feat: bodies attract

include/sims.hpp

@@ -1,21 +1,23 @@
 #ifndef SIMS_HPP
 #define SIMS_HPP
 
+#include "utils.hpp"
 #include <SDL3/SDL_pixels.h>
 #include <SDL3/SDL_render.h>
 #include <utility>
 
 class Body {
 private:
+        std::pair<float, float> old_positions[200];
+
+public:
     float x;
     float y;
     float vx = 1;
     float vy = 1;
-    std::pair<float, float> old_positions[200];
-
-public:
     SDL_Renderer* renderer;
     SDL_Color color = {255, 255, 255, 255}; // white
+    long double mass = EARTH_MASS;
 
     Body(int x_pos, int y_pos);
 
@@ -23,6 +25,7 @@ public:
     void live();
 };
 
-void two_bodies(SDL_Renderer* r, Body &a, Body &b);
+void two_bodies(SDL_Renderer* renderer, Body &a, Body &b);
+Body create_random_body();
 
 #endif

include/utils.hpp

@@ -0,0 +1,13 @@
+#ifndef UTILS_H
+#define UTILS_H
+
+#include <utility>
+
+// constants
+
+const long double EARTH_MASS =  5.972e24;
+const long double G = 6.67430e-11L;
+
+std::pair<int, int> get_random_position(int x_min, int x_max, int y_min, int y_max);
+
+#endif

src/main.cpp

@@ -2,6 +2,7 @@
 #include <SDL3/SDL_events.h>
 #include <SDL3/SDL_render.h>
 #include <SDL3/SDL_timer.h>
+#include <SDL3/SDL_video.h>
 #include <cstdio>
 #include <cstdlib>
 #include "../include/sims.hpp"
@@ -30,9 +31,9 @@ bool event_handling(SDL_Event e) {
 void sim(SDL_Renderer *r, SDL_Event e) {
     bool exit = false;
 
-    Body body1 = Body(50, 50);
+    Body body1 = create_random_body();
     body1.renderer = r;
-    Body body2 = Body(50, 100);
+    Body body2 = create_random_body();
     body2.renderer = r;
 
     while (!exit) {
@@ -52,7 +53,7 @@ int main() {
     }
 
     SDL_Window *window;
-    window = SDL_CreateWindow("Physics Sim", 720, 540, SDL_WINDOW_RESIZABLE);
+    window = SDL_CreateWindow("Physics Sim", 1920, 1080, SDL_WINDOW_FULLSCREEN);
     if (window == NULL) {
         std::printf("SDL_CreateWindow() failed");
         return EXIT_FAILURE;

src/sims/two_bodies.cpp

@@ -3,6 +3,7 @@
 #include <SDL3/SDL_render.h>
 #include "../../include/sims.hpp"
 #include "../../include/gfx.hpp"
+#include "../../include/utils.hpp"
 #include <algorithm>
 #include <cmath>
 #include <cstdint>
@@ -41,14 +42,50 @@ void Body::draw_tail() {
 
 
 void Body::live() {
-    x += vx;
-    y += sin(x * 0.1);
+    x += vx / 10000000000;
+    y += vy / 10000000000;
     draw_circle(renderer, x, y, 10, true);
     draw_tail();
 }
 
-void two_bodies(SDL_Renderer* r, Body& a, Body& b) {
+Body create_random_body() {
+    pair<int, int> pos = get_random_position(100, 700, 100, 700);
+
+    Body b = Body(pos.first, pos.second);
+    return b;
+}
+
+void two_bodies(SDL_Renderer* renderer, Body& a, Body& b) {
+    
+    float dt = 0.016; // 60 ticks per second, so the delta between one update and the next one is 0.016s
+
+    float dx = b.x - a.x;
+    float dy = b.y - a.y;
+    float r  = sqrtf(dx*dx + dy*dy);
+
+    double F  = G * (a.mass * b.mass) / (r*r);
+    double ux = dx / r;
+    double uy = dy / r;
+
+    double Fx = F * ux;
+    double Fy = F * uy;
+    a.vx += (Fx / a.mass) * dt;
+    a.vy += (Fy / a.mass) * dt;
+
+    b.vx -= (Fx / b.mass) * dt;
+    b.vy -= (Fy / b.mass) * dt;
+
+    if (r < 11) {
+        a.vx = 0;
+        b.vx = 0;
+        a.vy = 0;
+        b.vy = 0;
+    }
+
+    SDL_Log("F: %f", F);
+
     a.live();
     b.live();
 }
 
+

src/utils/rand.cpp

@@ -0,0 +1,15 @@
+#include <random>
+#include <utility>
+
+using namespace std;
+
+// no idea how it does it. gets a random {x,y} pair within a range
+pair<int, int> get_random_position(int x_min, int x_max, int y_min, int y_max) {
+    static random_device rd;
+    static mt19937 gen(rd());
+
+    uniform_int_distribution<> distrib_x(x_min, x_max);
+    uniform_int_distribution<> distrib_y(y_min, y_max);
+
+    return {distrib_x(gen), distrib_y(gen)};
+}