// Code ported from https://0fps.net/2012/06/30/meshing-in-a-minecraft-game/

// Note this implementation does not support different block types or block normals
// The original author describes how to do this here: https://0fps.net/2012/07/07/meshing-minecraft-part-2/



const int CHUNK_SIZE = 32;

// These variables store the location of the chunk in the world, e.g. (0,0,0), (32,0,0), (64,0,0)
int chunkPosX = 0;
int chunkPosY = 0;
int chunkPosZ = 0;

public void GreedyMesh()
{
    // Sweep over each axis (X, Y and Z)
    for (var d = 0; d < 3; ++d)
    {
        int i, j, k, l, w, h;
        int u = (d + 1) % 3;
        int v = (d + 2) % 3;
        var x = new int[3];
        var q = new int[3];

        var mask = new bool[CHUNK_SIZE * CHUNK_SIZE];
        q[d] = 1;

        // Check each slice of the chunk one at a time
        for (x[d] = -1; x[d] < CHUNK_SIZE;)
        {
            // Compute the mask
            var n = 0;
            for (x[v] = 0; x[v] < CHUNK_SIZE; ++x[v])
            {
                for (x[u] = 0; x[u] < CHUNK_SIZE; ++x[u])
                {                    
                    // q determines the direction (X, Y or Z) that we are searching
                    // m.IsBlockAt(x,y,z) takes global map positions and returns true if a block exists there
                    
                    bool blockCurrent = 0 <= x[d]             ? IsBlockAt(x[0] + chunkPosX,        x[1] + chunkPosY,        x[2] + chunkPosZ)        : true;
                    bool blockCompare = x[d] < CHUNK_SIZE - 1 ? IsBlockAt(x[0] + q[0] + chunkPosX, x[1] + q[1] + chunkPosY, x[2] + q[2] + chunkPosZ) : true;

                    // The mask is set to true if there is a visible face between two blocks,
                    //   i.e. both aren't empty and both aren't blocks
                    mask[n++] = blockCurrent != blockCompare;
                }
            }

            ++x[d];

            n = 0;
            
            // Generate a mesh from the mask using lexicographic ordering,      
            //   by looping over each block in this slice of the chunk
            for (j = 0; j < CHUNK_SIZE; ++j)
            {
                for (i = 0; i < CHUNK_SIZE;)
                {
                    if (mask[n])
                    {
                        // Compute the width of this quad and store it in w                        
                        //   This is done by searching along the current axis until mask[n + w] is false
                        for (w = 1; i + w < CHUNK_SIZE && mask[n + w]; w++) { }

                        // Compute the height of this quad and store it in h                        
                        //   This is done by checking if every block next to this row (range 0 to w) is also part of the mask.
                        //   For example, if w is 5 we currently have a quad of dimensions 1 x 5. To reduce triangle count,
                        //   greedy meshing will attempt to expand this quad out to CHUNK_SIZE x 5, but will stop if it reaches a hole in the mask
                        
                        var done = false;
                        for (h = 1; j + h < CHUNK_SIZE; h++)
                        {
                            // Check each block next to this quad
                            for (k = 0; k < w; ++k)
                            {
                                // If there's a hole in the mask, exit
                                if (!mask[n + k + h * CHUNK_SIZE])
                                {
                                    done = true;
                                    break;
                                }
                            }

                            if (done)
                                break;
                        }

                        x[u] = i;
                        x[v] = j;

                        // du and dv determine the size and orientation of this face
                        var du = new int[3];
                        du[u] = w;

                        var dv = new int[3];
                        dv[v] = h;

                        // Create a quad for this face. Colour, normal or textures are not stored in this block vertex format.
                        BlockVertex.AppendQuad(new Int3(x[0],                 x[1],                 x[2]),                 // Top-left vertice position
                                               new Int3(x[0] + du[0],         x[1] + du[1],         x[2] + du[2]),         // Top right vertice position
                                               new Int3(x[0] + dv[0],         x[1] + dv[1],         x[2] + dv[2]),         // Bottom left vertice position
                                               new Int3(x[0] + du[0] + dv[0], x[1] + du[1] + dv[1], x[2] + du[2] + dv[2])  // Bottom right vertice position
                                               );

                        // Clear this part of the mask, so we don't add duplicate faces
                        for (l = 0; l < h; ++l)
                            for (k = 0; k < w; ++k)
                                mask[n + k + l * CHUNK_SIZE] = false;

                        // Increment counters and continue
                        i += w;
                        n += w;
                    }
                    else
                    {
                        i++;
                        n++;
                    }
                }
            }
        }
    }
}