unit StrucAriEncoderUnit;
{-------------------------------------------------------------------------------
Structured Arithmetic Encoder Unit
----------------------------------
reSource v2.6
Copyright (C) 1998-2001 Victor Kasenda / gruv
http://go.to/gruv
email: vickas@singnet.com.sg

  uses the Group Arithmetic Model to encode the symbols

  first_level_symbol: 0-9
  second_level_symbol: 0 - (NumberOfEntries-1)

  Each entry in the AriModelList corresponds to the AriModel for the first_level_symbol.

-------------------------------------------------------------------------------}


(**) interface (**)
uses GroupAriModelUnit;


type
  TStrucAriEncoder = class
  private
    high, low: integer;     // ends of current code region
    bits_to_follow: integer;
    procedure BitPlusFollow(bit: byte);
  protected
    HeadAriModel: THeadAriModel;

    procedure OutputBit(bit: byte); virtual; abstract;
    procedure OutputBits(code: longint; count: byte); virtual; abstract;

    procedure StartEncoding;
    procedure DoneEncoding;

  public
    constructor Create;
    destructor Destroy; override;

    {procedure EncodeByte(a: byte);}
    procedure EncodeSymbol(symbol: integer);
  end;


(**) implementation (**)



constructor TStrucAriEncoder.Create;
begin
  inherited Create;
end;

destructor TStrucAriEncoder.Destroy;
begin
  inherited Destroy;
end;

{ At the end of the encoding process, there are still significant bits left
in the high and low registers. We output two bits, plus as many underflow
bits as are necessary }

procedure TStrucAriEncoder.BitPlusFollow(bit: byte);
begin
  OutputBit(bit);
  // output bits_to_follow opposite bits. Set bits_to_follow to zero.
  while (bits_to_follow > 0) do
  begin
    if bit = 0 then
      OutputBit(1)
    else
      OutputBit(0);

    dec(bits_to_follow);
  end;
end;

procedure TStrucAriEncoder.StartEncoding;
begin
  low := 0;               // full code region
  high := TOP_VALUE;
  bits_to_follow := 0;    // no bits to follow next
  HeadAriModel := THeadAriModel.Create;
end;

procedure TStrucAriEncoder.DoneEncoding;
begin
  // output two bits that select the quarter that the
  // current code range contains
  inc(bits_to_follow);
  if (low < FIRST_QTR) then
    BitPlusFollow(0)
  else
    BitPlusFollow(1);

  //OutputBits(0, 15);    //16 or 15 or none?
  HeadAriModel.Free;
end;

{-------------------------------------------------------------------------------
  EncodeSymbol
  ------------
  encodes the symbol 'symbol'.

  Algo:
  The encoding process is either 1 or 2 steps, depending on whether the group
  has several members.
  The design of the algo is such that the unique groups are zero and one.
  The symbols correspond to the unique group values.

  1) Get the group number for the symbol
  2) Encode the group number                 (step 1)
  3) If the group has residue, then
     a) Get the group symbol for the corresponding symbol in its group
     b) Encode the group symbol              (step 2)
-------------------------------------------------------------------------------}

procedure TStrucAriEncoder.EncodeSymbol(symbol: integer);

  procedure DoEncodeSymbol(symbol_index: integer; AriModel: TGroupAriModel);
  var
    range: integer;
  begin
    // narrow the code region to that alloted to this symbol
    range := high-low + 1;
    high := low + (((range * AriModel.cum_freq[symbol_index-1]) div AriModel.cum_freq[0]) -1);
    low := low + ((range * AriModel.cum_freq[symbol_index]) div AriModel.cum_freq[0]);

    // loop to output bits
    repeat
      if (high < HALF) then
        BitPlusFollow(0)                 // output 0 if in low half  (MSB=0)
      else if (low >= HALF) then
      begin
        BitPlusFollow(1);                // output 1 if in high half (MSB=1)
        dec(low, HALF);                  // set MSB to 0 for both low and high
        dec(high, HALF);
      end
      else if ((low >= FIRST_QTR) and (high < THIRD_QTR)) then
      begin
        inc(bits_to_follow);             // output an opposite bit later if in middle half
        dec(low, FIRST_QTR);             // substract offset to middle
        dec(high, FIRST_QTR);
      end
      else break;

      low := 2 * low;                    // scale up code region
      high := 2 * high + 1;
    until false;

    AriModel.UpdateModel(symbol_index);  // update the model with the symbol
  end;

var
  AriModel: TGroupAriModel;          // AriModel. reused through the levels
  symbol_index: integer;             // index for symbols. reused through the levels
  group_num, group_symbol: integer;  // 2nd and 3rd level symbols
begin
  // get the group number from the HeadAriModel
  group_num := HeadAriModel.GetGroupNum(symbol);
  // retrieve the AriModel and symbol_index for group_num
  HeadAriModel.GetSymbolInfo(group_num, AriModel, symbol_index);
  // encode the group number
  DoEncodeSymbol(symbol_index, AriModel);

  // encode any residue
  if HeadAriModel.HasResidue(group_num) then
  begin
    // convert the symbol to a group symbol in its respective group (group_num)
    group_symbol := HeadAriModel.SymbolToGroupSymbol(symbol, group_num);
    // get the AriModel and symbol_index for the group_symbol
    HeadAriModel.GetGroupSymbolInfo(group_symbol, group_num, AriModel, symbol_index);
    Assert(AriModel <> nil);
    // encode the group_symbol or residue
    DoEncodeSymbol(symbol_index, AriModel);
  end;
end;



end.