Changed root finder to use base-2 exponent of bound

Since the root isolation algorithm uses a power of two as bound anyway,
it makes sense to use it's exponent in method interfaces and throughout
the algorithm. This simplifies multiplications to cheap shifts and will
make it easier to detect when the isolating interval size is 1.

Also added some method documentation.

UPolynomialRoots.pas

@@ -29,10 +29,10 @@ type
   { TIsolatingInterval }
 
   // Represents an isolating interval of the form [C / 2^K, (C + H) / 2^K] in respect to [0, 1] or [A, B] in respect to
-  // [0, bound], with A = C * bound / 2^K and B = (C + H) * bound / 2^K.
+  // [0, 2^boundexp], with A = C * 2^boundexp / 2^K and B = (C + H) * 2^boundexp / 2^K.
   TIsolatingInterval = record
-    C, K, H: Cardinal;
-    Bound, A, B: TBigInt;
+    C, K, H, BoundExp: Cardinal;
+    A, B: TBigInt;
   end;
 
   TIsolatingIntervals = specialize TList<TIsolatingInterval>;
@@ -41,11 +41,16 @@ type
 
   TPolynomialRoots = class
   private
-    class function CalcUpperRootBound(constref APolynomial: TBigIntPolynomial): TBigInt;
-    class function CreateIsolatingInterval(const AC, AK, AH: Cardinal; constref ABound: TBigInt): TIsolatingInterval;
+    // Returns the exponent (base two) of an upper bound for the roots of the given polynomial, i.e. all real roots of
+    // the given polynomial are less or equal than 2^b, where b is the returned positive integer.
+    class function CalcUpperRootBound(constref APolynomial: TBigIntPolynomial): Cardinal;
+    class function CreateIsolatingInterval(const AC, AK, AH: Cardinal; constref ABoundExp: Cardinal):
+      TIsolatingInterval;
   public
+    // Returns root-isolating intervals for non-negative, non-multiple roots.
     class function BisectIsolation(constref APolynomial: TBigIntPolynomial): TIsolatingIntervals;
-    class function BisectIsolation(constref APolynomial: TBigIntPolynomial; constref ABound: TBigInt):
+    // Returns root-isolating intervals for non-multiple roots in the interval [0, 2^boundexp].
+    class function BisectIsolation(constref APolynomial: TBigIntPolynomial; constref ABoundExp: Cardinal):
       TIsolatingIntervals;
   end;
 
@@ -53,7 +58,7 @@ implementation
 
 { TPolynomialRoots }
 
-class function TPolynomialRoots.CalcUpperRootBound(constref APolynomial: TBigIntPolynomial): TBigInt;
+class function TPolynomialRoots.CalcUpperRootBound(constref APolynomial: TBigIntPolynomial): Cardinal;
 var
   i, sign: Integer;
   an, ai, max: TBigInt;
@@ -74,31 +79,38 @@ begin
   end;
   numeratorBit := max.GetMostSignificantBitIndex + 1;
   denominatorBit := an.GetMostSignificantBitIndex;
-  Result := TBigInt.One << (numeratorBit - denominatorBit);
+  Result := numeratorBit - denominatorBit;
 end;
 
-class function TPolynomialRoots.CreateIsolatingInterval(const AC, AK, AH: Cardinal; constref ABound: TBigInt):
+class function TPolynomialRoots.CreateIsolatingInterval(const AC, AK, AH: Cardinal; constref ABoundExp: Cardinal):
   TIsolatingInterval;
 begin
   Result.C := AC;
   Result.K := AK;
   Result.H := AH;
-  Result.Bound := ABound;
-  Result.A := (AC * ABound) >> AK;
-  Result.B := ((AC + AH) * ABound) >> AK;
+  Result.BoundExp := ABoundExp;
+  if ABoundExp >= AK then
+  begin
+    Result.A := AC << (ABoundExp - AK);
+    Result.B := (AC + AH) << (ABoundExp - AK);
+  end
+  else begin
+    Result.A := AC << (ABoundExp - AK);
+    Result.B := (AC + AH) << (ABoundExp - AK);
+  end;
 end;
 
 class function TPolynomialRoots.BisectIsolation(constref APolynomial: TBigIntPolynomial): TIsolatingIntervals;
 var
-  bound: TBigInt;
+  boundExp: Cardinal;
 begin
-  bound := CalcUpperRootBound(APolynomial);
-  Result := BisectIsolation(APolynomial, bound);
+  boundExp := CalcUpperRootBound(APolynomial);
+  Result := BisectIsolation(APolynomial, boundExp);
 end;
 
 // This is adapted from
 // https://en.wikipedia.org/wiki/Real-root_isolation#Bisection_method
-class function TPolynomialRoots.BisectIsolation(constref APolynomial: TBigIntPolynomial; constref ABound: TBigInt):
+class function TPolynomialRoots.BisectIsolation(constref APolynomial: TBigIntPolynomial; constref ABoundExp: Cardinal):
   TIsolatingIntervals;
 type
   TWorkItem = record
@@ -117,7 +129,7 @@ begin
 
   item.C := 0;
   item.K := 0;
-  item.P := APolynomial.ScaleVariable(ABound);
+  item.P := APolynomial.ScaleVariableByPowerOfTwo(ABoundExp);
   stack.Push(item);
   n := item.P.Degree;
 
@@ -129,7 +141,7 @@ begin
       // Found an integer root at 0.
       item.P := item.P.DivideByVariable;
       Dec(n);
-      Result.Add(CreateIsolatingInterval(item.C, item.K, 0, ABound));
+      Result.Add(CreateIsolatingInterval(item.C, item.K, 0, ABoundExp));
     end;
 
     varq := item.P.RevertOrderOfCoefficients.TranslateVariableByOne;
@@ -137,7 +149,7 @@ begin
     if v = 1 then
     begin
       // Found isolating interval.
-      Result.Add(CreateIsolatingInterval(item.C, item.K, 1, ABound));
+      Result.Add(CreateIsolatingInterval(item.C, item.K, 1, ABoundExp));
     end
     else if v > 1 then
     begin

tests/UPolynomialRootsTestCases.pas

@@ -89,7 +89,7 @@ begin
   // y = 3 * (x - 34000) * (x - 23017) * (x - 5) * (x^2 - 19) * (x + 112)
   //   = 3 * x^6 - 170730 * x^5 + 2329429920 * x^4 + 251300082690 * x^3 - 1270471872603 * x^2 + 4774763204640 * x - 24979889760000
   a := TBigIntPolynomial.Create([-24979889760000, 4774763204640, -1270471872603, 251300082690, 2329429920, -170730, 3]);
-  r := TPolynomialRoots.BisectIsolation(a, TBigInt.One << 15);
+  r := TPolynomialRoots.BisectIsolation(a, 15);
   AssertBisectResult(r, expRoots);
   r.Free;
 end;