Interpreter – behavioral design pattern – library MetaTrader 5

//+------------------------------------------------------------------+
//|                                                  Interpreter.mqh |
//|                                    2019-2020, dimitri pecheritsa |
//|                                                 792112@gmail.com |
//+------------------------------------------------------------------+
//| interpreter — behavioral design pattern                          |
//+------------------------------------------------------------------+
//   design patterns: elements of reusable object-oriented software
//   gof > erich gamma, richard helm, ralph johnson, john vlissides
//   published — 1994
//+------------------------------------------------------------------+
//| intent                                                           |
//+------------------------------------------------------------------+
//   given a language, define a represention for its grammar
//    along with an interpreter that uses the representation
//    to interpret sentences in the language
//+------------------------------------------------------------------+
//| benefits                                                         |
//+------------------------------------------------------------------+
//   aspect that may vary > grammar and interpretation of a language
//+------------------------------------------------------------------+
//| applicability                                                    |
//+------------------------------------------------------------------+
//   there is a language to interpret
//   statements can be abstract syntax trees
//   simple grammar
//      parser generators
//         for complex grammar
//         don't build abstract syntax trees
//   efficiency is not a critical concern
//      interpreters translate parse trees first (state machine)
//+------------------------------------------------------------------+
//| structure                                                        |
//+------------------------------------------------------------------+
//
//                +----->|Context|
//                |
// |Client|-------+
//                |
//                +----->|AbstractExpression|*<----------------+
//                       |------------------|                  |
//                       |Interpret(Context)|                  |
//                                ^                            |
//                                |                            |
//                    +-----------+-------------+              |
//                    |                         |              |
//           |TerminalExpression|    |NonterminalExpression|o--+
//           |------------------|    |---------------------|
//           |Interpret(Context)|    |Interpret(Context)   |
//
#include <SRCPatternsPatternOrganizer.mqh>
namespace Interpreter
{
//+------------------------------------------------------------------+
//| participants > abstract expression                               |
//+------------------------------------------------------------------+
//   declare abstract interpret method for all nodes
class Context;
class AbstractExpression
  {
public:
   virtual void      Interpret(Context&)=0;
  };
//+------------------------------------------------------------------+
//| participants > terminal expression                               |
//+------------------------------------------------------------------+
//   implement interpret method
//    associated with terminal symbols in the grammar
//   an instance is required for every terminal symbol in a sentence
class TerminalExpression:public AbstractExpression
  {
public:
   void              Interpret(Context&);
  };
//+------------------------------------------------------------------+
//| participants > terminal expression > interpret                   |
//+------------------------------------------------------------------+
void TerminalExpression::Interpret(Context& context)
  {
   context.m_result=
      StringSubstr(context.m_source,context.m_position,1)==
      CharToString(context.m_vocabulary);
  }
//+------------------------------------------------------------------+
//| participants > nonterminal expression                            |
//+------------------------------------------------------------------+
//   required for every rule r = r1, r2, ..., rn in the grammar
//   maintains instances of abstract expression
//    for each of the symbols r1 through rn
//   implements interpret method for nonterminal symbols in the grammar
//   interpret method calls itself recursively on r1 through rn
class NonterminalExpression:public AbstractExpression
  {
protected:
   AbstractExpression* m_nonterminal_expression;
   AbstractExpression* m_terminal_expression;
public:
   void              Interpret(Context&);
                    ~NonterminalExpression(void);
  };
//+------------------------------------------------------------------+
//| participants > nonterminal expression > destructor               |
//+------------------------------------------------------------------+
NonterminalExpression::~NonterminalExpression(void)
  {
   delete m_nonterminal_expression;
   delete m_terminal_expression;
  }
//+------------------------------------------------------------------+
//| participants > nonterminal expression > interpret                |
//+------------------------------------------------------------------+
void NonterminalExpression::Interpret(Context& context)
  {
   if(context.m_position<StringLen(context.m_source))
     {
      m_terminal_expression=new TerminalExpression;
      m_terminal_expression.Interpret(context);
      context.m_position++;
      if(context.m_result)
        {
         m_nonterminal_expression=new NonterminalExpression;
         m_nonterminal_expression.Interpret(context);
        }
     }
  }
//+------------------------------------------------------------------+
//| participants > context                                           |
//+------------------------------------------------------------------+
//   contains information that's global to the interpreter
