{-# LANGUAGE DataKinds #-}
-- | AI procedure for picking the best action for an actor.
module Game.LambdaHack.Client.AI.HandleAbilityM
  ( pickAction
#ifdef EXPOSE_INTERNAL
    -- * Internal operations
  , actionStrategy
  , waitBlockNow, pickup, equipItems, toShare, yieldUnneeded, unEquipItems
  , groupByEqpSlot, bestByEqpSlot, harmful, meleeBlocker, meleeAny
  , trigger, projectItem, ApplyItemGroup, applyItem, flee
  , displaceFoe, displaceBlocker, displaceTowards
  , chase, moveTowards, moveOrRunAid
#endif
  ) where

import Prelude ()

import Game.LambdaHack.Common.Prelude

import           Data.Either
import qualified Data.EnumMap.Lazy as LEM
import qualified Data.EnumMap.Strict as EM
import qualified Data.EnumSet as ES
import           Data.Function
import           Data.Ord
import           Data.Ratio

import           Game.LambdaHack.Client.AI.ConditionM
import           Game.LambdaHack.Client.AI.Strategy
import           Game.LambdaHack.Client.Bfs
import           Game.LambdaHack.Client.BfsM
import           Game.LambdaHack.Client.CommonM
import           Game.LambdaHack.Client.MonadClient
import           Game.LambdaHack.Client.Request
import           Game.LambdaHack.Client.State
import           Game.LambdaHack.Common.Ability
import           Game.LambdaHack.Common.Actor
import           Game.LambdaHack.Common.ActorState
import           Game.LambdaHack.Common.Faction
import           Game.LambdaHack.Common.Frequency
import           Game.LambdaHack.Common.Item
import qualified Game.LambdaHack.Common.ItemAspect as IA
import           Game.LambdaHack.Common.Kind
import           Game.LambdaHack.Common.Level
import           Game.LambdaHack.Common.Misc
import           Game.LambdaHack.Common.MonadStateRead
import           Game.LambdaHack.Common.Point
import qualified Game.LambdaHack.Common.PointArray as PointArray
import           Game.LambdaHack.Common.Random
import           Game.LambdaHack.Common.ReqFailure
import           Game.LambdaHack.Common.State
import qualified Game.LambdaHack.Common.Tile as Tile
import           Game.LambdaHack.Common.Time
import           Game.LambdaHack.Common.Vector
import qualified Game.LambdaHack.Content.ItemKind as IK
import           Game.LambdaHack.Content.ModeKind

-- | Pick the most desirable AI ation for the actor.
pickAction :: MonadClient m => ActorId -> Bool -> m RequestAnyAbility
{-# INLINE pickAction #-}
pickAction aid retry = do
  side <- getsClient sside
  body <- getsState $ getActorBody aid
  let !_A = assert (bfid body == side
                    `blame` "AI tries to move enemy actor"
                    `swith` (aid, bfid body, side)) ()
  let !_A = assert (isNothing (btrajectory body) && not (bproj body)
                    `blame` "AI gets to manually move its trajectory actors"
                    `swith` (aid, bfid body, side)) ()
  -- Reset fleeing flag. May then be set in @flee@.
  modifyClient $ \cli -> cli {sfleeD = EM.delete aid (sfleeD cli)}
  stratAction <- actionStrategy aid retry
  let bestAction = bestVariant stratAction
      !_A = assert (not (nullFreq bestAction)  -- equiv to nullStrategy
                    `blame` "no AI action for actor"
                    `swith` (stratAction, aid, body)) ()
  -- Run the AI: chose an action from those given by the AI strategy.
  rndToAction $ frequency bestAction

