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[Flags]
public enum MemoryState : byte
{
    Invalid = 0,
    Valid = 0x01,
    Changed = 0x02,

    InvalidUnchanged = 0,
    InvalidChanged = Changed,
    ValidUnchanged = Valid,

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class MyLibrary
{
    public MyLibrary()
    {
        pi = Math.PI;
    }

    public double pi { get; private set; }
    public double r { get; set; }

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public class ReflectionContext : IContext
{
    public ReflectionContext(object targetObject)
    {
        _targetObject = targetObject;
    }

    object _targetObject;

    public double ResolveVariable(string name)

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class MyFunctionContext : IContext
{
    public MyFunctionContext()
    {
    }

    public double ResolveVariable(string name)
    {
        throw new InvalidDataException($"Unknown variable: '{name}'");
    }

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// Variable
if (_tokenizer.Token == Token.Identifier)
{
    // Capture the name and skip it
    var name = _tokenizer.Identifier;
    _tokenizer.NextToken();

    // Parens indicate a function call, otherwise just a variable
    if (_tokenizer.Token != Token.OpenParens)
    {

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public interface IContext
{
    double ResolveVariable(string name);
    double CallFunction(string name, double[] arguments);
}

public class NodeFunctionCall : Node
{
    public NodeFunctionCall(string functionName, Node[] arguments)
    {

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class MyContext : IContext
{
    public MyContext(double r)
    {
        _r = r;
    }

    double _r;

    public double ResolveVariable(string name)

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// Variable
if (_tokenizer.Token == Token.Identifier)
{
    var node = new NodeVariable(_tokenizer.Identifier);
    _tokenizer.NextToken();
    return node;
}

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// Expression context
public interface IContext
{
    double ResolveVariable(string name);
}

// Represents a variable (or a constant) in an expression.  eg: "2 * pi"
public class NodeVariable : Node
{
    public NodeVariable(string variableName)

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// Identifier - starts with letter or underscore
if (char.IsLetter(_currentChar) || _currentChar == '_')
{
    var sb = new StringBuilder();

    // Accept letter, digit or underscore
    while (char.IsLetterOrDigit(_currentChar) || _currentChar == '_')
    {
        sb.Append(_currentChar);
        NextChar();