Solving a gamebook : chapters

Chapter modelisation

This article is a literate Haskell representation of the LoneWolf.Chapter module at the time of the writing.

It doesn’t include any code, only type definitions, and will certainly be the least interesting post of this serie. It however shows a possible encoding of the chapter’s descriptions, which the reader might find interesting on its own.

The last part discusses how the chapter definitions were partially generated.

Extensions and imports

Clearly not the best part :)

> {-# LANGUAGE GADTs #-}
> {-# LANGUAGE TemplateHaskell #-}
> {-# LANGUAGE DeriveDataTypeable #-}
> module LoneWolf.Chapter where
> import LoneWolf.Character
> import Control.Lens
> import Data.Data
> import Data.Data.Lens

Basic types

The following types should get a newtype, but to be honest, it is much more handy that way right now, because of the vicious code generation that is happening later …

> -- represents a probability
> type Proba = Rational
> -- each chapter has an identifier, with a value ranging from 1 to 350
> type ChapterId = Int
> -- some combat effects are timed, for example some fights can be evaded after a certain number of rounds
> type Rounds = Int
> -- price of stuff, in gold coins
> type Price = Int

What’s in a chapter?

Lone Wolf chapters usually have a descriptive text, some figures, a description of what happens to the payer, and a list of choices. The title of the chapter serves no real purpose here, as it is identical (in String form) to the associated ChapterId.

> data Chapter = Chapter { _title   :: String
>                        , _desc    :: String
>                        , _pchoice :: Decision
>                        } deriving (Show, Eq)

Here is what an actual chapter looks like:

 Chapter "346"
         "The driver nods and hands back the ticket. The inn is warm but poorly furnished..."
          [ ( "Buy a meal, and a room"
            , Conditional
                (HasItem Gold 2)
                (NoDecision (LoseItem Gold 2 (Goto 280))))
          , ( "Just the room"
            , Conditional
                (HasItem Gold 1)
                (NoDecision (LoseItem Gold 1 (MustEat NoHunt (Goto 280)))))
          , ("Nothing", NoDecision (MustEat NoHunt (Goto 280)))

The important part of this model is that the Decision type encodes the player choices, and contains ChapterOutcome fields that encode the outcome of the choices. This is a critical distinction, as it is structural to how the solver works.

The Decision type

In details, the Decision types has the following constructors:

> data Decision
>    = Decisions [(String, Decision)]
>    | CanTake Item Int Decision
>    | Canbuy Item Price Decision
>    | Cansell Item Price Decision
>    | Conditional BoolCond Decision
>    | Special SpecialChapter
>    | NoDecision ChapterOutcome
>    deriving (Show, Eq, Typeable, Data)
> data SpecialChapter = Cartwheel
>                     | Portholes
>                     deriving (Show, Eq, Typeable, Data)
  • The Decisions constructor represents a list of possible choices for the player, with, for each case, a textual and programmatic description.

  • The CanTake and CanBuy constructors represent situations where the player can take, or buy, items. CanSell serves an identical purpose, for selling items. These constructors are not necessary, as they could be encoded using other, simpler, primitives. The following are equivalent, with regards to the choices they encode:

    gaingold = CanTake Gold 10 (NoDecision (Goto 20))
    gaingold' = Decisions
      [ ("Take the gold", NoDecision (GainItem Gold 10 (Goto 20)))
      , ("Take 9 coins", NoDecision (GainItem Gold 9 (Goto 20)))
      , ("Take 8 coins", NoDecision (GainItem Gold 8 (Goto 20)))
      , ...
      , ("Leave the gold", NoDecision (Goto 20)

    Having specialized constructors helps a lot. First it makes it easier to write the chapter definitions, and makes them clearer. But, more importantly, it will help reducing the computational complexity by deciding locally with specific heuristics.

  • Some decisions are only accessible in certain conditions, for example:

     [ ( "If you have the Kai Discipline of Tracking, turn to 13."
         , Conditional (HasDiscipline Tracking) (NoDecision (Goto 13)))
     , ( "If you wish to take the left fork, turn to 155."
         , Conditional
             (Not (HasDiscipline Tracking))
             (NoDecision (Goto 155)))
     , ( "If you wish to take the right fork, turn to 293."
         , Conditional
             (Not (HasDiscipline Tracking))
             (NoDecision (Goto 293)))

    In this example, if the player has the Tracking discipline, he will be forced to jump to chapter 13. If not, he will be able to choose between chapters 155 and 293.

  • Some chapters are special, and will be handled specifically. More on this later!

  • The NoDecision constructor indicates that there is no further choice for the player, and is at the leaf of most Decision structures. I currently have flagged two chapters as special, but this might evolve in the future.

The ChapterOutcome type

A ChapterOutcome describe what happens to the player once he has made a decision.

> data ChapterOutcome
>         = Fight FightDetails ChapterOutcome
>         | EvadeFight Rounds ChapterId FightDetails ChapterOutcome
>         | DamagePlayer Int ChapterOutcome
>         | HealPlayer Int ChapterOutcome
>         | FullHeal ChapterOutcome
>         | HalfHeal ChapterOutcome
>         | GainItem Item Int ChapterOutcome
>         | LoseItem Item Int ChapterOutcome
>         | LoseItemKind [Slot] ChapterOutcome
>         | MustEat CanHunt ChapterOutcome

Most of the constructors are built like lists, in the sense that their last argument is a ChapterOutcome.