type ToAny a = Strategy (RequestTimed a) -> Strategy RequestAnyAbility

toAny :: ToAny a
toAny strat = RequestAnyAbility <$> strat

-- AI strategy based on actor's sight, smell, etc.
-- Never empty.
actionStrategy :: forall m. MonadClient m
               => ActorId -> Bool -> m (Strategy RequestAnyAbility)
{-# INLINE actionStrategy #-}
actionStrategy aid retry = do
  body <- getsState $ getActorBody aid
  scondInMelee <- getsClient scondInMelee
  let condInMelee = scondInMelee LEM.! blid body
  condAimEnemyPresent <- condAimEnemyPresentM aid
  condAimEnemyRemembered <- condAimEnemyRememberedM aid
  condAnyFoeAdj <- condAnyFoeAdjM aid
  threatDistL <- getsState $ meleeThreatDistList aid
  (fleeL, badVic) <- fleeList aid
  condSupport1 <- condSupport 1 aid
  condSupport3 <- condSupport 3 aid
  condSolo <- condSoloM aid  -- solo fighters aggresive
  canDeAmbientL <- getsState $ canDeAmbientList body
  actorSk <- currentSkillsClient aid
  condCanProject <- condCanProjectM (EM.findWithDefault 0 AbProject actorSk) aid
  condAdjTriggerable <- condAdjTriggerableM aid
  condBlocksFriends <- condBlocksFriendsM aid
  condNoEqpWeapon <- condNoEqpWeaponM aid
  condEnoughGear <- condEnoughGearM aid
  condFloorWeapon <- condFloorWeaponM aid
  condDesirableFloorItem <- condDesirableFloorItemM aid
  condTgtNonmoving <- condTgtNonmovingM aid
  explored <- getsClient sexplored
  actorAspect <- getsState sactorAspect
  let ar = actorAspect EM.! aid
      lidExplored = ES.member (blid body) explored
      panicFleeL = fleeL ++ badVic
      condHpTooLow = hpTooLow body ar
      condNotCalmEnough = not (calmEnough body ar)
      speed1_5 = speedScale (3%2) (gearSpeed ar)
      condCanMelee = actorCanMelee actorAspect aid body
      condMeleeBad = not ((condSolo || condSupport1) && condCanMelee)
      condThreat n = not $ null $ takeWhile ((<= n) . fst) threatDistL
      threatAdj = takeWhile ((== 1) . fst) threatDistL
      condManyThreatAdj = length threatAdj >= 2
      condFastThreatAdj =
        any (\(_, (aid2, _)) ->
              let ar2 = actorAspect EM.! aid2
              in gearSpeed ar2 > speed1_5)
        threatAdj
      heavilyDistressed =  -- actor hit by a proj or similarly distressed
        deltaSerious (bcalmDelta body)
      actorShines = IA.aShine ar > 0
      aCanDeLightL | actorShines = []
                   | otherwise = canDeAmbientL
      aCanDeLight = not $ null aCanDeLightL
      canFleeFromLight = not $ null $ aCanDeLightL `intersect` map snd fleeL
      actorMaxSk = IA.aSkills ar
      abInMaxSkill ab = EM.findWithDefault 0 ab actorMaxSk > 0
      stratToFreq :: Int -> m (Strategy RequestAnyAbility)
                  -> m (Frequency RequestAnyAbility)
      stratToFreq scale mstrat = do
        st <- mstrat
        return $! if scale == 0
                  then mzero
                  else scaleFreq scale $ bestVariant st
      -- Order matters within the list, because it's summed with .| after
      -- filtering. Also, the results of prefix, distant and suffix
      -- are summed with .| at the end.
      prefix, suffix :: [([Ability], m (Strategy RequestAnyAbility), Bool)]
      prefix =
        [ ( [AbApply], (toAny :: ToAny 'AbApply)
            <$> applyItem aid ApplyFirstAid
          , not condAnyFoeAdj && condHpTooLow)
        , ( [AbAlter], (toAny :: ToAny 'AbAlter)
            <$> trigger aid ViaStairs
              -- explore next or flee via stairs, even if to wrong level;
              -- in the latter case, may return via different stairs later on
          , condAdjTriggerable && not condAimEnemyPresent
            && ((condNotCalmEnough || condHpTooLow)  -- flee
                && condMeleeBad && condThreat 1
                || (lidExplored || condEnoughGear)  -- explore
                   && not condDesirableFloorItem) )
        , ( [AbDisplace]
          , displaceFoe aid  -- only swap with an enemy to expose him
          , condAnyFoeAdj && condBlocksFriends)  -- later checks foe eligible
        , ( [AbMoveItem], (toAny :: ToAny 'AbMoveItem)
            <$> pickup aid True
          , condNoEqpWeapon  -- we assume organ weapons usually inferior
            && condFloorWeapon && not condHpTooLow
            && abInMaxSkill AbMelee )
        , ( [AbAlter], (toAny :: ToAny 'AbAlter)
            <$> trigger aid ViaEscape
          , condAdjTriggerable && not condAimEnemyPresent
            && not condDesirableFloorItem )  -- collect the last loot
        , ( [AbMove]
          , flee aid fleeL
          , -- Flee either from melee, if our melee is bad and enemy close
            -- or from missiles, if hit and enemies are only far away,
            -- can fling at us and we can't well fling at them.
            not condFastThreatAdj
            && if | condThreat 1 ->
                      not condCanMelee
                      || condManyThreatAdj && not condSupport1 && not condSolo
                  | not condInMelee
                    && (condThreat 2 || condThreat 5 && canFleeFromLight) ->
                    -- Don't keep fleeing if just hit, because too close
                    -- to enemy to get out of his range, most likely,
                    -- and so melee him instead, unless can't melee at all.
                    not condCanMelee
                    || not condSupport3 && not condSolo && not heavilyDistressed
                  | condThreat 5 ->
                    -- Too far to flee from melee, too close from ranged,
                    -- not in ambient, so no point fleeing into dark; advance.
                    False
                  | otherwise ->
                    -- If I'm hit, they are still in range to fling at me,
                    -- even if I can't see them. And probably far away.
                    -- Too far to close in for melee; can't shoot; flee from
                    -- ranged attack and prepare ambush for later on.
                    not condInMelee
                    && heavilyDistressed
                    && (not condCanProject || canFleeFromLight) )
        , ( [AbMelee], (toAny :: ToAny 'AbMelee)
            <$> meleeBlocker aid  -- only melee blocker
          , condAnyFoeAdj  -- if foes, don't displace, otherwise friends:
            || not (abInMaxSkill AbDisplace)  -- displace friends, if possible
               && condAimEnemyPresent )  -- excited
                    -- So animals block each other until hero comes and then
                    -- the stronger makes a show for him and kills the weaker.
        , ( [AbAlter], (toAny :: ToAny 'AbAlter)
            <$> trigger aid ViaNothing
          , not condInMelee  -- don't incur overhead
            && condAdjTriggerable && not condAimEnemyPresent )
        , ( [AbDisplace]  -- prevents some looping movement
          , displaceBlocker aid retry  -- fires up only when path blocked
          , retry || not condDesirableFloorItem )
        , ( [AbMelee], (toAny :: ToAny 'AbMelee)
            <$> meleeAny aid
          , condAnyFoeAdj )  -- won't flee nor displace, so let it melee
        , ( [AbMove]
          , flee aid panicFleeL  -- ultimate panic mode, displaces foes
          , condAnyFoeAdj )
        ]
      -- Order doesn't matter, scaling does.
      -- These are flattened (taking only the best variant) and then summed,
      -- so if any of these can fire, it will fire. If none, @suffix@ is tried.
      -- Only the best variant of @chase@ is taken, but it's almost always
      -- good, and if not, the @chase@ in @suffix@ may fix that.
      distant :: [([Ability], m (Frequency RequestAnyAbility), Bool)]
      distant =
        [ ( [AbMoveItem]
          , stratToFreq (if condInMelee then 2 else 20000)
            $ (toAny :: ToAny 'AbMoveItem)
            <$> yieldUnneeded aid  -- 20000 to unequip ASAP, unless is thrown
          , True )
        , ( [AbMoveItem]
          , stratToFreq 1 $ (toAny :: ToAny 'AbMoveItem)
            <$> equipItems aid  -- doesn't take long, very useful if safe
          , not (condInMelee
                 || condDesirableFloorItem
                 || condNotCalmEnough
                 || heavilyDistressed) )
        , ( [AbProject]
          , stratToFreq (if condTgtNonmoving then 20 else 3)
              -- not too common, to leave missiles for pre-melee dance
            $ (toAny :: ToAny 'AbProject)
            <$> projectItem aid  -- equivalent of @condCanProject@ called inside
          , condAimEnemyPresent && not condInMelee )
        , ( [AbApply]
          , stratToFreq 1 $ (toAny :: ToAny 'AbApply)
            <$> applyItem aid ApplyAll  -- use any potion or scroll
          , condAimEnemyPresent || condThreat 9 )  -- can affect enemies
        , ( [AbMove]
          , stratToFreq (if | condInMelee ->
                              400  -- friends pummeled by target, go to help
                            | not condAimEnemyPresent ->
                              2  -- if enemy only remembered, investigate anyway
                            | otherwise ->
                              20)
            $ chase aid (not condInMelee
                         && (condThreat 12 || heavilyDistressed)
                         && aCanDeLight) retry
          , condCanMelee
            && (if condInMelee then condAimEnemyPresent
                else (condAimEnemyPresent || condAimEnemyRemembered)
                     && (not (condThreat 2)
                         || heavilyDistressed  -- if under fire, do something!
                         || not condMeleeBad)
                       -- this results in animals in corridor never attacking
                       -- (unless distressed by, e.g., being hit by missiles),
                       -- because they can't swarm opponent, which is logical,
                       -- and in rooms they do attack, so not too boring;
                       -- two aliens attack always, because more aggressive
                     && not condDesirableFloorItem) )
        ]
      -- Order matters again.
      suffix =
        [ ( [AbMoveItem], (toAny :: ToAny 'AbMoveItem)
            <$> pickup aid False  -- e.g., to give to other party members
          , not condInMelee )
        , ( [AbMoveItem], (toAny :: ToAny 'AbMoveItem)
            <$> unEquipItems aid  -- late, because these items not bad
          , not condInMelee )
        , ( [AbMove]
          , chase aid (not condInMelee
                       && heavilyDistressed
                       && aCanDeLight) retry
          , if condInMelee then condCanMelee && condAimEnemyPresent
            else not (condThreat 2) || not condMeleeBad )
        ]
      fallback =
        [ ( [AbWait], (toAny :: ToAny 'AbWait)
            <$> waitBlockNow
            -- Wait until friends sidestep; ensures strategy is never empty.
          , True )
        ]
  -- Check current, not maximal skills, since this can be a leader as well
  -- as non-leader action.
  let abInSkill ab = EM.findWithDefault 0 ab actorSk > 0
      checkAction :: ([Ability], m a, Bool) -> Bool
      checkAction (abts, _, cond) = all abInSkill abts && cond
      sumS abAction = do
        let as = filter checkAction abAction
        strats <- mapM (\(_, m, _) -> m) as
        return $! msum strats
      sumF abFreq = do
        let as = filter checkAction abFreq
        strats <- mapM (\(_, m, _) -> m) as
        return $! msum strats
      combineDistant as = liftFrequency <$> sumF as
  sumPrefix <- sumS prefix
  comDistant <- combineDistant distant
  sumSuffix <- sumS suffix
  sumFallback <- sumS fallback
  return $! sumPrefix .| comDistant .| sumSuffix .| sumFallback

