Fix: Schelling Model Neighbor Similarity Calculation

[Sahil-Chhoker]

Summary

Fixed #2515 Schelling segregation model to calculate agent happiness using neighbor similarity fraction, aligning with Wikipedia and NetLogo standards.

Bug / Issue

Current model incorrectly counts empty spaces as neighbors, leading to inaccurate agent happiness determination.

Implementation

Modified SchellingAgent.step() to:

• Filter out empty cells
• Calculate similar neighbor fraction
• Scale homophily threshold

similar_neighbors = [
    neighbor for neighbor in neighbors 
    if hasattr(neighbor, 'type') and neighbor.type == self.type
]
total_neighbors = [
    neighbor for neighbor in neighbors 
    if hasattr(neighbor, 'type')
]

if len(total_neighbors) > 0:
    similarity_fraction = len(similar_neighbors) / len(total_neighbors)
    
    if similarity_fraction < self.model.homophily / 8.0:
        self.model.grid.move_to_empty(self)
    else:
        self.model.happy += 1

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔴 +5.4% [+3.5%, +7.2%]	🔵 +0.4% [+0.3%, +0.5%]
BoltzmannWealth	large	🔵 -0.3% [-0.9%, +0.2%]	🔵 -3.5% [-6.0%, -1.0%]
Schelling	small	🔵 +0.6% [+0.3%, +0.9%]	🟢 -16.1% [-16.3%, -15.8%]
Schelling	large	🔵 +0.4% [-0.0%, +0.9%]	🟢 -6.5% [-7.6%, -5.4%]
WolfSheep	small	🔵 +0.2% [-0.1%, +0.5%]	🔵 -0.0% [-0.2%, +0.1%]
WolfSheep	large	🔵 -1.4% [-2.3%, -0.5%]	🔵 -3.2% [-5.9%, +0.1%]
BoidFlockers	small	🔵 +0.9% [+0.4%, +1.5%]	🔵 +0.7% [-0.2%, +1.7%]
BoidFlockers	large	🔵 +0.4% [-0.4%, +1.2%]	🔵 +0.1% [-0.3%, +0.5%]

[quaquel]

why not change homopily at the model level to be a fraction? That makes the model independent of the neighborhood size.

[quaquel]

you iterate twice, with one check being done in both. You can make this more efficient by looping only once.