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Optimal_Format_Bits :=
Count_Type'Min
(Count_Type'Min (Stored_Format_Bits, Fixed_Format_Bits),
Count_Type'Min
(Count_Type'Min (Dynamic_Format_Bits, Dynamic_Format_Bits_2),
Recycled_Format_Bits));
-- Selection of the block format with smallest size
if Fixed_Format_Bits = Optimal_Format_Bits then
Send_Fixed_Block;
elsif Dynamic_Format_Bits = Optimal_Format_Bits then
Send_Dynamic_Block (New_Descr);
elsif Dynamic_Format_Bits_2 = Optimal_Format_Bits then
Send_Dynamic_Block (New_Descr_2);
elsif Recycled_Format_Bits = Optimal_Format_Bits then
Put_Lz_Buffer (Lzb);
else -- We have stored_format_bits = optimal_format_bits
Expand_Lz_Buffer (Lzb, Last_Block);
end if;
end Send_As_Block;
subtype Full_Range_Lz_Buffer_Type is Lz_Buffer_Type (Lz_Buffer_Index_Type);
type P_Full_Range_Lz_Buffer_Type is access Full_Range_Lz_Buffer_Type;
procedure Dispose is new Ada.Unchecked_Deallocation
(Full_Range_Lz_Buffer_Type,
P_Full_Range_Lz_Buffer_Type);
-- This is the main, big, fat, circular buffer containing LZ codes,
-- each LZ code being a literal or a DL code.
-- Heap allocation is needed because default stack is too small on some targets.
Lz_Buffer : P_Full_Range_Lz_Buffer_Type;
Lz_Buffer_Index : Lz_Buffer_Index_Type := 0;
Past_Lz_Data : Boolean := False;
-- When True: some LZ_buffer_size data before lz_buffer_index (modulo!) are real, past data
---------------------------------------------------------------------------------
-- Scanning and sampling: the really sexy part of the Taillaule algorithm... --
---------------------------------------------------------------------------------
-- We examine similarities in the LZ data flow at different step sizes.
-- If the optimal Huffman encoding for this portion is very different, we choose to
-- cut current block and start a new one. The shorter the step, the higher the threshold
-- for starting a dynamic block, since the compression header is taking some room each time.
-- *Tuned* (a bit...)
Min_Step : constant := 750;
type Step_Threshold_Metric is record
Slider_Step : Lz_Buffer_Index_Type; -- Should be a multiple of Min_Step
Cutting_Threshold : Positive;
Metric : Distance_Type;
end record;
-- *Tuned* thresholds
-- NB: the enwik8, then silesia, then others tests are tough for lowering any!
Step_Choice : constant array (Positive range <>) of Step_Threshold_Metric :=
((8 * Min_Step, 420, L1_Tweaked), -- Deflate_1, Deflate_2, Deflate_3 (enwik8)
(4 * Min_Step, 430, L1_Tweaked), -- Deflate_2, Deflate_3 (silesia)
(Min_Step, 2050, L1_Tweaked)); -- Deflate_3 (DB test)
Max_Choice : constant array (Taillaule_Deflation_Method) of Positive :=
(Deflate_1 => 1,
Deflate_2 => 2,
Deflate_3 => Step_Choice'Last);
Slider_Size : constant := 4096;
Half_Slider_Size : constant := Slider_Size / 2;
Slider_Max : constant := Slider_Size - 1;
-- Phases (A) and (B) are done in a single function: we get Huffman
-- descriptors that should be good for encoding a given sequence of LZ atoms.
function Build_Descriptors (Lzb : Lz_Buffer_Type) return Deflate_Huff_Descriptors is
Stats_Lit_Len : Stats_Lit_Len_Type;
Stats_Dis : Stats_Dis_Type;
begin
Get_Statistics (Lzb, Stats_Lit_Len, Stats_Dis);
return Build_Descriptors (Stats_Lit_Len, Stats_Dis);
end Build_Descriptors;
procedure Scan_And_Send_From_Main_Buffer
(From, To : Lz_Buffer_Index_Type;
Last_Flush : Boolean)
is
-- The following descriptors are *not* used for compressing, but for detecting similarities.
Initial_Hd, Sliding_Hd : Deflate_Huff_Descriptors;
Start, Slide_Mid, Send_From : Lz_Buffer_Index_Type;
Sliding_Hd_Computed : Boolean;
begin
if To - From < Slider_Max then
Send_As_Block (Lz_Buffer (From .. To), Last_Flush);
return;
end if;
-- For further comments: n := LZ_buffer_size
if Past_Lz_Data then -- We have n / 2 previous data before 'from'.
Start := From - Lz_Buffer_Index_Type (Half_Slider_Size);
else
Start := From; -- Cannot have past data
end if;
-- Looped over, (mod n). Slider data are in two chunks in main buffer