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if I /= Length then
Sum := Sum + Counts (I);
end if;
end loop;
end Tweak_For_Better_Rle;
subtype Stats_Lit_Len_Type is Stats_Type (Alphabet_Lit_Len);
subtype Stats_Dis_Type is Stats_Type (Alphabet_Dis);
-- Phase (B) : we turn statistics into Huffman bit lengths
function Build_Descriptors
(Stats_Lit_Len : Stats_Lit_Len_Type;
Stats_Dis : Stats_Dis_Type) return Deflate_Huff_Descriptors
is
Bl_For_Lit_Len : Bit_Length_Array_Lit_Len;
Bl_For_Dis : Bit_Length_Array_Dis;
procedure Llhcl_Lit_Len is new Length_Limited_Huffman_Code_Lengths
(Alphabet_Lit_Len,
Count_Type,
Stats_Lit_Len_Type,
Bit_Length_Array_Lit_Len,
15);
procedure Llhcl_Dis is new Length_Limited_Huffman_Code_Lengths
(Alphabet_Dis,
Count_Type,
Stats_Dis_Type,
Bit_Length_Array_Dis,
15);
Stats_Dis_Copy : Stats_Dis_Type := Stats_Dis;
Used : Natural := 0;
begin
-- See "PatchDistanceCodesForBuggyDecoders" in Zopfli's deflate.c
-- NB: here, we patch the occurrences and not the bit lengths, to avoid invalid codes.
-- The decoding bug concerns Zlib v.<= 1.2.1, UnZip v.<= 6.0, WinZip v.10.0.
for I in Stats_Dis_Copy'Range loop
if Stats_Dis_Copy (I) /= 0 then
Used := Used + 1;
end if;
end loop;
if Used < 2 then
if Used = 0 then -- No distance code used at all (data must be almost random)
Stats_Dis_Copy (0) := 1;
Stats_Dis_Copy (1) := 1;
elsif Stats_Dis_Copy (0) = 0 then
Stats_Dis_Copy (0) := 1; -- now code 0 and some other code have non-zero counts
else
Stats_Dis_Copy (1) := 1; -- now codes 0 and 1 have non-zero counts
end if;
end if;
Llhcl_Lit_Len (Stats_Lit_Len, Bl_For_Lit_Len); -- Call the magic algorithm for setting
Llhcl_Dis (Stats_Dis_Copy, Bl_For_Dis); -- up Huffman lengths of both trees
return Build_Descriptors (Bl_For_Lit_Len, Bl_For_Dis);
end Build_Descriptors;
-- Here is one original part in the Taillaule algorithm: use of basic
-- topology (L1, L2 distances) to check similarities between Huffman code sets.
-- Bit length vector. Convention: 16 is unused bit length (close to the bit length for the
-- rarest symbols, 15, and far from the bit length for the most frequent symbols, 1).
-- Deflate uses 0 for unused.
subtype Bl_Code is Integer_M32 range 1 .. 16;
type Bl_Vector is array (1 .. 288 + 32) of Bl_Code;
function Convert (H : Deflate_Huff_Descriptors) return Bl_Vector is
Bv : Bl_Vector;
J : Positive := 1;
begin
for I in H.Lit_Len'Range loop
if H.Lit_Len (I).Bit_Length = 0 then
Bv (J) := 16;
else
Bv (J) := Integer_M32 (H.Lit_Len (I).Bit_Length);
end if;
J := J + 1;
end loop;
for I in H.Dis'Range loop
if H.Dis (I).Bit_Length = 0 then
Bv (J) := 16;
else
Bv (J) := Integer_M32 (H.Dis (I).Bit_Length);
end if;
J := J + 1;
end loop;
return Bv;
end Convert;
-- L1 or Manhattan distance
function L1_Distance (B1, B2 : Bl_Vector) return Natural_M32 is
S : Natural_M32 := 0;
begin
for I in B1'Range loop
S := S + abs (B1 (I) - B2 (I));
end loop;
return S;
end L1_Distance;
-- L1, tweaked