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with FZ_Mul;   use FZ_Mul;
with FZ_Sqr;   use FZ_Sqr;
with FZ_Divis; use FZ_Divis;
with FZ_Barr;  use FZ_Barr;


package body FZ_ModEx is
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   end FZ_Mod_Sqr;
   
   
   -- (Barrettronic) Modular Exponent: Result := Base^Exponent mod Modulus
   procedure FZ_Mod_Exp(Base     : in  FZ;
                        Exponent : in  FZ;
                        Modulus  : in  FZ;
                        Result   : out FZ) is
   -- (Barrettronic) Modular Squaring, using given Barrettoid
   procedure FZ_Mod_Sqr_Barrett(X        : in  FZ;
                                Bar      : in  Barretoid;
                                Product  : out FZ) is
      
      -- The wordness of both operands is equal:
      L     : constant Indices := X'Length;
      
      -- Double-width register for squaring and modulus operations
      XX    : FZ(1 .. L * 2);
      
      -- To refer to the lower and upper halves of the working register:
      XX_Lo : FZ renames XX(1     .. L);
      XX_Hi : FZ renames XX(L + 1 .. XX'Last);
   
   begin
      
      -- XX_Lo:XX_Hi := X^2
      FZ_Square_Buffered(X, XX_Lo, XX_Hi);
      
      -- Product := XX mod M
      FZ_Barrett_Reduce(X => XX, Bar => Bar, XReduced => Product);
      
   end FZ_Mod_Sqr_Barrett;
   
   
   -- Barrettronic Modular Exponent, using given Barrettoid
   procedure FZ_Mod_Exp_Barrett(Base     : in  FZ;
                                Exponent : in  FZ;
                                Bar      : in  Barretoid;
                                Result   : out FZ) is
      
      -- Double-width scratch buffer for the modular operations
      D   : FZ(1 .. Base'Length * 2);
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      -- Buffer register for the Result
      R   : FZ(Result'Range);
      
      -- Space for Barrettoid
      Bar : Barretoid(ZXMLength       => Modulus'Length + 1,
                      BarretoidLength => 2 * B'Length);
      
   begin
      
      -- First, pre-compute the Barretoid for the given Modulus:
      FZ_Make_Barrettoid(Modulus => Modulus, Result => Bar);
      
      -- Result := 1
      WBool_To_FZ(1, R);
      
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      -- Output the Result:
      Result := R;
      
   end FZ_Mod_Exp_Barrett;
   
   
   -- (Barrettronic) Modular Exponent: Result := Base^Exponent mod Modulus
   procedure FZ_Mod_Exp(Base     : in  FZ;
                        Exponent : in  FZ;
                        Modulus  : in  FZ;
                        Result   : out FZ) is
      
      -- Space for Barrettoid
      Bar : Barretoid(ZXMLength       => Modulus'Length + 1,
                      BarretoidLength => 2 * Base'Length);
      
   begin
      
      -- First, pre-compute the Barretoid for the given Modulus:
      FZ_Make_Barrettoid(Modulus => Modulus, Result => Bar);
      
      -- Compute the modular exponentiation using the above Barrettoid:
      FZ_Mod_Exp_Barrett(Base     => Base,
                         Exponent => Exponent,
                         Bar      => Bar,
                         Result   => Result);
      
   end FZ_Mod_Exp;
   
end FZ_ModEx;