SharpC-A-C-Interpreter-In-C-0110

jiezi

5 年前

勿庸质疑，说到控制结构，首先会想到 if。其实现应如下所示：

public class IfThen : ControlFlow
{
    public Expression.ExpressionNode Condition;

    public Block ThenClause
    {get { return Children.Count > 0 ? Children.First() as Block: null; }
    }

    public Block ElseClause
    {get { return Children.Count > 1 ? Children.Last() as Block: null; }
    }

运行时处理则应该如下：

public override void Run(Context ctx)
{Debug.WriteLine("if(" + Condition.ToString() + ")");

    Expression.Operand.Operand condVal = Condition.Evaluate(this);

    Debug.WriteLine("Condition=" + condVal.GetValue(this).ToString());

    if (condVal.GetValue(this).AsInt != 0)
    {if (ThenClause != null)
        {Debug.WriteLine("Then");
            ThenClause.Run(this);
        }
    }
    else
    {if (ElseClause != null)
        {Debug.WriteLine("Else");
            ElseClause.Run(this);
        }
    }
}

看起来相当简单。

循环具有类似的结构：条件判断，循环体。其基类如下所示：

public class Loop : ControlFlow
{
    public Expression.ExpressionNode Condition;

    public virtual Block Body
    {get { return Children.Count > 0 ? Children.Last() as Block : null; }
    }

    public bool NeedBreak = false;
    public bool NeedContinue = false;

最简单的循环莫过于 while，或 do..while。好了，已经将 while 循环都列举了。
先看 while 循环的运行代码：

 public override void Run(Context ctx)
{Debug.WriteLine("Begin While.");

    base.Run(ctx);

    while (true)
    {if (Condition != null)
        {Expression.Operand.Value condVal = Condition.Evaluate(this).GetValue(this);

            Debug.WriteLine(string.Format("Condition: [{0}] = {1}", Condition.ToString(), condVal.ToString()));

            if (condVal.AsInt == 0)
            {break;}
        }

        Body.Run(this);

        if (NeedBreak)
            break;
    } // while

    Debug.WriteLine("End While.");
}

do…while 与 while 不同的部分在于，先运行循环体，再运行判断：

public override void Run(Context ctx)
{Debug.WriteLine("Begin Do...While.");

    base.Run(ctx);

    while(true)
    {Body.Run(this);

        if (NeedBreak)
            break;

        if (Condition != null)
        {Expression.Operand.Value val = Condition.Evaluate(this).GetValue(this);

            Debug.WriteLine(string.Format("Condition : {0} = {1}", Condition.ToString(), val.ToString()));

            if (val.AsInt == 0)
            {break;}
        }
    } // while

    Debug.WriteLine("End Do...While.");
}

复杂一点的是 for 循环，比其它循环结构多出初始化部分和迭代部分：

public class ForLoop : Loop
{
    public Context Initializer
    {get { return Children.Count > 0 ? Children.First() : null; }
    }

    public Expression.ExpressionNode Iterator;

由于初始化部分可以定义循环变量，形如：for(int i = 0; ….)。所以，还需要重载以下两个方法：

public override bool HasDefined(string str)
{if (FindByName(str) == null)
        return base.HasDefined(str);
    else
        return true;
}

public override Context FindByName(string str)
{
    Context res = null;

    if (Initializer != null)
    {
        Initializer.Parent = null;

        res = Initializer.FindByName(str);

        Initializer.Parent = this;
    }

    if (res == null)
        return this.Parent.FindByName(str);
    else
        return res;
}

然后就可以确定 for 循环的运行代码：

public override void Run(Context ctx)
{Debug.WriteLine("Begin For.");

    if (Initializer != null)
        Initializer.Run(this);

    base.Run(ctx);

    while (true)
    {if (Condition != null)
        {Expression.Operand.Value val = Condition.Evaluate(this).GetValue(this);

            Debug.WriteLine(string.Format("Condition: [{0}] = {1}", Condition.ToString(), val.ToString()));

            if (val.AsInt == 0)
            {break;}
        }

        Body.Run(this);

        if (NeedBreak)
            break;

        if (Iterator != null)
            Iterator.Evaluate(this);
    } // while

    if (Initializer != null)
    {Initializer.FreeLocalVariables();
    }

    Debug.WriteLine("End For.");
}

鲜有解释器（或编译器）实现了 switch 控制。无它，唯复杂尔。SharpC 将 switch 划分为两个部分实现：switch(表达式）和 case。先看 Case 的实现，由条件值和执行部分组成：

public class Case : ControlFlow
{
    public Expression.Operand.Value Value;
    public Block Body
    {get { return Children.Count > 0 ? Children.First() as Block : null; }
    }
}

