JUCE/modules/juce_core/maths/juce_Expression.h

/*
  ==============================================================================

   This file is part of the JUCE library.
   Copyright (c) 2020 - Raw Material Software Limited

   JUCE is an open source library subject to commercial or open-source
   licensing.

   The code included in this file is provided under the terms of the ISC license
   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
   To use, copy, modify, and/or distribute this software for any purpose with or
   without fee is hereby granted provided that the above copyright notice and
   this permission notice appear in all copies.

   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.

  ==============================================================================
*/

namespace juce
{

//==============================================================================
/**
    A class for dynamically evaluating simple numeric expressions.

    This class can parse a simple C-style string expression involving floating point
    numbers, named symbols and functions. The basic arithmetic operations of +, -, *, /
    are supported, as well as parentheses, and any alphanumeric identifiers are
    assumed to be named symbols which will be resolved when the expression is
    evaluated.

    Expressions which use identifiers and functions require a subclass of
    Expression::Scope to be supplied when evaluating them, and this object
    is expected to be able to resolve the symbol names and perform the functions that
    are used.

    @tags{Core}
*/
class JUCE_API  Expression
{
public:
    //==============================================================================
    /** Creates a simple expression with a value of 0. */
    Expression();

    /** Destructor. */
    ~Expression();

    /** Creates a copy of an expression. */
    Expression (const Expression&);

    /** Copies another expression. */
    Expression& operator= (const Expression&);

    /** Move constructor */
    Expression (Expression&&) noexcept;

    /** Move assignment operator */
    Expression& operator= (Expression&&) noexcept;

    /** Creates a simple expression with a specified constant value. */
    explicit Expression (double constant);

    /** Attempts to create an expression by parsing a string.
        Any errors are returned in the parseError argument provided.
    */
    Expression (const String& stringToParse, String& parseError);

    /** Returns a string version of the expression. */
    String toString() const;

    /** Returns an expression which is an addition operation of two existing expressions. */
    Expression operator+ (const Expression&) const;
    /** Returns an expression which is a subtraction operation of two existing expressions. */
    Expression operator- (const Expression&) const;
    /** Returns an expression which is a multiplication operation of two existing expressions. */
    Expression operator* (const Expression&) const;
    /** Returns an expression which is a division operation of two existing expressions. */
    Expression operator/ (const Expression&) const;
    /** Returns an expression which performs a negation operation on an existing expression. */
    Expression operator-() const;

    /** Returns an Expression which is an identifier reference. */
    static Expression symbol (const String& symbol);

    /** Returns an Expression which is a function call. */
    static Expression function (const String& functionName, const Array<Expression>& parameters);

    /** Returns an Expression which parses a string from a character pointer, and updates the pointer
        to indicate where it finished.

        The pointer is incremented so that on return, it indicates the character that follows
        the end of the expression that was parsed.

        If there's a syntax error in parsing, the parseError argument will be set
        to a description of the problem.
    */
    static Expression parse (String::CharPointerType& stringToParse, String& parseError);

    //==============================================================================
    /** When evaluating an Expression object, this class is used to resolve symbols and
        perform functions that the expression uses.
    */
    class JUCE_API  Scope
    {
    public:
        Scope();
        virtual ~Scope();

        /** Returns some kind of globally unique ID that identifies this scope. */
        virtual String getScopeUID() const;

        /** Returns the value of a symbol.
            If the symbol is unknown, this can throw an Expression::EvaluationError exception.
            The member value is set to the part of the symbol that followed the dot, if there is
            one, e.g. for "foo.bar", symbol = "foo" and member = "bar".
            @throws Expression::EvaluationError
        */
        virtual Expression getSymbolValue (const String& symbol) const;

        /** Executes a named function.
            If the function name is unknown, this can throw an Expression::EvaluationError exception.
            @throws Expression::EvaluationError
        */
        virtual double evaluateFunction (const String& functionName,
                                         const double* parameters, int numParameters) const;

        /** Used as a callback by the Scope::visitRelativeScope() method.
            You should never create an instance of this class yourself, it's used by the
            expression evaluation code.
        */
        class Visitor
        {
        public:
            virtual ~Visitor() = default;
            virtual void visit (const Scope&) = 0;
        };

        /** Creates a Scope object for a named scope, and then calls a visitor
            to do some kind of processing with this new scope.

