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// Copyright (c) .NET Foundation. All rights reserved.

// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

namespace System.Web.Razor

{

public abstract class StateMachine<TReturn>

{

protected delegate StateResult State();

protected abstract State StartState { get; }

protected State CurrentState { get; set; }

protected virtual TReturn Turn()

{

if (CurrentState != null)

{

StateResult result;

do

{

// Keep running until we get a null result or output

result = CurrentState();

CurrentState = result.Next;

}

while (result != null && !result.HasOutput);

if (result == null)

{

return default(TReturn); // Terminated

}

return result.Output;

}

return default(TReturn);

}

/// <summary>

/// Returns a result indicating that the machine should stop executing and return null output.

/// </summary>

protected StateResult Stop()

{

return null;

}

/// <summary>

/// Returns a result indicating that this state has no output and the machine should immediately invoke the specified state

/// </summary>

/// <remarks>

/// By returning no output, the state machine will invoke the next state immediately, before returning

/// controller to the caller of <see cref="Turn"/>

/// </remarks>

protected StateResult Transition(State newState)

{

return new StateResult(newState);

}

/// <summary>

/// Returns a result containing the specified output and indicating that the next call to

/// <see cref="Turn"/> should invoke the provided state.

/// </summary>

protected StateResult Transition(TReturn output, State newState)

{

return new StateResult(output, newState);

}

/// <summary>

/// Returns a result indicating that this state has no output and the machine should remain in this state

/// </summary>

/// <remarks>

/// By returning no output, the state machine will re-invoke the current state again before returning

/// controller to the caller of <see cref="Turn"/>

/// </remarks>

protected StateResult Stay()

{

return new StateResult(CurrentState);

}

/// <summary>

/// Returns a result containing the specified output and indicating that the next call to

/// <see cref="Turn"/> should re-invoke the current state.

/// </summary>

protected StateResult Stay(TReturn output)

{

return new StateResult(output, CurrentState);

}

protected class StateResult

{

public StateResult(State next)

{

HasOutput = false;

Next = next;

}

public StateResult(TReturn output, State next)

{

HasOutput = true;

Output = output;

Next = next;

}

public bool HasOutput { get; set; }

public TReturn Output { get; set; }

public State Next { get; set; }

}

}

}