ludic/docs/language/phases/phase-fixedupdate.md

id	name	category	kind	tokens	sig	tip	order
phase-fixedupdate	FixedUpdate	phases	phase	FixedUpdate	phase FixedUpdate	The deterministic simulation step — physics and gameplay meant to be reproducible.	2

FixedUpdate runs each frame after Input and before Update. It is the home for the deterministic core of the simulation — movement, collisions, physics — the logic you want to produce the identical result given the same inputs and RNG seed. The runtime groups Input, FixedUpdate, Update, and LateUpdate into the "sim" step (exposed together for replay, rollback, and headless tests), while Render is kept separate so drawing never affects the outcome. Put anything that must stay reproducible here and mark the handler @deterministic; put frame-dressing and non-critical logic in Update instead.

program FixedUpdateExample {
  property Position { column: int = 0, row: int = 0 }
  property Velocity { delta_x: int = 0, delta_y: int = 0 }
  model Projectile { Position, Velocity }

  handler SpawnShot phase Start {
    spawn Projectile { Position { column: 0, row: 8 };  Velocity { delta_x: 1, delta_y: 0 } }
  }

  @deterministic
  handler AdvancePositions phase FixedUpdate {
    for (current_position, current_velocity) in query [Position, Velocity] {
      current_position.column = current_position.column + current_velocity.delta_x
      current_position.row = current_position.row + current_velocity.delta_y
    }
  }

  handler DrawWorld phase Render {
    Screen.clear(Color.MidnightBlue)
    for (current_position) in query [Position, {Projectile}] {
      Screen.fill_rectangle(x: current_position.column * 16, y: current_position.row * 16, width: 16, height: 16, color: Color.Crimson)
    }
    Screen.show()
  }
}