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TextFile

separate (Generic_Elementary_Functions)

function Kf_Em1 (Y : Common_Float) return Common_Float is

-- On input, Y is a floating point value in Common_Float;
-- On output, a Common_Float value is returned that represents
--   exp(Y) - 1  for general cases.

   X, X2, Z1, Z2, Z1_Prime : Common_Float;
   Z2_Prime, Cond, Result  : Common_Float;
   M, L, L_Rem, D          : Common_Int;

   Two_To : constant array (Common_Int range -3 .. 3) of Common_Float :=
      (0.125, 0.25, 0.5, 1.0, 2.0, 4.0, 8.0);

   Log2 : constant := 16#0.B17217F7D1CF79ABC9E3B39803F2F6AF40#;

   Log2_By_64 : constant := 16#0.02C5C85FDF473DE6AF278ECE600FCBDAB#;

begin

   X    := Y;
   D    := Float_Type'Base'Digits;
   Cond := -6.0 * Common_Float (D) * Log2;

   if (X < Cond) then
      return (-1.0);
   end if;


   if (abs (X) < Log2_By_64) then
      X2 := 0.0;
      return Kf_Em1sm (X, X2);
   end if;

-- Get M, Z1, and Z2 so that  Exp(X) = 2**M * (Z1 + Z2)

   Kp_Exp (X, M, Z1, Z2);

-- Calculate L, Z1_Prime, and Z2_Prime such that
--     2**M * (Z1 + Z2) = Radix**L * (Z1_Prime + Z2_Prime)

   case Common_Float'Machine_Radix is

      when 2 =>
         L        := M;
         Z1_Prime := Z1;
         Z2_Prime := Z2;

      when others =>     -- Radix is 16
         L_Rem    := M rem 4;
         L        := (M - L_Rem) / 4;
         Z1_Prime := Z1 * Two_To (L_Rem);
         Z2_Prime := Z2 * Two_To (L_Rem);

   end case;


-- Now Exp(X) - 1 = Radix**L * (Z1_Prime + Z2_Prime) - 1

-- The following is the computation of Exp(X) - 1

   if (M = 0) then
      Result := (Z1_Prime - 1.0) + Z2_Prime;

   elsif (abs (M) <= D) then
      Result := Scale (Z2_Prime, L);
      Result := (Scale (Z1_Prime, L) - 1.0) + Result;

   elsif (M < -D) then
      Result := Z1_Prime + Z2_Prime;
      Result := Scale (Result, L) - 1.0;

   else
      Result := Z2_Prime - Scale (1.0, -L);
      Result := Z1_Prime + Result;
      Result := Scale (Result, L);
   end if;


   return (Result);


end Kf_Em1;