waitBlockNow :: MonadClient m => m (Strategy (RequestTimed 'AbWait))
waitBlockNow = return $! returN "wait" ReqWait

pickup :: MonadClient m
       => ActorId -> Bool -> m (Strategy (RequestTimed 'AbMoveItem))
pickup aid onlyWeapon = do
  benItemL <- benGroundItems aid
  b <- getsState $ getActorBody aid
  -- This calmE is outdated when one of the items increases max Calm
  -- (e.g., in pickup, which handles many items at once), but this is OK,
  -- the server accepts item movement based on calm at the start, not end
  -- or in the middle.
  -- The calmE is inaccurate also if an item not IDed, but that's intended
  -- and the server will ignore and warn (and content may avoid that,
  -- e.g., making all rings identified)
  ar <- getsState $ getActorAspect aid
  let calmE = calmEnough b ar
      isWeapon (_, _, _, itemFull, _) = IK.isMelee $ itemKind itemFull
      filterWeapon | onlyWeapon = filter isWeapon
                   | otherwise = id
      prepareOne (oldN, l4)
                 (Benefit{benInEqp}, _, iid, ItemFull{itemKind}, (itemK, _)) =
        let prep newN toCStore = (newN, (iid, itemK, CGround, toCStore) : l4)
            n = oldN + itemK
        in if | calmE && IK.goesIntoSha itemKind && not onlyWeapon ->
                prep oldN CSha
              | benInEqp && eqpOverfull b n ->
                if onlyWeapon then (oldN, l4)
                else prep oldN (if calmE then CSha else CInv)
              | benInEqp ->
                prep n CEqp
              | not onlyWeapon ->
                prep oldN CInv
              | otherwise -> (oldN, l4)
      (_, prepared) = foldl' prepareOne (0, []) $ filterWeapon benItemL
  return $! if null prepared then reject
            else returN "pickup" $ ReqMoveItems prepared

-- This only concerns items that can be equipped, that is with a slot
-- and with @inEqp@ (which implies @goesIntoEqp@).
-- Such items are moved between any stores, as needed. In this case,
-- from inv or sha to eqp.
equipItems :: MonadClient m
           => ActorId -> m (Strategy (RequestTimed 'AbMoveItem))
equipItems aid = do
  body <- getsState $ getActorBody aid
  ar <- getsState $ getActorAspect aid
  let calmE = calmEnough body ar
  eqpAssocs <- getsState $ kitAssocs aid [CEqp]
  invAssocs <- getsState $ kitAssocs aid [CInv]
  shaAssocs <- getsState $ kitAssocs aid [CSha]
  condShineWouldBetray <- condShineWouldBetrayM aid
  condAimEnemyPresent <- condAimEnemyPresentM aid
  discoBenefit <- getsClient sdiscoBenefit
  let improve :: CStore
              -> (Int, [(ItemId, Int, CStore, CStore)])
              -> ( IA.EqpSlot
                 , ( [(Int, (ItemId, ItemFullKit))]
                   , [(Int, (ItemId, ItemFullKit))] ) )
              -> (Int, [(ItemId, Int, CStore, CStore)])
      improve fromCStore (oldN, l4) (slot, (bestInv, bestEqp)) =
        let n = 1 + oldN
        in case (bestInv, bestEqp) of
          ((_, (iidInv, _)) : _, []) | not (eqpOverfull body n) ->
            (n, (iidInv, 1, fromCStore, CEqp) : l4)
          ((vInv, (iidInv, _)) : _, (vEqp, _) : _)
            | vInv > vEqp && not (eqpOverfull body n)
              -- Only add random minor boosts if one slot remains free
              -- for major boosts, to avoid wield/unwield loops.
              -- All slots may be taken only via adding major boosts.
              || not (toShare slot) && not (eqpOverfull body (n + 1)) ->
                (n, (iidInv, 1, fromCStore, CEqp) : l4)
          _ -> (oldN, l4)
      heavilyDistressed =  -- Actor hit by a projectile or similarly distressed.
        deltaSerious (bcalmDelta body)
      -- We filter out unneeded items. In particular, we ignore them in eqp
      -- when comparing to items we may want to equip, so that the unneeded
      -- but powerful items don't fool us.
      -- In any case, the unneeded items should be removed from equip
      -- in @yieldUnneeded@ earlier or soon after this check.
      -- In other stores we need to filter, for otherwise we'd have
      -- a loop of equip/yield.
      filterNeeded (_, (itemFull, _)) =
        not $ hinders condShineWouldBetray condAimEnemyPresent
                      heavilyDistressed (not calmE) ar itemFull
      bestThree = bestByEqpSlot discoBenefit
                                (filter filterNeeded eqpAssocs)
                                (filter filterNeeded invAssocs)
                                (filter filterNeeded shaAssocs)
      bEqpInv = foldl' (improve CInv) (0, [])
                $ map (\(slot, (eqp, inv, _)) ->
                        (slot, (inv, eqp))) bestThree
      bEqpBoth | calmE =
                   foldl' (improve CSha) bEqpInv
                   $ map (\(slot, (eqp, _, sha)) ->
                           (slot, (sha, eqp))) bestThree
               | otherwise = bEqpInv
      (_, prepared) = bEqpBoth
  return $! if null prepared
            then reject
            else returN "equipItems" $ ReqMoveItems prepared

