[Crystal] AI

Pokémon Crystal - AI calculations
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NOTE:
- Some behaviours are wrongly explained here. TODO




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LINKS :
(1) AI layers : https://github.com/pret/pokecrystal/blob/master/data/trainers/attributes.asm
(2) AI constant flags : https://github.com/pret/pokecrystal/blob/4fb008844030283ad34cb0802b088b4dd7c9891c/constants/trainer_data_constants.asm
(3) Scoring : https://github.com/pret/pokecrystal/blob/650686476c47d03148dfb93de1388b1a7294e385/engine/battle/ai/scoring.asm

WHEN AI IS CALLED :
- Just before the battle menu shows up.

INITIALISATION :
- Each usable move is given a score of 20.
- Each unusable move is given a score of 80. An unusable move is defined as follows :
  - Disabled moves.
  - Moves with 0 PP left.

SCORES MODIFICATION :
- Each AI layer will juggle with each move score.
- Encouraging a move decreases its score (gets closer to 0).
- Discouraging a move increases its score (gets away from 0).
- A call to `AIDiscourageMove` will add 10 to a move score. It almost always means the move is discarded.
  - the term "dismiss" will be used when this function is called.
- Otherwise, (en/dis)couraging describes a score variation of 1.
- Greatly (en/dis)couraging describes a score variation of 2.

IMPACT OF RANDOMNESS :
- Many AI layers involve randomness thresholds, which can create variance in the results.
- Variance functions are
  - `AI_50_50` : 50/50 outcome.
  - `AI_80_20` : 80/20 outcome (the odds of an outcome is context-specific).

ADRESSES :
- `wBuffer1` up to `wBuffer1 + 3` store enemy move scores.

SCORE COMPILATION :
- Once all AI layers have been applied, all moves with the minimum score are candidates for the AI choice.
- If more than 1 move is candidate, the selection is done at random, each move having the same chance to be picked.


EXAMPLE :
- Blue's AI is defined as follows :
  AI_BASIC | AI_SETUP | AI_SMART | AI_AGGRESSIVE | AI_CAUTIOUS | AI_STATUS | AI_RISKY

- AI_BASIC will be the first layer, AI_SETUP will be the second, etc.
- Let's investigate a setup turn on Pidgeot against a neutral faster player Pokémon :
  Moveset : QUICK_ATTACK, WHIRLWIND, WING_ATTACK, MIRROR_MOVE

(0) Initialisation : scores set to 20/20/20/20
(1) AI_BASIC :
    - Dismiss all moves in `AI_Redundant` if they are useless.
    - Dismiss status-only moves if the user is already statused.
    - Dismiss Safeguard if already active.

    Since none apply here, scores are unchanged : 20/20/20/20

(2) AI_SETUP :
    - 50% chance to greatly encourage stat-up/down moves during first enemy turn.

    Since none apply here, scores are unchanged : 20/20/20/20

(3) AI_SMART :
    - Context-specific scoring : it could be seen as a combo-search layer.
    - It will search moves in `AI_Smart_EffectHandlers` and update their scoring.

    MIRROR_MOVE (EFFECT_MIRROR_MOVE) and WHIRLWIND (EFFECT_FORCE_SWITCH) are in this table.
    a) MIRROR_MOVE :
                                   ┌─ enemy is faster ────────── dismiss
       - Player did not use a move ┴─ enemy is slower or tied ── nothing
       - Player used a move ┬─ not in `v` ──────────── nothing
                            └─ in `UsefulMoves` ─┬─50%────────── nothing
                                                 └─50%─ encourage ─┬─ enemy slower or tied ── nothing
                                                                   └─ enemy is faster ─┬─10%─ nothing
                                                                                       └─90%- encourage

       The player didn't use a move, the enemy is slower, so MIRROR_MOVE score won't be dismissed.

    b) WHIRLWIND :
       - Discourage if the player has not shown a super-effective move, or its typing is super-effective.
       The player didn't use a move, and is neutral typing, so the move is discouraged.

    Scores are now : 20/21/20/20

(4) AI_AGGRESSIVE :
    - Discourage all damaging moves but the one that does the most damage, unless such a move :
      - has 0 or 1 base power (moves like Seismic Toss, Hidden Power, Counter, Fissure, etc.)
      - is "reckless" (Selfdestruct, Thrash, Double Slap, Double Kick, etc.)
    - If no damaging move deals damage to the player (immune), no move will be discouraged

    The most damaging move is WING_ATTACK and the only other damaging move is QUICK_ATTACK,
    so the latter is discouraged.

    Scores are now 21/21/20/20

(5) AI_CAUTIOUS :
    - 90% chance to discourage moves in `ResidualMoves` after the first turn
    - Such moves are among Mist, Leech Seed, Poison Gas, etc.

    Since we're still in turn 1, this clause doesn't apply.
    Scores are unchanged : 21/21/20/20

(6) AI_STATUS :
    - Dismiss status moves that don't affect the player.
    - If the player can't be poisoned, put to sleep, or paralyzed.

    Since none apply here, scores are unchanged : 21/21/20/20

(7) AI_RISKY :
    - Use any move that will KO the target.
    - Enemy never uses moves like Selfdestruct or Horn Drill at full health (effect in `RiskyEffects`).
    - If effect in `RiskyEffects` and enemy is not at full health,
        20% chance to reduce score by 5.

    Since none apply here, scores are unchanged : 21/21/20/20

(8) Now that all layers have been encountered, the AI chooses a move among the lowest scores, at random.
    In this case, WING_ATTACK and MIRROR_MOVE have 20.
    Pidgeot will be doing either of those 50% of the time.

MANIP BLUE's PIDGEOT :
- Since wLastPlayerCounterMove is not reset when using X Items,
    you can use a move in `UsefulMoves` to get 75% Mirror Move odds.
- This can be a life-saver for specific Pokémon like Heracross with Earthquake,
    since Wing Attack is a 4x effective move.