View source for AI Recruitment

A new [[RCA_AI#The_Candidate_Actions_.28CAs.29_of_the_main_loop_Stage_in_the_RCA_AI|recruitment candidate action]] (CA) was introduced and made the default in Wesnoth 1.11.  It is much more configurable than previous versions and supports multiple leader recruiting.  This page describes how it works and the parameters available for customizing it.

==How it works==
===Map analysis===
In a first step, the CA analyzes the map and tries to find "important hexes". Those are locations on the map where a fight will occur with high probability. The important hexes are later used in combat simulations to calculate an average defense rating for the combatants.

When the game is running in debug mode ''and'' the game is started with the parameter <code>--log-info=ai/recruitment</code> the important hexes are marked with white 'x' markers on the map.

===Score Map===

A score map is created for all leaders who are able to recruit. All unit types from the recruit and recall lists are mapped to a score. 

In the end, the scores represent the desired unit ratios on the map. This means the AI will ''not'' just recruit the unit with the highest score, but in such a way that the mix of units on the map comes as close as possible to the mix represented by the scores.
The scores are filled with values coming from combat analyses (see below).

After that the AI modifies the scores in several ways:
* Similar units get a penalty for being similar. Similar units are units in one advancement tree.
 Example (Archer can advance to Ranger):
 			before	after
 Elvish Fighter: 	  50	 50
 Elvish Archer:		  50	 25
 Elvish Ranger:		  50	 25
* The aspects ''recruitment_more'', ''recruitment_diversity'' and ''recruitment_randomness'' are handled ([[#Other_Recruitment_Related_Aspects|see below]]).

===Combat Analysis===
For each unit type the AI leaders can recruit, the CA simulates fights against all enemy units on the map. If there are less then 5 enemy units, the enemy recruitment list(s) are also taken into account.

After combat analysis the score map may look like this:

 Goblin Spearman score:		16.1252
 Naga Fighter score:		0
 Orcish Archer score:		61.6442
 Orcish Assassin score:		88.0737
 Orcish Grunt score:		100
 Troll Whelp score:		26.7646
 Wolf Rider score:		0

====Implementation details for interested readers====

The combat analysis uses the function ''compare_unit_types(A, B)''. It takes two unit types and simulates a fight, using a cache because simulation is expensive. The function returns a positive value if unit type ''A'' is better than ''B'' and a negative value if unit type ''B'' is better than ''A''. If the return value is 2.0, it means that unit type A is twice as good as unit type B.

Here is a important code snippet from ''compare_unit_types()'':
<syntaxhighlight lang=cpp>
double value_of_a = damage_to_b / (b_max_hp * a_cost);
double value_of_b = damage_to_a / (a_max_hp * b_cost);

if (value_of_a > value_of_b) {
  return value_of_a / value_of_b;
} else if (value_of_a < value_of_b) {
  return -value_of_b / value_of_a;
} else {
  return 0.;
}
</syntaxhighlight>
''damage_to_b'' comes from simulations and is the average damage dealt by ''A'' when ''A'' attacks (''A'' chooses weapon) plus the average damage dealt by ''A'' when ''B'' attacks (''B'' chooses weapon). ''damage_to_a'' is equivalent.

The return value of ''compare_unit_types()'' is multiplied by the enemy's current hitpoints and added to the score map.

Included in the damage calculations are average defense (coming from "important hexes"), resistance, resistance abilities, average time of day, drain, poison, berserk, swarm and slows.

At this point the scores can be quite inconvenient for a generalized treatment. They could all be negative, very high or too similar. Thus, the scores are transformed linearly. The resulting scores are numbers between 0.0 and 100.0.
The minimum depends on the aspect ''recruitment_diversity''.

Simplified code snippet:
<syntaxhighlight lang=cpp>
double new_100 = max_score;
double new_0 = max_score - (recruitment_diversity * (max_score - average_score));

for (double& score : scores) {
  score = 100 * ((score - new_0) / (new_100 - new_0)); // linear transformation
  if (score < 0.) {
    score = 0.;
  }
}
</syntaxhighlight>

==Aspect ''recruitment_instructions''==

This is a powerful aspect for controlling recruitment via WML.  Its syntax is quite complex and requires the [[Wesnoth_AI_Framework#The_.5Bai.5D_Tag_.E2.80.94_Aspects|''composite'' aspect configuration]].  At its core are the ''[recruit]'' tags, which are the instructions to the candidate action defining which units are to be recruited.
<syntaxhighlight lang=wml>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
       turns=
       [value]
         #(here multiple [recruit] and/or [limit]-tags)
         [recruit]
           #(attributes)
         [/recruit]
         [limit]
           #(attributes)
         [/limit]          
       [value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>

Attributes inside '''[facet]''':
* '''turns'''="": (string) This key takes a comma separated list containing numbers specifying the turns. '-' can be used between two values to define a range. An empty string means ''all turns''.
* The [[Wesnoth_AI_Framework#The_.5Bai.5D_Tag_.E2.80.94_Aspects|other keys possible in [facet] tags]] can, in principle, also be used, but note that ''invalidate_on_gamestate_change'' and ''invalidate_on_minor_gamestate_change'' have no effect on [recruit] tags.  They are read at the beginning of the turn and are immutable for the remainder of the turn.

