OOP Battleship Part 4: AI Strategies

If you recall the AIPlayer class from Part 2, the Create factory method takes an IGameStrategy parameter:

Public Function Create(ByVal gridId As Byte, ByVal GameStrategy As IGameStrategy) As IPlayer
    With New AIPlayer
        .PlayerType = ComputerControlled
        .GridIndex = gridId
        Set .Strategy = GameStrategy
        Set .PlayGrid = PlayerGrid.Create(gridId)
        Set Create = .Self
    End With
End Function

An AIPlayer can be created with an instance of any class that implements the IGameStrategy interface.

In any OOP language that supports class inheritance, we could have a base class e.g. GameStrategyBase, from which we could derive the various implementations, and with that we would have a place to write all the code that’s common to all implementations, …or that all implementations would possibly need to use… or not. See, class inheritance is the most important language feature that the “VBA can’t do OOP” or “VBA is not a real language” crowd love to bring up. And yet, more often than not, class inheritance isn’t the ideal solution – composition is.

And we’re going to do exactly that, by composing all IGameStrategy implementations with a GameStrategyBase class:

Coupling a game strategy with this “base” class isn’t an issue: the class is specifically meant to be used by IGameStrategy implementations. So we can shamelessly do this:

Option Explicit
Implements IGameStrategy
Private base As GameStrategyBase

Private Sub Class_Initialize()
    Set base = New GameStrategyBase
End Sub

And then proceed with implementing the PlaceShip method, given that AI player’s own PlayerGrid and the IShip the game controller is asking us to place on the grid. The base.PlaceShip method simply returns the first legal position+direction it can find.

Then we can implement the Play function to return an IGridCoord position and let the controller know what position this player is shooting at. We have a number of helper functions in GameStrategyBase we can use for that.

Random

The RandomShotStrategy shoots at random coordinates until it has located all enemy ships …then proceeds to sink them all, one after the other. It also places its ships randomly, regardless of whether the ships are adjacent or not.

Private Sub IGameStrategy_PlaceShip(ByVal grid As PlayerGrid, ByVal currentShip As IShip)

    Dim direction As ShipOrientation
    Dim position As IGridCoord
    Set position = base.PlaceShip(Random, grid, currentShip, direction)

    grid.AddShip Ship.Create(currentShip.ShipKind, direction, position)
    If grid.shipCount = PlayerGrid.ShipsPerGrid Then grid.Scramble

End Sub

Private Function IGameStrategy_Play(ByVal enemyGrid As PlayerGrid) As IGridCoord
    Dim position As IGridCoord
    Do
        If EnemyShipsNotAcquired(enemyGrid)  0 Then
            Set position = base.ShootRandomPosition(Random, enemyGrid)
        Else
            Set position = base.DestroyTarget(Random, enemyGrid, enemyGrid.FindHitArea)
        End If
    Loop Until base.IsLegalPosition(enemyGrid, position)
    Set IGameStrategy_Play = position
End Function

Here the double-negative in the statement “the number of enemy ships not acquired, is not equal to zero” (WordPress is having a hard time with rendering that  operator, apparently), will probably be end up being inverted into a positive statement, which would make it read better. Perhaps If EnemyShipsToFind = 0 Then, and invert the Else logic. Or…

Private Function IGameStrategy_Play(ByVal enemyGrid As PlayerGrid) As IGridCoord
    Dim position As IGridCoord
    Do
        If EnemyShipsToFind(enemyGrid) > 0 Then
            Set position = base.ShootRandomPosition(Random, enemyGrid)
enemyGrid.FindHitArea)
        Else
            Set position = base.DestroyTarget(Random, enemyGrid,
        End If
    Loop Until base.IsLegalPosition(enemyGrid, position)
    Set IGameStrategy_Play = position
End Function

That EnemyShipsToFind function should probably be a member of the PlayerGrid class.

FairPlay

The FairPlayStrategy is similar, except it will proceed to destroy an enemy ship as soon as it’s located. It also takes care to avoid placing ships adjacent to each other.

Private Sub IGameStrategy_PlaceShip(ByVal grid As PlayerGrid, ByVal currentShip As IShip)
    Do
        Dim direction As ShipOrientation
        Dim position As IGridCoord
        Set position = base.PlaceShip(Random, grid, currentShip, direction)

    Loop Until Not grid.HasAdjacentShip(position, direction, currentShip.Size)

    grid.AddShip Ship.Create(currentShip.ShipKind, direction, position)
    If grid.shipCount = PlayerGrid.ShipsPerGrid Then grid.Scramble
End Sub

Private Function IGameStrategy_Play(ByVal enemyGrid As PlayerGrid) As IGridCoord
    Dim position As GridCoord
    Do
        Dim area As Collection
        Set area = enemyGrid.FindHitArea

        If Not area Is Nothing Then
            Set position = base.DestroyTarget(Random, enemyGrid, area)
        Else
            Set position = base.ShootRandomPosition(Random, enemyGrid)
        End If
    Loop Until base.IsLegalPosition(enemyGrid, position)
    Set IGameStrategy_Play = position
End Function

Merciless

The MercilessStrategy is more elaborate: it doesn’t just shoot at random – it shoots in patterns, targeting the edges and/or the center areas of the grid. It will destroy an enemy ship as soon as it’s found, and will avoid shooting in an area that couldn’t possibly host the smallest enemy ship that’s still afloat. And yet, it’s possible it just shoots a random position, too:

Private Sub IGameStrategy_PlaceShip(ByVal grid As PlayerGrid, ByVal currentShip As IShip)
    Do
        Dim direction As ShipOrientation
        Dim position As IGridCoord
        Set position = base.PlaceShip(Random, grid, currentShip, direction)
    Loop Until Not grid.HasAdjacentShip(position, direction, currentShip.Size)

    grid.AddShip Ship.Create(currentShip.ShipKind, direction, position)
    If grid.shipCount = PlayerGrid.ShipsPerGrid Then grid.Scramble
End Sub

Private Function IGameStrategy_Play(ByVal enemyGrid As PlayerGrid) As IGridCoord
    Dim position As GridCoord
    Do
        Dim area As Collection
        Set area = enemyGrid.FindHitArea

        If Not area Is Nothing Then
            Set position = base.DestroyTarget(Random, enemyGrid, area)
        Else
            If this.Random.NextSingle < 0.1 Then
                Set position = base.ShootRandomPosition(this.Random, enemyGrid)
            ElseIf this.Random.NextSingle < 0.6 Then
                Set position = ScanCenter(enemyGrid)
            Else
                Set position = ScanEdges(enemyGrid)
            End If
        End If

    Loop Until base.IsLegalPosition(enemyGrid, position) And _
               base.VerifyShipFits(enemyGrid, position, enemyGrid.SmallestShipSize) And _
               AvoidAdjacentHitPosition(enemyGrid, position)
    Set IGameStrategy_Play = position
End Function

In most cases (ScanCenter and ScanEdges do), the AI doesn’t even care to “remember” the last hit it made: instead, it asks the enemy grid to give it a “hit area”. It then proceeds to analyze whether that area is horizontal or vertical, and then attempts to extend it further.

It’s Open-Source!

I uploaded the complete code to GitHub: https://github.com/rubberduck-vba/Battleship.