Skills and Spells

Skills and Spells

The central organizing table for skills and spells is skill_table, an array of type 'struct skill_type' (merc.h), and is defined in const.c. Skills include spells as a special case. Indexes into this table are 'skill numbers' or 'spell numbers', abbreviated 'sn'. An 'sn' is a purely internal value, and can vary over time as skills and spells are added. This way, the skill table can be expanded conveniently.

The 'practice' command and player saving/loading are completely table-driven from skill_table. Skills and spells may be freely added or deleted without changing these functions.

The fields of skill_table are:

    char *	name;

The name is used for practicing the skill or spell and by the 'cast'
command.  Mobiles which cast spells also often invoke the spell by
name.  Save files contain this name.

    int		skill_level [ MAX_CLASS ];

This array is indexed by character class, GET_CLASS( ch ).  It contains
the minimum level which each class needs in order to practice the skill
or spell.  If a given class cannot use this skill or spell, the level
is set to L_APP, so that all immortals can use all skills and spells.
Mobiles ignore this table; generally their skill percentages are a
simple function of level.

    SPELL_FUN *	spell_fun;

This is the function which implements the spell.  Spell functions
take four arguments: an 'sn', a level, a caster, and a pointer to a
target victim or object.  This field is unused for skills.

    int		target;

This identifies the targets of the spell.  The legal values are:

	    TAR_IGNORE			Spell chooses its own targets.
	    TAR_CHAR_OFFENSIVE		Spell is offensive (starts combat).
	    TAR_CHAR_DEFENSIVE		Spell is defensive (any char is legal).
	    TAR_CHAR_SELF		Spell is personal-effect only.
	    TAR_OBJ_INV			Spell is used on an object.

PC's may not cast TAR_CHAR_OFFENSIVE spells on other PC's.  These
spells also cause the victim to attack the caster.

This field is unused for skills.

    int		minimum_position;

This is the minimum position needed to cast the spell.  It is always
either POSITION_FIGHTING or POSITION_STANDING.  This field is unused
for skills.  Instead, the minimum position is set in cmd_table in
interp.c.

    int *	pgsn;

This is the address of a gsn (global skill/spell number) associated
with this skill or spell.  Almost all skills have gsn's; a few spells
do.  gsn's are helpful for spells that need to be referenced outside
the spell_fun itself.

    int		min_mana;

This is the minimum mana required to cast this spell by a sufficiently
high level spell caster.  A lower level spell caster will need more
mana than the minimum; see 'do_cast' in 'magic.c' for the algorithm.
This field is unused for skills.

    int		beats;

This is the delay time, in pulses, imposed on the user of a spell or
caster of a spell.  One pulse is 0.25 second (this is derived from
PULSE_PER_SECOND in merc.h).

    char *	noun_damage;

This is a noun or noun phrase containing the damage message for skills
or spells that perform damage.

    char *	msg_off;

This is the message sent to the character when the skill or spell wears
off.

Examples

    {
	"fireball",		{    15, L_APP, L_APP, L_APP, L_APP },
	spell_fireball,		TAR_CHAR_OFFENSIVE,	POS_FIGHTING,
	NULL,			15,	12,
	"fireball",		"!Fireball!"
    },

This is a fifteenth level magic-user spell.  It costs 15 mana, and
imposes 12 pulses (3 seconds) of delay time on the caster.  It's an
offensive spell, and can be used either fighting or standing.  There
is no gsn.  The damage message will say 'fireball', and the wear-off
message has a strange form to indicate errors because this spell is
instantaneous.

    {
	"armor",		{     5,     1, L_APP, L_APP, L_APP },
	spell_armor,		TAR_CHAR_DEFENSIVE,	POS_STANDING,
	NULL,			 5,	12,
	"",			"You feel less protected."
    },

This is a fifth level magic-user or first level cleric spell.  It can
be cast on any char without starting a fight.  It has no damage message
but does have a wear-off message.

    {
	"gas breath",		{    35, L_APP, L_APP, L_APP, L_APP },
	spell_gas_breath,	TAR_IGNORE,		POS_FIGHTING,
	NULL,			50,	12,
	"blast of gas",		"!Gas Breath!"
    },

This is one of the dragon-breath spells.  It is accessible only to
35th level magic users.  The spell selects its own targets (TAR_IGNORE)
rather than using the common spell-driver target selection code.