Attributes inside '''[recruit]''':
* '''id''': (string) {{DevFeature1.13|5}} By specifying an ID it becomes possible to manipulate the [recruit] tags using [[DirectActionsWML#.5Bmodify_ai.5D|modify_ai]]. This is entirely optional.
* '''type='''"": (string) This key takes a comma separated list containing unit types, usages or levels. Common values for ''usage'' are 'scout', 'fighter', 'archer', 'healer' and 'mixed fighter'. An empty string means 'all units'. If more than one unit is specified, the AI decides what to recruit according to the score map.
* '''number'''=-1: (integer) A number ''n'' greater than 0 instructs the AI to recruit ''n'' units. '-1' means 'as much as possible'. '0' means 'do not recruit'.
* '''importance'''=1: (integer) Priority of the recruitment.  The AI first recruits units with the highest importance, until the specified limits are reached. If gold is lacking or the castle is full, only the most important units are recruited, all other [recruit] jobs are dropped.
* '''leader_id'''="": (string) ''id'' of the leader to execute this recruitment. Empty string means 'all leaders'.  Note that this is only a recommendation to the AI. If the specified leader has no free hexes available, the AI uses other leaders to do the job.
* '''total'''=no: (boolean) If set to 'yes', the AI counts the own units on the map which match at least one of the given types and then recruits the difference between ''number'' and the counted amount.
* '''blocker'''=yes: (boolean) If set to 'yes', the AI stops recruiting if this job cannot be done (for example, because of lack of gold or a [limit]). If set to 'no', the AI drops this job and tries to continue with less important ones.
* '''pattern'''=no: (boolean) If set to 'yes', the unit to recruit is not chosen by the AI, but randomly according to the frequency in the type attribute. For example, when <code>type=Orcish Grunt, Orcish Grunt, scout</code> and <code>number=6</code>, the AI recruits 6 units, with the probability that one unit is a grunt being twice as high as that of a scout. If the ''type'' attribute is empty, the AI recruits randomly. (See also [[#Other_Recruitment_Related_Aspects|''recruitment_pattern'']])

Attributes inside '''[limit]''':
* '''id''': (string) {{DevFeature1.13|5}} By specifying an ID it becomes possible to manipulate the [limit] tags using [[DirectActionsWML#.5Bmodify_ai.5D|modify_ai]]. This is entirely optional.
* '''type'''="": (string) This key takes a comma separated list containing unit types, usages or levels. An empty string means 'all units'.
* '''max'''=0: (int) The maximum of units of the given type(s).

'''Notes:'''
* [limit] has higher priority than [recruit].
* The ''recruitment_save_gold'' aspect has higher priority than [recruit]. So if exact recruiting is wished, deactivate ''recruitment_save_gold'' ([[#Aspect_recruitment_save_gold|see below]]).
* If recruitment is specified for a turn, the AI does '''not recruit any other unit''' by default. (So if the AI is told to recruit 1 scout in turn 1, then the AI will only recruit 1 scout and no other units). To prevent this behavior one can add this [recruit] tag:
<syntaxhighlight lang=wml>
[recruit]
   importance=0
[/recruit]
</syntaxhighlight>
The AI then falls back to normal recruiting when all other [recruit] tags are dealt with. This is also the content of the [default] facet for the aspect.
* Even if only a [limit] tag is used in the aspect declaration, the [default] facet will get overwritten. Include the [recruit] tag mentioned in the previous point to let the AI recruit something. {{DevFeature1.13|5}} As of 1.13.5, if there is a [limit] tag but no [recruit] tag, an empty [recruit] tag is implied, allowing the AI to recruit. If both [limit] and [recruit] tags are present, no additional [recruit] tag is ever implied.
* Only one facet at a time can be active. When there are more [facet]s defined in a turn, only one will be used (usually the first one). Use several [recruit] and/or [limit] tags in the same facet instead.
* Do not define more than one [recruit] tag with the same importance. Also note that the aspect ''recruitment_pattern'' expands into a [recruit] tag with importance=1.
* {{DevFeature1.13|5}} The tags '''[total]''' and '''[pattern]''' may be used instead of [recruit]. These have basically the same meaning and contents as [recruit], but imply either total=yes or pattern=yes. They are converted to [recruit] tags and can be manipulated as such by modify_ai.

