fool-cardgame/assets/scripts/core/bot/BasicBot.ts

import { Card } from '../cards/Card';
import { Action, ActionType, AttackAction, DefendAction } from '../rules/Action';
import { Phase } from '../rules/GameState';
import { Bot } from './Bot';
import { isTrump, PlayerView } from './PlayerView';

/**
 * The bot described in GAMEPLAY.md, implemented literally.
 *
 * - Opening a turn: the cheapest non-trump; a trump only if that is all it has.
 * - Adding to an attack: every eligible non-trump it holds. Trumps go in only
 *   when the deck is empty and its hand is nothing but trumps.
 * - Defending: the cheapest card that does the job, non-trumps first, and a
 *   take only when nothing in hand can beat the attack.
 *
 * Deliberately simple and entirely deterministic — it is the baseline a smarter
 * opponent will be measured against, and the workhorse of the soak tests.
 *
 * Two points GAMEPLAY.md leaves open, decided here: cards are added one per
 * decision, cheapest first (the rules core serialises additions anyway), and
 * when several attacks are unbeaten the lowest slot is answered first.
 */
export class BasicBot implements Bot {
    readonly name: string;

    constructor(name = 'basic') {
        this.name = name;
    }

    decide(view: PlayerView): Action {
        if (!view.onMove || view.legalActions.length === 0) {
            throw new Error(`${this.name} was asked to move out of turn`);
        }

        switch (view.phase) {
            case Phase.Attack:
                return this._openTurn(view);
            case Phase.Reinforce:
                return this._reinforce(view);
            case Phase.Defence:
                return this._defend(view);
            default:
                return view.legalActions[0];
        }
    }

    /** Cheapest non-trump, or the cheapest trump when the hand is all trumps. */
    private _openTurn(view: PlayerView): Action {
        const attacks = view.legalActions.filter(isAttack);
        return bestBy(attacks, (a) => cost(view, a.card));
    }

    private _reinforce(view: PlayerView): Action {
        const attacks = view.legalActions.filter(isAttack);
        const nonTrump = attacks.filter((a) => !isTrump(view, a.card));

        if (nonTrump.length > 0) {
            return bestBy(nonTrump, (a) => a.card.rank);
        }

        // Throwing trumps away is only worth it at the very end of a game, when
        // the deck is exhausted and there is nothing else left to throw.
        const holdingOnlyTrumps = view.hand.every((card) => isTrump(view, card));
        if (attacks.length > 0 && view.deckRemaining === 0 && holdingOnlyTrumps) {
            return bestBy(attacks, (a) => a.card.rank);
        }

        return pass(view);
    }

    private _defend(view: PlayerView): Action {
        const defences = view.legalActions.filter(isDefend);
        if (defences.length === 0) {
            return take(view);
        }
        const firstSlot = Math.min(...defences.map((d) => d.slot));
        const forSlot = defences.filter((d) => d.slot === firstSlot);
        return bestBy(forSlot, (d) => cost(view, d.card));
    }
}

/** Ranks a card for disposal: non-trumps first, then by value. */
function cost(view: PlayerView, card: Card): number {
    return (isTrump(view, card) ? 100 : 0) + card.rank;
}

function isAttack(action: Action): action is AttackAction {
    return action.type === ActionType.Attack;
}

function isDefend(action: Action): action is DefendAction {
    return action.type === ActionType.Defend;
}

function pass(view: PlayerView): Action {
    const action = view.legalActions.find((a) => a.type === ActionType.Pass);
    if (!action) throw new Error('Expected passing to be legal');
    return action;
}

function take(view: PlayerView): Action {
    const action = view.legalActions.find((a) => a.type === ActionType.Take);
    if (!action) throw new Error('Expected taking to be legal');
    return action;
}

/** Lowest score wins; ties break on card id so the choice is reproducible. */
function bestBy<T extends AttackAction | DefendAction>(items: readonly T[], score: (item: T) => number): T {
    let best = items[0];
    let bestScore = score(best);
    for (const item of items.slice(1)) {
        const s = score(item);
        if (s < bestScore || (s === bestScore && item.card.id < best.card.id)) {
            best = item;
            bestScore = s;
        }
    }
    return best;
}