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Interpreter.cs
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Interpreter.cs
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using System;
namespace Parlot.Tests.Calc
{
/*
* Grammar:
* expression => factor ( ( "-" | "+" ) factor )* ;
* factor => unary ( ( "/" | "*" ) unary )* ;
* unary => ( "-" ) unary
* | primary ;
* primary => NUMBER
* | "(" expression ")" ;
*/
/// <summary>
/// This version of the Interpreter evaluates the value while it parses the expression.
/// </summary>
public class Interpreter
{
private Scanner _scanner;
public decimal Evaluate(string text)
{
_scanner = new Scanner(text);
return ParseExpression();
}
private decimal ParseExpression()
{
var value = ParseFactor();
_scanner.SkipWhiteSpace();
while (true)
{
if (_scanner.ReadChar('+'))
{
_scanner.SkipWhiteSpace();
value += ParseFactor();
}
else if (_scanner.ReadChar('-'))
{
_scanner.SkipWhiteSpace();
value -= ParseFactor();
}
else
{
break;
}
}
return value;
}
private decimal ParseFactor()
{
var value = ParseUnaryExpression();
_scanner.SkipWhiteSpace();
while (true)
{
if (_scanner.ReadChar('*'))
{
_scanner.SkipWhiteSpace();
value *= ParseUnaryExpression();
}
else if (_scanner.ReadChar('/'))
{
_scanner.SkipWhiteSpace();
value /= ParseUnaryExpression();
}
else
{
break;
}
}
return value;
}
/*
unary = ( "-" ) unary
| primary ;
*/
private decimal ParseUnaryExpression()
{
_scanner.SkipWhiteSpace();
if (_scanner.ReadChar('-'))
{
return -1 * ParseUnaryExpression();
}
return ParsePrimaryExpression();
}
/*
primary = NUMBER
| "(" expression ")" ;
*/
private decimal ParsePrimaryExpression()
{
_scanner.SkipWhiteSpace();
var number = new TokenResult();
if (_scanner.ReadDecimal(number))
{
return decimal.Parse(number.Span);
}
if (_scanner.ReadChar('('))
{
var value = ParseExpression();
if (!_scanner.ReadChar(')'))
{
throw new ParseException("Expected ')'", _scanner.Cursor.Position);
}
return value;
}
throw new ParseException("Expected primary expression", _scanner.Cursor.Position);
}
}
}