/// <summary>
/// Distribute products across sums.
/// </summary>
/// <param name="f"></param>
/// <param name="x"></param>
/// <returns></returns>
public static Expression Factor(this Expression f, Expression x)
{
// If f is a product, just factor its terms.
if (f is Product)
{
return(Product.New(((Product)f).Terms.Select(i => i.Factor(x))));
}
// If if is l^r, factor l and distribute r.
if (f is Power)
{
Expression l = ((Power)f).Left.Factor(x);
Expression r = ((Power)f).Right;
return(Product.New(Product.TermsOf(l).Select(i => Power.New(i, r))));
}
// If f is a polynomial of x, use polynomial factoring methods.
if (f is Polynomial && (((Polynomial)f).Variable.Equals(x) || ReferenceEquals(x, null)))
{
return(((Polynomial)f).Factor());
}
// Try interpreting f as a polynomial of x.
if (!ReferenceEquals(x, null))
{
// If f is a polynomial of x, factor it.
try
{
return(Polynomial.New(f, x).Factor());
}
catch (Exception) { }
}
// Just factor out common sub-expressions.
if (f is Sum)
{
Sum s = (Sum)f;
IEnumerable <Expression> terms = s.Terms.Select(i => i.Factor()).Buffer();
// All of the distinct factors.
IEnumerable <Expression> factors = terms.SelectMany(i => FactorsOf(i).Except(1, -1)).Distinct();
// Choose the most common factor to use.
Expression factor = factors.ArgMax(i => terms.Count(j => FactorsOf(j).Contains(i)));
// Find the terms that contain the factor.
IEnumerable <Expression> contains = terms.Where(i => FactorsOf(i).Contains(factor)).Buffer();
// If more than one term contains the factor, pull it out and factor the resulting expression (again).
if (contains.Count() > 1)
{
return(Sum.New(
Product.New(factor, Sum.New(contains.Select(i => Binary.Divide(i, factor))).Evaluate()),
Sum.New(terms.Except(contains, Expression.RefComparer))).Factor(null));
}
}
return(f);
}
public static Expression operator /(Polynomial N, Polynomial D)
{
if (Equals(N.Variable, D.Variable))
{
Polynomial R;
Polynomial Q = Divide(N, D, out R);
return(Sum.New(Q, Binary.Divide(R, D)));
}
else
{
return((Expression)N / (Expression)D);
}
}
public override bool Matches(Expression E, MatchContext Matched)
{
Expression matched;
if (Matched.TryGetValue(Right, out matched))
{
if (Left.Matches(E ^ Binary.Divide(1, matched), Matched))
{
return(true);
}
}
// x^0 = 1.
if (E.EqualsOne() && Right.Matches(0, Matched))
{
return(true);
}
// 0^x = 0.
if (E.EqualsZero() && Left.Matches(0, Matched))
{
return(true);
}
Binary PE = E as Power;
if (!ReferenceEquals(PE, null) && Matched.TryMatch(() => Left.Matches(PE.Left, Matched) && Right.Matches(PE.Right, Matched)))
{
return(true);
}
// If the exponent matches 1, E can match left.
if (Matched.TryMatch(() => Right.Matches(1, Matched) && Left.Matches(E, Matched)))
{
return(true);
}
if (Right.IsInteger() && Left.Matches(ComputerAlgebra.Power.New(E, Binary.Divide(1, Right)).Evaluate(), Matched))
{
return(true);
}
return(false);
}
protected override Expression VisitPower(Power P)
{
Expression f = P.Left;
Expression g = P.Right;
if (g.DependsOn(x))
{
// f(x)^g(x)
return(Product.New(P,
Sum.New(
Product.New(Visit(f), Binary.Divide(g, f)),
Product.New(Visit(g), Call.Ln(f)))).Evaluate());
}
else
{
// f(x)^g
return(Product.New(
g,
Power.New(f, Binary.Subtract(g, 1)),
Visit(f)).Evaluate());
}
}
public override bool Matches(Expression E, MatchContext Matched)
{
// if E is zero, any term can match to zero to succeed.
if (E.EqualsZero())
{
return(Terms.Any(i => i.Matches(0, Matched)));
}
// Move the constants in this pattern to E.
IEnumerable <Expression> PTerms = Terms;
IEnumerable <Expression> Constants = PTerms.OfType <Constant>();
if (Constants.Any())
{
E = Binary.Divide(E, New(Constants)).Evaluate();
PTerms = PTerms.Except(Constants, RefComparer);
}
IEnumerable <Expression> ETerms = TermsOf(E);
// Try starting the match at each term of the pattern.
foreach (Expression p in PTerms)
{
// Remaining terms of the pattern.
Expression P = New(PTerms.ExceptUnique(p, RefComparer));
// If p is a variable, we have to handle the possibility that more than one term of E might match this term.
if (p is Variable)
{
// Check if p has already been matched. If it has, treat it as a constant and match the rest of the terms.
Expression matched;
if (Matched.TryGetValue(p, out matched))
{
// p has already been matched. Remove it out of E and match the remainder of the pattern.
if (P.Matches(E / matched, Matched))
{
return(true);
}
}
else
{
// Try matching p to the various combinations of the terms of E.
for (int i = 1; i <= ETerms.Count(); ++i)
{
foreach (IEnumerable <Expression> e in ETerms.Combinations(i))
{
if (Matched.TryMatch(() =>
p.Matches(New(e), Matched) &&
P.Matches(New(ETerms.ExceptUnique(e, RefComparer)), Matched)))
{
return(true);
}
}
}
// Try matching p to identity.
if (Matched.TryMatch(() => p.Matches(1, Matched) && P.Matches(E, Matched)))
{
return(true);
}
}
}
else
{
// If p is not a variable, try matching it to any of the terms of E.
foreach (Expression e in ETerms)
{
if (Matched.TryMatch(() =>
p.Matches(e, Matched) &&
P.Matches(New(ETerms.ExceptUnique(e, RefComparer)), Matched)))
{
return(true);
}
}
}
}
return(false);
}
