using System;

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public enum Currency

{

    None,

    GBP,

    EUR,

    USD

}

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public struct Money

{

    public decimal Amount;

    public Currency Currency;

    private static Money none = new Money {Amount = 0, Currency = Currency.None};

    public static Money None => none;

    public static Money Free => none;

}

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public struct Cdr

{

    public string Iccid;

    public ulong Bytes;

}

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public interface IProduct

{

    IProductUse BuildProductUse( Cdr cdr );

}

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public interface IDataContext

{

    void Save();

}

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public interface IProductUse

{

    Money Cost { get; }

    void Apply( IDataContext database );

}

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public class PerMBProduct : IProduct

{

    private string name;

    private Money costPerMB;

    public PerMBProduct( string name, Money costPerMB )

    {

        this.name = name;

        this.costPerMB = costPerMB;

    }

    IProductUse IProduct.BuildProductUse( Cdr cdr )

    {

        return new Use( this, cdr.Bytes ); 

    }

    private class Use : IProductUse

    {

        private readonly PerMBProduct product;

        private readonly ulong bytes;

        public Use( PerMBProduct product, ulong bytes )

        {

            this.product = product;

            this.bytes = bytes;

            Cost = new Money {Amount = product.costPerMB.Amount * bytes, Currency = product.costPerMB.Currency};

        }

        public Money Cost { get; }

        public void Apply( IDataContext database )

        {

            database.Save();

        }

    }

}

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public class PoolProduct : IProduct

{

    private string name;

    private ulong size;

    public PoolProduct( string name, ulong size )

    {

        this.name = name;

        this.size = size;

    }

    IProductUse IProduct.BuildProductUse( Cdr cdr )

    {

        Use result = null;

        if( cdr.Bytes <= this.size )

        {

            result = new Use( this, cdr.Bytes );

            size -= cdr.Bytes;

            cdr.Bytes = 0;

        }

        else if( size <= cdr.Bytes )

        {

            result = new Use( this, size );

            cdr.Bytes -= size;

            size = 0;

        }

        return result;

    }

    private class Use : IProductUse

    {

        private readonly PoolProduct pool;

        private readonly ulong bytes;

        public Use( PoolProduct pool, ulong bytes )

        {

            this.pool = pool;

            this.bytes = bytes;

        }

        public Money Cost => Money.Free;

        public void Apply( IDataContext database )

        {

            database.Save();

        }

    }

}

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Console.WriteLine("Hello World");

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int i = 2 + 2;

i