ADR 0005: a zap and a throw resolve before the mobs move

Status         Accepted
Audience       Anyone reordering the turn schedule, or wondering why
               `item_system` and `throw_system` sit ahead of `ai`.
Supersedes     --
Related        ../reference/input-and-turn-loop.md

item_system and throw_system run before ai. throw_system is not ordered after trap_system.

Context

A zapped wand is aimed at the dungeon as it stood when the key was pressed. For a while the schedule resolved it after ai, so it read a tile the target had already walked off. That made the aiming reticle look broken.

The bolt wands hid it: a bolt sweeps a line and a blast a disc, so they still caught somebody. But every wand that reads one exact tile (teleport away, teleport to, polymorph, haste, slow, cancellation) reported finding nothing there while the reticle had been on the monster the whole time.

throw_system showed the same fault from the other side. Aimed at the empty tile in front of an approaching orc, the dagger arrived after the orc had stepped onto it and hit one it was never thrown at.

throw_system was also ordered after trap_system. Nothing needed that.

Decision

Order both before ai, and drop the edge after trap_system.

1. A shot that comes down on a trap sets the trap off through detonate_at, inside the throw’s own resolution, so it never waits for the trap step.
2. The two orderings are pinned by a_zap_lands_on_the_tile_the_player_aimed_at_not_the_one_the_target_left and a_throw_lands_where_the_floor_was_when_the_player_let_go in update.rs.
3. The schedule is built by turn_schedule() in models/src/schedule.rs rather than inline in main, so those tests can run it.

Consequences

Accepted costs:

• A turn order with load-bearing edges. Moving either system later breaks aiming, and only the two tests say so.

Benefits realised:

• What the reticle sat on is what the zap or the throw hits.

When to revisit

Reopen this decision if any of these becomes true:

• Aiming becomes real-time. If the target can move between the key and the resolution on purpose, the ordering is a design question.
• Another system needs the post-ai world. Then it needs its own step, not a move of these two.

See also

../reference/input-and-turn-loop.md the schedule, step by step ../how-to/work-with-the-ecs.md adding a step with an .after() edge