Add headless mode for terminal-based simulation

- Add --headless/-h flag to run without graphics
- Add --days N flag to specify simulation duration
- Print initial state, periodic updates, and final state
- Useful for testing, debugging, and headless environments

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/main.cpp

@@ -4,16 +4,30 @@
 #include "renderer.h"
 #include <cstdio>
 #include <cstring>
+#include <cstdlib>
 
 int main(int argc, char** argv) {
     // Parse command line arguments
     const char* config_file = "configs/solar_system.txt";
-    if (argc > 1) {
-        config_file = argv[1];
+    bool headless = false;
+    int sim_duration_days = 365;  // Default: 1 year for headless mode
+
+    for (int i = 1; i < argc; i++) {
+        if (strcmp(argv[i], "--headless") == 0 || strcmp(argv[i], "-h") == 0) {
+            headless = true;
+        } else if (strcmp(argv[i], "--days") == 0 && i + 1 < argc) {
+            sim_duration_days = atoi(argv[++i]);
+        } else if (argv[i][0] != '-') {
+            config_file = argv[i];
+        }
     }
 
     printf("=== Orbital Mechanics Simulation ===\n");
     printf("Loading configuration: %s\n", config_file);
+    if (headless) {
+        printf("Mode: Headless (terminal output only)\n");
+        printf("Duration: %d days\n", sim_duration_days);
+    }
 
     // Create simulation with time step of 60 seconds
     const int MAX_BODIES = 100;
@@ -27,6 +41,53 @@ int main(int argc, char** argv) {
         return 1;
     }
 
+    // If headless mode, run simulation without rendering
+    if (headless) {
+        const double SECONDS_PER_DAY = 86400.0;
+        const double total_time = sim_duration_days * SECONDS_PER_DAY;
+        const double output_interval = SECONDS_PER_DAY;  // Print status once per day
+        double next_output_time = 0.0;
+
+        printf("\n=== Initial State ===\n");
+        for (int i = 0; i < sim->body_count; i++) {
+            CelestialBody* body = &sim->bodies[i];
+            printf("%s:\n", body->name);
+            printf("  Position: (%.3e, %.3e, %.3e) m\n",
+                   body->position.x, body->position.y, body->position.z);
+            printf("  Velocity: (%.3e, %.3e, %.3e) m/s\n",
+                   body->velocity.x, body->velocity.y, body->velocity.z);
+        }
+
+        printf("\n=== Running Simulation ===\n");
+        while (sim->time < total_time) {
+            update_simulation(sim);
+
+            // Print periodic updates
+            if (sim->time >= next_output_time) {
+                printf("Day %.1f: ", sim->time / SECONDS_PER_DAY);
+                for (int i = 0; i < sim->body_count && i < 3; i++) {
+                    double dist = vec3_magnitude(sim->bodies[i].position);
+                    printf("%s=%.3e m  ", sim->bodies[i].name, dist);
+                }
+                printf("\n");
+                next_output_time += output_interval;
+            }
+        }
+
+        printf("\n=== Final State (Day %.1f) ===\n", sim->time / SECONDS_PER_DAY);
+        for (int i = 0; i < sim->body_count; i++) {
+            CelestialBody* body = &sim->bodies[i];
+            printf("%s:\n", body->name);
+            printf("  Position: (%.3e, %.3e, %.3e) m\n",
+                   body->position.x, body->position.y, body->position.z);
+            printf("  Velocity: (%.3e, %.3e, %.3e) m/s\n",
+                   body->velocity.x, body->velocity.y, body->velocity.z);
+        }
+
+        destroy_simulation(sim);
+        return 0;
+    } else {
+        // Graphical mode
         // Initialize renderer
         init_renderer(1280, 720, "Orbital Mechanics Simulation");
 
@@ -87,4 +148,5 @@ int main(int argc, char** argv) {
 
         printf("\nSimulation ended. Final time: %.2f days\n", sim->time / 86400.0);
         return 0;
+    }
 }