From 051b02ed115d31bdc26a5a9b526e07210a21ca4a Mon Sep 17 00:00:00 2001
From: Jacques Lucke <jacques@blender.org>
Date: Fri, 20 Oct 2023 16:47:15 +0200
Subject: [PATCH] Fix #113851: viscoelastic springs do not work

Caused by 425b871607.

The root issue was that these two states of the hash table were
mixed up in 425b871607: (1) the hash table exists and (2) the hash
table is empty.

The use of `std::optional` restores these two different states again.
---
 source/blender/blenkernel/BKE_particle.h            | 4 +++-
 source/blender/blenkernel/intern/particle_system.cc | 6 +++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/source/blender/blenkernel/BKE_particle.h b/source/blender/blenkernel/BKE_particle.h
index 8be441d047e..15b353aeab6 100644
--- a/source/blender/blenkernel/BKE_particle.h
+++ b/source/blender/blenkernel/BKE_particle.h
@@ -9,6 +9,8 @@
  * \ingroup bke
  */
 
+#include <optional>
+
 #include "BLI_buffer.h"
 #include "BLI_compiler_attrs.h"
 #include "BLI_map.hh"
@@ -85,7 +87,7 @@ typedef struct SPHData {
   ParticleSystem *psys[10];
   ParticleData *pa;
   float mass;
-  blender::Map<blender::OrderedEdge, int> eh;
+  std::optional<blender::Map<blender::OrderedEdge, int>> eh;
   float *gravity;
   float hfac;
   /* Average distance to neighbors (other particles in the support domain),
diff --git a/source/blender/blenkernel/intern/particle_system.cc b/source/blender/blenkernel/intern/particle_system.cc
index a7b8172a0d8..ca26b6a1cbd 100644
--- a/source/blender/blenkernel/intern/particle_system.cc
+++ b/source/blender/blenkernel/intern/particle_system.cc
@@ -1806,7 +1806,7 @@ static void sph_force_cb(void *sphdata_v, ParticleKey *state, float *force, floa
   SPHRangeData pfr;
   SPHNeighbor *pfn;
   float *gravity = sphdata->gravity;
-  const blender::Map<blender::OrderedEdge, int> &springhash = sphdata->eh;
+  const std::optional<blender::Map<blender::OrderedEdge, int>> &springhash = sphdata->eh;
 
   float q, u, rij, dv[3];
   float pressure, near_pressure;
@@ -1890,9 +1890,9 @@ static void sph_force_cb(void *sphdata_v, ParticleKey *state, float *force, floa
 
     if (spring_constant > 0.0f) {
       /* Viscoelastic spring force */
-      if (pfn->psys == psys[0] && fluid->flag & SPH_VISCOELASTIC_SPRINGS && !springhash.is_empty())
+      if (pfn->psys == psys[0] && fluid->flag & SPH_VISCOELASTIC_SPRINGS && springhash.has_value())
       {
-        spring_index = springhash.lookup({index, pfn->index});
+        spring_index = springhash->lookup_default({index, pfn->index}, 0);
 
         if (spring_index) {
           spring = psys[0]->fluid_springs + spring_index - 1;