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docs/session_summaries/2026-01-07-rename-bodies-to-simulation-and-refactor-velocities.md

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+# Rename bodies.h/cpp to simulation.h and Refactor Velocities (2026-01-07)
+
+## Changes Made
+
+Refactored simulation module and velocity initialization:
+
+1. **Renamed files**
+   - src/bodies.h → src/simulation.h
+   - src/bodies.cpp → src/simulation.cpp
+   - Updated all include references in src/ and tests/
+   - Updated Makefile to reference simulation.cpp
+   - Updated documentation references
+
+2. **Moved simulation logic from config_loader to simulation**
+   - Moved calculate_initial_velocities() to simulation.cpp
+   - Moved calculate_soi_radii() to simulation.cpp
+   - Removed OrbitParams struct - use sim->bodies[i].fields directly
+   - Config loader now only handles TOML parsing
+
+3. **Refactored calculate_initial_velocities()**
+   - Broke up 114-line function into 8 focused functions with descriptive names
+   - Added vec3_cross() utility to physics.cpp for reusable cross product calculation
+   - Made helper functions static in simulation.cpp (internal implementation detail)
+   - Removed unnecessary helper declarations from simulation.h (cleaner public API)
+
+New helper functions:
+- compute_perpendicular_orbital_velocity() - root body orbits (26 lines)
+- compute_orbital_velocity_from_vis_viva() - child body orbits (37 lines)
+- compute_system_barycenter() - barycenter calculation (28 lines)
+- compute_total_root_mass() - total mass calculation (8 lines)
+- set_root_bodies_velocity() - sets all root velocities (15 lines)
+- set_child_bodies_velocity() - sets all child velocities (13 lines)
+- print_system_info_if_multiple_roots() - debug output (7 lines)
+
+Main compute_initial_velocities() reduced from 114 to 10 lines.
+
+4. **Cleaned up public API**
+   - Removed helper function declarations from simulation.h
+   - Only public API remains: compute_initial_velocities(), calculate_initial_velocities(), calculate_soi_radii()
+   - Removed deprecated comment
+
+## Commits
+
+1. `81e799c` Rename bodies.h/cpp to simulation.h/cpp
+2. `9aafbd5` Refactor velocity initialization into smaller functions
+3. `aa8a6b9` Merge branch 'loading-refactor' - refactor velocity initialization
+
+## Results
+
+16 files changed, +200/-212 lines (net -12 lines)
+
+**Net line changes:**
+- config_loader.cpp: 330 → 143 lines (-187)
+- config_loader.h: 22 → 16 lines (-6)
+- physics.cpp: 123 → 131 lines (+8)
+- physics.h: 38 → 39 lines (+1)
+- simulation.cpp: 187 → 350 lines (+163)
+- simulation.h: 56 → 62 lines (+6)
+
+All builds and tests pass (24 assertions, 5 test cases).
+
+**Benefits:**
+- Code reuse: vec3_cross() used 2x (eliminates duplicate perpendicular velocity logic)
+- Testability: smaller functions easier to test individually
+- Readability: descriptive function names for each body type scenario
+- Maintainability: main function reduced from 114 to 10 lines (91% reduction)
+- Better separation of concerns: config_loader only parses TOML, simulation handles physics