# Rules and dice rolling

In EvAdventure we have decided to use the Knave RPG ruleset. This is commercial, but released under Creative Commons 4.0, meaning it’s okay to share and adapt Knave for any purpose, even commercially. If you don’t want to buy it but still follow along, you can find a free fan-version here.

## Summary of Knave rules

Knave, being inspired by early Dungeons & Dragons, is very simple.

- It uses six Ability bonuses Strength (STR), Dexterity (DEX), Constitution (CON), Intelligence (INT), Wisdom (WIS) and Charisma (CHA). These are rated from `+1` to `+10`.

- Rolls are made with a twenty-sided die (`1d20`), usually adding a suitable Ability bonus to the roll.

- If you roll with advantage, you roll `2d20` and pick the highest value, If you roll with disadvantage, you roll `2d20` and pick the lowest.

- Rolling a natural `1` is a critical failure. A natural `20` is a critical success. Rolling such in combat means your weapon or armor loses quality, which will eventually destroy it.

- A saving throw (trying to succeed against the environment) means making a roll to beat `15` (always). So if you are lifting a heavy stone and have `STR +2`, you’d roll `1d20 + 2` and hope the result is higher than `15`.

- An opposed saving throw means beating the enemy’s suitable Ability ‘defense’, which is always their `Ability bonus + 10`. So if you have `STR +1` and are arm wrestling someone with `STR +2`, you roll `1d20 + 1` and hope to roll higher than `2 + 10 = 12`.

- A special bonus is `Armor`, `+1` is unarmored, additional armor is given by equipment. Melee attacks test `STR` versus the `Armor` defense value while ranged attacks uses `WIS` vs `Armor`.

- Knave has no skills or classes. Everyone can use all items and using magic means having a special ‘rune stone’ in your hands; one spell per stone and day.

- A character has `CON + 10` carry ‘slots’. Most normal items uses one slot, armor and large weapons uses two or three.

- Healing is random, `1d8 + CON` health healed after food and sleep.

- Monster difficulty is listed by hy many 1d8 HP they have; this is called their “hit die” or HD. If needing to test Abilities, monsters have HD bonus in every Ability.

- Monsters have a morale rating. When things go bad, they have a chance to panic and flee if rolling `2d6` over their morale rating.

- All Characters in Knave are mostly randomly generated. HP is `<level>d8` but we give every new character max HP to start.

- Knave also have random tables, such as for starting equipment and to see if dying when hitting 0. Death, if it happens, is permanent.

## Making a rule module

> Create a new module mygame/evadventure/rules.py

There are three broad sets of rules for most RPGS:

- Character generation rules, often only used during character creation

- Regular gameplay rules - rolling dice and resolving game situations

- Character improvement - getting and spending experience to improve the character

We want our `rules` module to cover as many aspeects of what we’d otherwise would have to look up in a rulebook.

## Rolling dice

We will start by making a dice roller. Let’s group all of our dice rolling into a structure like this (not functional code yet):

    class EvAdventureRollEngine:

       def roll(...):
           # get result of one generic roll, for any type and number of dice

       def roll_with_advantage_or_disadvantage(...)
           # get result of normal d20 roll, with advantage/disadvantage (or not)

       def saving_throw(...):
           # do a saving throw against a specific target number

       def opposed_saving_throw(...):
           # do an opposed saving throw against a target's defense

       def roll_random_table(...):
           # make a roll against a random table (loaded elsewere)

       def morale_check(...):
           # roll a 2d6 morale check for a target

       def heal_from_rest(...):
           # heal 1d8 when resting+eating, but not more than max value.

       def roll_death(...):
           # roll to determine penalty when hitting 0 HP.

    dice = EvAdventureRollEngine()

This structure (called a singleton) means we group all dice rolls into one class that we then initiate into a variable `dice` at the end of the module. This means that we can do the following from other modules:

        from .rules import dice

        dice.roll("1d8")

### Generic dice roller

We want to be able to do `roll("1d20")` and get a random result back from the roll.

    # in mygame/evadventure/rules.py

    from random import randint

    class EvAdventureRollEngine:

        def roll(self, roll_string):
            """
            Roll XdY dice, where X is the number of dice
            and Y the number of sides per die.

            Args:
                roll_string (str): A dice string on the form XdY.
            Returns:
                int: The result of the roll.