toShare :: IA.EqpSlot -> Bool
toShare IA.EqpSlotMiscBonus = False
toShare IA.EqpSlotMiscAbility = False
toShare _ = True

yieldUnneeded :: MonadClient m
              => ActorId -> m (Strategy (RequestTimed 'AbMoveItem))
yieldUnneeded aid = do
  body <- getsState $ getActorBody aid
  ar <- getsState $ getActorAspect aid
  let calmE = calmEnough body ar
  eqpAssocs <- getsState $ kitAssocs aid [CEqp]
  condShineWouldBetray <- condShineWouldBetrayM aid
  condAimEnemyPresent <- condAimEnemyPresentM aid
  discoBenefit <- getsClient sdiscoBenefit
  -- Here and in @unEquipItems@ AI may hide from the human player,
  -- in shared stash, the Ring of Speed And Bleeding,
  -- which is a bit harsh, but fair. However any subsequent such
  -- rings will not be picked up at all, so the human player
  -- doesn't lose much fun. Additionally, if AI learns alchemy later on,
  -- they can repair the ring, wield it, drop at death and it's
  -- in play again.
  let heavilyDistressed =  -- Actor hit by a projectile or similarly distressed.
        deltaSerious (bcalmDelta body)
      csha = if calmE then CSha else CInv
      yieldSingleUnneeded (iidEqp, (itemEqp, (itemK, _))) =
        if | harmful discoBenefit iidEqp ->
             [(iidEqp, itemK, CEqp, CInv)]  -- harmful not shared
           | hinders condShineWouldBetray condAimEnemyPresent
                     heavilyDistressed (not calmE) ar itemEqp ->
             [(iidEqp, itemK, CEqp, csha)]
           | otherwise -> []
      yieldAllUnneeded = concatMap yieldSingleUnneeded eqpAssocs
  return $! if null yieldAllUnneeded
            then reject
            else returN "yieldUnneeded" $ ReqMoveItems yieldAllUnneeded

-- This only concerns items that can be equipped, that is with a slot
-- and with @inEqp@ (which implies @goesIntoEqp@).
-- Such items are moved between any stores, as needed. In this case,
-- from inv or eqp to sha.
unEquipItems :: MonadClient m
             => ActorId -> m (Strategy (RequestTimed 'AbMoveItem))
unEquipItems aid = do
  body <- getsState $ getActorBody aid
  ar <- getsState $ getActorAspect aid
  let calmE = calmEnough body ar
  eqpAssocs <- getsState $ kitAssocs aid [CEqp]
  invAssocs <- getsState $ kitAssocs aid [CInv]
  shaAssocs <- getsState $ kitAssocs aid [CSha]
  condShineWouldBetray <- condShineWouldBetrayM aid
  condAimEnemyPresent <- condAimEnemyPresentM aid
  discoBenefit <- getsClient sdiscoBenefit
  let improve :: CStore -> ( IA.EqpSlot
                           , ( [(Int, (ItemId, ItemFullKit))]
                             , [(Int, (ItemId, ItemFullKit))] ) )
              -> [(ItemId, Int, CStore, CStore)]
      improve fromCStore (slot, (bestSha, bestEOrI)) =
        case bestEOrI of
          _ | not (toShare slot)
              && fromCStore == CEqp
              && not (eqpOverfull body 1) ->  -- keep minor boosts up to M-1
            []
          ((vEOrI, (iidEOrI, bei)) : _) | (toShare slot || fromCStore == CInv)
                                          && getK bei > 1
                                          && betterThanSha vEOrI bestSha ->
            -- To share the best items with others, if they care.
            [(iidEOrI, getK bei - 1, fromCStore, CSha)]
          (_ : (vEOrI, (iidEOrI, bei)) : _) | (toShare slot
                                               || fromCStore == CInv)
                                              && betterThanSha vEOrI bestSha ->
            -- To share the second best items with others, if they care.
            [(iidEOrI, getK bei, fromCStore, CSha)]
          ((vEOrI, (_, _)) : _) | fromCStore == CEqp
                                  && eqpOverfull body 1
                                  && worseThanSha vEOrI bestSha ->
            -- To make place in eqp for an item better than any ours.
            -- Even a minor boost is removed only if sha has a better one.
            [(fst $ snd $ last bestEOrI, 1, fromCStore, CSha)]
          _ -> []
      getK (_, (itemK, _)) = itemK
      betterThanSha _ [] = True
      betterThanSha vEOrI ((vSha, _) : _) = vEOrI > vSha
      worseThanSha _ [] = False
      worseThanSha vEOrI ((vSha, _) : _) = vEOrI < vSha
      heavilyDistressed =  -- Actor hit by a projectile or similarly distressed.
        deltaSerious (bcalmDelta body)
      -- Here we don't need to filter out items that hinder (except in sha)
      -- because they are moved to sha and will be equipped by another actor
      -- at another time, where hindering will be completely different.
      -- If they hinder and we unequip them, all the better.
      -- We filter sha to consider only eligible items in @worseThanSha@.
      filterNeeded (_, (itemFull, _)) =
        not $ hinders condShineWouldBetray condAimEnemyPresent
                      heavilyDistressed (not calmE) ar itemFull
      bestThree = bestByEqpSlot discoBenefit eqpAssocs invAssocs
                                (filter filterNeeded shaAssocs)
      bInvSha = concatMap
                  (improve CInv . (\(slot, (_, inv, sha)) ->
                                    (slot, (sha, inv)))) bestThree
      bEqpSha = concatMap
                  (improve CEqp . (\(slot, (eqp, _, sha)) ->
                                    (slot, (sha, eqp)))) bestThree
      prepared = if calmE then bInvSha ++ bEqpSha else []
  return $! if null prepared
            then reject
            else returN "unEquipItems" $ ReqMoveItems prepared

