//@ ts-check

const Region = 
{
    /** @type {Region.CoordsToIndex} */
    I(inX, inY, inZ)
    {
        return inX + inY*16 + inZ*256;
    },
    /** @type {Region.IndexToCoords} */
    XYZ(inI)
    {
        let zWhole = Math.floor(inI / 256);
        let zPart = inI % 256;
        let yWhole = Math.floor(zPart / 16);
        let yPart = zPart % 16;
        return [yPart, yWhole, zWhole];
    },

    /** @type {Region.BufferBuilder} */
    Surface(inVoxels)
    {
        var vertices = []; // output position buffer
        var triPointVert = []; // output index buffer

        var vertPointNeighbors = []; // vertex array, contains arrays of integer pointers to vertices
        var voxPointVert = []; // for each inVoxel; integer pointer to vertices output buffer

        /*  

        inVoxels
            |
        [1 to 1]
            |
        voxPointVert---[optional pointers to]---> vertices <---- triPointVert
                                                     |
                                                 [1 to 1]
                                                     |
                                                 vertPointNeighbors
                     
        */

        /** @type {(voxIndex:number)=>[number, boolean]} */
        function Vert(inVoxIndex)
        {
            /*
                If not preexisting, create a vertex with the coordinates of the voxel at `voxPointVert[inVoxIndex]`.
                Add the new vertex to `vertices`.
                Initialize empty neighbor connections.
                Put the *index* of new vertex back in the voxel array at `inVoxIndex`
                Return the vertex *index*, plus a flag if a vertex was already there.

                Impure refs: voxPointVert, vertPointNeighbors, vertices
            */
            
            var vertPointer = voxPointVert[inVoxIndex] 
            if(vertPointer === undefined)// is there no vertex for this voxel?
            {
                vertPointer = vertices.length;
                voxPointVert[inVoxIndex] = vertPointer;

                vertPointNeighbors.push([]);
                vertices.push(Region.XYZ(inVoxIndex));

                return [vertPointer, false];
            }
            else
            {
                return [vertPointer, true];
            }
        }
        function Edge(vert1, vert1WasFull, vert2, vert2WasFull)
        {
            /*
                Sets up neighbor relationships between vert1 and vert2.
                Contains a check for if they have already been connected.

                Impure refs: vertPointNeighbors
            */

            let p1Neighbors = vertPointNeighbors[vert1];
            let p2Neighbors = vertPointNeighbors[vert2];

            // if the two verts were already filled
            if((vert1WasFull && vert2WasFull))
            {
                // it's possible that they are already connected
                if(p1Neighbors.indexOf(vert2) !== -1)
                {
                    // quit if they are
                    return;
                }
            }

            p1Neighbors.push(vert2);
            p2Neighbors.push(vert1);
        }
        function Face(a, b, c, d /* vox indices */)
        {
            let [p1, p1WasFull] = Vert(a);
            let [p2, p2WasFull] = Vert(b);
            let [p3, p3WasFull] = Vert(c);
            let [p4, p4WasFull] = Vert(d);

            triPointVert.push(p1, p2, p3, p3, p4, p1);

            Edge(p1, p1WasFull, p2, p2WasFull);
            Edge(p2, p2WasFull, p3, p3WasFull);
            Edge(p3, p3WasFull, p4, p4WasFull);
            Edge(p4, p4WasFull, p1, p1WasFull);
        }

        var i;
        var x, y, z;
        var BA, BU, BC, BR,  TA, TU, TC, TR,  CL, CD, CB;
        for(i=0; i<inVoxels.length; i++)
        {
            if(inVoxels[i])
            {
                [x, y, z] = Region.XYZ(i);

                // bottom: at, "up", corner, right
                BA = i;
                BU = Region.I(x,   y+1, z  );
                BC = Region.I(x+1, y+1, z  );
                BR = Region.I(x+1, y,   z  );

                // top: at, "up" corner, right
                TA = Region.I(x,   y,   z+1);
                TU = Region.I(x,   y+1, z+1);
                TC = Region.I(x+1, y+1, z+1);
                TR = Region.I(x+1, y,   z+1);

                // check: left, "down", below
                CL = Region.I(x-1, y,   z  );
                CD = Region.I(x,   y-1, z  );
                CB = Region.I(x,   y,   z-1);

                if(!inVoxels[BU]){ Face(BU, BC, TC, TU); } // XZ (away from origin)
                if(!inVoxels[BR]){ Face(BC, BR, TR, TC); } // YZ (away from origin)
                if(!inVoxels[TA]){ Face(TA, TU, TC, TR); } // XY (away from origin)

                if(!inVoxels[CL]){ Face(BA, BU, TU, TA); } // YZ (touches the origin)
                if(!inVoxels[CD]){ Face(BR, BA, TA, TR); } // XZ (touches the origin)
                if(!inVoxels[CB]){ Face(BU, BA, BR, BC); } // XY (touches the origin)
            }
        }

        return {
            pos: new Float32Array(vertices.flat()),
            ind: triPointVert
        };
    }
};

export default Region;