//Copyright (c) 2019 by Jason Southwell
//
//Permission is hereby granted, free of charge, to any person obtaining a copy
//of this software and associated documentation files (the "Software"), to deal
//in the Software without restriction, including without limitation the rights
//to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//copies of the Software, and to permit persons to whom the Software is
//furnished to do so, subject to the following conditions:
//
//The above copyright notice and this permission notice shall be included in all
//copies or substantial portions of the Software.
//
//THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//SOFTWARE.

unit qr.rs;

interface

uses System.SysUtils, System.Classes;

type
  EReedSolomonException = exception;
  TReedSolomonGenerator = class(TObject)
  private
    FCoefficients : TArray<Byte>;
  public
    constructor Create(Degree : integer);
    function GetRemaineder(const Data : TArray<Byte>) : TArray<Byte>;
    function Multiply(x, y : Byte) : Byte;
  end;

implementation

{ TReedSolomonGenerator }

constructor TReedSolomonGenerator.Create(Degree: integer);
var
  i, j : integer;
  root : Byte;
begin
  if (Degree < 1) or (Degree > 255)  then
    raise EReedSolomonException.Create('Degree out of range');
  inherited Create;
  SetLength(FCoefficients,Degree);
  for i := 0 to degree-2 do
    FCoefficients[i] := 0;
  FCoefficients[Degree-1] := 1;

  root := 1;

  for i := 0 to degree-1 do
  begin
    for j := 0 to Length(FCoefficients)-1 do
    begin
      FCoefficients[j] := Multiply(FCoefficients[j], root);
      if (j + 1 < length(FCoefficients)) then
        FCoefficients[j] := FCoefficients[j] xor FCoefficients[j + 1];
    end;
    root := Multiply(root, 2);
  end;
end;

function TReedSolomonGenerator.GetRemaineder(const Data: TArray<Byte>): TArray<Byte>;
var
  i: Integer;
  ary : TArray<Byte>;
  b, factor : Byte;
  j: Integer;
begin
  SetLength(Result, Length(FCoefficients));
  for i := 0 to Length(FCoefficients)-1 do
    Result[i] := 0;

  ary := Data;
  for i := 0 to Length(ary)-1 do
  begin
    b := ary[i];
    factor := b xor Result[0];
    Delete(Result,0,1);
    SetLength(Result, Length(Result)+1);
    Result[Length(Result)-1] := 0;
    for j := 0 to Length(FCoefficients)-1 do
    begin
      Result[j] := Result[j] xor Multiply(FCoefficients[j], factor);
    end;
  end;
end;

function TReedSolomonGenerator.Multiply(x, y : Byte): Byte;
var
  i : integer;
  bResult : Byte;
begin
  if (x shr 8 <> 0) or (y shr 8 <> 0) then
    raise EReedSolomonException.Create('Byte out of range.');

  bResult := 0;
  for i := 7 downto 0 do
  begin
    bResult := (bResult shl 1) xor ((bResult shr 7) * $11D);
    bResult := (bResult xor (((y shr i) and 1) * x));
  end;

  if bResult shr 8 <> 0 then
    raise EReedSolomonException.Create('Assertion Error');
  Result := bResult;
end;

end.