    {
	"pick lock",		{ L_APP, L_APP,     1, L_APP, L_APP },
	spell_null,		TAR_IGNORE,		POS_STANDING,
	&gsn_pick_lock,		 0,	12,
	"",			"!Pick!"
    },

This is a first-level thief skill.  Many of the fields are unused.
This skill has a gsn, so that do_pick_lock does not have to call
'skill_lookup' to find the sn.

Indexing skill_table

'skill_table' is indexed by an 'sn' or 'gsn'. The function 'skill_lookup' takes a string and returns the appropriate sn, or -1 if the name is not found.

Some skills, such as 'second attack', are referenced very frequently. For these skills, a global variable such as 'gsn_second_attack' is initialized to the sn of this skill. Most of the fighter and thief skills have gsn's; only a few spells (poison, charm, domination) need gsn's.

Gsn's are initialized at boot time in boot_db, which follows the 'pgsn' field in skill_table. This takes slightly more time and space than using '#define' to define them as constants, but is much more error-proof.

Magic items (potions, scrolls, pills, wands, and staves) have charges of magical spells. These spells are referenced by the spell name in area files. This is a change from previous Merc and Diku releases, which used slot numbers for each spell. The function 'skill_lookup' converts these spell names to internal skill numbers.

The spell driver

The spell driver level is the first half of 'magic.c'. It consists of 'do_cast' for the 'cast' command and 'obj_cast_spell' which is called from 'do_brandish', 'do_eat', 'do_quaff', 'do_recite', and 'do_zap'.

The spell driver takes care of level checking, position checking, target selection, target validation (such as preventing PC's from casting offensive spells on other PC's), staff area effect (just multi-target selection), mana costs, wait states, and starting combat for offensive spells. All of these things are table-driven by 'skill_table'.

The spell function itself takes care of calling functions like 'damage' and 'affect_to_char'. Read the existing ones to see what is needed for a new one.

Some spells, such as 'earthquake', 'locate object' or 'summon', perform their own target selection. These spells are target type TAR_IGNORE. Within the spell function, the variable 'target_name' is available (set by the spell driver).

Adding a new spell

1. Choose appropriate values for the other 'skill_table' fields, and write your new paragraph into 'const.c'. Order is immaterial.

2. Add the 'spell_*' function declaration to the 'DECLARE_SPELL_FUN' section of 'merc.h'.

3. Drop the code for the spell function into 'magic.c'.

4. If your spell needs 'AFF_' bits, create new 'AFF_ bits' in 'merc.h'.

5. You may need to increase 'MAX_SKILL' in 'merc.h'.

You may wish to take a look at 'template.txt' for a template on what to do when you add a new skill or spell to EnvyMud.

Adding a new skill

For skills, only some of the fields are relevant: 'name', 'skill_level', 'beats', 'pgsn', 'noun_damage', and 'msg_off' are used; 'min_mana', 'spell_fun', 'target', and 'minimum_position' are not used.

Skills are either automatic (such as 'parry') or manual (such as 'rescue'). Many automatic skills are referenced in 'fight.c'; look at 'multi_hit' for examples of the use of 'gsn_second_attack' and 'gsn_third_attack'.

An explicit skill such as 'rescue' has its associated command 'do_rescue'. This command is defined and called in the normal way in interp.c. Within 'do_rescue', 'gsn_rescue' is used to find fields such as 'beats'.

To add a new skill:

1. Choose appropriate values for the 'skill_table' fields, and write your new paragraph into 'const.c'. Order is immaterial.

2. You'll probably need a 'gsn'. Declare it in 'merc.h' and 'db.c'. Drop its address into the 'pgsn' field of the paragraph in 'const.c'.

3. If the skill is automatic, just use 'ch->pcdata->learned[gsn_new_skill]' in your code. If your skill is manual, you need to write an appropriate command function for it.

4. You may need to increase 'MAX_SKILL' in 'merc.h'.

You may wish to take a look at 'template.txt' for a template on what to do when you add a new skill or spell to EnvyMud.