groupByEqpSlot :: [(ItemId, ItemFullKit)]
               -> EM.EnumMap IA.EqpSlot [(ItemId, ItemFullKit)]
groupByEqpSlot is =
  let f (iid, itemFullKit) = case IK.getEqpSlot $ itemKind $ fst itemFullKit of
        Nothing -> Nothing
        Just es -> Just (es, [(iid, itemFullKit)])
      withES = mapMaybe f is
  in EM.fromListWith (++) withES

bestByEqpSlot :: DiscoveryBenefit
              -> [(ItemId, ItemFullKit)]
              -> [(ItemId, ItemFullKit)]
              -> [(ItemId, ItemFullKit)]
              -> [(IA.EqpSlot
                  , ( [(Int, (ItemId, ItemFullKit))]
                    , [(Int, (ItemId, ItemFullKit))]
                    , [(Int, (ItemId, ItemFullKit))] ) )]
bestByEqpSlot discoBenefit eqpAssocs invAssocs shaAssocs =
  let eqpMap = EM.map (\g -> (g, [], [])) $ groupByEqpSlot eqpAssocs
      invMap = EM.map (\g -> ([], g, [])) $ groupByEqpSlot invAssocs
      shaMap = EM.map (\g -> ([], [], g)) $ groupByEqpSlot shaAssocs
      appendThree (g1, g2, g3) (h1, h2, h3) = (g1 ++ h1, g2 ++ h2, g3 ++ h3)
      eqpInvShaMap = EM.unionsWith appendThree [eqpMap, invMap, shaMap]
      bestSingle = strongestSlot discoBenefit
      bestThree eqpSlot (g1, g2, g3) = (bestSingle eqpSlot g1,
                                        bestSingle eqpSlot g2,
                                        bestSingle eqpSlot g3)
  in EM.assocs $ EM.mapWithKey bestThree eqpInvShaMap

harmful :: DiscoveryBenefit -> ItemId -> Bool
harmful discoBenefit iid =
  -- Items that are known, perhaps recently discovered, and it's now revealed
  -- they should not be kept in equipment, should be unequipped
  -- (either they are harmful or they waste eqp space).
  not $ benInEqp $ discoBenefit EM.! iid

-- Everybody melees in a pinch, even though some prefer ranged attacks.
meleeBlocker :: MonadClient m => ActorId -> m (Strategy (RequestTimed 'AbMelee))
meleeBlocker aid = do
  b <- getsState $ getActorBody aid
  actorAspect <- getsState sactorAspect
  let ar = actorAspect EM.! aid
  fact <- getsState $ (EM.! bfid b) . sfactionD
  actorSk <- currentSkillsClient aid
  mtgtMPath <- getsClient $ EM.lookup aid . stargetD
  case mtgtMPath of
    Just TgtAndPath{ tapTgt=TEnemy{}
                   , tapPath=AndPath{pathList=q : _, pathGoal} }
      | q == pathGoal -> return reject  -- not a real blocker, but goal enemy
    Just TgtAndPath{tapPath=AndPath{pathList=q : _, pathGoal}} -> do
      -- We prefer the goal position, so that we can kill the foe and enter it,
      -- but we accept any @q@ as well.
      let maim | adjacent (bpos b) pathGoal = Just pathGoal
               | adjacent (bpos b) q = Just q
               | otherwise = Nothing  -- MeleeDistant
      lBlocker <- case maim of
        Nothing -> return []
        Just aim -> getsState $ posToAssocs aim (blid b)
      case lBlocker of
        (aid2, body2) : _ -> do
          let ar2 = actorAspect EM.! aid2
          -- No problem if there are many projectiles at the spot. We just
          -- attack the first one.
          if | actorDying body2
               || bproj body2  -- displacing saves a move
                  && EM.findWithDefault 0 AbDisplace actorSk <= 0 ->
               return reject
             | isFoe (bfid b) fact (bfid body2)
                 -- at war with us, so hit, not displace
               || isFriend (bfid b) fact (bfid body2) -- don't start a war
                  && EM.findWithDefault 0 AbDisplace actorSk <= 0  -- can't disp
                  && EM.findWithDefault 0 AbMove actorSk > 0  -- blocked move
                  && 3 * bhp body2 < bhp b  -- only get rid of weak friends
                  && gearSpeed ar2 <= gearSpeed ar -> do
               mel <- maybeToList <$> pickWeaponClient aid aid2
               return $! liftFrequency $ uniformFreq "melee in the way" mel
             | otherwise -> return reject
        [] -> return reject
    _ -> return reject  -- probably no path to the enemy, if any

-- Everybody melees in a pinch, skills and weapons allowing,
-- even though some prefer ranged attacks.
meleeAny :: MonadClient m => ActorId -> m (Strategy (RequestTimed 'AbMelee))
meleeAny aid = do
  b <- getsState $ getActorBody aid
  fact <- getsState $ (EM.! bfid b) . sfactionD
  adjacentAssocs <- getsState $ actorAdjacentAssocs b
  let foe (_, b2) =
        not (bproj b2) && isFoe (bfid b) fact (bfid b2) && bhp b2 > 0
      adjFoes = filter foe adjacentAssocs
  btarget <- getsClient $ getTarget aid
  mtarget <- case btarget of
    Just (TEnemy aid2 _) -> do
      b2 <- getsState $ getActorBody aid2
      return $! if adjacent (bpos b2) (bpos b) && foe (aid2, b2)
                then Just (aid2, b2)
                else Nothing
    _ -> return Nothing
  let adjTargets = maybe adjFoes return mtarget
  mels <- mapM (pickWeaponClient aid . fst) adjTargets
  let freq = uniformFreq "melee adjacent" $ catMaybes mels
  return $! liftFrequency freq

