I am making a turn based hex-grid game. The player selects units and moves them across the hex grid. Each tile in the grid is of a particular terrain type (eg desert, hills, mountains, etc) and each unit type has different abilities when it comes to moving over the terrain (e.g. some can move over mountains easily, some with difficulty and some not at all).

Each unit has a movement value and each tile takes a certain amount of movement based on its terrain type and the unit type. E.g it costs a tank 1 to move over desert, 4 over swamp and cant move at all over mountains. Where as a flying unit moves over everything at a cost of 1.

The issue I have is that when a unit is selected, I want to highlight an area around it showing where it can move, this means working out all the possible paths through the surrounding hexes, how much movement each path will take and lighting up the tiles based on that information.

I got this working with a recursive function and found it took too long to calculate, I moved the function into a thread so that it didn't block the game but still it takes around 2 seconds for the thread to calculate the moveable area for a unit with a move of 8. Its over a million recursions which obviously is problematic.

I'm wondering if anyone has an clever ideas on how I can optimize this problem.

Here's the recursive function I'm currently using (its C# btw):

private void CalcMoveGridRecursive(int nCenterIndex, int nMoveRemaining)
{
    //List of the 6 tiles adjacent to the center tile
    int[] anAdjacentTiles = m_ThreadData.m_aHexData[nCenterIndex].m_anAdjacentTiles;

    foreach(int tileIndex in anAdjacentTiles)
    {
        //make sure this adjacent tile exists
        if(tileIndex == -1)
            continue;

        //How much would it cost the unit to move onto this adjacent tile
        int nMoveCost = m_ThreadData.m_anTerrainMoveCost[(int)m_ThreadData.m_aHexData[tileIndex].m_eTileType];

        if(nMoveCost != -1 && nMoveCost <= nMoveRemaining)
        {
            //Make sure the adjacent tile isnt already in our list.
            if(!m_ThreadData.m_lPassableTiles.Contains(tileIndex))
                m_ThreadData.m_lPassableTiles.Add(tileIndex);

            //Now check the 6 tiles surrounding the adjacent tile we just checked (it becomes the new center).
            CalcMoveGridRecursive(tileIndex, nMoveRemaining - nMoveCost);
        }
    }
}

At the end of the recursion, m_lPassableTiles contains a list of the indexes of all the tiles that the unit can possibly reach and they are made to glow.

This all works, it just takes too long. Does anyone know a better approach to this?