# Config File Improvements & Main.cpp Refactoring

**Date:** 2026-01-03

## Overview

This session focused on refactoring main.cpp into smaller functions and adding support for eccentric orbits through an extended configuration file format.

## Changes Made

### 1. Main.cpp Refactoring

Broke down 215-line main() function into focused, reusable functions:

**New Functions:**
- `parse_command_line_args()` - Parse CLI args into ProgramArgs struct
- `print_startup_info()` - Display simulation configuration
- `run_headless_simulation()` - Terminal-based simulation with state capture
- `run_gui_simulation()` - 3D visualization mode

**Result:** main() now only ~25 lines (setup and dispatch)

**Additional Improvements:**
- Added initial state capture in headless mode
- Initial state now printed both at start AND end for comparison
- Cleaner separation of concerns

### 2. Eccentric Orbit Support

Extended config format from 10 to 12 fields:

**Old Format (10 fields):**
```
name mass radius x y z parent_index r g b
```

**New Format (12 fields):**
```
name mass radius x y z parent_index r g b eccentricity semi_major_axis
```

**Implementation:**
- Uses vis-viva equation: `v = sqrt(G*M*(2/r - 1/a))`
- Circular orbits: e=0, a=orbital_radius
- Elliptical orbits: e>0, a=semi_major_axis
- Unified code path for both orbit types

**Example Configurations:**
```
# Circular orbit
Earth 5.972e24 6.371e6 1.496e11 0 0 0 0.0 0.5 1.0 0 1.496e11

# Eccentric orbit (e=0.7)
Comet 1e14 5e3 1.122e11 0 0 0 0.5 0.8 1.0 0.7 3.74e11
```

### 3. Code Simplification

- Removed dual format parsing (now single unified format)
- Shortened function name: `calculate_initial_velocities_with_params()` → `calculate_velocities()`
- Removed `OrbitParams.has_params` field (always have params now)
- Cleaner velocity calculation logic

### 4. Testing

Created test_simple.txt with eccentric comet:
- Eccentricity: 0.7
- Semi-major axis: 2.5 AU
- Perihelion: 0.75 AU
- Aphelion: 4.25 AU

**Test Results:**
- ✓ Initial velocity: 44.849 km/s (matches theoretical)
- ✓ Position varies from 0.75 AU to ~3.8 AU over simulation
- ✓ Velocity varies from 44.8 km/s (perihelion) to 10.6 km/s (aphelion)
- ✓ Energy conservation maintained

### 5. Documentation Updates

**README.md:**
- Updated config format section with 12-field format
- Added examples for both circular and eccentric orbits
- Added "Eccentric orbit support" to features list
- Updated field descriptions

**docs/config_assumptions.md:**
- Documented fixed issues (eccentric orbits, main.cpp refactoring)
- Updated format assumptions (10 → 12 fields)
- Added prioritized TODO list
- Added session notes with detailed change log

## Git Commits

1. `e5da7aa` - Refactor main.cpp into focused functions
   - Extracted argument parsing, headless sim, GUI sim
   - Added initial state capture for comparison

2. `17f11bb` - Add eccentric orbit support with unified config format
   - Extended format to 12 fields
   - Implemented vis-viva equation
   - Added example comet configuration

3. `8b9a06e` - Update documentation for eccentric orbit support
   - Updated README with new format
   - Updated config_assumptions.md

4. `61ac1c5` - Add session notes and TODOs to config documentation
   - Documented all session changes
   - Created prioritized action items

## Files Modified

**Source Code:**
- src/main.cpp (refactored into functions)
- src/config_loader.cpp (added orbit parameters support)

**Configuration:**
- configs/test_simple.txt (updated to 12-field format + comet)

**Documentation:**
- README.md (new format examples)
- docs/config_assumptions.md (status updates + TODOs)

## Known Issues & TODOs

**HIGH PRIORITY:**
- [ ] Update configs/solar_system.txt to new 12-field format
- [ ] Update configs/example_binary_star.txt to new 12-field format
- [ ] Fix Moon position in solar_system.txt
  - Current: (1.500e11, 0, 0) - 4 million km from Earth
  - Should: (~1.496e11 + 3.844e8, 0, 0) - 384,400 km from Earth
- [ ] Fix Jupiter's moons positions (same issue as Moon)

**MEDIUM PRIORITY:**
- [ ] Add config file validation
- [ ] Document coordinate system clearly (absolute vs relative)

**LOW PRIORITY:**
- [ ] Consider relative coordinate option for child bodies
- [ ] Add 3D orbit support (currently XY plane only)

## Technical Notes

### Vis-Viva Equation

```
v = sqrt(G*M*(2/r - 1/a))
```

Where:
- v = orbital velocity at current position
- G = gravitational constant
- M = parent body mass
- r = current distance from parent
- a = semi-major axis of orbit

**For circular orbits (e=0):**
- r = a (constant)
- Equation simplifies to: `v = sqrt(G*M/r)`

**For eccentric orbits (e>0):**
- At perihelion (r = a(1-e)): velocity is maximum
- At aphelion (r = a(1+e)): velocity is minimum
- Demonstrates Kepler's 2nd law (equal areas in equal times)

**Orbital Period:**
```
T = 2π * sqrt(a³/GM)
```

Example for comet (a=2.5 AU): T ≈ 1444 days

## Testing Commands

**Build:**
```bash
make
```

**Run with default config (currently test_simple.txt):**
```bash
./orbit_sim
```

**Run headless for 1000 days:**
```bash
./orbit_sim --days 1000
```

**Run with specific config:**
```bash
./orbit_sim configs/test_simple.txt
```

**Run headless with readable units:**
```bash
./orbit_sim --headless --readable --days 365
```

## Next Session Recommendations

1. Update remaining config files to 12-field format
2. Fix moon positions with correct physics
3. Test updated configurations
4. Consider adding config validation warnings
5. Possibly add argument of periapsis parameter for 3D orbit orientation