source: branches/HeuristicLab.Problems.GrammaticalOptimization/HeuristicLab.Problems.GrammaticalOptimization/ExpressionInterpreter.cs @ 11727

using System;
using System.Collections.Generic;
using System.Diagnostics.Eventing.Reader;
using System.Linq;
using System.Runtime.Remoting.Messaging;
using System.Text;
using System.Threading.Tasks;
using HeuristicLab.Common;

namespace HeuristicLab.Problems.GrammaticalOptimization {
  public class ExpressionInterpreter {
    private string sentence;
    private int syIdx;
    // interprets sentences from L(G(Expr)):
    // Expr -> Term { ('+' | '-' | '^' ) Term } 
    // Term -> Fact { ('*' | '/') Fact }
    // Fact -> '!' Expr | '(' Expr ')' | Var
    // Var -> 'a'..'z'

    // for boolean problems the symbol * representes the AND operator, the symbol + represents the OR operator, ! = NOT, ^ = XOR

    public bool Interpret(string sentence, bool[] vars) {
      InitLex(sentence);
      var d = new double[vars.Length];
      for (int i = 0; i < vars.Length; i++) d[i] = vars[i] ? 1.0 : 0.0;
      return !(Expr(d).IsAlmost(0));
    }

    public double Interpret(string sentence, double[] vars) {
      InitLex(sentence);
      return Expr(vars);
    }


    private void InitLex(string sentence) {
      this.sentence = sentence;
      this.syIdx = 0;
    }

    private char CurSy() {
      if (syIdx >= sentence.Length) return '\0';
      return sentence[syIdx];
    }
    private void NewSy() {
      if (syIdx < sentence.Length) syIdx++;
    }

    // helper for xor
    private double Not(double x) {
      return x.IsAlmost(0) ? 1.0 : 0.0;
    }

    private double Expr(double[] d) {
      var r = 0.0;
      r = Term(d);
      while (CurSy() == '+' || CurSy() == '-' || CurSy() == '^') {
        if (CurSy() == '+') {
          NewSy();
          r += Expr(d);
        } else if (CurSy() == '-') {
          NewSy();
          r -= Expr(d);
        } else if (CurSy() == '^') {
          NewSy();
          var e = Expr(d);
          r = Not(r) * e + r * Not(e); // xor = (!x AND y) OR (x AND !y)
        }
      }
      return r;
    }

    private double Term(double[] d) {
      var r = 0.0;
      r = Fact(d);
      while (CurSy() == '*' || CurSy() == '/') {
        if (CurSy() == '*') {
          NewSy();
          r *= Term(d);
        } else if (CurSy() == '/') {
          NewSy();
          r /= Term(d);
        }
      }
      return r;
    }

    private double Fact(double[] d) {
      double r = 0.0;
      if (CurSy() == '!') {
        NewSy();
        r = Not(Expr(d));
      } else if (CurSy() == '(') {
        NewSy();
        r = Expr(d);
        if (CurSy() != ')') throw new ArgumentException();
        NewSy();
      } else if (CurSy() >= 'a' && CurSy() <= 'z') {
        int o = Convert.ToByte(CurSy()) - Convert.ToByte('a');
        if (o < 0 || o >= d.Length) throw new ArgumentException();
        r = d[o];
        NewSy();
      } else throw new ArgumentException();
      return r;
    }

  }
}