As an introduction to the project, I would like to implement water physics. I have a few thoughts on how to go about it and would like some feedback.

First flowing fluid scan in a 5 block radius to find any air block on a lower y level to try to form a waterfall; I have implemented this scanning as a BFS.
Next placing water blocks with a higher level (source block has a level of 0) around the source at the position which reduces the manhattan distance to the air block on a lower y level.

However, this scanning would be repeated on every fluid spread and a lot of the same blocks would be scanned. I don't know if there's any way to avoid this since fluid spread is delayed and does not happen instantaneously.

When a water block is placed, a new entity is added to the world with a Fluid tag, Position and counter component. A system then query and checks if the counter has been reached and fluid phystics for the block is processed and the entity is removed from the world.

Attached is BFS which locates the nearest air block on a lower y level.

fn bfs(state: &State, start: BlockPosition) -> Vec<BlockPosition> {
    use std::collections::{VecDeque, HashSet};

    let mut visisted: HashSet<BlockPosition> = HashSet::new();
    let mut queue: VecDeque<BlockPosition> = VecDeque::new();

    let mut depth = 0;
    let mut until_next_depth = 1;
    let mut blocks_in_next_depth = 0;

    let mut drain: Vec<BlockPosition> = Vec::new();

    queue.push_back(start);

    while let Some(pos) = queue.pop_front() {
        // Track visited blocks
        if visisted.contains(&pos) {
            continue;
        }
        visisted.insert(pos);
        
        // If the block below is air, we have reached the depth which contains the first airblock.
        if let Some(Block::Air) = state.block_at(pos + DOWN) {
            drain.push(pos + DOWN);
        }

        // Add all air surrounding air blocks to the queue
        for dir in PLANE {
            if let Some(Block::Air) = state.block_at(pos + *dir) {
                blocks_in_next_depth += 1;
                queue.push_back(pos + *dir);
            }
        }

        until_next_depth -= 1;
        // When until next depth reaches zero we have checked all blocks at this depth 
        if until_next_depth == 0 {
            // If the drain is not empty we should break since we do not want to go deeper
            if !drain.is_empty() {
                break;
            }
            depth += 1;

            // We have reached the max depth
            if depth >= 5 {
                break;
            }
            until_next_depth = blocks_in_next_depth;
            blocks_in_next_depth = 0;
        }
    }
    drain
}

Fluid physics #160

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