claudes_game/docs/find_dominant_body_bug.md

Find Dominant Body Bug: Mutual SOI Issue

Overview

This documents the remaining issue with find_dominant_body() when two similar-mass bodies are positioned within each other's sphere of influence (SOI).

Date: January 20, 2026 Status: Failing Test (Test 4 from test_plan_invalid_parent_assignment.md) Related: docs/test_plan_invalid_parent_assignment.md

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Test Case 4: Mutual SOI - Similar Mass Planets

Purpose: Edge case: two Earth-like planets positioned within each other's SOI

Actual Result: ❌ STILL FAILS (separate issue, not fixed by spacecraft validation)

Config: tests/configs/mutual_soi_close.toml

Setup:

• PlanetA: mass = 5.972e24 kg, position = {1.496e11, 0, 0}
• PlanetB: mass = 5.972e24 kg, position = {1.501e11, 0, 0}
• Separation: 5e8 meters (500 million km)
• Planet SOI: ~9.25e8 meters (925 million km)
• Both planets within each other's SOI

Why This Fails:

• Both planets start with parent=0 (Sun)
• Both are within each other's SOI
• find_dominant_body() logic selects closest body within SOI
• Result: PlanetA selects PlanetB as parent, PlanetB selects PlanetA as parent
• Config validation passes (both are ≥ parent.radius + body.radius from Sun)

Key Assertions:

// Both should orbit Sun (not each other)
REQUIRE(sim->bodies[PLANET_A_IDX].parent_index == SUN_IDX);
REQUIRE(sim->bodies[PLANET_B_IDX].parent_index == SUN_IDX);

// Planets should never have each other as parents
for (int parent : history.planet_a_parents) {
    REQUIRE(parent != PLANET_B_IDX);
}
for (int parent : history.planet_b_parents) {
    REQUIRE(parent != PLANET_A_IDX);
}

Expected Behavior (Option A):

• Both planets should continue orbiting Sun
• Neither should become other's parent
• This requires future fix to find_dominant_body() logic (mass hierarchy check)

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Future Work

Current Status

Tests 1-3 from the parent assignment test suite are now passing. Test 4 continues to fail, documenting a separate mutual SOI issue.

Potential Fixes for Test 4

1. Add mass hierarchy check to find_dominant_body()
2. Prevent mutual SOI assignments for similar-mass bodies
3. Detect and reject invalid configs at load time
4. Implement proper N-body interaction or restricted 3-body solution

Enhanced Detection

• SOI overlap detection at config load time
• Automatic correction of invalid parent assignments
• Validation warnings for edge cases

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References

• src/simulation.cpp:64-107 - find_dominant_body() implementation
• tests/configs/mutual_soi_close.toml - Test configuration for mutual SOI case
• tests/test_invalid_parent_assignment.cpp - Test suite implementation