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====== Skills/experience documentation for developers ======

===== Introduction =====

Skills were redone to a large extent in April 2003.  This document has been
updated to reflect how the skills work.

The main change is that experience categories were removed from the game.
Instead, experience goes to the skill itself.  Thus. how good a player is at
the skills is directly proportional to how good they are at that skill, and
not the category itself.

===== Sketch of system =====

In the skills/experience code, players gain experience for the activities
which they perform in the game ("You are what you do").  The activities a
player may engage in are controlled by the skills they possess. All players
start with a basic set of skills which they may expand through adventuring.
While monsters do not gain experience from the use of skills, they may use any
skills which exist in their inventory if they have the can_use_skill flag set.

In the code, skills are objects which exist in the player/monster inventory.
Both NPC/monsters and players use the same skill archetypes. Not all skills
are however enabled for monster use. Check the Skills_players.doc for
available NPC skills.

The experience one gets for a skill is greatly simplified.  No longer
is it weighted based on the stats of the player.  Rather, the experience is
based on what the skill was used for - opening a tough trap gets more
exp than opening an easy trap.  The stats the player has will improve
the chances of success in most cases - this is bonus enough without also
gaining additional experience.

The chracters total experience is no longer related to the sum of experience
in the players skills - A player could for example only of 1000 exp, but have
skills with 2500 exp, 300 exp, etc.  Removing the tie between skills and total
experience allows for reasonable skill advancement - you can allow a player
to easily get to level 20 in a skill without them now being level 20 player.

Note also that the only tunables are now in the code or in the archetypes -
if the exp for disarming is out of whack, the code would need to be changed
to give more appropriate amounts.

===== How to add new skills =====

Adding a new skill to Crossfire is not overly difficult, it is little more difficult
than adding new archetypes and a spell to the game.

==== Creation of new skill: outline of needed steps ====

A number of steps are required to create a new skill. 

  - Edit a new skills archetype. See below for appropriate parameters.  If you desire the skill to be a skill tool, edit a "face" for the new skill. If you want to have the skill to be learned via a skill scroll, edit a skillscroll for the skill.  Place the new archetype(s) in the lib/arch/skills directory.  Remember to name your new skill appropriately (ie skill_<new skill name>).  Make sure you select a unique subtype for your new skill.
  - Edit skill_util.c. Add an entry for the skill in do_skill() (so that it may be used as a "long-range attack"). If the new skill is a hth attack take a look at the attack_hth_skills[] table in do_skill_attack() -- where does the hth attack rank? The most useful attacks should occur earlier in the table.
  - Create the skill code. If you created a hth attack, you probably can get away with just using attack_hth. For other skills, put the skill code in skills.c. If your new skill is to be an "associated" skill, then make sure    that it returns the value of calc_skill_exp().
  - Edit treasures/artifacts file as needed (esp. if your skill will become one of the starting skills, or will show up in shops.)

===== Detail of skill archetype values =====

This section details the various object/archetype values.

First, we detail skill objects:
  * type: SKILL (43)
  * subtype: subtype of skill
  * invisible: 1
  * no_drop: 1
  * name: Name of the skill, used by things like 'use_skill', as well as output of 'skills' command.
  * skill: Same as name - this simplifies code, so that we can look at op->skill for both skills and skill tools. It also means that if a skill named is passed, we can verify we have the matching entry.
  * Stats (Str, Dex, sp, grace, etc): These modify the abilities of the player, in a sense giving bonuses.
  * expmul: this is the ratio of experience the players total should increase by when this skill is use.  If this is zero, then experience only goes to to the skill.  Values higher than 1 are allowed.  Note that experience rewarded to the players total is in addition to that given to the skill.  Eg, if player should get 500 exp for using a skill, and expmul is 1, the player will get 500 added to that skill as well as 500 to their total.
  * exp: The exp the player has in the skill (object).  If this is an archetype, this contains the base amount the player gets for using the skill. exp will be set to 0 when the skill is given to the player.
  * level: for an object, the level of this skill - this is just determined from the exp above based on the experience table. For an archetype, this is a percentage value that determines how the level difference effects experience rewards (like the old lexp value). Basically, if it is set to 100, the ratio is normal (eg, if opponent is twice level of player, player would get twice as much normal exp).  If level is 200, player would get 4 times.  If level if 50, player would half normal.  If level is 0, then we don't adjust exp reward based on level.
  * can_use_skill (flag): If this is set, the player knows the skill natively (eg, does not need a skill tool, see below).  If this is not set, then this skill object is acting as a container for experience. For example, if a player is using a holy symbol in order to get his praying skill, we still need to have skill_praying in the players inventory to store the experience in.  However, the player can't use that praying skill without a holy symbole until they learn it from a skill scroll.