            If the name is valid, this method must create a suitable (temporary) Scope
            object to represent it, and must call the Visitor::visit() method with this
            new scope.
        */
        virtual void visitRelativeScope (const String& scopeName, Visitor& visitor) const;
    };

    /** Evaluates this expression, without using a Scope.
        Without a Scope, no symbols can be used, and only basic functions such as sin, cos, tan,
        min, max are available.
        To find out about any errors during evaluation, use the other version of this method which
        takes a String parameter.
    */
    double evaluate() const;

    /** Evaluates this expression, providing a scope that should be able to evaluate any symbols
        or functions that it uses.
        To find out about any errors during evaluation, use the other version of this method which
        takes a String parameter.
    */
    double evaluate (const Scope& scope) const;

    /** Evaluates this expression, providing a scope that should be able to evaluate any symbols
        or functions that it uses.
    */
    double evaluate (const Scope& scope, String& evaluationError) const;

    /** Attempts to return an expression which is a copy of this one, but with a constant adjusted
        to make the expression resolve to a target value.

        E.g. if the expression is "x + 10" and x is 5, then asking for a target value of 8 will return
        the expression "x + 3". Obviously some expressions can't be reversed in this way, in which
        case they might just be adjusted by adding a constant to the original expression.

        @throws Expression::EvaluationError
    */
    Expression adjustedToGiveNewResult (double targetValue, const Scope& scope) const;

    /** Represents a symbol that is used in an Expression. */
    struct Symbol
    {
        Symbol (const String& scopeUID, const String& symbolName);
        bool operator== (const Symbol&) const noexcept;
        bool operator!= (const Symbol&) const noexcept;

        String scopeUID;    /**< The unique ID of the Scope that contains this symbol. */
        String symbolName;  /**< The name of the symbol. */
    };

    /** Returns a copy of this expression in which all instances of a given symbol have been renamed. */
    Expression withRenamedSymbol (const Symbol& oldSymbol, const String& newName, const Scope& scope) const;

    /** Returns true if this expression makes use of the specified symbol.
        If a suitable scope is supplied, the search will dereference and recursively check
        all symbols, so that it can be determined whether this expression relies on the given
        symbol at any level in its evaluation. If the scope parameter is null, this just checks
        whether the expression contains any direct references to the symbol.

        @throws Expression::EvaluationError
    */
    bool referencesSymbol (const Symbol& symbol, const Scope& scope) const;

    /** Returns true if this expression contains any symbols. */
    bool usesAnySymbols() const;

    /** Returns a list of all symbols that may be needed to resolve this expression in the given scope. */
    void findReferencedSymbols (Array<Symbol>& results, const Scope& scope) const;

    //==============================================================================
    /** Expression type.
        @see Expression::getType()
    */
    enum Type
    {
        constantType,
        functionType,
        operatorType,
        symbolType
    };

    /** Returns the type of this expression. */
    Type getType() const noexcept;

    /** If this expression is a symbol, function or operator, this returns its identifier. */
    String getSymbolOrFunction() const;

    /** Returns the number of inputs to this expression.
        @see getInput
    */
    int getNumInputs() const;

    /** Retrieves one of the inputs to this expression.
        @see getNumInputs
    */
    Expression getInput (int index) const;

private:
    //==============================================================================
    class Term;
    struct Helpers;
    ReferenceCountedObjectPtr<Term> term;

    explicit Expression (Term*);
};

} // namespace juce