This is the longest method in the game, and again, most of the logic happens inside callbacks. First we tag the gems being removed by setting their type data to -1. All the gems inside matchArray will be removed:

function GameScene:collapseGrid ()
    for i = 1, #self.gridController.matchArray do
        self.grid[self.gridController.matchArray[i].x]
        [self.gridController.matchArray[i].y] = -1
    end

    local column = nil
    local newColumn = nil
    local i

Next, we traverse the grid's columns and rearrange the gems whose type is not equal to -1 inside the column arrays. Essentially, we update the data here so that gems above the ones removed "fall down". The actual change will take place in the animateCollapse method:

    for c = 1, constants...