Add a variety of masses and radii

* Resolve: Add a variety of masses and radii (#6)

* (Revert changes to .idea/codeStyles)

demo/CMakeLists.txt

@@ -30,7 +30,8 @@ endif ()
 set(CMAKE_WIN32_EXECUTABLE TRUE)
 
 add_executable(grass main.cpp
-        Table.h)
+        Table.h
+        irhall.h)
 
 target_link_libraries(grass raylib)
 target_link_libraries(grass dyn)

demo/irhall.h

@@ -0,0 +1,20 @@
+#ifndef GRASS_IRHALL_H
+#define GRASS_IRHALL_H
+
+// Irwin-Hall approximation to the uniform distribution.
+
+namespace phy {
+
+/// @brief Approximate a normal distribution using a function that generates a
+/// uniform (0,1) variate each time it is called using the Irwin-Hall
+/// approximation (and the central limit theorem).
+template <typename F = float, typename A> F irhnormal(A u01) {
+  F s{};
+  for (unsigned char i = 0; i < 12; i++)
+    s += u01();
+  return s - F(6);
+}
+
+} // namespace phy
+
+#endif // GRASS_IRHALL_H

demo/main.cpp

@@ -7,7 +7,10 @@
 #include <sstream>
 #include <vector>
 
+#include <halton.h>
+
 #include "Table.h"
+#include "irhall.h"
 
 using namespace phy;
 
@@ -19,6 +22,26 @@ static int do_main() {
     return std::abs(p.kin.y0) > 5'000.0f;
   };
 
+  // Quasi-random number generator.
+  // - Quality not so important.
+  // - Possibly reduce binary size by reusing code.
+  // - Possibly help with instruction cache by having simple code.
+  // - Definitely small state (1 short int vs. compared to, say, mt19937).
+  dyn::Halton qrng;
+  auto normal_variate = [&qrng]() {
+    auto &q = qrng;
+    // Approximate normal distribution using uniform random variates.
+    return phy::irhnormal([&q]() { return q.x01(); });
+  };
+  // Make a particle at rest at the origin with a random mass and radius.
+  auto random_particle = [&normal_variate]() {
+    // Random mass and radius, but not other properties.
+    auto mass = std::exp(normal_variate());
+    auto radius = std::exp(2.0f * normal_variate() - 3.0f);
+    // xy, v, m, r.
+    return Particle{{0}, {0}, mass, radius};
+  };
+
   InitWindow(600, 600, "Basic 1,000 particle demo (click to add particles)");
 
   // The physical table (store particles, etc.); backup.
@@ -158,8 +181,10 @@ static int do_main() {
       auto m = GetMousePosition();
       auto n = GetScreenToWorld2D(m, cam);
       auto o = std::complex<float>(n.x, n.y);
-      // Position, velocity, mass, radius
-      table.emplace_back(o, 0.0f, MASS, RADIUS);
+      auto p = random_particle();
+      // Set location
+      p.kin.y0 = o;
+      table.push_back(p);
 
       interactive.spawned_last_frame = true;
     } else {