            """

            # split the XdY input on the 'd' one time
            number, diesize = roll_string.split("d", 1)

            # convert from string to integers
            number = int(number)
            diesize = int(diesize)

            # make the roll
            return sum(randint(1, diesize) for _ in range(number))

The `randint` standard Python library module produces a random integer
in a specific range. The line

    sum(randint(1, diesize) for _ in range(number))

works like this:

- For a certain `number` of times …

- … create a random integer between `1` and `diesize` …

- … and `sum` all those integers together.

You could write the same thing less compactly like this:

    rolls = []
    for _ in range(number):
       random_result = randint(1, diesize)
       rolls.append(random_result)
    return sum(rolls)

We don’t ever expect end users to call this method; if we did, we would have to validate the inputs much more - We would have to make sure that `number` or `diesize` are valid inputs and not crazy big so the loop takes forever!

### Rolling with advantage

Now that we have the generic roller, we can start using it to do a more complex roll.

    # in mygame/evadventure/rules.py

    # ...

    class EvAdventureRollEngine:

        def roll(roll_string):
            # ...

        def roll_with_advantage_or_disadvantage(self, advantage=False, disadvantage=False):

            if not (advantage or disadvantage) or (advantage and disadvantage):
                # normal roll - advantage/disadvantage not set or they cancel
                # each other out
                return self.roll("1d20")
            elif advantage:
                 # highest of two d20 rolls
                 return max(self.roll("1d20"), self.roll("1d20"))
            else:
                 # disadvantage - lowest of two d20 rolls
                 return min(self.roll("1d20"), self.roll("1d20"))

The `min()` and `max()` functions are standard Python fare for getting the biggest/smallest of two arguments.

### Saving throws

We want the saving throw to itself figure out if it succeeded or not. This means it needs to know the Ability bonus (like STR `+1`). It would be convenient if we could just pass the entity doing the saving throw to this method, tell it what type of save was needed, and then have it figure things out:

    result, quality = dice.saving_throw(character, Ability.STR)

The return will be a boolean `True/False` if they pass, as well as a `quality` that tells us if a perfect fail/success was rolled or not.

To make the saving throw method this clever, we need to think some more about how we want to store our data on the character.

For our purposes it sounds reasonable that we will be using Attributes for storing the Ability scores. To make it easy, we will name them the same as the Enum values we set up in the previous lesson. So if we have an enum `STR = "strength"`, we want to store the Ability on the character as an Attribute `strength`.

From the Attribute documentation, we can see that we can use `AttributeProperty` to make it so the Attribute is available as `character.strength`, and this is what we will do.

So, in short, we’ll create the saving throws method with the assumption that we will be able to do `character.strength`, `character.constitution`, `character.charisma` etc to get the relevant Abilities.

    # in mygame/evadventure/rules.py
    # ...
    from .enums import Ability

    class EvAdventureRollEngine:

       def roll(...)
           # ...

       def roll_with_advantage_or_disadvantage(...)
           # ...

       def saving_throw(self, character, bonus_type=Ability.STR, target=15,
                        advantage=False, disadvantage=False):
           """
           Do a saving throw, trying to beat a target.

           Args:
              character (Character): A character (assumed to have Ability bonuses
                  stored on itself as Attributes).
              bonus_type (Ability): A valid Ability bonus enum.
              target (int): The target number to beat. Always 15 in Knave.
              advantage (bool): If character has advantage on this roll.
              disadvantage (bool): If character has disadvantage on this roll.

           Returns:
               tuple: A tuple (bool, Ability), showing if the throw succeeded and
                   the quality is one of None or Ability.CRITICAL_FAILURE/SUCCESS

           """

           # make a roll
           dice_roll = self.roll_with_advantage_or_disadvantage(advantage, disadvantage)

           # figure out if we had critical failure/success
           quality = None
           if dice_roll == 1:
               quality = Ability.CRITICAL_FAILURE
           elif dice_roll == 20:
               quality = Ability.CRITICAL_SUCCESS

           # figure out bonus
           bonus = getattr(character, bonus_type.value, 1)

           # return a tuple (bool, quality)
           return (dice_roll + bonus) > target, quality

The `getattr(obj, attrname, default)` function is a very useful Python tool for getting an attribute off an object and getting a default value if the attribute is not defined.

### Opposed saving throw

With the building pieces we already created, this method is simple. Remember that the defense you have to beat is always the relevant bonus + 10 in Knave. So if the enemy defends with `STR +3`, you must roll higher than `13`.