为简化实现，用 Block 作为 case 的执行部分。副作用是多了一层，略微影响效率。再看 switch 的实现：

public class Switch : ControlFlow
{
    private Dictionary<int, int> m_caseDict;

    public Expression.ExpressionNode Condition;

    public Block Body
    {get { return Children.Count > 0 ? Children.First() as Block : null; }
    }

    public Case Default
    {get { return Body != null ? Body.Children.Last() as Case : null; }
    }

    public List<Context> CaseSet
    {get { return Body != null ? Body.Children.Count > 1 ? Body.Children.GetRange(0, Body.Children.Count - 1) : null : null; }
    }

switch 的执行部分亦为一个 block。同时 Default 是最后一个 Case。再看 switch 的执行部分：

public override void Run(Context ctx)
{Debug.WriteLine("Begin Switch.");

    bool needBreak = false;

    if (IsFirstRunning)
    {m_caseDict = new Dictionary<int, int>();

        int idx = 0;

        Body.OnReturn += delegate(Context stx, Return ret)
        {Debug.WriteLine("Return");

            needBreak = true;

            Block parentBlock = this.ParentBlock;

            if (parentBlock != null && parentBlock.OnReturn != null)
                parentBlock.OnReturn(stx, ret);
        };

        Body.OnBreak = delegate(Context stx)
        {Debug.WriteLine("Break.");

            needBreak = true;
        };

        foreach (Context item in CaseSet)
        {
            Case stxCase = item as Case;

            m_caseDict.Add(stxCase.Value.AsInt, idx++);
            stxCase.Body.OnBreak += Body.OnBreak;
        }

        IsFirstRunning = false;
    }

    Expression.Operand.Value val = Condition.Evaluate(this).GetValue(ctx);

    Debug.WriteLine(string.Format("Condition: [{0}] = {1}", Condition, val));


    if (m_caseDict.ContainsKey(val.AsInt))
    {int idx = m_caseDict[val.AsInt];
        IEnumerator<Context> caseEnum = CaseSet.GetEnumerator();
        int i = 0;

        while (caseEnum.MoveNext())
        {if (i++ < idx)
                continue;

            Debug.WriteLine(string.Format("case {0}", (caseEnum.Current as Case).Value.AsInt));

            (caseEnum.Current as Case).Body.Run(this);

            if (needBreak)
                break;
        } // while
    }
    else
    {Debug.WriteLine("Default");

        Default.Run(this);
    }
}

在第一次执行的时候先建立 case 字典，希望下次运行的时候能够加快速度。然后，运行与表达式相匹配的 case，并从该 case 开始，依次运行下一个 case，直到运行到 break 为止。

这三种控制结构都必须结合上下文环境共同作用。看起来它们更像是一种标记。因此上，它们的实现也相当简单：

public class Break : ControlFlow
{public override void Print(int tabs = 0)
    {Debug.WriteLine(new string('\t', tabs) + "break;");            
    }
}

public class Continue : ControlFlow
{public override void Print(int tabs = 0)
    {Debug.WriteLine(new string('\t', tabs) + "continue;");
    }
}

return 多了一个返回值的表达式：

public class Return : ControlFlow
{
    public Expression.ExpressionNode Expression;

    public override void Print(int tabs = 0)
    {if (Expression == null)
            Debug.WriteLine(new string('\t', tabs) + "return ;");
        else
        {Debug.WriteLine(new string('\t', tabs) + "return" + Expression.ToString());
        }
    }
}

一切需要回溯到 Block，在 Block 运行时，会检测这三种控制结构：

public override void Run(Context ctx)
{if (IsFirstRunning)
    {
        Block parentBlock = this.ParentBlock;

        if (parentBlock != null)
        {
            this.OnReturn += parentBlock.OnReturn;
            this.OnBreak += parentBlock.OnBreak;
            this.OnContinue += parentBlock.OnContinue;
        }

        IsFirstRunning = false;
    }

    foreach (Context stx in Children)
    {if (stx is ControlFlow.ControlFlow)
        {if (!ExecuteControlFlow(stx))
                break;

再看 ExecuteControlFlow()：

private bool ExecuteControlFlow(Context stx)
{if (stx is ControlFlow.Return)
    {if (OnReturn != null)
            OnReturn(this, stx as ControlFlow.Return);

        return false;
    }
    else
    {if (stx is ControlFlow.Break)
        {if (OnBreak != null)
                OnBreak(this);

            return false;
        }
        else
            if (stx is ControlFlow.Continue)
            {if (OnContinue != null)
                    OnContinue(this);

                return false;
            }
            else
                stx.Run(this);
    }

    return true;
}

通过返回 false 来立即中止上层 Block 的运行循环。

SharpC-A-C-Interpreter-In-C-0110

if

loop

switch

Break, Continue and Return

break

continue

return