMS, update_strategy,
                                            update_strategy = nothing) where {NDIMS}
         ELTYPE = typeof(search_radius)
 
-        if isnothing(update_strategy)
+        if isnothing(update_strategy) && Threads.nthreads == 1
+            # Use serial update on one thread to avoid a second loop over all particles
+            # when `:parallel` is picked.
+            update_strategy = :serial
+        elseif isnothing(update_strategy)
+            # Automatically choose best available update option for this cell list
             update_strategy = first(supported_update_strategies(cell_list))
         elseif !(update_strategy in supported_update_strategies(cell_list))
             throw(ArgumentError("$update_strategy is not a valid update strategy for " *
diff --git a/test/neighborhood_search.jl b/test/neighborhood_search.jl
index ae21441..45a0b8d 100644
--- a/test/neighborhood_search.jl
+++ b/test/neighborhood_search.jl
@@ -158,13 +158,19 @@
             max_corner = maximum(coords, dims = 2) .+ search_radius
 
             neighborhood_searches = [
-                GridNeighborhoodSearch{NDIMS}(; search_radius, n_points),
+                GridNeighborhoodSearch{NDIMS}(; search_radius, n_points,
+                                              # Note that `:semi_parallel` is only the
+                                              # default on multiple threads.
+                                              update_strategy = :semi_parallel),
                 GridNeighborhoodSearch{NDIMS}(; search_radius, n_points,
                                               update_strategy = :serial),
                 GridNeighborhoodSearch{NDIMS}(; search_radius, n_points,
                                               cell_list = FullGridCellList(; min_corner,
                                                                            max_corner,
-                                                                           search_radius)),
+                                                                           search_radius),
+                                              # Note that `:parallel` is only the
+                                              # default on multiple threads.
+                                              update_strategy = :parallel),
                 GridNeighborhoodSearch{NDIMS}(; search_radius, n_points,
                                               cell_list = FullGridCellList(; min_corner,
                                                                            max_corner,
@@ -179,9 +185,9 @@
             ]
 
             names = [
-                "`GridNeighborhoodSearch`",
+                "`GridNeighborhoodSearch` with `:semi_parallel` update",
                 "`GridNeighborhoodSearch` with `:serial` update",
-                "`GridNeighborhoodSearch` with `FullGridCellList` with `DynamicVectorOfVectors`",
+                "`GridNeighborhoodSearch` with `FullGridCellList` with `DynamicVectorOfVectors` and `:parallel` update",
                 "`GridNeighborhoodSearch` with `FullGridCellList` with `DynamicVectorOfVectors` and `:semi_parallel` update",
                 "`GridNeighborhoodSearch` with `FullGridCellList` with `Vector{Vector}`",
                 "`PrecomputedNeighborhoodSearch`",