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Swarm missiles: stop at conventional infantry, and consider every unit in an adjacent hex #8958

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Swarm missiles: stop at conventional infantry, and consider every unit in an adjacent hex #8958

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b532eb5
Fix #8911: swarm missiles must not pick infantry sheltering in a buil…
HammerGS Sep 12, 2026
dd00dd9
Address review: scan every unit in an adjacent hex, add coverage
HammerGS Sep 12, 2026
9b74b03
Merge remote-tracking branch 'origin/main' into Fix-8911-swarm-second…
HammerGS Sep 12, 2026
be4c54c
Follow the swarm rules: no line of sight, and stop at infantry
HammerGS Sep 13, 2026
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37 changes: 35 additions & 2 deletions megamek/src/megamek/common/compute/Compute.java
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Original file line number Diff line number Diff line change
Expand Up @@ -6449,12 +6449,14 @@ public static Coords scatterAssaultDrop(Coords coords, int margin) {
public static @Nullable Entity getSwarmMissileTarget(Game game, int aeId, Coords coords,
int weaponId) {
Entity tempEntity;
Entity attacker = game.getEntity(aeId);
// first, check the hex of the original target
Iterator<Entity> entities = game.getEntities(coords);
Vector<Entity> possibleTargets = new Vector<>();
while (entities.hasNext()) {
tempEntity = entities.next();
if (!tempEntity.getTargetedBySwarm(aeId, weaponId)) {
if (!tempEntity.getTargetedBySwarm(aeId, weaponId)
&& canBeSwarmedFrom(game, attacker, tempEntity)) {
// we found a target
possibleTargets.add(tempEntity);
}
Expand All @@ -6476,7 +6478,8 @@ public static Coords scatterAssaultDrop(Coords coords, int margin) {
entities = game.getEntities(tempcoords);
if (entities.hasNext()) {
tempEntity = entities.next();
if (!tempEntity.getTargetedBySwarm(aeId, weaponId)) {
if (!tempEntity.getTargetedBySwarm(aeId, weaponId)
&& canBeSwarmedFrom(game, attacker, tempEntity)) {

@HammerGS HammerGS Sep 12, 2026

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Good catch, and it matters more because of this PR. The single-unit scan was already there and was mostly harmless, but this branch adds a reason to reject a unit, so a hex whose first unit is sheltering infantry now contributes nothing and hides whatever else is standing in it. Changed to walk the hex in dd00dd9, matching the original-target scan a few lines above.

// we found a target
possibleTargets.add(tempEntity);
}
Expand All @@ -6490,6 +6493,36 @@ public static Coords scatterAssaultDrop(Coords coords, int margin) {
return null;
}

/**
* Whether leftover swarm missiles may pick this unit as their secondary target.
*
* <p>Conventional infantry inside a building cannot be fired on directly from outside it; the attacker has to
* shoot the building hex instead (TW p.172). Leftover swarm missiles look for a target among the units nearby
* (TO:AUE p.183), and that search has to obey the same rule, or a volley aimed at a building ends up hitting
* the platoon sheltering in it.</p>
*
* <p>Everything else is fair game, including infantry in the open and non-infantry inside the building, which
* the building absorption rules already cover.</p>
*
* @param game the game being played
* @param attacker the unit that fired the volley, or {@code null} if it has left the board
* @param target the unit being considered as a secondary target
*
* @return {@code true} if the missiles may attack this unit
*/
private static boolean canBeSwarmedFrom(Game game, @Nullable Entity attacker, Entity target) {
boolean isShelteringInfantry = (target instanceof Infantry) && isInBuilding(game, target);

@HammerGS HammerGS Sep 12, 2026

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Added in dd00dd9 as SwarmMissileTargetTest. Four cases: infantry in a building skipped from outside, infantry in the open still picked, a non-infantry unit in the same building still picked, and a legal target standing behind a shelterer still found.

The last one fails with the loop change reverted. The shelter case failed while the building terrain was absent, so it is genuinely reading the rule rather than passing by accident.

Worth noting for anyone reusing this: the board is built in code rather than from a board string, because the test board loader fills hexes in file order rather than at the coordinates written, which put the building in the wrong hex.

if (!isShelteringInfantry) {
return true;
}
if (attacker == null) {
return false;
}
// Fire that reaches them through the building itself is legal, which is how a unit already inside shoots
// the platoon it shares the building with.
return LosEffects.calculateLOS(game, attacker, target).getThruBldg() != null;
}

public static @Nullable Coords getFinalPosition(Coords currentPosition, int... v) {
if ((v == null) || (v.length != 6) || (currentPosition == null)) {
return currentPosition;
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