-- The level the actor is on is either explored or the actor already
-- has a weapon equipped, so no need to explore further, he tries to find
-- enemies on other levels.
-- We don't verify any embedded item is targeted by the actor, but at least
-- the actor doesn't target a visible enemy at this point.
trigger :: MonadClient m
        => ActorId -> FleeViaStairsOrEscape
        -> m (Strategy (RequestTimed 'AbAlter))
trigger aid fleeVia = do
  b <- getsState $ getActorBody aid
  lvl <- getLevel (blid b)
  let f pos = case EM.lookup pos $ lembed lvl of
        Nothing -> Nothing
        Just bag -> Just (pos, bag)
      pbags = mapMaybe f $ vicinityUnsafe (bpos b)
  efeat <- embedBenefit fleeVia aid pbags
  return $! liftFrequency $ toFreq "trigger"
    [ (ceiling benefit, ReqAlter pos)
    | (benefit, (pos, _)) <- efeat ]

projectItem :: MonadClient m
            => ActorId -> m (Strategy (RequestTimed 'AbProject))
projectItem aid = do
  btarget <- getsClient $ getTarget aid
  b <- getsState $ getActorBody aid
  mfpos <- case btarget of
    Nothing -> return Nothing
    Just target -> getsState $ aidTgtToPos aid (blid b) target
  seps <- getsClient seps
  case (btarget, mfpos) of
    (_, Just fpos) | adjacent (bpos b) fpos -> return reject
    (Just TEnemy{}, Just fpos) -> do
      mnewEps <- makeLine False b fpos seps
      case mnewEps of
        Just newEps -> do
          actorSk <- currentSkillsClient aid
          let skill = EM.findWithDefault 0 AbProject actorSk
          -- ProjectAimOnself, ProjectBlockActor, ProjectBlockTerrain
          -- and no actors or obstacles along the path.
          benList <- condProjectListM skill aid
          localTime <- getsState $ getLocalTime (blid b)
          let coeff CGround = 2  -- pickup turn saved
              coeff COrgan = error $ "" `showFailure` benList
              coeff CEqp = 100000  -- must hinder currently
              coeff CInv = 1
              coeff CSha = 1
              fRanged (Benefit{benFling}, cstore, iid, itemFull, kit) =
                -- We assume if the item has a timeout, most effects are under
                -- Recharging, so no point projecting if not recharged.
                -- This changes in time, so recharging is not included
                -- in @condProjectListM@, but checked here, just before fling.
                let recharged = hasCharge localTime itemFull kit
                    trange = IK.totalRange $ itemKind itemFull
                    bestRange =
                      chessDist (bpos b) fpos + 2  -- margin for fleeing
                    rangeMult =  -- penalize wasted or unsafely low range
                      10 + max 0 (10 - abs (trange - bestRange))
                    benR = coeff cstore * benFling
                in if trange >= chessDist (bpos b) fpos && recharged
                   then Just ( - ceiling (benR * fromIntegral rangeMult / 10)
                             , ReqProject fpos newEps iid cstore )
                   else Nothing
              benRanged = mapMaybe fRanged benList
          return $! liftFrequency $ toFreq "projectItem" benRanged
        _ -> return reject
    _ -> return reject

data ApplyItemGroup = ApplyAll | ApplyFirstAid
  deriving Eq

applyItem :: MonadClient m
          => ActorId -> ApplyItemGroup -> m (Strategy (RequestTimed 'AbApply))
applyItem aid applyGroup = do
  actorSk <- currentSkillsClient aid
  b <- getsState $ getActorBody aid
  condShineWouldBetray <- condShineWouldBetrayM aid
  condAimEnemyPresent <- condAimEnemyPresentM aid
  localTime <- getsState $ getLocalTime (blid b)
  ar <- getsState $ getActorAspect aid
  let calmE = calmEnough b ar
      condNotCalmEnough = not calmE
      heavilyDistressed =  -- Actor hit by a projectile or similarly distressed.
        deltaSerious (bcalmDelta b)
      skill = EM.findWithDefault 0 AbApply actorSk
      -- This detects if the value of keeping the item in eqp is in fact < 0.
      hind = hinders condShineWouldBetray condAimEnemyPresent
                     heavilyDistressed condNotCalmEnough ar
      permittedActor itemFull kit =
        either (const False) id
        $ permittedApply localTime skill calmE itemFull kit
      -- Both effects tweak items, which is only situationally beneficial
      -- and not really the best idea while in combat.
      getTweak IK.PolyItem = True
      getTweak IK.Identify = True
      getTweak (IK.OneOf l) = any getTweak l
      getTweak (IK.Recharging eff) = getTweak eff
      getTweak (IK.Composite l) = any getTweak l
      getTweak _ = False
      q (Benefit{benInEqp}, _, _, itemFull@ItemFull{itemKind}, kit) =
        let durable = IK.Durable `elem` IK.ifeature itemKind
        in (not benInEqp  -- can't wear, so OK to break
            || durable  -- can wear, but can't break, even better
            || not (IK.isMelee itemKind)  -- anything else expendable
               && hind itemFull)  -- hinders now, so possibly often, so away!
           && permittedActor itemFull kit
           && not (any getTweak $ IK.ieffects itemKind)
           && not (IK.isHumanTrinket itemKind)  -- hack for elixir of youth
      -- Organs are not taken into account, because usually they are either
      -- melee items, so harmful, or periodic, so charging between activations.
      -- The case of a weak weapon curing poison is too rare to incur overhead.
      stores = [CEqp, CInv, CGround] ++ [CSha | calmE]
  discoBenefit <- getsClient sdiscoBenefit
  benList <- getsState $ benAvailableItems discoBenefit aid stores
  getKind <- getsState $ flip getIidKind
  let (myBadGrps, myGoodGrps) = partitionEithers $ mapMaybe (\iid ->
        let itemKind = getKind iid
        in if IK.isTmpCondition itemKind
           then Just $ if benInEqp (discoBenefit EM.! iid)
                       then Left $ toGroupName $ IK.iname itemKind
                         -- conveniently, @iname@ matches @ifreq@
                       else Right $ toGroupName $ IK.iname itemKind
           else Nothing) (EM.keys $ borgan b)
      coeff CGround = 2  -- pickup turn saved
      coeff COrgan = error $ "" `showFailure` benList
      coeff CEqp = 1
      coeff CInv = 1
      coeff CSha = 1
      fTool benAv@(Benefit{benApply}, cstore, iid, ItemFull{itemKind}, _) =
        let -- Don't include @Ascend@ nor @Teleport@, because maybe no foe near.
            -- Don't include @OneOf@ because other effects may kill you.
            getHP (IK.RefillHP p) | p > 0 = True
            getHP (IK.Recharging eff) = getHP eff
            getHP (IK.Composite l) = any getHP l
            getHP _ = False
            heals = any getHP $ IK.ieffects itemKind
            dropsGrps = IK.getDropOrgans itemKind
            dropsBadOrgans =
              not (null myBadGrps)
              && toGroupName "condition" `elem` dropsGrps
                 || not (null (dropsGrps `intersect` myBadGrps))
            dropsGoodOrgans =
              not (null myGoodGrps)
              && toGroupName "condition" `elem` dropsGrps
                 || not (null (dropsGrps `intersect` myGoodGrps))
            wastesDrop = null myBadGrps && not (null dropsGrps)
            durable = IK.Durable `elem` IK.ifeature itemKind
            situationalBenApply | dropsBadOrgans = benApply + 20
                                | wastesDrop = benApply - 10
                                | otherwise = benApply
            benR = ceiling situationalBenApply
                   * if cstore == CEqp && not durable
                     then 1000  -- must hinder currently
                     else coeff cstore
            canApply = situationalBenApply > 0 && case applyGroup of
              ApplyFirstAid -> q benAv && heals
              ApplyAll -> q benAv
                          && not dropsGoodOrgans
                          && (dropsBadOrgans
                              || not (hpEnough b ar && heals))
        in if canApply
           then Just (benR, ReqApply iid cstore)
           else Nothing
      benTool = mapMaybe fTool benList
  return $! liftFrequency $ toFreq "applyItem" benTool

