java-topology/defects/minecraft/patch/minecraft-0004-village-goal-visited-hashset.md

UNDF: UNDF-2026-000000393

Classification

Field	Value
CWE	CWE-407 Inefficient Algorithmic Complexity
Severity	MEDIUM
Component	net/minecraft/world/entity/ai/goal/MoveThroughVillageGoal.java
Function	hasNotVisited(BlockPos)
Hot path	Called per AI tick for every village-dwelling mob

Defect

MoveThroughVillageGoal.visited is a List<BlockPos>. The method hasNotVisited(pos) calls this.visited.contains(pos) — O(V) linear scan per call, called inside the goal's tick loop: O(V²) per mob per tick.

// BEFORE — O(V²): List.contains() inside per-tick loop
private final List<BlockPos> visited = Lists.newArrayList();

private boolean hasNotVisited(BlockPos pos) {
    return !this.visited.contains(pos);  // O(V) scan
}

Bounded at V≤15 by vanilla cap. Structural defect pattern; higher caps in mods produce measurable overhead. Affects all villages mobs: villagers, iron golems, cats.

Fix

Maintain a parallel HashSet<BlockPos> for O(1) membership testing.

// AFTER — O(1): HashSet for contains(), List retained for clearing/iteration
private final List<BlockPos> visited    = Lists.newArrayList();
private final Set<BlockPos>  visitedSet = new HashSet<>();

private boolean hasNotVisited(BlockPos pos) {
    return !this.visitedSet.contains(pos);  // O(1)
}

// On clear: visited.clear(); visitedSet.clear();
// On add:   visited.add(pos); visitedSet.add(pos);