===Examples===
Recruit 3 grunts (and nothing else) on turns 3-5.
<syntaxhighlight lang=wml highlight=5,8,9>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      turns=3-5
      [value] 
        [recruit]
          type=Orcish Grunt
          number=3
        [/recruit]
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Recruit as many scouts as possible until there are 4 scouts in total, then recruit other units.
<syntaxhighlight lang=wml highlight=9,12-14>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      [value] 
        [recruit]
          type=scout
          number=4
          total=yes
          importance=1
        [/recruit]
        [recruit]
          importance=0
        [/recruit]
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Recruit 6 grunts or level 2 units (whatever seems better to the AI) and nothing else.
<syntaxhighlight lang=wml highlight=7>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      [value] 
        [recruit]
          type=Orcish Grunt, 2
          number=6
        [/recruit]
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Recruit 5 scouts with 'leader1' and 5 grunts with 'leader2'. Try to recruit the grunts even if 'leader1' cannot recruit all of the scouts.
<syntaxhighlight lang=wml highlight=9-11,16,17>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      [value] 
        [recruit]
          type=scout
          number=5
          importance=2
          leader_id=leader1
          blocker=no
        [/recruit]
        [recruit]
          type=Orcish Grunt
          number=5
          importance=1
          leader_id=leader2
        [/recruit]
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Always recruit random units.
<syntaxhighlight lang=wml highlight=7,8>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      [value] 
        [recruit]
          type=
          pattern=yes
          importance=1
        [/recruit]
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Do not recruit on turn 4.
<syntaxhighlight lang=wml highlight=5,9>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      turns=4
      [value] 
        [recruit]
          importance=1
          number=0
        [/recruit]
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Do not have more than 6 units on the map.
<syntaxhighlight lang=wml>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      [value] 
        [limit]
          type=    #empty type means all units
          max=6
        [/limit]
        [recruit]         #Do not forget to include the default
          importance=0    #[recruit]-tag. Otherwise the AI won't
        [/recruit]        #recruit anything.
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
The following does not work because only one [facet] can be active.
<syntaxhighlight lang=wml highlight=5,10,15,20>
[ai]
  [aspect]
    id=recruitment_instructions
    [facet]
      turns=1-10
      [value] 
        [recruit]
          type=scout
          number=5
          importance=1
        [/recruit]
      [/value]
    [/facet]
    [facet]
      turns=3
      [value] 
        [recruit]
          type=Orcish Grunt
          number=3
          importance=2
        [/recruit]
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Instead, one has to use <code>turns=1-2,4-10</code> in the first facet.

==Aspect ''recruitment_save_gold''==

===How it works===

For several reasons it can be a good idea not to spend all gold at once:
* To save upkeep
* To wait for the enemy to recruit first and thus to counter more easily
* To save gold for the next scenario when the enemy is almost defeated
It can also be a good idea (especially for the AI) to recruit in waves (instead of recruiting one unit or so each turn).  If this type of behavior is desired, there is the aspect ''recruitment_save_gold''.

The Recruitment CA is always in one of the following states:
* NORMAL
* SAVE_GOLD
* SPEND_ALL_GOLD
* LEADER_IN_DANGER
Except for state 'LEADER_IN_DANGER', the states are persistent over turns.

The AI keeps track of a ''unit_ratio''. This is <code>our_total_unit_costs / enemy_total_unit_costs</code>, where the costs are the sum of the cost of all units on the map weighted by their HP.  The AI also attempts to estimate the total income over the next 5 turns.

If the AI is in state NORMAL '''and''' the unit_ratio exceeds a predefined threshold (defined in ''begin'', see below) '''and''' the estimated income is positive, the AI switches to state SAVE_GOLD. Now the AI does not recruit units anymore until ''unit_ratio'' falls below another predefined threshold (defined in ''end''). Then the AI switches back to state NORMAL and recruits again.

With this behavior the AI can save quite a lot of gold. If the gold then accumulates to more than another predefined value (''spend_all_gold''), it switches into state SPEND_ALL_GOLD and recruits a big army to start an offensive wave.

The AI ignores all those gold saving strategies when it is in state LEADER_IN_DANGER. This happens when an enemy is close to the AI leader (within 5 hexes).

'''Important note:''' While one might assume that the AI plays better when it tries to save some gold, tests show that this is generally not the case. They indicate that, for the current AI, it is always better for the AI to have as many units on the map as possible. This aspect is therefore deactivated by default.

