source: branches/HeuristicLab.Problems.GrammaticalOptimization/HeuristicLab.Common/ExpressionExtender.cs @ 12014

using System;
using System.CodeDom;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Security.Cryptography.X509Certificates;
using System.Text;
using System.Threading.Tasks;
using HeuristicLab.Common;

namespace HeuristicLab.Algorithms.Bandits {
  // helper to create canonical forms of expressions
  // NOTE: Not implemented yet (see unit test for divisions)
  // TODO: change symbolicregressionpoly10problem to use this class
  // this does not support expressions with constants (in transformations we assume constant opt is used)
  // (e.g. we transform all negative signs to + because it is assumed a negative constant can be produced for the term)
  public class ExpressionExtender {

    // supports the grammar
    // G(E):
    //     E -> V | V+E | V-E | V*E | V%E | V/E | (E) 
    //     V -> <variables>
    //     ";

    // might produce expressions of the form |/x
    // the pipe symbol | is used for the constant one (in comparison to other constants)
    private string sentence;
    private int syIdx;

    public string CanonicalRepresentation(string phrase) {
      InitLex(phrase);
      var e = CanonicalExpr();
      return e.ToString(); 
    }


    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++;
    }

    // an expression is a sorted set of terms 
    // a term is an (ordered) list of factors
    // a factor is either a single symbol or an inverted expression
    // CanonicalExpression reads multiple terms (expressions) and must merge the terms in the expression (ordering and duplicates)
    // CanonicalTerm reads multiple factors and must expand factors whenever it reads a combined factor (expression) it produces an expression
    // CanonicalFactor produces an expression 


    // canonical expression returns either a single term or a set of terms
    private Expr CanonicalExpr() {
      var terms = new List<Expr>();
      terms.Add(CanonicalTerm());
      var curSy = CurSy();
      while (curSy == '+' || curSy == '-' || curSy == '^') {
        if (curSy == '+') {
          NewSy();
          terms.Add(CanonicalTerm());
        } else if (curSy == '-') {
          NewSy();
          terms.Add(CanonicalTerm()); // minus is the same as plus assuming constant opt
        } else {
          NewSy();
          throw new NotImplementedException();
          // var e = Expr(variables, constants);
          // r = Not(r) * e + r * Not(e); // xor = (!x AND y) OR (x AND !y)
        }
        curSy = CurSy();
      }

      return new Expr(terms.SelectMany(t => t.Terms));
    }

    // canonical term returns either a single term (product of factors) or a set of terms 
    private Expr CanonicalTerm() {
      var factors = new List<Factor>();
      var f = CanonicalFact();
      if (f != null) factors.Add(f);
      var curSy = CurSy();
      while (curSy == '*' || curSy == '%') {
        if (curSy == '*') {
          NewSy();
          f = CanonicalFact();
          if (f != null) factors.Add(f);
        } else {
          NewSy();
          f = CanonicalFact();
          // if there is only one term we can add multiple inverted simple factors instead of the whole inverted expression
          if (!f.IsSimpleFactor && f.Expr.Terms.Count == 1) {
            foreach (var invF in f.Expr.Terms.First().Factors) {
              if (invF.ToString() == "|") continue;
              invF.Invert();
              factors.Add(invF);
            }
          } else {
            f.Invert();
            if (f != null) factors.Add(f);
          }
        }
        curSy = CurSy();
      }

      factors = CancelFactors(factors).ToList();
      return ExpandFactors(factors);
    }

    // canonical fact returns a factor (either a singe variable, or a set of terms)
    private Factor CanonicalFact() {
      var curSy = CurSy();
      if (curSy == '!') {
        throw new NotSupportedException();
      } else if (curSy == '(') {
        NewSy();
        Expr r = CanonicalExpr(); // this is already simplified
        if (CurSy() != ')') throw new ArgumentException();
        NewSy();
        return new Factor(r);
      } else if (curSy >= 'a' && curSy <= 'z') {
        NewSy();
        return new Factor(curSy);
        //       } else if (curSy >= '0' && curSy <= '9') {
      } else if (curSy >= 'A' && curSy <= 'Z') {
        // treat nonterminals in the same way as variables
        NewSy();
        return new Factor(curSy);

