kiln/kiln-core/src/scripts/transporter.rhai

// Built-in script for the `transporter_*` archetypes (see builtin).
//
// A transporter is a solid, see-through, unpushable machine that teleports
// whatever bumps into it from its facing side. It animates through a 4-frame
// loop (500 ms/frame) and, on a front bump, moves the bumper either to the cell
// just past it or — if that's blocked — out of the nearest opposite-facing
// transporter further along its axis (skipping any whose entrance is blocked).
//
// The direction is not baked into this source: every transporter shares one
// compiled copy and reads its `BUILTIN_transporter_<dir>` tag. The world is
// reached through the global `Board`/`Player` constants. The bumper is moved by
// coordinate via `shift([[from], [to]])`, which relocates whatever solid sits at
// the entrance — player, object, or a pushed crate — onto the empty destination.

// This transporter's facing unit vector, from its direction tag.
fn facing(me) {
    if me.has_tag("BUILTIN_transporter_north") { [0, -1] }
    else if me.has_tag("BUILTIN_transporter_south") { [0, 1] }
    else if me.has_tag("BUILTIN_transporter_east") { [1, 0] }
    else { [-1, 0] }
}

// The tag of the opposite-facing transporter this one pairs with.
fn opposite_tag(me) {
    if me.has_tag("BUILTIN_transporter_north") { "BUILTIN_transporter_south" }
    else if me.has_tag("BUILTIN_transporter_south") { "BUILTIN_transporter_north" }
    else if me.has_tag("BUILTIN_transporter_east") { "BUILTIN_transporter_west" }
    else { "BUILTIN_transporter_east" }
}

// The 4-frame animation loop for this direction.
fn frames(me) {
    if me.has_tag("BUILTIN_transporter_north") { ["^", "-", "^", "~"] }
    else if me.has_tag("BUILTIN_transporter_south") { ["v", "_", "v", "-"] }
    else if me.has_tag("BUILTIN_transporter_east") { [")", "|", ")", ">"] }
    else { ["(", "|", "(", "<"] }
}

fn tick(me, dt) {
    // Advance one animation frame every 0.5s, like the spinner: frame state lives
    // in the board Registry (script scope resets each tick), keyed per instance.
    if me.waiting { return; }
    let fs = frames(me);
    let fkey = `xport_${me.id}`;
    let f = Board.registry.get_or(fkey, 0);
    set_tile(fs[f % 4]);
    Board.registry.set(fkey, (f + 1) % 4);
    me.delay(0.15);
}

// Move whatever solid sits at (sx, sy) onto the empty cell (tx, ty) immediately.
// A two-cell `shift` relocates the source solid — player, object, or crate — and
// leaves its old cell empty (the destination is checked empty by the caller). Kept
// as a helper so the nested-array literal stays at a shallow expression depth.
fn transport(sx, sy, tx, ty) {
    shift([[sx, sy], [tx, ty]]); now();
}

fn bump(me, dir) {
    let d = facing(me);
    let dx = d[0];
    let dy = d[1];

    // Only transport things that hit us from the front — the entrance side (-facing).
    // `dir` is the side the bump came from, so its offset must be the opposite of our
    // facing; a bump from any other side does nothing.
    if dir.dx != -dx || dir.dy != -dy { return; }

    // Whatever solid is pressed against our entrance cell (me - d) gets moved:
    // the player, another object, or a pushed crate — all handled uniformly by
    // shifting the solid at that coordinate onto an empty destination.
    let ex = me.x - dx;
    let ey = me.y - dy;

    // 1. The cell just past us (the opposite side): if nothing solid is there,
    //    drop the bumper onto it.
    let fx = me.x + dx;
    let fy = me.y + dy;
    if Board.passable(fx, fy) {
        transport(ex, ey, fx, fy);
        return;
    }

    // 2. Otherwise scan along our axis for the nearest opposite-facing transporter
    //    whose far side (the cell just past it) is free, and drop the bumper there.
    //    A blocked pair is skipped; give up at the board edge.
    let opp = Board.tagged(opposite_tag(me));
    let cx = fx;
    let cy = fy;
    loop {
        cx += dx;
        cy += dy;
        if cx < 0 || cy < 0 || cx >= Board.width || cy >= Board.height {
            return;
        }
        // Is one of the opposite transporters sitting at (cx, cy)?
        let here = false;
        for o in opp {
            if o.x == cx && o.y == cy { here = true; }
        }
        if here {
            // Emerge on the paired transporter's far side — the cell just past it
            // along our scan (cx + d), out its back.
            let tx = cx + dx;
            let ty = cy + dy;
            if Board.passable(tx, ty) {
                transport(ex, ey, tx, ty);
                return;
            }
        }
    }
}