private static bool ParseMulDiv(ArraySegment <char> str, out Expr?result)
{
result = null;
var mulIndex = str.LastIndexOf(out var sign, '*', '/');
if (mulIndex < 0)
{
return(false);
}
if (!ParseExprMulDiv(str.Slice(0, mulIndex), out var left))
{
throw ParserException.Build("Could not recognize any left expression", str, mulIndex + 1);
}
if (!ParseExprMulDiv(str.Slice(mulIndex + 1), out var right))
{
throw ParserException.Build("Could not recognize any right expression", str, mulIndex + 1);
}
Debug.Assert(left != null, nameof(left) + " != null");
Debug.Assert(right != null, nameof(right) + " != null");
result = sign == '*' ? (Expr) new ExprMul(left, right) : new ExprDiv(left, right);
return(true);
}
private static bool ParsePlusMinus(ArraySegment <char> str, out Expr?result)
{
result = null;
var plusIndex = str.LastIndexOf(out var sign, '+', '-');
if (plusIndex < 0)
{
return(false);
}
if (!ParseExprAll(str.Slice(0, plusIndex), out var left))
{
throw ParserException.Build("Could not recognize any left expression", str, 0);
}
if (!ParseExprAll(str.Slice(plusIndex + 1), out var right))
{
throw ParserException.Build("Could not recognize any right expression", str, plusIndex + 1);
}
Debug.Assert(left != null, nameof(left) + " != null");
Debug.Assert(right != null, nameof(right) + " != null");
result = sign == '+' ? (Expr) new ExprPlus(left, right) : new ExprMinus(left, right);
return(true);
}
private static bool ParseNum(ArraySegment <char> str, out Expr result)
{
if (!int.TryParse(str, out var value))
{
throw ParserException.Build("Could not recognize number", str, 0);
}
result = new ExprNum(value);
return(true);
}
public static Expr Parse(string str)
{
var arraySegment = str.ToCharArray();
if (!ParseExpr(arraySegment, out var result))
{
throw ParserException.Build("Could not find an arithmetic expression", arraySegment, 0);
}
return(result ?? throw new Exception("Result cannot be null"));
}
private static void LexCheck(ArraySegment <char> str)
{
for (int i = 0; i < str.Count; i++)
{
var ch = str[i];
bool valid = char.IsDigit(ch) || ch == '+' || ch == '-' || ch == '*' || ch == '/' || ch == '(' || ch == ')';
if (!valid)
{
throw ParserException.Build("Incorrect char", str, i);
}
}
}
private static bool ParseSubExpr(ArraySegment <char> str, out Expr?result)
{
result = null;
if (str.Count > 0 && str[0] == '(')
{
int sum = 1;
for (int i = 1; i < str.Count; i++)
{
if (str[i] == '(')
{
sum++;
}
if (str[i] == ')')
{
sum--;
}
if (sum == 0)
{
if (i + 1 != str.Count)// (....)....
{
return(false);
}
if (!ParseExprAll(str.Slice(1, i - 1), out var sub))
{
throw ParserException.Build("Could not recognize any sub expression", str, i);
}
Debug.Assert(sub != null, nameof(sub) + " != null");
result = new ExprSubExpr(sub);
return(true);
}
}
throw ParserException.Build("Could not find closing ')'", str, 0);
}
return(false);
}