Convert config improvements session notes to markdown

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docs/sessions/2026-01-03-config-improvements.md

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+# 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