#include <iostream>
#include <numeric>
#include <iostream>
#include <fstream>
#include <memory>

#include "src/vector.h"

struct DataCell {
	double data[3][3];

	inline double& get(int x, int y) {
		return data[1+x][1-y];
	}

	inline double get(int x, int y) const {
		return data[1+x][1-y];
	}
};

using Velocity = Vector;
using Force = Vector;
using Density = double;

constexpr DataCell weight{
	1./36., 1./9., 1./36.,
	1./9.,  4./9., 1./9.,
	1./36., 1./9., 1./36
};

struct Cell : DataCell {
	void equilibrize(Density d, Velocity v) {
		for ( int i = -1; i <= 1; ++i ) {
			for ( int j = -1; j <= 1; ++j ) {
				get(i,j) = weight.get(i,j) * d * (1 + 3*v.comp(i,j) + 4.5*sq(v.comp(i,j)) - 1.5*sq(v.norm()));
			}
		}
	}

	double sum() const {
		return get(-1, 1) + get( 0, 1) + get( 1, 1) + get(-1, 0) + get( 0, 0) + get( 1, 0) + get(-1,-1) + get( 0,-1) + get( 1,-1);
	}

	Velocity velocity(Density d) const {
		return 1./d * Velocity{
			get( 1, 0) - get(-1, 0) + get( 1, 1) - get(-1,-1) + get( 1,-1) - get(-1,1),
			get( 0, 1) - get( 0,-1) + get( 1, 1) - get(-1,-1) - get( 1,-1) + get(-1,1)
		};
	}
};

class CellBuffer {
	private:
		const std::size_t dim_x_;
		const std::size_t dim_y_;

		std::unique_ptr<Cell[]> curr_;
		std::unique_ptr<Cell[]> prev_;

	public:
		CellBuffer(std::size_t dimX, std::size_t dimY):
			dim_x_(dimX),
			dim_y_(dimY),
			curr_(new Cell[dimX*dimY]),
			prev_(new Cell[dimX*dimY]) { }

		void swap() {
			curr_.swap(prev_);
		}

		inline Cell& curr(std::size_t x, std::size_t y) {
			return curr_[y*dim_x_ + x];
		}

		inline Cell& prev(std::size_t x, std::size_t y) {
			return prev_[y*dim_x_ + x];
		}

};

constexpr std::size_t dimX = 128;
constexpr std::size_t dimY = 128;

constexpr double tau = 0.6;

CellBuffer pop(dimX, dimY);

Density  density [dimX][dimY];
Velocity velocity[dimX][dimY];
Force    force   [dimX][dimY];

void computeLbmStep(std::size_t t) {
	pop.swap();

	for ( std::size_t x = 1; x < dimX - 1; ++x ) {
		for ( std::size_t y = 1; y < dimY - 1; ++y ) {
			// update velocity (force coupling)
			//velocity[x][y] += tau / density[x][y] * force[x][y];

			// compute equilibrium
			Cell eq;
			eq.equilibrize(density[x][y], velocity[x][y]);

			// collide (BGK, relax towards equilibrium) & stream
			for ( int i = -1; i <= 1; ++i ) {
				for ( int j = -1; j <= 1; ++j ) {
					pop.curr(x+i,y+j).get(i,j) = pop.prev(x,y).get(i,j) + 1./tau * (eq.get(i,j) - pop.prev(x,y).get(i,j));
				}
			}
		}
	}

	// straight wall cell bounce back
	for ( std::size_t x = 2; x < dimX - 2; ++x ) {
		pop.curr(x,1).get( 0, 1) = pop.curr(x,0).get( 0,-1);
		pop.curr(x,1).get( 1, 1) = pop.curr(x,0).get(-1,-1);
		pop.curr(x,1).get(-1, 1) = pop.curr(x,0).get( 1,-1);

		pop.curr(x,dimY-2).get( 0,-1) = pop.curr(x,dimY-2).get( 0, 1);
		pop.curr(x,dimY-2).get(-1,-1) = pop.curr(x,dimY-2).get( 1, 1);
		pop.curr(x,dimY-2).get( 1,-1) = pop.curr(x,dimY-2).get(-1, 1);
	}
	for ( std::size_t y = 2; y < dimY - 2; ++y ) {
		pop.curr(1,y).get( 1, 0) = pop.curr(0,y).get(-1, 0);
		pop.curr(1,y).get( 1,-1) = pop.curr(0,y).get(-1, 1);
		pop.curr(1,y).get( 1, 1) = pop.curr(0,y).get(-1,-1);

		pop.curr(dimX-2,y).get(-1, 0) = pop.curr(dimX-1,y).get( 1, 0);
		pop.curr(dimX-2,y).get(-1, 1) = pop.curr(dimX-1,y).get( 1,-1);
		pop.curr(dimX-2,y).get(-1,-1) = pop.curr(dimX-1,y).get( 1, 1);
	}