      } else throw new ArgumentException("found symbol " + curSy);
    }

    // a list of factors (symbols, or expressions, and possibly inverses are read
    // a list to factors symbols or expressions and possibly inverses are produced
    // all non-inverse expression factors are expanded
    private Expr ExpandFactors(IEnumerable<Factor> factors) {
      // if (invFactors.Count > 0) throw new NotImplementedException();
      Debug.Assert(!factors.First().IsInverse); // the first factor is never an inverted factor

      // each factor could be a list of terms (expression)

      Expr currentFact = null;
      var firstFactor = factors.First();
      if (firstFactor.IsSimpleFactor || firstFactor.IsInverse) currentFact = new Expr(new Term(firstFactor));
      else currentFact = firstFactor.Expr;

      foreach (var fact in factors.Skip(1)) {
        Expr curExpr = null;
        if (fact.IsSimpleFactor || fact.IsInverse) curExpr = new Expr(new Term(fact));
        else curExpr = fact.Expr;
        currentFact = AllProducts(currentFact, curExpr);
      }
      return currentFact;
    }

    private Expr AllProducts(Expr a, Expr b) {
      var aTerms = a.Terms.ToArray();
      var bTerms = b.Terms.ToArray();
      var combs = from aT in aTerms
                  from bT in bTerms
                  let factors = CancelFactors(aT.Factors.Concat(bT.Factors))
                  select new Term(factors);
      return new Expr(combs);
    }

    private IEnumerable<Factor> CancelFactors(IEnumerable<Factor> factors) {
      var factorsArr = factors.ToArray();
      var results = new List<Factor>(factors);
      foreach (var f in factorsArr) {
        if (f.ToString() == "|") results.Remove(f);
        if (f.ToString() == "|/(|)") results.Remove(f);
        if (f.IsInverse) {
          // find matching
          Factor match;
          match = factorsArr.FirstOrDefault(other => !other.IsInverse && f.Cancels(other));
          if (match != null) {
            results.Remove(f);
            var idx = results.IndexOf(match);

            results.Remove(match);
            if (!results.Any())
              results.Insert(idx, new Factor('|')); // when the factor is the last one then insert a one

            // also mark as cancelled in the factorsArr
            idx = Array.IndexOf(factorsArr, match);
            factorsArr[idx] = new Factor('|');
          }
        }
      }
      // remove all unnecessary "1* factors"

      if (results.Count == 0) results.Add(new Factor('|'));
      return results;
    }

    #region term
    // term can be merged (essentially an ordered list of factors)
    internal class Term : IComparable<Term> {
      private readonly SortedList<Factor, int> factors; // factor symbol and the number of occurrences

      public IEnumerable<Factor> Factors {
        get {
          return factors.SelectMany(p => Enumerable.Repeat(p.Key, p.Value));
        }
      }

      public Term(Factor f) {
        factors = new SortedList<Factor, int>();
        factors.Add(f, 1);
      }
      public Term(IEnumerable<Factor> factors) {
        this.factors = new SortedList<Factor, int>();
        foreach (var f in factors) {
          if (this.factors.ContainsKey(f)) this.factors[f] += 1;
          else this.factors.Add(f, 1);
        }
      }

      public int CompareTo(Term other) {
        if (ContainsNonTerminal(Factors) && !ContainsNonTerminal(other.Factors)) {
          return 1;
        } else if (!ContainsNonTerminal(Factors) && ContainsNonTerminal(other.Factors)) {
          return -1;
        } else {
          var countComp = Factors.Count().CompareTo(other.Factors.Count());
          if (countComp != 0) return countComp;
          return string.Join("", Factors).CompareTo(string.Join("", other.Factors));
        }
      }

      public override string ToString() {

        return string.Join("*", Factors);
      }
      public override bool Equals(object obj) {
        var other = obj as Term;
        if (other == null) return false;
        if (this.Factors.Count() != other.Factors.Count()) return false;
        if (this.Factors.Zip(other.Factors, Tuple.Create).Any(t => t.Item1 != t.Item2)) return false;
        return true;
      }
      public override int GetHashCode() {
        var h = 31415;
        foreach (var v in Factors) {
          h ^= v.GetHashCode();
        }
        return h;
      }
    }
    #endregion