===== Skill Tools =====

Skill tools are items that let a player use a skill they do not otherwise
know.  Skill tools may also have advantages, eg, spellpaths they grant to the
caster, stat bonuses, etc.

Most of the values for the skill tools are just like normal equipment
(value, weight, face, body_..., ) fields.

  * type: skill_tool (74)
  * skill: Name of the skill this object allows the user of.

Note - the old skill code used 'sp' to denote the skill to use.

===== Skill Scrolls =====

  * type: SKILLSCROLL
  * skill: Name of the skill to be learned

Rest of the values are per normal equipment (weight, value, identified, etc).


===== Other Objects =====

Many other objects will use the 'skill' field in their object to denote
what skill is needed to use this object.  Thus, for examples, readable
objects have 'skill literacy' to denote that the literacy skill is
needed to read them.  Weapons have 'skill' values to denote what
skill is needed to use the weapon.  Same for spells.
 

===== Workings of the Skill System =====

This section attempts to briefly explain how this all works.

Due to the addition of the skill pointer, it is no longer required
that a skill be in the ready_skill position to gain experience.

Whenever a player tries to use skill either directly (ready_skill ..)
or indirectly (cast a spell which requires knowledge of the skill), the
code will examine the players inventory to see if they an in fact
use the skill.  This first checks to see if the player has the appropriate
skill archetype in their object.  If they do, and can_use_skill is set
to 1, nothing more is done.  If that is not the case, we then look for
a skill tool.  If none is found, we tell the player the can't use the
skill.  If one is found, we try to apply the skill tool - if this can not
be done, we also error out.

Only if the player explicitly activates a skill with ready_skill do
we change the players range pointer.  Otherwise, it will remain as is
(but not that casting a spell might also change the range pointer).

add_exp has been modified to take two additional parameters -
skill_name and flag.

skill_name is the skill to add the experience to.  By passing this
to add exp, a lot of the code no longer needs to change chosen_skill,
then reset it back again.


flag determines what to do if the player does not currently have the
skill pointer in their inventory.  This can arise if the player
is using a skill tool, or part of a party.

In the default case of flag being 0, if the player does not currently
have the skill in their inventory, this is added (with can_use_skill 0).
If flag is 1, we add the exp to the players total exp, but don't
give them any in the skill.  If it is 2, the player gets nothing.

This fixes many of the abuses of party combat - if a member of your
party is killing things with wizardry, you'll get wizardry exp.  If
you don't have wizardry, you'll get some general exp, but you can't
funnel it into things like alchemy anymore.

The effect of flag 1 to add exp is so that that a player can't have
thousands of exp in a skill and never have used in themselves - for a player
to have any exp in a skill, he will have had to use it at least once.  Note
however that a player could have used the skill just once (to say kill a
kobold) and yet get a bunch more exp from party members that are actually good
wizards.

The handling of add_exp with skill_name is pretty simple.  In most cases, we
know immediately the skill that was used (eg, melee combat, search, disarm,
etc).  In cases of indirect death (spells), we set the skill in the spell
object to the skill that it should get awarded to.

===== Changes & Limitations =====

The old skill system had the idea of stats that effect the skill.  There
is no good way to do that within the new system - instead, the code
the executes the skill will do this.  This really isn't much worse
than the old system anyways - the old code still did things like
'stat average * 3' or otherwise determine how important the stats are.