    # in mygame/evadventure/rules.py

    from .enums import Ability

    class EvAdventureRollEngine:

        def roll(...):
            # ...

        def roll_with_advantage_or_disadvantage(...):
            # ...

        def saving_throw(...):
            # ...

        def opposed_saving_throw(self, attacker, defender,
                                 attack_type=Ability.STR, defense_type=Ability.ARMOR,
                                 advantage=False, disadvantage=False):
            defender_defense = getattr(defender, defense_type.value, 1) + 10
            result, quality = self.saving_throw(attacker, bonus_type=attack_type,
                                                target=defender_defense,
                                                advantage=advantave, disadvantage=disadvantage)

            return result, quality

### Morale check

We will make the assumption that the `morale` value is available from the creature simply as `monster.morale` - we need to remember to make this so later!

In Knave, a creature have roll with `2d6` equal or under its morale to not flee or surrender when things go south. The standard morale value is 9.

    # in mygame/evadventure/rules.py

    class EvAdventureRollEngine:

        # ...

        def morale_check(self, defender):
            return self.roll("2d6") <= getattr(defender, "morale", 9)

### Roll for Healing

To be able to handle healing, we need to make some more assumptions about how we store health on game entities. We will need `hp_max` (the total amount of available HP) and `hp` (the current health value). We again assume these will be available as `obj.hp` and `obj.hp_max`.

According to the rules, after consuming a ration and having a full night’s sleep, a character regains `1d8 + CON` HP.

    # in mygame/evadventure/rules.py

    from .enums import Ability

    class EvAdventureRollEngine:

        # ...

        def heal_from_rest(self, character):
            """
            A night's rest retains 1d8 + CON HP

            """
            con_bonus = getattr(character, Ability.CON.value, 1)
            character.heal(self.roll("1d8") + con_bonus)

We make another assumption here - that `character.heal()` is a thing. We tell this function how much the character should heal, and it will do so, making sure to not heal more than its max number of HPs

> Knowing what is available on the character and what rule rolls we need is a bit of a chicken-and-egg problem. We will make sure to implement the matching Character class next lesson.

### Rolling on a table

We occasionally need to roll on a ‘table’ - a selection of choices. There are two main table-types we need to support:

Simply one element per row of the table (same odds to get each result).

Result
item1
item2
item3
item4

This we will simply represent as a plain list

    ["item1", "item2", "item3", "item4"]

Ranges per item (varying odds per result):

Range | Result
--- | ---
1-5 | item1
6-15 | item2
16-19 | item3
20 | item4

This we will represent as a list of tuples:

    [("1-5", "item1"), ("6-15", "item2"), ("16-19", "item4"), ("20", "item5")]

We also need to know what die to roll to get a result on the table (it may not always be obvious, and in some games you could be asked to roll a lower dice to only get early table results, for example).

    # in mygame/evadventure/rules.py

    from random import randint, choice

    class EvAdventureRollEngine:

        # ...

        def roll_random_table(self, dieroll, table_choices):
            """
            Args:
                 dieroll (str): A die roll string, like "1d20".
                 table_choices (iterable): A list of either single elements or
                    of tuples.
            Returns:
                Any: A random result from the given list of choices.

            Raises:
                RuntimeError: If rolling dice giving results outside the table.

            """
            roll_result = self.roll(dieroll)

            if isinstance(table_choices[0], (tuple, list)):
                # the first element is a tuple/list; treat as on the form [("1-5", "item"),...]
                for (valrange, choice) in table_choices:
                    minval, *maxval = valrange.split("-", 1)
                    minval = abs(int(minval))
                    maxval = abs(int(maxval[0]) if maxval else minval)

                    if minval <= roll_result <= maxval:
                        return choice

                # if we get here we must have set a dieroll producing a value
                # outside of the table boundaries - raise error
                raise RuntimeError("roll_random_table: Invalid die roll")
            else:
                # a simple regular list
                roll_result = max(1, min(len(table_choices), roll_result))
                return table_choices[roll_result - 1]

Check that you understand what this does.

This may be confusing:

    minval, *maxval = valrange.split("-", 1)
    minval = abs(int(minval))
    maxval = abs(int(maxval[0]) if maxval else minval)

If `valrange` is the string `1-5`, then `valrange.split("-", 1)` would result in a tuple `("1", "5")`. But if the string was in fact just `"20"` (possible for a single entry in an RPG table), this would lead to an error since it would only split out a single element - and we expected two.

