Drag and drop

/// What a slot's item looks like while it is dragged: a copy of the slot's
/// picture, which the slot's Drag Source names as its Preview and draws under
/// the pointer while the item is carried. It stays hidden, so it is drawn
/// nowhere else.
component SlotGhost {
    /// The slot whose item it shows, which is kept by that name: hotbar_1 to
    /// hotbar_4, or bag_1 to bag_16.
    slot: String = "bag_1",

    fn loop() {
        // The satchel and the slots keep what each slot holds: 1 a potion, 2
        // the elixir, 3 a key, 0 nothing. An empty slot is never picked up,
        // so it needs no picture.
        let item = kept_number(slot, 0.0);
        if item == 1.0 {
            set_text("PanelComponent", "region_name", "potion");
        } else if item == 2.0 {
            set_text("PanelComponent", "region_name", "elixir");
        } else if item == 3.0 {
            set_text("PanelComponent", "region_name", "key");
        }
    }
}

/// A slot's two ends of a drag, on a slot of the hotbar or the bag: the item
/// it holds can be picked up and carried to another slot, and an item carried
/// here lands in it. An empty slot has nothing to pick up, and a full one
/// takes nothing, so an item let go over it stays where it was.
component SlotDrag {
    /// The slot it is, which is kept by that name: hotbar_1 to hotbar_4, or
    /// bag_1 to bag_16. Its Drag Source presses this name, and its Drop
    /// Target this name with _in after it.
    slot: String = "bag_1",

    fn loop() {
        // The satchel and the slots keep what each slot holds: 1 a potion, 2
        // the elixir, 3 a key, 0 nothing.
        let item = kept_number(slot, 0.0);

        // What a drag from here carries: its kind, which a Drop Target takes
        // only if it is on the target's list, and the item's number, which
        // the slot it lands in keeps.
        let kind = "";
        if item == 1.0 {
            kind = "potion";
        } else if item == 2.0 {
            kind = "elixir";
        } else if item == 3.0 {
            kind = "key";
        }
        set_text("DragSourceComponent", "kind", kind);
        set_number("DragSourceComponent", "payload", item);
        // Switched off, an empty slot is not picked up, so a press on it is
        // only a press, and a full one takes no drop.
        set_flag("DragSourceComponent", "disabled", item == 0.0);
        set_flag("DropTargetComponent", "disabled", item != 0.0);

        // An item let go over a slot that takes it presses that slot's _in
        // action, with the item's number as its amount, and this slot's own
        // action with 1, on the game's next step. Let go anywhere else, over
        // nothing or over a slot that refuses it, it presses this slot's
        // action with 0, and nothing needs doing: the item never left.
        if pressed(slot) && amount(slot) == 1.0 {
            // It landed, and the slot it landed in keeps it now.
            keep_number(slot, 0.0);
        }
        if pressed(slot + "_in") {
            // An item landed here, and the amount is its number.
            keep_number(slot, amount(slot + "_in"));
        }
    }
}

/// The hero's satchel: it files what the hero carries into the slots, the
/// hotbar's four and the bag's sixteen. What the hero owns is kept in
/// potions, elixirs and keys. Each slot is kept too, hotbar_1 to hotbar_4 and
/// bag_1 to bag_16, as the item in it: 1 a potion, 2 the elixir, 3 a key, 0
/// nothing. Only the satchel files, one item a step, so two pickups taken on
/// the same step never go in the same slot, and it files nothing into a slot
/// an item is being dropped in.
component Satchel {
    fn loop() {
        // What the slots hold as this step began: how many of each item, how
        // many slots are empty, the first empty one of the hotbar and of the
        // bag that no drop is filling, and the last one holding each item.
        let potions = 0.0;
        let elixirs = 0.0;
        let keys = 0.0;
        let empty_hotbar = 0.0;
        let empty_bag = 0.0;
        let first_hotbar = "";
        let first_bag = "";
        let last_potion = "";
        let last_elixir = "";
        let last_key = "";
        for i in 1..5 {
            let slot = "hotbar_" + text(i);
            let item = kept_number(slot, 0.0);
            if item == 0.0 {
                empty_hotbar += 1.0;
                // An item let go over an empty slot presses the slot's _in
                // action on the game's next step, and the slot takes it on
                // that step: it is filling, so nothing is filed into it.
                if first_hotbar == "" && !pressed(slot + "_in") {
                    first_hotbar = slot;
                }
            } else if item == 1.0 {
                potions += 1.0;
                last_potion = slot;
            } else if item == 2.0 {
                elixirs += 1.0;
                last_elixir = slot;
            } else if item == 3.0 {
                keys += 1.0;
                last_key = slot;
            }
        }
        for i in 1..17 {
            let slot = "bag_" + text(i);
            let item = kept_number(slot, 0.0);
            if item == 0.0 {
                empty_bag += 1.0;
                if first_bag == "" && !pressed(slot + "_in") {
                    first_bag = slot;
                }
            } else if item == 1.0 {
                potions += 1.0;
                last_potion = slot;
            } else if item == 2.0 {
                elixirs += 1.0;
                last_elixir = slot;
            } else if item == 3.0 {
                keys += 1.0;
                last_key = slot;
            }
        }

        // What the hero owns that no slot holds yet waits to be filed; less
        // than none is an item a slot holds that the hero no longer owns.
        let waiting_potions = kept_number("potions", 0.0) - potions;
        let waiting_elixirs = kept_number("elixirs", 0.0) - elixirs;
        let waiting_keys = kept_number("keys", 0.0) - keys;

        // One item a step, into the first empty slot. A potion or the elixir,
        // the two things the hero drinks, goes in the hotbar while it has
        // room, then in the bag: drink_slot is where it goes. A key goes only
        // in the bag.
        let drink_slot = first_hotbar;
        if drink_slot == "" {
            drink_slot = first_bag;
        }
        if waiting_potions > 0.0 && drink_slot != "" {
            keep_number(drink_slot, 1.0);
        } else if waiting_elixirs > 0.0 && drink_slot != "" {
            keep_number(drink_slot, 2.0);
        } else if waiting_keys > 0.0 && first_bag != "" {
            keep_number(first_bag, 3.0);
        }

        // An item the hero no longer owns, as the key the door took, leaves
        // the last slot holding it.
        if waiting_potions < 0.0 {
            keep_number(last_potion, 0.0);
        }
        if waiting_elixirs < 0.0 {
            keep_number(last_elixir, 0.0);
        }
        if waiting_keys < 0.0 {
            keep_number(last_key, 0.0);
        }

        // The room left once what waits is filed, which a pickup reads before
        // it lets the hero take it. What the hotbar has no room for waits for
        // the bag.
        let room_hotbar = empty_hotbar - max(waiting_potions, 0.0) - max(waiting_elixirs, 0.0);
        let room_bag = empty_bag - max(waiting_keys, 0.0);
        if room_hotbar < 0.0 {
            room_bag += room_hotbar;
            room_hotbar = 0.0;
        }
        keep_number("room_hotbar", room_hotbar);
        keep_number("room_bag", room_bag);
    }
}