	// edge wall cell bounce back
	pop.curr(1,1).get( 0, 1) = pop.curr(0,0).get( 0,-1);
	pop.curr(1,1).get( 1, 1) = pop.curr(0,0).get(-1,-1);
	pop.curr(1,1).get( 1, 0) = pop.curr(0,0).get(-1, 0);

	pop.curr(1,dimY-2).get( 0,-1) = pop.curr(0,dimY-1).get( 0, 1);
	pop.curr(1,dimY-2).get( 1,-1) = pop.curr(0,dimY-1).get(-1, 1);
	pop.curr(1,dimY-2).get( 1, 0) = pop.curr(0,dimY-1).get(-1, 0);

	pop.curr(dimX-2,1).get( 0, 1) = pop.curr(dimX-1,0).get( 0,-1);
	pop.curr(dimX-2,1).get(-1, 1) = pop.curr(dimX-1,0).get( 1,-1);
	pop.curr(dimX-2,1).get(-1, 0) = pop.curr(dimX-1,0).get( 1, 0);

	pop.curr(dimX-2,dimY-2).get( 0,-1) = pop.curr(dimX-1,dimY-1).get( 0, 1);
	pop.curr(dimX-2,dimY-2).get(-1,-1) = pop.curr(dimX-1,dimY-1).get( 1, 1);
	pop.curr(dimX-2,dimY-2).get(-1, 0) = pop.curr(dimX-1,dimY-1).get( 1, 0);

	// update density, velocity field
	for ( std::size_t x = 0; x < dimX; ++x ) {
		for ( std::size_t y = 0; y < dimY; ++y ) {
			density[x][y]  = pop.curr(x,y).sum();
			velocity[x][y] = pop.curr(x,y).velocity(density[x][y]);
		}
	}
}

void writeCurrentStateAsVTK(int time) {
	std::ofstream fout;
	fout.open(("result/data_t" + std::to_string(time) + ".vtk").c_str());

	fout << "# vtk DataFile Version 3.0\n";
	fout << "lbm_output\n";
	fout << "ASCII\n";
	fout << "DATASET RECTILINEAR_GRID\n";
	fout << "DIMENSIONS " << dimX - 2 << " " << dimY - 2 << " 1" << "\n";

	fout << "X_COORDINATES " << dimX - 2 << " float\n";
	for( std::size_t x = 1; x < dimX - 1; ++x ) {
		fout << x << " ";
	}

	fout << "\nY_COORDINATES " << dimY - 2 << " float\n";
	for( std::size_t y = 1; y < dimY - 1; ++y ) {
		fout << y << " ";
	}

	fout << "\nZ_COORDINATES " << 1 << " float\n";
	fout << 0 << "\n";
	fout << "POINT_DATA " << (dimX - 2) * (dimY - 2) << "\n";

	fout << "VECTORS velocity float\n";
	for ( std::size_t y = 1; y < dimY - 1; ++y ) {
		for ( std::size_t x = 1; x < dimX - 1; ++x ) {
			fout << velocity[x][y][0] << " " << velocity[x][y][1] << " 0\n";
		}
	}

	fout << "SCALARS density float 1\n";
	fout << "LOOKUP_TABLE default\n";
	for ( std::size_t y = 1; y < dimY - 1; ++y ) {
		for ( std::size_t x = 1; x < dimX - 1; ++x ) {
			fout << density[x][y] << "\n";
		}
	}

	fout.close();
}

void init() {
	for ( std::size_t x = 0; x < dimX; ++x ) {
		for ( std::size_t y = 0; y < dimY; ++y ) {
			density[x][y] = 1.0;
			velocity[x][y] = { 0.0, 0.0 };
			force[x][y] = { 0.0, 0.0 };

			pop.curr(x,y).equilibrize(density[x][y], velocity[x][y]);
			pop.prev(x,y).equilibrize(density[x][y], velocity[x][y]);
		}
	}

	for ( std::size_t y = 55; y < dimY-55; ++y ) {
		for ( std::size_t x = 35; x < dimX-75; ++x ) {
			density[x][y] = 0.8;
		}
		for ( std::size_t x = 75; x < dimX-35; ++x ) {
			density[x][y] = 0.8;
		}
	}
}

int main() {
	init();

	for ( std::size_t t = 0; t < 500; ++t ) {
		computeLbmStep(t);

		if ( t % 2 == 0 ) {
			writeCurrentStateAsVTK(t);
		}
	}
}