-- If low on health or alone, flee in panic, close to the path to target
-- and as far from the attackers, as possible. Usually fleeing from
-- foes will lead towards friends, but we don't insist on that.
flee :: MonadClient m
     => ActorId -> [(Int, Point)] -> m (Strategy RequestAnyAbility)
flee aid fleeL = do
  b <- getsState $ getActorBody aid
  -- Regardless if fleeing accomplished, mark the need.
  modifyClient $ \cli -> cli {sfleeD = EM.insert aid (bpos b) (sfleeD cli)}
  let vVic = map (second (`vectorToFrom` bpos b)) fleeL
      str = liftFrequency $ toFreq "flee" vVic
  mapStrategyM (moveOrRunAid aid) str

-- The result of all these conditions is that AI displaces rarely,
-- but it can't be helped as long as the enemy is smart enough to form fronts.
displaceFoe :: MonadClient m => ActorId -> m (Strategy RequestAnyAbility)
displaceFoe aid = do
  COps{coTileSpeedup} <- getsState scops
  b <- getsState $ getActorBody aid
  lvl <- getLevel $ blid b
  fact <- getsState $ (EM.! bfid b) . sfactionD
  friends <- getsState $ friendRegularList (bfid b) (blid b)
  adjacentAssocs <- getsState $ actorAdjacentAssocs b
  let foe (_, b2) =
        not (bproj b2) && isFoe (bfid b) fact (bfid b2) && bhp b2 > 0
      adjFoes = filter foe adjacentAssocs
      displaceable body =  -- DisplaceAccess
        Tile.isWalkable coTileSpeedup (lvl `at` bpos body)
      nFriends body = length $ filter (adjacent (bpos body) . bpos) friends
      nFrNew = nFriends b + 1
      qualifyActor (aid2, body2) = do
        actorMaxSk <- maxActorSkillsClient aid2
        dEnemy <- getsState $ dispEnemy aid aid2 actorMaxSk
          -- DisplaceDying, DisplaceBraced, DisplaceImmobile, DisplaceSupported
        let nFrOld = nFriends body2
        return $! if displaceable body2 && dEnemy && nFrOld < nFrNew
                  then Just (nFrOld * nFrOld, bpos body2 `vectorToFrom` bpos b)
                  else Nothing
  vFoes <- mapM qualifyActor adjFoes
  let str = liftFrequency $ toFreq "displaceFoe" $ catMaybes vFoes
  mapStrategyM (moveOrRunAid aid) str

displaceBlocker :: MonadClient m
                => ActorId -> Bool -> m (Strategy RequestAnyAbility)
displaceBlocker aid retry = do
  b <- getsState $ getActorBody aid
  mtgtMPath <- getsClient $ EM.lookup aid . stargetD
  str <- case mtgtMPath of
    Just TgtAndPath{ tapTgt=TEnemy{}
                   , tapPath=AndPath{pathList=q : _, pathGoal} }
      | q == pathGoal && not retry ->
        return reject  -- not a real blocker but goal, possibly enemy to melee
    Just TgtAndPath{tapPath=AndPath{pathList=q : _}}
      | adjacent (bpos b) q ->  -- not veered off target
      displaceTowards aid q retry
    _ -> return reject  -- goal reached
  mapStrategyM (moveOrRunAid aid) str

displaceTowards :: MonadClient m
                => ActorId -> Point -> Bool -> m (Strategy Vector)
displaceTowards aid target retry = do
  COps{coTileSpeedup} <- getsState scops
  b <- getsState $ getActorBody aid
  let source = bpos b
  let !_A = assert (adjacent source target) ()
  lvl <- getLevel $ blid b
  if boldpos b /= Just target -- avoid trivial loops
     && Tile.isWalkable coTileSpeedup (lvl `at` target) then do
       -- DisplaceAccess
    mleader <- getsClient sleader
    mBlocker <- getsState $ posToAssocs target (blid b)
    case mBlocker of
      [] -> return reject
      [(aid2, b2)] | Just aid2 /= mleader -> do
        mtgtMPath <- getsClient $ EM.lookup aid2 . stargetD
        enemyTgt <- condAimEnemyPresentM aid
        enemyPos <- condAimEnemyRememberedM aid
        enemyTgt2 <- condAimEnemyPresentM aid2
        enemyPos2 <- condAimEnemyRememberedM aid2
        case mtgtMPath of
          Just TgtAndPath{tapPath=AndPath{pathList=q : _}}
            | q == source  -- friend wants to swap
              || retry  -- desperate
                 && not (boldpos b == Just target  -- and no displace loop
                         && not (waitedLastTurn b))
              || (enemyTgt || enemyPos) && not (enemyTgt2 || enemyPos2) ->
                 -- he doesn't have Enemy target and I have, so push him aside,
                 -- because, for heroes, he will never be a leader, so he can't
                 -- step aside himself
              return $! returN "displace friend" $ target `vectorToFrom` source
          Just _ -> return reject
          Nothing -> do  -- an enemy or ally or disoriented friend --- swap
            tfact <- getsState $ (EM.! bfid b2) . sfactionD
            actorMaxSk <- maxActorSkillsClient aid2
            dEnemy <- getsState $ dispEnemy aid aid2 actorMaxSk
            if not (isFoe (bfid b2) tfact (bfid b)) || dEnemy then
              return $! returN "displace other" $ target `vectorToFrom` source
            else return reject  -- DisplaceDying, etc.
      _ -> return reject  -- DisplaceProjectiles or trying to displace leader
  else return reject

