From 3c6372901bd9c94c4a3c7d3f97a0e363d4951823 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Luk=C3=A1=C5=A1=20Hejl?= <hejl.lukas@gmail.com>
Date: Tue, 8 Oct 2024 18:07:12 +0200
Subject: [PATCH] SPE-2486: Fix weak randomness of fuzzy skin.

Co-authored-by: Ryan Cavanaugh <SeaRyanC@users.noreply.github.com>
Co-authored-by: Noisyfox <timemanager.rick@gmail.com>
---
 src/libslic3r/PerimeterGenerator.cpp | 28 +++++++++++++++++++---------
 1 file changed, 19 insertions(+), 9 deletions(-)

diff --git a/src/libslic3r/PerimeterGenerator.cpp b/src/libslic3r/PerimeterGenerator.cpp
index 1b9a000bb3..709c75401d 100644
--- a/src/libslic3r/PerimeterGenerator.cpp
+++ b/src/libslic3r/PerimeterGenerator.cpp
@@ -14,6 +14,7 @@
 #include <cstdlib>
 #include <iterator>
 #include <limits>
+#include <random>
 #include <unordered_map>
 #include <unordered_set>
 #include <utility>
@@ -52,6 +53,15 @@
 
 namespace Slic3r {
 
+// Produces a random value between 0 and 1. Thread-safe.
+static double random_value() {
+    thread_local std::random_device rd;
+    // Hash thread ID for random number seed if no hardware rng seed is available
+    thread_local std::mt19937 gen(rd.entropy() > 0 ? rd() : std::hash<std::thread::id>()(std::this_thread::get_id()));
+    thread_local std::uniform_real_distribution<double> dist(0.0, 1.0);
+    return dist(gen);
+}
+
 ExtrusionMultiPath PerimeterGenerator::thick_polyline_to_multi_path(const ThickPolyline &thick_polyline, ExtrusionRole role, const Flow &flow, const float tolerance, const float merge_tolerance)
 {
     ExtrusionMultiPath multi_path;
@@ -196,11 +206,11 @@ public:
 };
 
 // Thanks Cura developers for this function.
-static void fuzzy_polygon(Polygon &poly, double fuzzy_skin_thickness, double fuzzy_skin_point_dist)
+static void fuzzy_polygon(Polygon &poly, double fuzzy_skin_thickness, double fuzzy_skin_point_distance)
 {
-    const double min_dist_between_points = fuzzy_skin_point_dist * 3. / 4.; // hardcoded: the point distance may vary between 3/4 and 5/4 the supplied value
-    const double range_random_point_dist = fuzzy_skin_point_dist / 2.;
-    double dist_left_over = double(rand()) * (min_dist_between_points / 2) / double(RAND_MAX); // the distance to be traversed on the line before making the first new point
+    const double min_dist_between_points = fuzzy_skin_point_distance * 3. / 4.; // hardcoded: the point distance may vary between 3/4 and 5/4 the supplied value
+    const double range_random_point_dist = fuzzy_skin_point_distance / 2.;
+    double dist_left_over = random_value() * (min_dist_between_points / 2.); // the distance to be traversed on the line before making the first new point
     Point* p0 = &poly.points.back();
     Points out;
     out.reserve(poly.points.size());
@@ -210,9 +220,9 @@ static void fuzzy_polygon(Polygon &poly, double fuzzy_skin_thickness, double fuz
         double p0p1_size = p0p1.norm();
         double p0pa_dist = dist_left_over;
         for (; p0pa_dist < p0p1_size;
-            p0pa_dist += min_dist_between_points + double(rand()) * range_random_point_dist / double(RAND_MAX))
+            p0pa_dist += min_dist_between_points + random_value() * range_random_point_dist)
         {
-            double r = double(rand()) * (fuzzy_skin_thickness * 2.) / double(RAND_MAX) - fuzzy_skin_thickness;
+            double r = random_value() * (fuzzy_skin_thickness * 2.) - fuzzy_skin_thickness;
             out.emplace_back(*p0 + (p0p1 * (p0pa_dist / p0p1_size) + perp(p0p1).cast<double>().normalized() * r).cast<coord_t>());
         }
         dist_left_over = p0pa_dist - p0p1_size;
@@ -234,7 +244,7 @@ static void fuzzy_extrusion_line(Arachne::ExtrusionLine &ext_lines, double fuzzy
 {
     const double min_dist_between_points = fuzzy_skin_point_dist * 3. / 4.; // hardcoded: the point distance may vary between 3/4 and 5/4 the supplied value
     const double range_random_point_dist = fuzzy_skin_point_dist / 2.;
-    double       dist_left_over          = double(rand()) * (min_dist_between_points / 2) / double(RAND_MAX); // the distance to be traversed on the line before making the first new point
+    double       dist_left_over          = random_value() * (min_dist_between_points / 2.); // the distance to be traversed on the line before making the first new point
 
     auto                                   *p0 = &ext_lines.front();
     std::vector<Arachne::ExtrusionJunction> out;
@@ -249,8 +259,8 @@ static void fuzzy_extrusion_line(Arachne::ExtrusionLine &ext_lines, double fuzzy
         Vec2d  p0p1      = (p1.p - p0->p).cast<double>();
         double p0p1_size = p0p1.norm();
         double p0pa_dist = dist_left_over;
-        for (; p0pa_dist < p0p1_size; p0pa_dist += min_dist_between_points + double(rand()) * range_random_point_dist / double(RAND_MAX)) {
-            double r = double(rand()) * (fuzzy_skin_thickness * 2.) / double(RAND_MAX) - fuzzy_skin_thickness;
+        for (; p0pa_dist < p0p1_size; p0pa_dist += min_dist_between_points + random_value() * range_random_point_dist) {
+            double r = random_value() * (fuzzy_skin_thickness * 2.) - fuzzy_skin_thickness;
             out.emplace_back(p0->p + (p0p1 * (p0pa_dist / p0p1_size) + perp(p0p1).cast<double>().normalized() * r).cast<coord_t>(), p1.w, p1.perimeter_index);
         }
         dist_left_over = p0pa_dist - p0p1_size;