source: branches/HeuristicLab.Problems.GrammaticalOptimization/HeuristicLab.Problems.GrammaticalOptimization/Problems/RoyalPairProblem.cs @ 11847

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using HeuristicLab.Encodings.SymbolicExpressionTreeEncoding;

namespace HeuristicLab.Problems.GrammaticalOptimization {
  // counts the number of times a pair of symbols occurs in a sentence
  public class RoyalPairProblem : ISymbolicExpressionTreeProblem {
    private const string grammarString = @"
G(S):
S -> a | aS | b | bS
";

    private const string hlGrammarString = @"
G(S):
S -> a | b | SS
";

    private readonly IGrammar grammar;
    public RoyalPairProblem() {
      this.grammar = new Grammar(grammarString);
      this.SymbolicExpressionGrammar = new GenericSymbExprGrammar(new Grammar(hlGrammarString));
      // TODO: allow configuration of the number of symbols
    }

    public double BestKnownQuality(int maxLen) {
      return maxLen - 1;
    }

    public IGrammar Grammar {
      get { return grammar; }
    }

    private Regex regex = new Regex("(?=ab)|(?=ba)"); // count the number of "ab" and "ba" pairs
    public double Evaluate(string sentence) {
      // sentence must contain only terminal symbols, we are not checking if the sentence is syntactically valid here because it would be too slow!
      Debug.Assert(sentence.Any(c => grammar.IsTerminal(c)));
      return regex.Matches(sentence).Count;
    }

    public string CanonicalRepresentation(string phrase) {
      return phrase;
    }

    public IEnumerable<Feature> GetFeatures(string phrase)
    {
      throw new NotImplementedException();
    }
    public ISymbolicExpressionGrammar SymbolicExpressionGrammar { get; private set; }
    public string ConvertTreeToSentence(ISymbolicExpressionTree tree) {
      var sb = new StringBuilder();
      foreach (var s in tree.Root.GetSubtree(0).GetSubtree(0).IterateNodesPrefix()) {
        if (s.Symbol.Name == "S") continue;
        sb.Append(s.Symbol.Name);
      }
      return sb.ToString();
    }
  }
}