I know I will have trouble with the EvadeFight element. It represents a fight that the player can evade after a certain number of rounds. It is really problematic, because such fights can have a lot of outcomes during each round, and will make the game unsolvable if implemented naïvely. This will certainly require some form of approximation, but I’ll take my time to commit to one.

Another constructor of interest is the MustEat constructor. It represents chapters where the player loses some hit points if he doesn’t have a meal in his backpack. In the Lone Wolf series, the Hunting discipline let the player ignore these events. But in this specific book, there are a couple of chapters where Hunting can’t be used! In a previous tentative, I used a flag in the game state to indicate whether or not hunting was permitted (it’s explicitely told when it stops and when it starts). In general, it is better to make the game state as simple as possible, so as to get good memoization performance, so I decided to manually track the chapters where hunting was disabled, and mark them as such.

>         | Goto ChapterId
>         | GameLost
>         | GameWon

Those three constructors are the “leaves” of this type, as they conclude an outcome. They work in the same way as the [] constructor that concludes a list.

>         | Randomly [(Proba, ChapterOutcome)]
>         | Conditionally [(BoolCond, ChapterOutcome)]
>         deriving (Show, Eq, Typeable, Data)

Finally, those work like trees. They represent random outcomes, or cases where multiple choices are provided to the player, but in reality only one is ever possible depending on the situation. Here are example of the latter, taken from chapter 36:

 [ (HasItem Laumspur 1, Goto 145)
 , (HasDiscipline Healing, Goto 210)
 , (Always True, Goto 275)

Note how it works like Haskell’s guards, and ends up with the equivalent of otherwise, which I called … botherwise:

> botherwise :: BoolCond
> botherwise = Always True
> data CanHunt = Hunt | NoHunt
>              deriving (Show, Eq, Typeable, Data)
> data BoolCond = HasDiscipline Discipline
>               | Not BoolCond
>               | HasEndurance Int
>               | COr BoolCond BoolCond
>               | CAnd BoolCond BoolCond
>               | HasItem Item Int
>               | Always Bool
>               deriving (Show, Eq, Typeable, Data)
> (.&&.) :: BoolCond -> BoolCond -> BoolCond
> (.&&.) = CAnd

The boolean conditions type (BoolCond) is pretty self-explanatory.

Combat description

The three main parts of a combat descriptions are the name of the opponent, its combat skill, and its endurance. There are however many combat situations that involve special modifiers which are detailled here. Fights against consecutive opponents are handled by chaining the Fight and EvadeFight constructors.

> data FightDetails = FightDetails
>           { _opponent     :: String
>           , _fcombatSkill :: CombatSkill
>           , _fendurance   :: Endurance
>           , _fightMod     :: [FightModifier]
>           } deriving (Show, Eq, Typeable, Data)
> data FightModifier = Undead
>                    | MindblastImmune
>                    | Timed Int FightModifier
>                    | CombatBonus CombatSkill
>                    | BareHanded
>                    | FakeFight ChapterId
>                    | EnemyMindblast
>                    | PlayerInvulnerable
>                    | DoubleDamage -- chapter 306
>                    deriving (Show, Eq, Typeable, Data)
  • Undead: undead creatures take double damage from the Sommerswerd.
  • MindblastImmune: the opponent is not affected by the MindBlast ability of the player.
  • CombatBonus: the player receives a combat bonus for this fight.
  • BareHanded: the player is forced to fight bare handed, reducing his combat skill by 4 points.
  • FakeFight: this isn’t a real fight, and the player will have his endurance restored at the end of the fight. The ChapterId is provided as the destination if he loses the fight. This happens in chapter 276.
  • EnemyMindblast: the enemy has the MindBlast ability!
  • PlayerInvulnerable the player cannot be harmed.
  • DoubleDamage: happens in chapter 306, where the player inflicts double damage.
  • Timed: the effect only lasts a given number of rounds.

Lenses, plates and utilities

> makePrisms ''ChapterOutcome
> makePrisms ''Decision
> makeLenses ''FightDetails
> moneyCond :: Int -> ChapterOutcome -> Decision
> moneyCond price = Conditional (HasItem Gold price) . NoDecision . LoseItem Gold price
> outcomePlate :: Traversal' Decision ChapterOutcome
> outcomePlate = biplate

All of these are mainly useful in the LoneWolf.XML module.

The actual chapters data

The chapters are described in the LoneWolf.Book02 module, but it’s all generated code! The LoneWolf.XML module contains all the code that is necessary to parse the XML file provided by project Aon, compute a baseline, adapt all chapters that have special rules and generate code. The code is horrendous, as it’s a one-use program.

In previous iterations of this project, I used to manually generate all the chapter’s definition, and chapter 346, that could be seen at the beginning of this article, would look like that:

(346, UnsetFlag CanHunt
    ( Decisions [ ("buy meal", ItemPay Gold 1 (Outcome (Goto 1104)))
                , ("eat own meal", ItemPay Meal 1 (Outcome (Goto 1104)))
                , ("starve", Outcome (ODamage 3 (Goto 1104))) ] ))

Compared to the current version, the chapter’s text is missing, and the choice descriptions are not as nice. It was also harder to debug, and one can see the UnsetFlag CanHunt situation I mentioned earlier.

The current situation is a net gain, as it has all the original text content, and also wrote most of the code for me. It was however still a lot of work to read all the chapters and make sure they were properly encoded. I also realized my previous encoding was imperfect in parts (I never realized there was a chapter where you could sell your weapons!), and I have no illusions there are mistakes in the current definitions.

Next time will have actual code, with a rules interpreter!