    #region factor
    // factors is either a single symbol or an inverted expression
    internal class Factor : IComparable<Factor> {
      public bool IsSimpleFactor { get { return Expr == null; } }
      public char Symbol { get { return symbol; } }
      public Expr Expr { get { return expr; } }
      public bool IsInverse { get { return inv; } }
      private readonly char symbol = '\0';
      private readonly Expr expr;
      private bool inv;

      public Factor(char f) {
        this.symbol = f;
      }
      public Factor(Expr expr) {
        this.expr = expr;
      }

      public void Invert() {
        this.inv = !inv;
      }
      public bool Cancels(Factor other) {
        if (this.inv == other.inv) return false;
        if (this.Expr != null && other.Expr == null) return false;
        if (this.Expr == null && other.Expr != null) return false;
        if (Expr == null) return this.Symbol.Equals(other.symbol);
        else return this.Expr.CompareTo(other.Expr) == 0;
      }

      public int CompareTo(Factor other) {
        // 1) single symbol factors first
        // 2) expression factors by expression compare
        var crit1 = ContainsNonTerminal(this).CompareTo(ContainsNonTerminal(other));
        if (crit1 != 0) return crit1;

        var crit2 = this.IsInverse.CompareTo(other.IsInverse);
        if (crit2 != 0) return crit2;

        var crit3 = this.IsSimpleFactor.CompareTo(other.IsSimpleFactor);
        if (crit3 != 0) return crit3;

        // both are simple or expressions
        if (IsSimpleFactor) return this.symbol.CompareTo(other.symbol);
        else return this.Expr.CompareTo(other.Expr);
      }

      public override string ToString() {
        var s = Expr == null ? symbol.ToString() : "(" + expr.ToString() + ")";
        if (IsInverse) {
          return "|/" + s;
        } else return s;
      }
      public override bool Equals(object obj) {
        var other = obj as Factor;
        if (other == null) return false;
        if (IsInverse != other.IsInverse) return false;
        if (this.symbol != other.symbol) return false;
        if (this.Expr != other.Expr) return false;
        return true;
      }
      public override int GetHashCode() {
        var h = 31415;
        h ^= symbol.GetHashCode();
        if (Expr != null) h ^= Expr.GetHashCode();
        return h;
      }
    }
    #endregion

    #region expr

    internal class Expr : IComparable<Expr> {
      public readonly SortedSet<Term> Terms; // only set for Kind == Expr 
      //public bool Inverse;
      public Expr(Term t) {
        Terms = new SortedSet<Term>();
        Terms.Add(t);
      }
      public Expr(IEnumerable<Term> exprTerms) {
        Terms = new SortedSet<Term>();
        foreach (var t in exprTerms) {
          Terms.Add(t);
        }
      }

      public void Merge(Expr other) {
        this.Terms.UnionWith(other.Terms);
      }

      public int CompareTo(Expr other) {
        var sizeComp = this.Terms.Count.CompareTo(other.Terms.Count);
        if (sizeComp != 0) return sizeComp;
        // same size => compare terms
        foreach (var pair in Terms.Zip(other.Terms, Tuple.Create)) {
          var termComp = pair.Item1.CompareTo(pair.Item2);
          if (termComp != 0) return termComp;
        }
        return 0;
      }

      public override string ToString() {
        // if (Inverse && Terms.Count > 1)
        //   return "(" + string.Join("+", Terms) + ")";
        // else if (Inverse && Terms.Count == 1) {
        //   return Terms.First().ToString().Replace("%", "#").Replace("*", "%").Replace("#", "*");
        // } else
        return string.Join("+", Terms);
      }
      public override bool Equals(object obj) {
        var other = obj as Expr;
        if (other == null) return false;
        if (this.Terms.Count() != other.Terms.Count()) return false;
        return this.Terms.Intersect(other.Terms).Count() == this.Terms.Count;
      }

      public override int GetHashCode() {
        var h = 31415;
        if (Terms != null)
          foreach (var t in Terms) {
            h ^= t.GetHashCode();
          }
        return h;
      }
    }
    #endregion
    internal static bool IsNonTerminal(char symb) {
      return symb >= 'A' && symb <= 'Z';
    }
    internal static bool ContainsNonTerminal(IEnumerable<Factor> factors) {
      return factors.Any(ContainsNonTerminal);
    }
    internal static bool ContainsNonTerminal(Factor f) {
      if (f.Expr == null) return IsNonTerminal(f.Symbol);
      else return ContainsNonTerminal(f.Expr);
    }

    private static bool ContainsNonTerminal(Expr expr) {
      return expr.Terms.Any(ContainsNonTerminal);
    }

    private static bool ContainsNonTerminal(Term term) {
      return ContainsNonTerminal(term.Factors);
    }
  }
}