module PongBall (
  input i_Clk,
  input i_IsGameActive,
  input [5:0] i_CellX,
  input [5:0] i_CellY,

  output reg o_DoDrawBall,
  output reg [5:0] o_BallX,
  output reg [5:0] o_BallY
);
  parameter SCREEN_WIDTH_CELLS = 40;
  parameter SCREEN_HEIGHT_CELLS = 30;

  parameter CLOCKS_PER_SECOND = 25000000;
  parameter BALL_MOVE_TIME_MS = 50;
  parameter BALL_SPEED = 1250000;

  reg [$clog2(BALL_SPEED)-1:0] r_ballMoveCount;

  reg [5:0] r_PrevBallX;
  reg [5:0] r_PrevBallY;

  always @(posedge i_Clk) begin
    if (!i_IsGameActive) begin
      o_BallX <= SCREEN_WIDTH_CELLS / 2;
      o_BallY <= SCREEN_HEIGHT_CELLS / 2;
      r_PrevBallX <= SCREEN_WIDTH_CELLS / 2 + 1;
      r_PrevBallY <= SCREEN_HEIGHT_CELLS / 2 - 2;
    end else begin
      if (r_ballMoveCount < BALL_SPEED) begin
        r_ballMoveCount <= r_ballMoveCount + 1;
      end else begin
        r_ballMoveCount <= 0;

        r_PrevBallX <= o_BallX;
        r_PrevBallY <= o_BallY;

        if (
          // at right edge, travelling right
          (r_PrevBallX < o_BallX && o_BallX == SCREEN_WIDTH_CELLS - 1) ||
          // not at left edge
          (o_BallX < r_PrevBallX && o_BallX != 0)
        ) begin
          o_BallX <= o_BallX - 1;
        end else begin
          o_BallX <= o_BallX + 1;
        end

        if (
          // at bottom edge, travelling up
          (r_PrevBallY < o_BallY && o_BallY == SCREEN_HEIGHT_CELLS - 1) ||
          // not at left edge
          (o_BallY < r_PrevBallY && o_BallY != 0)
        ) begin
          o_BallY <= o_BallY - 1;
        end else begin
          o_BallY <= o_BallY + 1;
        end
      end
    end
  end

  always @(posedge i_Clk) begin
    if (i_CellX == o_BallX && i_CellY == o_BallY) begin
      o_DoDrawBall <= 1;
    end else begin
      o_DoDrawBall <= 0;
    end
  end
endmodule