Hunting monster approaches stairs faster than expected

[brturn]

Reported by Bentwing on Discord

The reported behavior is that a burning Zombie moves toward the stairs 5 "steps" in the time it takes the player to go up, then back down the stairs.

This is difficult to reproduce, so the following analysis is based on reading the code, not reproducing the behavior seen.

ANALYSIS

When a monster is hunting the player, and the player leaves the level, the monster gains a new status STATUS_ENTERS_LEVEL_IN:

BrogueCE/src/brogue/RogueMain.c

Line 609 in 2e0ea9a

monst->status[STATUS_ENTERS_LEVEL_IN] = clamp(mapToStairs[monst->loc.x][monst->loc.y] * monst->movementSpeed / 100 + 1, 1, 150);

This status tracks the number of steps the monster will take while the player is on another level. In the normal case, the status will count down and the monster will enter the level:

BrogueCE/src/brogue/Time.c

Lines 1955 to 1956 in 2e0ea9a

	} else if (monst->status[STATUS_ENTERS_LEVEL_IN] == 1) {
	monsterEntersLevel(monst, n);

If the player returns to the level before the monster makes it all the way to the stairs, monsters on that level are restored to their locations and states via restoreMosnter():

BrogueCE/src/brogue/Architect.c

Line 3501 in 2e0ea9a

void restoreMonster(creature *monst, short **mapToStairs, short **mapToPit) {

A monster that has STATUS_ENTERS_LEVEL_IN must be handled differently, since it will have traversed some distance while the player was away:

BrogueCE/src/brogue/Architect.c

Line 3510 in 2e0ea9a

if (monst->status[STATUS_ENTERS_LEVEL_IN] > 0) {

Once a new location is computed for the monster, the final placement location is chosen via getQualifyingPathLocNear(), which picks a legal location for the monster given a set of flags:

BrogueCE/src/brogue/Architect.c

Lines 3542 to 3543 in 2e0ea9a

	pos qualifiedPosition = getQualifyingPathLocNear((pos){ *x, *y }, true, T_DIVIDES_LEVEL & avoidedFlagsForMonster(&(monst->info)), 0,
	avoidedFlagsForMonster(&(monst->info)), (HAS_MONSTER | HAS_PLAYER | HAS_STAIRS | IS_IN_MACHINE), true);

This is unproven, but I believe what is happening is the following:

1. A burning zombie generates gas which is ignited around it. The zombie moves slowly, but the gas is generated quickly. So on any given turn, the zombie is often surrounded by a halo of 1-2 tiles of burning gas.
2. Burning gas has the terrain flag T_IS_FIRE.
3. The call to getQualifyingPathLocNear() passes a set of flags returned from avoidedFlagsForMonster()
4. Zombies are not immune to fire, so the flag set forces the selection of a tile which is NOT on fire.
5. The closest tile to the desired location, which is not on fire, is several spaces closer to the player than what would be expected in normal gameplay.

POTENTIAL FIX:

Allow tiles which the monster would already be willing to step into (eg. monsters on fire will continue to walk through fire).

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A good example of this bug was reported by BaroqueBickle on Discord

bugs.broguerec.zip

Turn 3279 - goblin mystic is seen caught in web, min 13 steps from stairs. Web, other monsters, and spectral blades fill the intervening space.

Turn 3288 - (9 turns later) player has gone up stairs, changed weapons, and went back down stairs. Goblin mystic is now adjacent to the player.