there's about 2 * 3 * 6 = 36 different logics to evaluate
We can apply divide and conquer technique.
- you have 6 states. It is possible to use
Chain of Responibility pattern here to choose appropriate state handler
- when desired state handler is found, then we can apply desired function. The appropriate function can be considered as strategy. So it is a place where
Strategy pattern can be applied.
- we can separate strategies by appropriate states and put them in simple factory to get desired strategy by key.
This is what we will do. So let's see it more thoroughly.
Chain of responsibility pattern
If you have a lot if else statements, it is possible to use Chain of Responsibility pattern. As wiki says about Chain of Responsibility:
The chain-of-responsibility pattern is a behavioral design pattern
consisting of a source of command objects and a series of processing
objects. Each processing object contains logic that defines the
types of command objects that it can handle; the rest are passed to
the next processing object in the chain. A mechanism also exists for
adding new processing objects to the end of this chain
So let's dive in code. Let me show an example via C#.
So this is our Argument class which has Left and Right operands:
public class Arguments
{
public string Left { get; private set; }
public string Right { get; private set; }
public MyState MyState { get; private set; }
public MyKey MyKey => new MyKey(MyState, Left);
public Arguments(string left, string right, MyState myState)
{
Left = left;
Right = right;
MyState = myState;
}
}
And this is your 6 states:
public enum MyState
{
One, Two, Three, Four, Five, Six
}
This is start of Decorator pattern. This is an abstraction of StateHandler which defines behaviour to to set next handler:
public abstract class StateHandler
{
public abstract MyState State { get; }
private StateHandler _nextStateHandler;
public void SetSuccessor(StateHandler nextStateHandler)
{
_nextStateHandler = nextStateHandler;
}
public virtual IDifferentLogicStrategy Execute(Arguments arguments)
{
if (_nextStateHandler != null)
return _nextStateHandler.Execute(arguments);
return null;
}
}
and its concrete implementations of StateHandler:
public class OneStateHandler : StateHandler
{
public override MyState State => MyState.One;
public override IDifferentLogicStrategy Execute(Arguments arguments)
{
if (arguments.MyState == State)
return new StrategyStateFactory().GetInstanceByMyKey(arguments.MyKey);
return base.Execute(arguments);
}
}
public class TwoStateHandler : StateHandler
{
public override MyState State => MyState.Two;
public override IDifferentLogicStrategy Execute(Arguments arguments)
{
if (arguments.MyState == State)
return new StrategyStateFactory().GetInstanceByMyKey(arguments.MyKey);
return base.Execute(arguments);
}
}
and the third state handler looks like this:
public class ThreeStateHandler : StateHandler
{
public override MyState State => MyState.Three;
public override IDifferentLogicStrategy Execute(Arguments arguments)
{
if (arguments.MyState == State)
return new StrategyStateFactory().GetInstanceByMyKey(arguments.MyKey);
return base.Execute(arguments);
}
}
Strategy pattern
Let's pay attention to the following row of code:
return new StrategyStateFactory().GetInstanceByMyKey(arguments.MyKey);
The above code is an example of using Strategy pattern. We have different ways or strategies to handle
your cases. Let me show a code of strategies of evaluation of your expressions.
This is an abstraction of strategy:
public interface IDifferentLogicStrategy
{
string Evaluate(Arguments arguments);
}
And its concrete implementations:
public class StrategyWildCardStateOne : IDifferentLogicStrategy
{
public string Evaluate(Arguments arguments)
{
// your logic here to evaluate "_" (a wildcard)
return "StrategyWildCardStateOne";
}
}
public class StrategyIntegerStringStateOne : IDifferentLogicStrategy
{
public string Evaluate(Arguments arguments)
{
// your logic here to evaluate "1" (an integer string)
return "StrategyIntegerStringStateOne";
}
}
And the third strategy:
public class StrategyNormalStringStateOne : IDifferentLogicStrategy
{
public string Evaluate(Arguments arguments)
{
// your logic here to evaluate "abc" (a normal string)
return "StrategyNormalStringStateOne";
}
}
Simple factory
There is no pattern like simple factory. However, it is a place where we can get instances of strategies by key. So by doing this we avoided to use multiple if else statements to choose correct strategy.
So, we need a place where we can store strategies by state and argument value. At first, let's create MyKey struct. It will have help us to differentiate State and arguments:
public struct MyKey
{
public readonly MyState MyState { get; }
public readonly string ArgumentValue { get; } // your three cases: "_",
// an integer string, a normal string
public MyKey(MyState myState, string argumentValue)
{
MyState = myState;
ArgumentValue = argumentValue;
}
public override bool Equals([NotNullWhen(true)] object? obj)
{
return obj is MyKey mys
&& mys.MyState == MyState
&& mys.ArgumentValue == ArgumentValue;
}
public override int GetHashCode()
{
unchecked // Overflow is fine, just wrap
{
int hash = 17;
hash = hash * 23 + MyState.GetHashCode();
hash = hash * 23 + ArgumentValue.GetHashCode();
return hash;
}
}
}
and then we can create a simple factory:
public class StrategyStateFactory
{
private Dictionary<MyKey, IDifferentLogicStrategy>
_differentLogicStrategyByStateAndValue =
new Dictionary<MyKey, IDifferentLogicStrategy>()
{
{ new MyKey(MyState.One, "_"), new StrategyWildCardStateOne() },
{ new MyKey(MyState.One, "intString"),
new StrategyIntegerStringStateOne() },
{ new MyKey(MyState.One, "normalString"),
new StrategyNormalStringStateOne() }
};
public IDifferentLogicStrategy GetInstanceByMyKey(MyKey myKey)
{
return _differentLogicStrategyByStateAndValue[myKey];
}
}
So we've written our strategies and we've stored these strategies in simple factory StrategyStateFactory.
Then we need to check the above implementation:
StateHandler chain = new OneStateHandler();
StateHandler secondStateHandler = new TwoStateHandler();
StateHandler thirdStateHandler = new ThreeStateHandler();
chain.SetSuccessor(secondStateHandler);
secondStateHandler.SetSuccessor(thirdStateHandler);
Arguments arguments = new Arguments("_", "_", MyState.One);
IDifferentLogicStrategy differentLogicStrategy = chain.Execute(arguments);
string evaluatedResult =
differentLogicStrategy.Evaluate(arguments); // output: "StrategyWildCardStateOne"
I believe I gave basic idea how it can be done.