class Context
  {
public:
   string            m_source;
   char              m_vocabulary;
   int               m_position;
   bool              m_result;
   //---
                     Context(char,string);
   void              Result(void);
  };
//+------------------------------------------------------------------+
//| participants > context > constructor                             |
//+------------------------------------------------------------------+
Context::Context(char vocabulary,string source):
   m_source(source),
   m_vocabulary(vocabulary),
   m_position(0),
   m_result(false)
  {
  }
//+------------------------------------------------------------------+
//| participants > context > result                                  |
//+------------------------------------------------------------------+
void Context::Result(void)
  {
   printf("result is %s for m_source:'%s' with m_vocabulary:'%s'",
          (string)m_result,m_source,CharToString(m_vocabulary));
  }
//+------------------------------------------------------------------+
//| participants > client                                            |
//+------------------------------------------------------------------+
//   builds (or is given) an abstract syntax tree
//   tree represents a sentence in the language
//   grammar defines language
//   tree is assembled from instances of the nonterminal expression and
//    terminal expression classes
//   call interpret method
class Client:public ClientExample
  {
public:
   string            Output(void);
   void              Run(void);
  };
string Client::Output(void) {return __FUNCTION__;}
//+------------------------------------------------------------------+
//| collaborations                                                   |
//+------------------------------------------------------------------+
//   client
//      builds (or is given) the sentence
//      as an abstract syntax tree of nonterminal expression and
//       terminal expression
//      initializes the context and invokes the interpret operation
//   nonterminal expression
//      defines interpret method on each subexpression
//   terminal expression
//      interpret method defines the base case in the recursion
//   node
//      interpret method uses the context to store and access
//       the state of the interpreter
void Client::Run(void)
  {
//---language recognition grammar context (example)
//      V = {a}; L = V*; where V-m_vocabulary, L-language
//      correct chains: a, aa, aaa, ...
//      wrong chains: b, ab, aba, ...
   Context context_1('a',"aaa"); //chain ok
   Context context_2('a',"aba"); //wrong chain
   AbstractExpression* expression;
//---
   expression=new NonterminalExpression;
   expression.Interpret(context_1);
   context_1.Result(); //result ok
   delete expression;
//---
   expression=new NonterminalExpression;
   expression.Interpret(context_2);
   context_2.Result(); //bad result
   delete expression;
  }
}
//+------------------------------------------------------------------+
//| output                                                           |
//+------------------------------------------------------------------+
//   Interpreter::Client::Output
//   result is true for m_source:'aaa' with m_vocabulary:'a'
//   result is false for m_source:'aba' with m_vocabulary:'a'
//+------------------------------------------------------------------+
//| consequences                                                     |
//+------------------------------------------------------------------+
//   easy to change and extend the grammar
//      inherit to change or extend the grammar
//      modify expressions incrementally
//      change old expressions to define new ones
//   easy to implement grammar
//      nodes
//         have similar implementations
//         easy to write
//         can be generated with a compiler/parser generator
//   complex grammars are hard to maintain
//      at least one class for every rule
//      hard to manage and maintain
//      parser or compiler generators are more appropriate
//   adding new ways to interpret expressions
//      easier to evaluate an expression
//         define a new operation on the expression
//            support pretty printing/type-checking
//      use visitor to avoid changing the grammar classes
//+------------------------------------------------------------------+
//| implementation                                                   |
//+------------------------------------------------------------------+
//   creating the abstract syntax tree
//      doesn't address parsing
//      the tree can be created by
//         table-driven parser
//         hand-crafted (recursive) parser
//         client
//   defining the interpret operation
//      not in the expression classes
//      put interpret in a separate visitor object
//         grammar for a programming language operations
//            type-checking
//            optimization
//            code generation
//   terminal symbols with the flyweight pattern
//      grammars
//         whose sentences contain many occurrences
//          of a terminal symbol
//         might benefit from sharing a single copy of that symbol
//         for computer programs
//            each program variable will appear in many places
//             throughout the code
//      terminal nodes
//         generally don't store their tree m_position
//      parent nodes
//         pass them the context for interpretation
//      flyweight pattern applies
//         shared (intrinsic) state and passed-in (extrinsic) state
//          are distinct
//+------------------------------------------------------------------+
//| related patterns                                                 |
//+------------------------------------------------------------------+
//   composite > the abstract syntax tree
//   flyweight > share terminal symbols within the abstract syntax tree
//   iterator > traverse the structure
//   visitor > maintain the behavior in each node
//    in the abstract syntax tree in one class
//+------------------------------------------------------------------+