By using `*maxval` (with the `*`), `maxval` is told to expect 0 or more elements in a tuple. So the result for `1-5` will be `("1", ("5",))` and for `20` it will become `("20", ())`. In the line

    maxval = abs(int(maxval[0]) if maxval else minval)

we check if `maxval` actually has a value `("5",)` or if its empty `()`. The result is either `"5"` or the value of `minval`.

### Roll for death

While original Knave suggests hitting 0 HP means insta-death, we will grab the optional “death table” from the “prettified” Knave’s optional rules to make it a little less punishing. We also changed the result of `2` to ‘dead’ since we don’t simulate ‘dismemberment’ in this tutorial:

Roll | Result | -1d4 Loss of Ability
--- | --- | ---
1-2 | dead | -
3 | weakened | STR
4 | unsteady | DEX
5 | sickly | CON
6 | addled | INT
7 | rattled | WIS
8 | disfigured | CHA

All the non-dead values map to a loss of 1d4 in one of the six Abilities (but you get HP back). We need to map back to this from the above table. One also cannot have less than -10 Ability bonus, if you do, you die too.

    # in mygame/evadventure/rules.py

    death_table = (
        ("1-2", "dead"),
        ("3": "strength",
        ("4": "dexterity"),
        ("5": "constitution"),
        ("6": "intelligence"),
        ("7": "wisdom"),
        ("8": "charisma"),
    )

    class EvAdventureRollEngine:

        # ...

        def roll_random_table(...)
            # ...

        def roll_death(self, character):
            ability_name = self.roll_random_table("1d8", death_table)

            if ability_name == "dead":
                # TODO - kill the character!
                pass
            else:
                loss = self.roll("1d4")

                current_ability = getattr(character, ability_name)
                current_ability -= loss

                if current_ability < -10:
                    # TODO - kill the character!
                    pass
                else:
                    # refresh 1d4 health, but suffer 1d4 ability loss
                    self.heal(character, self.roll("1d4")
                    setattr(character, ability_name, current_ability)

                    character.msg(
                        "You survive your brush with death, and while you recover "
                        f"some health, you permanently lose {loss} {ability_name} instead."
                    )

    dice = EvAdventureRollEngine()

Here we roll on the ‘death table’ from the rules to see what happens. We give the character a message if they survive, to let them know what happened.

We don’t yet know what ‘killing the character’ technically means, so we mark this as `TODO` and return to it in a later lesson. We just know that we need to do something here to kill off the character!

## Testing

> Make a new module `mygame/evadventure/tests/test_rules.py`

Testing the `rules` module will also showcase some very useful tools when testing.

    # mygame/evadventure/tests/test_rules.py

    from unittest.mock import patch
    from evennia.utils.test_resources import BaseEvenniaTest
    from .. import rules

    class TestEvAdventureRuleEngine(BaseEvenniaTest):

        def setUp(self):
            """Called before every test method"""
            super().setUp()
            self.roll_engine = rules.EvAdventureRollEngine()

        @patch("evadventure.rules.randint")
        def test_roll(self, mock_randint):
            mock_randint.return_value = 4
            self.assertEqual(self.roll_engine.roll("1d6", 4)
            self.assertEqual(self.roll_engine.roll("2d6", 2 * 4)

        # test of the other rule methods below ...

As before, run the specific test with

    evennia test --settings settings.py .evadventure.tests.test_rules

### Mocking and patching

The `setUp` method is a special method of the testing class. It will be run before every test method. We use `super().setUp()` to make sure the parent class’ version of this method always fire. Then we create a fresh `EvAdventureRollEngine` we can test with.

In our test, we import `patch` from the `unittest.mock` library. This is a very useful tool for testing. Normally the `randint` function we imported in `rules` will return a random value. That’s very hard to test for, since the value will be different every test.

With `@patch` (this is called a decorator), we temporarily replace `rules.randint` with a ‘mock’ - a dummy entity. This mock is passed into the testing method. We then take this `mock_randint` and set `.return_value = 4` on it.

Adding `return_value` to the mock means that every time this mock is called, it will return 4. For the duration of the test we can now check with `self.assertEqual` that our `roll` method always returns a result as-if the random result was 4.

There are many resources for understanding mock, refer to them for further help.

> The `EvAdventureRollEngine` have many methods to test. We leave this as an extra exercise!

## Summary

This concludes all the core rule mechanics of Knave - the rules used during play. We noticed here that we are going to soon need to establish how our Character actually stores data. So we will address that next.