chase :: MonadClient m
      => ActorId -> Bool -> Bool -> m (Strategy RequestAnyAbility)
chase aid avoidAmbient retry = do
  COps{coTileSpeedup} <- getsState scops
  body <- getsState $ getActorBody aid
  fact <- getsState $ (EM.! bfid body) . sfactionD
  mtgtMPath <- getsClient $ EM.lookup aid . stargetD
  lvl <- getLevel $ blid body
  let isAmbient pos = Tile.isLit coTileSpeedup (lvl `at` pos)
                      && Tile.isWalkable coTileSpeedup (lvl `at` pos)
                        -- if solid, will be altered and perhaps darkened
  str <- case mtgtMPath of
    Just TgtAndPath{tapPath=AndPath{pathList=q : _, ..}}
      | pathGoal == bpos body -> return reject  -- shortcut and just to be sure
      | not $ avoidAmbient && isAmbient q ->
      -- With no leader, the goal is vague, so permit arbitrary detours.
      moveTowards aid q pathGoal (fleaderMode (gplayer fact) == LeaderNull
                                  || retry)
    _ -> return reject  -- goal reached or banned ambient lit tile
  if avoidAmbient && nullStrategy str
  then chase aid False retry
  else mapStrategyM (moveOrRunAid aid) str

moveTowards :: MonadClient m
            => ActorId -> Point -> Point -> Bool -> m (Strategy Vector)
moveTowards aid target goal relaxed = do
  b <- getsState $ getActorBody aid
  actorSk <- currentSkillsClient aid
  let source = bpos b
      alterSkill = EM.findWithDefault 0 AbAlter actorSk
      !_A = assert (source == bpos b
                    `blame` (source, bpos b, aid, b, goal)) ()
      !_B = assert (adjacent source target
                    `blame` (source, target, aid, b, goal)) ()
  fact <- getsState $ (EM.! bfid b) . sfactionD
  salter <- getsClient salter
  noFriends <- getsState $ \s p -> all (isFoe (bfid b) fact . bfid . snd)
                                       (posToAssocs p (blid b) s)
  let lalter = salter EM.! blid b
      -- Only actors with AbAlter can search for hidden doors, etc.
      enterableHere p = alterSkill >= fromEnum (lalter PointArray.! p)
  if noFriends target && enterableHere target then
    return $! returN "moveTowards adjacent" $ target `vectorToFrom` source
  else do
    let goesBack p = Just p == boldpos b
        nonincreasing p = chessDist source goal >= chessDist p goal
        isSensible | relaxed = \p -> noFriends p
                                     && enterableHere p
                   | otherwise = \p -> nonincreasing p
                                       && not (goesBack p)
                                       && noFriends p
                                       && enterableHere p
        sensible = [ ((goesBack p, chessDist p goal), v)
                   | v <- moves, let p = source `shift` v, isSensible p ]
        sorted = sortBy (comparing fst) sensible
        groups = map (map snd) $ groupBy ((==) `on` fst) sorted
        freqs = map (liftFrequency . uniformFreq "moveTowards") groups
    return $! foldr (.|) reject freqs

-- Actor moves or searches or alters or attacks.
-- This function is very general, even though it's often used in contexts
-- when only one or two of the many cases can possibly occur.
moveOrRunAid :: MonadClient m
             => ActorId -> Vector -> m (Maybe RequestAnyAbility)
moveOrRunAid source dir = do
  COps{coTileSpeedup} <- getsState scops
  sb <- getsState $ getActorBody source
  actorSk <- currentSkillsClient source
  let lid = blid sb
  lvl <- getLevel lid
  let alterSkill = EM.findWithDefault 0 AbAlter actorSk
      spos = bpos sb           -- source position
      tpos = spos `shift` dir  -- target position
      t = lvl `at` tpos
  -- We start by checking actors at the target position,
  -- which gives a partial information (actors can be invisible),
  -- as opposed to accessibility (and items) which are always accurate
  -- (tiles can't be invisible).
  tgts <- getsState $ posToAssocs tpos lid
  case tgts of
    [(target, b2)] -> do
      -- @target@ can be a foe, as well as a friend.
      tfact <- getsState $ (EM.! bfid b2) . sfactionD
      actorMaxSk <- maxActorSkillsClient target
      dEnemy <- getsState $ dispEnemy source target actorMaxSk
      if | boldpos sb == Just tpos && not (waitedLastTurn sb)
             -- avoid displace loops
           || not (Tile.isWalkable coTileSpeedup $ lvl `at` tpos) ->
             -- DisplaceAccess
           return Nothing
         | isFoe (bfid b2) tfact (bfid sb)
           && not dEnemy -> do  -- DisplaceDying, etc.
           -- If really can't displace, melee.
           wps <- pickWeaponClient source target
           case wps of
             Nothing -> return Nothing
             Just wp -> return $ Just $ RequestAnyAbility wp
         | otherwise ->
           return $ Just $ RequestAnyAbility $ ReqDisplace target
    (target, _) : _ -> do  -- can be a foe, as well as friend (e.g., projectile)
      -- If really can't displace, melee.
      -- No problem if there are many projectiles at the spot. We just
      -- attack the first one.
      -- Attacking does not require full access, adjacency is enough.
      wps <- pickWeaponClient source target
      case wps of
        Nothing -> return Nothing
        Just wp -> return $ Just $ RequestAnyAbility wp
    [] -- move or search or alter
       | Tile.isWalkable coTileSpeedup $ lvl `at` tpos ->
         -- Movement requires full access.
         return $ Just $ RequestAnyAbility $ ReqMove dir
         -- The potential invisible actor is hit.
       | alterSkill < Tile.alterMinWalk coTileSpeedup t ->
         error $ "AI causes AlterUnwalked" `showFailure` (source, dir)
       | EM.member tpos $ lfloor lvl ->
         -- Only possible if items allowed inside unwalkable tiles.
         error $ "AI causes AlterBlockItem" `showFailure` (source, dir)
       | otherwise ->
         -- Not walkable, but alter skill suffices, so search or alter the tile.
         return $ Just $ RequestAnyAbility $ ReqAlter tpos