===The Aspect===
This aspect also requires the [[Wesnoth_AI_Framework#The_.5Bai.5D_Tag_.E2.80.94_Aspects|composite syntax]].
<syntaxhighlight lang=wml>
[ai]
  [aspect]
    id=recruitment_save_gold
    [facet]
      [value]
        #(attributes)
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>
Attributes inside '''[value]''':
* '''active'''=0: (int) From this turn on the aspect is active. The AI always spends all gold until then. <code>active=0</code> can be used to deactivate gold saving.
* '''begin''': (double) See explanation above.
* '''end''': (double) See explanation above.
* '''spend_all_gold''': (int) See explanation above. If set to '-1', the AI never switches to state SPEND_ALL_GOLD.
* '''save_on_negative_income''': (boolean) If this is set to yes, the AI switches to state SAVE_GOLD even if the income is negative (estimated over 5 turns, village gain and unit loss estimation included).

===Example===
<syntaxhighlight lang=wml>
[ai]
  [aspect]
    id=recruitment_save_gold
    [facet]
      [value]
        active=2
        begin=1.5
        end=1.1
        spend_all_gold=-1
        save_on_negative_income=no
      [/value]
    [/facet]
  [/aspect]
[/ai]
</syntaxhighlight>

A macro can be used to deactivate recruitment_save_gold:

<syntaxhighlight lang=wml>
{AI_ASPECT recruitment_save_gold {AI_DEACTIVATE_SAVE_GOLD} }
</syntaxhighlight>

==Other Recruitment Related Aspects==

*'''recruitment_diversity'''=2.0: (double) When this value is high, the AI recruits more units which are currently rare on the map. When the value is 0.0, the AI only recruits the best unit. See [[#Implementation_details_for_interested_readers|above]] for implementation details.
*'''recruitment_more'''="": (string) This key takes a comma separated list containing unit types, usages or levels. This is meant to let a scenario editor make an easy hack for making the AI recruit more units of a specific type. (25 will be added to the unit type's score.) The same unit type can also be used several time.  For example, with <code>recruitment-more="Orcish Grunt, Orcish Grunt"</code> it is possible to add a total of 50 to the Grunt's score.
*'''recruitment_randomness'''=50: (int) A random value between 0 and ''recruitment_randomness'' is added to each score. Use a high value, such as '200', to increase randomness.
*'''recruitment_pattern'''="": (string) This key takes a comma separated list containing unit types, usages or levels. Common values for ''usage'' are 'scout', 'fighter', 'archer', 'healer' and 'mixed fighter'. This tells the AI with what probability it should recruit different types of units. The usage is listed in the unit type config files (see data/core/units/ for mainline units; see also [[UnitTypeWML]]).
**For example, <code>recruitment_pattern=fighter,fighter,2</code> means that the AI recruits on average twice as many fighters as level 2 units. It does not mean that it recruits two fighters first, then a level 2 unit, then two fighters again, etc.
**Internally ''recruitment_pattern'' expands into a ''recruitment_instructions'' facet with <code>pattern=yes</code> and <code>importance=1</code>. Make sure that there is no ''recruitment_instructions'' facet with <code>importance=1</code> when using ''recruitment_pattern''.
*'''villages_per_scout'''=4: (int) A number 0 or higher which determines how many scouts the AI recruits. If set to '0', the AI does not recruit scouts to capture villages.

==Making recruitment strong==

The default values for the recruitment aspects are chosen to provide variety in the units the AI recruits.  Tests have shown that it plays slightly better (against other AIs, at least) when it mostly recruits the units that score highest in the evaluation explained above.  The best values found are:
* '''recruitment_diversity=0.8''': Let the AI recruit the best units only.
* '''recruitment_randomness=0''': Let the recruitment not be random.
* '''villages_per_scout=0''': The AI plays slightly better with fewer or no scouts.
* '''deactivate recruitment_save_gold''': The AI plays better when it spends all money.

In Multiplayer games, the AI 'Strong AI (RCA)' can be chosen with the settings as described here.  {{DevFeature1.13|5}}  As this is only a very minor modification of the default AI, this AI has been changed to a [[Wesnoth_AI#AIs_Available_in_the_Multiplayer_Setup_Menu|Dev AI]] (available in debug mode only) and has been renamed to 'Default AI (RCA) with Alternate Recruiting'.

==Some notes on multiple leaders==

The new recruitment CA is able to recruit with multiple leaders — even with different recruitment lists for each leader.  However there are some drawbacks:
* In the current version, the leaders recruit almost the same amount of units by default. This can partly be configured with the ''recruitment_instructions'' aspect and its attribute ''leader_id''.
** Note however that, as mentioned above, ''leader_id'' in ''recruitment_instructions'' is only a recommendation. For example, a leader recruits more units when enemies are near.
* Multiple leaders with different recruitment lists in combination with a <code>recruitment_pattern</code> may not result in the expected behavior. For the AI it is more important that the leaders recruit the same amount of units than fulfilling the ratios given in ''recruitment_pattern''.


'''See also:''' [[Wesnoth AI]]
[[Category:AI]]