use crate::sim::cell_sim::sim::UpdateCtx;

#[inline]
pub fn sim_update(ctx: &mut UpdateCtx) {
    // if the water can fall, do so
    if ctx.candidates_swap(&[
        (0, 1),
        (-1 + 2 * ctx.seqno_parity as i32, 1),
        (1 - 2 * ctx.seqno_parity as i32, 1),
    ]) {
        return;
    }

    // if the water can't fall, check if we can move left or right
    // these are inverted on parity so that we don't preference a direction
    let left_target = ctx.get_cell(-1, 0);
    let can_move_left =
        left_target.is_some_and(|c| c.material.def().density < ctx.material.density);
    let right_target = ctx.get_cell(1, 0);
    let can_move_right =
        right_target.is_some_and(|c| c.material.def().density < ctx.material.density);

    // we can't move down or to other side, so we're stuck
    if !can_move_left && !can_move_right {
        return;
    }

    // if we can't move left, just move right
    if !can_move_left {
        ctx.candidates_swap(&[(1, 0)]);
        return;
    }
    // and vice versa
    if !can_move_right {
        ctx.candidates_swap(&[(-1, 0)]);
        return;
    }

    // find the closest hole within 20 pixels (TODO optimize)
    // a hole is any space below us with a lesser density
    // prevents equidistance stuck state
    let starting_side = if ctx.seqno_parity == 0 { 1 } else { -1 };
    for i in 0..20 {
        let side = if i % 2 == 0 {
            starting_side
        } else {
            -starting_side
        };

        let offset = side * (1 + i / 2);
        let hole_target = ctx.get_cell(offset, 1);
        if let Some(target) = hole_target
            && target.material.def().density < ctx.material.density
        {
            // we identified a hole and we know that the space on this side is open
            // move toward the hole
            // new_target.flags = new_target.flags ^ 0b1;
            ctx.candidates_swap(&[(side, 0)]);
            return;
        }
    }

    // we didn't find a hole, so just move "randomly" on the same surface
    // TODO when to settle?
    // let target_x = if !can_move_left {
    //     1
    // } else if !can_move_right {
    //     -1
    // } else if ctx.seqno_parity % 2 == 1 {
    //     1
    // } else {
    //     -1
    // };

    // ctx.candidates_swap(&[(target_x, 0)]);
}