feat(ai): add monster behaviours

src/ai.lua

@@ -0,0 +1,79 @@
+-- Monster behaviours, one per ai tag. Each takes (game, monster) and fully
+-- resolves that monster's turn through game helpers, so all state mutation and
+-- rng use stays centralised and deterministic.
+
+local Ai = {}
+
+local AGGRO = { chase = 10, pack = 13, ranged = 12, flee = 8, erratic = 8 }
+local DIRS = { { 1, 0 }, { -1, 0 }, { 0, 1 }, { 0, -1 } }
+
+local function chase(game, monster)
+  if game:playerAdjacent(monster) then
+    game:monsterMelee(monster)
+  elseif game:canSeePlayer(monster, AGGRO.chase) then
+    game:stepMonsterToward(monster, game.player.x, game.player.y)
+  end
+end
+
+local function pack(game, monster)
+  if game:playerAdjacent(monster) then
+    game:monsterMelee(monster)
+    return
+  end
+  if game:canSeePlayer(monster, AGGRO.pack) then
+    game:stepMonsterToward(monster, game.player.x, game.player.y)
+    return
+  end
+  local ally = game:nearestAlly(monster)
+  if ally then
+    game:stepMonsterToward(monster, ally.x, ally.y)
+  end
+end
+
+local function ranged(game, monster)
+  if game:canSeePlayer(monster, monster.range) and game:hasClearShot(monster) then
+    if game:playerAdjacent(monster) then
+      game:stepMonsterAway(monster, game.player.x, game.player.y)
+    else
+      game:monsterShoot(monster)
+    end
+    return
+  end
+  if game:canSeePlayer(monster, AGGRO.ranged) then
+    game:stepMonsterToward(monster, game.player.x, game.player.y)
+  end
+end
+
+local function flee(game, monster)
+  if game:canSeePlayer(monster, AGGRO.flee) then
+    game:stepMonsterAway(monster, game.player.x, game.player.y)
+  end
+end
+
+local function erratic(game, monster)
+  if game:playerAdjacent(monster) then
+    game:monsterMelee(monster)
+    return
+  end
+  if game:canSeePlayer(monster, AGGRO.erratic) and game.rng:chance(1, 2) then
+    game:stepMonsterToward(monster, game.player.x, game.player.y)
+  else
+    local dir = DIRS[game.rng:range(1, 4)]
+    game:moveMonster(monster, dir[1], dir[2])
+  end
+end
+
+Ai.behaviours = {
+  chase = chase,
+  pack = pack,
+  ranged = ranged,
+  flee = flee,
+  erratic = erratic,
+}
+
+function Ai.act(game, monster)
+  local behaviour = Ai.behaviours[monster.ai] or chase
+  behaviour(game, monster)
+end
+
+return Ai