card-game

Card Game

A playbook for card games — card data, the deck/hand/discard zones, the turn structure, and how card effects resolve. This is a compositional skill: it models cards as data and wires them to UI. It does not re-teach data assets or UI nodes; it defines the zone model, the draw machinery, and the effect-resolution rules that keep a card game correct and bug-free.

When to use

• Use when building any game where the core objects are cards moving between zones (deck → hand → play → discard): deckbuilder, TCG/CCG, solitaire, roguelike deckbuilder.
• Use when designing draw/shuffle/reshuffle, turn structure, card costs, or how effects resolve.

When not to use: board/tile state with matching rules → puzzle. RPG with an incidental card battler → start from rpg. For defining cards as assets, use godot-resources / unity-scriptableobjects; for the hand/drag UI, use godot-ui-control.

Core loop

Draw to your hand → spend resources to play cards → effects resolve and change the board → end the turn (cleanup/discard) → opponent/next phase → repeat until a win condition. Depth comes from the combinations a hand allows; the engine's job is to resolve them unambiguously.

Must-have systems

1. Card data — id, name, cost, type, text, and an effect spec (data, not code).
2. Zones — deck (draw pile), hand, play/board, discard, exile/removed; cards live in exactly one.
3. Draw + shuffle + reshuffle — draw from deck to hand; reshuffle discard into deck when empty.
4. Turn structure — phases (untap/draw/main/combat/end) as a state machine.
5. Resource system — mana/energy/actions that gate how much you do per turn.
6. Effect resolution — apply a card's effects in a defined order; handle targets and triggers.
7. Win/loss condition — life total, deck-out, objective.
8. UI — hand layout, drag/drop or tap-to-play, zone counts, targeting affordances.

Design knobs

Patterns

1. Zones + draw with automatic reshuffle

# Pseudocode. A card is in exactly one zone at a time; moving = remove here, add there.
def draw(n):
    for _ in range(n):
        if not deck:
            if not discard:          # truly empty: deck-out (lose, or take fatigue)
                on_deck_out(); return
            deck.extend(discard)     # reshuffle discard into deck
            discard.clear()
            shuffle(deck, rng)       # use a seeded RNG (see save-systems for replays)
        hand.append(deck.pop())

2. Card as data + effect resolution

# Pseudocode. Effects are a data list interpreted by the engine — not bespoke code per card.
card = {
    "id": "fireball", "cost": 3, "type": "spell",
    "effects": [ {"op": "damage", "amount": 6, "target": "chosen_enemy"} ],
}
def play(card, caster, chosen_targets):
    # Validate everything first; a failed check must leave zones and resources untouched.
    if resources[caster] < card.cost: return False      # can't afford
    if not targets_legal(card, chosen_targets): return False
    resources[caster] -= card.cost                      # commit: pay, then move, then resolve
    move(card, from_zone=hand, to_zone=play_or_discard(card))
    for fx in card.effects:
        resolve_effect(fx, caster, chosen_targets)      # one interpreter handles every card
    return True

3. Turn structure as a phase machine

# Pseudocode. Fixed phases keep timing windows (triggers, priority) unambiguous.
PHASES = ["untap", "draw", "main", "combat", "end"]
def take_turn(player):
    for phase in PHASES:
        enter_phase(player, phase)        # fire "on_phase" triggers here
        if phase == "draw":   draw(1)
        if phase == "main":   await player_plays_cards()
        if phase == "combat": resolve_combat()
        if phase == "end":    discard_to_hand_limit(player); clear_temporary_effects()

Pitfalls / failure modes

• A card existing in two zones at once → duplication/loss bugs. Enforce "exactly one zone"; move = remove-then-add, and assert no card appears twice.
• Forgetting to reshuffle → draws silently fail or crash on empty deck. Reshuffle discard, or define deck-out/fatigue explicitly (Pattern 1).
• One function per card → unmaintainable and untestable. Make effects data interpreted by a small set of operations (Pattern 2).
• Ambiguous effect order / simultaneous triggers → nondeterministic outcomes. Resolve in a defined order (a queue or stack); document LIFO vs. FIFO (refs).
• Unseeded shuffle in a game that needs replays/undo → can't reproduce. Use a seeded RNG instance (not the engine's global one) and save its state with the run; see refs §6 for the per-engine calls.
• Off-by-one in the shuffle's random index → some orders never occur (a biased deck). Engine ranges differ: Godot randi_range(a, b) and Phaser RND.between(a, b) include b; Unity Random.Range(int, int) excludes it.
• No hand limit / no answers → degenerate hoarding or unbeatable threats. Add a hand cap and ensure removal exists for every threat archetype.
• Targeting state leaks → a cancelled play leaves the board mid-targeting. Make play atomic: validate cost + targets first, then commit.

Composition (build it from these skills)

• Card content: godot-resources / unity-scriptableobjects — define each card as a data asset.
• UI: game-ui-ux for layout, scaling, and focus navigation; godot-ui-control for hand layout, drag/drop, zone counts, and targeting prompts.
• Persistence/replays: save-systems for collection, run state (roguelike deckbuilder), and seeded replays.
• Opponent AI: game-ai for an AI that evaluates playable cards and picks targets.
• Animation/feedback: the engine animation skill for card movement; audio-design for cues.
• Scripting: godot-gdscript / unity-csharp-scripting for the effect interpreter.

References

• For the effect queue/stack, keywords/triggers, targeting, deckbuilder vs. constructed archetypes, and shuffle fairness, read references/effect-resolution.md.