 In addition, this allows for more flexibility for multi faceted
skills.  For example, the identification portion of some skills should
probably use int, but the creation portion should perhaps be dex and strength.

There is no more SK_level function - while this could still be useful
to replace code like 

<code>
level= op->chosen_skill?op->chosen_skill->level:op->level;
</code>

the use of automatically de-referencing the op->chosen_skill is not
suggested - the new skill system is much more lenient on passing the
appropriate skill object to functions that need it, so calls to
SK_level in most cases may not really be the right approach - in many
cases, the chosen_skill may not be what the code is really expecting.

===== Skill subtypes =====
Here is the description of the available skill subtypes, with their specific values. List is in ''include/skills.h''

{FIXME} describe/expand

==== SK_LOCKPICKING ====
==== SK_HIDING ====
==== SK_SMITHERY ====
==== SK_BOWYER ====
==== SK_JEWELER ====
==== SK_ALCHEMY ====
==== SK_STEALING ====
==== SK_LITERACY ====
==== SK_BARGAINING ====
==== SK_JUMPING ====
==== SK_DET_MAGIC ====
==== SK_ORATORY ====
==== SK_SINGING ====
==== SK_DET_CURSE ====
==== SK_FIND_TRAPS ====
==== SK_MEDITATION ====
==== SK_PUNCHING ====
==== SK_FLAME_TOUCH ====
==== SK_KARATE ====
==== SK_CLIMBING ====
==== SK_WOODSMAN ====
==== SK_INSCRIPTION ====
==== SK_ONE_HANDED_WEAPON ====
==== SK_MISSILE_WEAPON ====
==== SK_THROWING ====
==== SK_USE_MAGIC_ITEM ====
==== SK_DISARM_TRAPS ====
==== SK_SET_TRAP ====
==== SK_THAUMATURGY ====
==== SK_PRAYING ====
==== SK_CLAWING ====
==== SK_LEVITATION ====
==== SK_SUMMONING ====
==== SK_PYROMANCY ====
==== SK_EVOCATION ====
==== SK_SORCERY ====
==== SK_TWO_HANDED_WEAPON ====
==== SK_WRAITH_FEED ====

==== SK_HARVESTING ====
This skill lets a player "grab" items from a spot. Example is fishing, where the player "grabs" fish from the sea/a river.

Main function is ''server/c_misc.c:do_harvest()''.

The skill object should have the following attributes:
  * ''slaying'': the raw verb that will be displayed to the player. Example: ''fish''.

This skill can only be used through a skill tool, which should have the following parameters set:
  * ''harvest_race'': an identifier used to link the skill to what it can grab. Example: ''fish''
  * ''harvest_tool'': a subtype for the skill. Used to limit what this tool can grab. Example: ''pole'', ''fishingnet'', ...
  * ''harvest_speed'': speed cost of using the tool. If positive, absolute speed. If negative, ratio of player's speed

In order for the player to grab items, spots where the skill applies should contain in their inventory items the following values:
  * ''harvest_race'': should match the tool's
  * ''harvest_exp'': experience gain when successfully harvesting
  * ''harvest_level'': base level the item can be harvested at
  * ''harvestable'': any value, as long as the key exists. This will be removed when the item is harvested, and ensures you can't harvest already harvested items or skill/skill tools.
  * ''harvest_tool'': optional, if set can only be harvested by a tool with the same value

If multiple items are available, a random one in possible ones is chosen (with a maximum of 10 objects to choose from).

The harvesting probability on a 100 basis is (with ''item_level'' the target item's ''harvest_level'' and ''skill_level'' the player's level in the skill):
  * if item_level >= skill_level + 10, can't get
  * if item_level >= skill_level, 10 + skill_level - item_level
  * if item_level >= skill_level - 10, 10 + (skill_level - item_level) * 2;
  * maximum 30

If harvested item is a monster, it'll be inserted in the map instead of player's inventory (suggestion by Lalo)
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