expression-evaluator.h File Reference

TODO: insert short description here. More...

Go to the source code of this file.

Classes
class	Inkscape::Util::EvaluatorQuantity
	EvaluatorQuantity: More...
class	Inkscape::Util::EvaluatorToken
	EvaluatorToken. More...
class	Inkscape::Util::ExpressionEvaluator
	ExpressionEvaluator. More...
class	Inkscape::Util::EvaluatorException
	Special exception class for the expression evaluator. More...

Namespaces
namespace	Inkscape
	Helper class to stream background task notifications as a series of messages.
namespace	Inkscape::Util
	Miscellaneous supporting code.

Typedefs
typedef int	Inkscape::Util::TokenType

Enumerations
enum	{ Inkscape::Util::TOKEN_NUM = 30000 , Inkscape::Util::TOKEN_IDENTIFIER = 30001 , Inkscape::Util::TOKEN_ANY = 40000 , Inkscape::Util::TOKEN_END = 50000 }
	TokenType. More...

Detailed Description

TODO: insert short description here.

Expression evaluator: A straightforward recursive descent parser, no fuss, no new dependencies.

The lexer is hand coded, tedious, not extremely fast but works. It evaluates the expression as it goes along, and does not create a parse tree or anything, and will not optimize anything. It uses doubles for precision, with the given use case, that's enough to combat any rounding errors (as opposed to optimizing the evaluation).

It relies on external unit resolving through a callback and does elementary dimensionality constraint check (e.g. "2 mm + 3 px * 4 in" is an error, as L + L^2 is a mismatch). It uses g_strtod() for numeric conversions and it's non-destructive in terms of the parameters, and it's reentrant.

EBNF:

expression ::= term { ('+' | '-') term }* | <empty string> ;

term ::= exponent { ( '*' | '/' ) exponent }* ;

exponent ::= signed factor { '^' signed factor }* ;

signed factor ::= ( '+' | '-' )? factor ;

unit factor ::= factor unit? ;

factor ::= number | '(' expression ')' ;

number ::= ? what g_strtod() consumes ? ;

unit ::= ? what not g_strtod() consumes and not whitespace ? ;

The code should match the EBNF rather closely (except for the non-terminal unit factor, which is inlined into factor) for maintainability reasons.

It will allow 1++1 and 1+-1 (resulting in 2 and 0, respectively), but I figured one might want that, and I don't think it's going to throw anyone off.

Definition in file expression-evaluator.h.