3D fluid flow from unnamed game

flow.cpp

#include <api.h>
using namespace api;
void simulate_fluid_flow(int x, int y, int z, Block origin);

/// @brief Max flow distance of fluid
static constexpr uint16_t FLOW_MAX  = 15;
/// @brief Time between flow changes
static constexpr float    FLOW_TIME = 0.15;


static inline bool isOverwritable(Block blk, Block fluid)
{
	if (blk.isAir())
		return true;
	if (blk.getID() == fluid.getID())
		return fluid.getExtra() < blk.getExtra();
	return !blk.isLiquid() && blk.isOverwritable() && !blk.isInGroup("seaweed");
}

static int fluid_flow_down(GridWalker walker, Block blk)
{
	// Check destination
	walker.move(0, -1, 0);
	if (!walker.good())
		return 0;

	auto newblock = walker.get();
	if (isOverwritable(newblock, blk))
	{
		walker.set(blk);
		simulate_fluid_flow(walker.getX(), walker.getY(), walker.getZ(), blk);
		return 1;
	}
	// Also return the level of the other block when the IDs match
	if(newblock.getID() == blk.getID())
		return newblock.getExtra();
	return 15;
}

template <int dx, int dy, int dz>
static int fluid_check_side(GridWalker walker, Block blk)
{
	walker.move(dx, dy, dz);
	auto newblock = walker.get();
	if (isOverwritable(newblock, blk))
	{
		auto under = walker.below().get();
		if (isOverwritable(under, blk))
			return 2;
		return 1;
	}
	return 0;
}
template <int dx, int dy, int dz>
static void fluid_flow(GridWalker walker, Block blk)
{
	walker.move(dx, dy, dz);
	walker.set(blk);
	simulate_fluid_flow(walker.getX(), walker.getY(), walker.getZ(), blk);
}

void simulate_fluid_flow(int x, int y, int z, Block origin)
{
	auto level = origin.getExtra();
	if (level < FLOW_MAX)
	{
		block_t fluid_id = origin.getID();
		// Start flowing
		Timer::oneshot(FLOW_TIME,
		[x, y, z, fluid_id] (auto)
		{
			GridWalker walker{x, y, z};
			auto origin = walker.get();
			// Check source and flow level
			if (origin.getID() == fluid_id && origin.getExtra() < FLOW_MAX)
			{
				Block blk = origin;
				// Raise flow level when going down
				int flow =
					fluid_flow_down(walker, Block{fluid_id, 1});
				if (flow <= 2)
					return;

				// Increase flow level on the sides
				blk.setExtra(origin.getExtra() + 1);

				int px = fluid_check_side< 1, 0,  0>(walker, blk);
				int nx = fluid_check_side<-1, 0,  0>(walker, blk);
				int pz = fluid_check_side< 0, 0,  1>(walker, blk);
				int nz = fluid_check_side< 0, 0, -1>(walker, blk);

				int flow_target = 1;
				if (px == 2 || nx == 2 || pz == 2 || nz == 2)
					flow_target = 2;

				if (px == flow_target)
					fluid_flow< 1,  0,  0>(walker, blk);
				if (nx == flow_target)
					fluid_flow<-1,  0,  0>(walker, blk);
				if (pz == flow_target)
					fluid_flow< 0,  0,  1>(walker, blk);
				if (nz == flow_target)
					fluid_flow< 0,  0, -1>(walker, blk);
			}
		});
	}
}