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+// SPDX-License-Identifier: LGPL-3.0-or-later
+/** @file
+ * TODO: insert short description here
+ */
+/* LIBGIMP - The GIMP Library
+ * Copyright (C) 1995-1997 Peter Mattis and Spencer Kimball
+ *
+ * Original file from libgimpwidgets: gimpeevl.h
+ * Copyright (C) 2008-2009 Fredrik Alstromer <roe@excu.se>
+ * Copyright (C) 2008-2009 Martin Nordholts <martinn@svn.gnome.org>
+ * Modified for Inkscape by Johan Engelen
+ * Copyright (C) 2011 Johan Engelen
+ * Copyright (C) 2013 Matthew Petroff
+ *
+ * This library is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 3 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see
+ * <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef INKSCAPE_UTIL_EXPRESSION_EVALUATOR_H
+#define INKSCAPE_UTIL_EXPRESSION_EVALUATOR_H
+
+#include "util/units.h"
+
+#include <exception>
+#include <sstream>
+#include <string>
+
+/**
+ * @file
+ * 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.
+ */
+
+namespace Inkscape {
+namespace Util {
+
+class Unit;
+
+/**
+ * EvaluatorQuantity:
+ * @param value         In reference units.
+ * @param dimension     mm has a dimension of 1, mm^2 has a dimension of 2, etc.
+ */
+class EvaluatorQuantity
+{
+public:
+    EvaluatorQuantity(double value = 0, unsigned int dimension = 0);
+    
+    double value;
+    unsigned int dimension;
+};
+
+/**
+ * TokenType
+ */
+enum {
+  TOKEN_NUM        = 30000,
+  TOKEN_IDENTIFIER = 30001,
+  TOKEN_ANY        = 40000,
+  TOKEN_END        = 50000
+};
+typedef int TokenType;
+
+/**
+ * EvaluatorToken
+ */
+class EvaluatorToken
+{
+public:
+    EvaluatorToken();
+    
+    TokenType type;
+    
+    union {
+        double fl;
+        struct {
+            const char *c;
+            int size;
+        };
+    } value;
+};
+
+/**
+ * ExpressionEvaluator
+ * @param string    NULL terminated input string to evaluate
+ * @param unit      Unit output should be in
+ */
+class ExpressionEvaluator
+{
+public:
+    ExpressionEvaluator(const char *string, Unit const *unit = nullptr);
+    
+    EvaluatorQuantity evaluate();
+
+private:
+    const char *string;
+    Unit const *unit;
+    
+    EvaluatorToken current_token;
+    const char *start_of_current_token;
+    
+    EvaluatorQuantity evaluateExpression();
+    EvaluatorQuantity evaluateTerm();
+    EvaluatorQuantity evaluateExpTerm();
+    EvaluatorQuantity evaluateSignedFactor();
+    EvaluatorQuantity evaluateFactor();
+    
+    bool acceptToken(TokenType token_type, EvaluatorToken *consumed_token);
+    void parseNextToken();
+    void acceptTokenCount(int count, TokenType token_type);
+    void isExpected(TokenType token_type, EvaluatorToken *value);
+    
+    void movePastWhiteSpace();
+    
+    static bool isUnitIdentifierStart(gunichar c);
+    static int getIdentifierSize(const char *s, int start);
+    
+    static bool resolveUnit(const char *identifier, EvaluatorQuantity *result, Unit const *unit);
+    
+    void throwError(const char *msg);
+};
+
+/**
+ * Special exception class for the expression evaluator.
+ */
+class EvaluatorException : public std::exception {
+public:
+    EvaluatorException(const char *message, const char *at_position) {
+        std::ostringstream os;
+        const char *token = at_position ? at_position : "<End of input>";
+        os << "Expression evaluator error: " << message << " at '" << token << "'";
+        msgstr = os.str();
+    }
+
+    ~EvaluatorException() noexcept override = default; // necessary to destroy the string object!!!
+
+    const char *what() const noexcept override {
+        return msgstr.c_str();
+    }
+protected:
+    std::string msgstr;
+};
+
+}
+}
+
+#endif // INKSCAPE_UTIL_EXPRESSION_EVALUATOR_H