text stringlengths 0 234 |
|---|
-- for various distance and length ranges, plus extra bits for specifying the |
-- exact values. The codes are sent as Huffman codes with variable bit lengths |
-- (nothing to do with the lengths of LZ distance-length tokens). |
-- Length Codes |
-- ------------ |
-- Extra Extra Extra Extra |
-- Code Bits Length Code Bits Lengths Code Bits Lengths Code Bits Length(s) |
-- ---- ---- ------ ---- ---- ------- ---- ---- ------- ---- ---- --------- |
-- 257 0 3 265 1 11,12 273 3 35-42 281 5 131-162 |
-- 258 0 4 266 1 13,14 274 3 43-50 282 5 163-194 |
-- 259 0 5 267 1 15,16 275 3 51-58 283 5 195-226 |
-- 260 0 6 268 1 17,18 276 3 59-66 284 5 227-257 |
-- 261 0 7 269 2 19-22 277 4 67-82 285 0 258 |
-- 262 0 8 270 2 23-26 278 4 83-98 |
-- 263 0 9 271 2 27-30 279 4 99-114 |
-- 264 0 10 272 2 31-34 280 4 115-130 |
-- |
-- Example: the code # 266 means the LZ length (# of message bytes to be copied) |
-- shall be 13 or 14, depending on the extra bit value. |
Deflate_Code_For_Lz_Length : constant array (Length_Range) of Natural := |
(3 => 257, -- Codes 257..264, with no extra bit |
4 => 258, |
5 => 259, |
6 => 260, |
7 => 261, |
8 => 262, |
9 => 263, |
10 => 264, |
11 .. 12 => 265, -- Codes 265..268, with 1 extra bit |
13 .. 14 => 266, |
15 .. 16 => 267, |
17 .. 18 => 268, |
19 .. 22 => 269, -- Codes 269..272, with 2 extra bits |
23 .. 26 => 270, |
27 .. 30 => 271, |
31 .. 34 => 272, |
35 .. 42 => 273, -- Codes 273..276, with 3 extra bits |
43 .. 50 => 274, |
51 .. 58 => 275, |
59 .. 66 => 276, |
67 .. 82 => 277, -- Codes 277..280, with 4 extra bits |
83 .. 98 => 278, |
99 .. 114 => 279, |
115 .. 130 => 280, |
131 .. 162 => 281, -- Codes 281..284, with 5 extra bits |
163 .. 194 => 282, |
195 .. 226 => 283, |
227 .. 257 => 284, |
258 => 285); -- Code 285, with no extra bit |
Extra_Bits_For_Lz_Length_Offset : constant array (Length_Range) of Integer := |
(3 .. 10 | 258 => Invalid, -- just placeholder, there is no extra bit there! |
11 .. 18 => 11, |
19 .. 34 => 19, |
35 .. 66 => 35, |
67 .. 130 => 67, |
131 .. 257 => 131); |
Extra_Bits_For_Lz_Length : constant array (Length_Range) of Natural := |
(3 .. 10 | 258 => 0, |
11 .. 18 => 1, |
19 .. 34 => 2, |
35 .. 66 => 3, |
67 .. 130 => 4, |
131 .. 257 => 5); |
procedure Put_Dl_Code (Distance : Distance_Range; Length : Length_Range) is |
Extra_Bits : Natural; |
begin |
Put_Huffman_Code (Curr_Descr.Lit_Len (Deflate_Code_For_Lz_Length (Length))); |
-- Extra bits are needed to differentiate lengths sharing the same code. |
Extra_Bits := Extra_Bits_For_Lz_Length (Length); |
if Extra_Bits > 0 then |
-- We keep only the last extra_bits bits of the length (minus given offset). |
-- Example: if extra_bits = 1, only the parity is sent (0 or 1); |
-- the rest has been already sent with Put_Huffman_code above. |
-- Equivalent: x:= x mod (2 ** extra_bits); |
Put_Code |
(U32 (Length - Extra_Bits_For_Lz_Length_Offset (Length)) and |
(Shift_Left (U32'(1), Extra_Bits) - 1), |
Extra_Bits); |
end if; |
-- Distance Codes |
-- -------------- |
-- Extra Extra Extra Extra |
-- Code Bits Dist Code Bits Dist Code Bits Distance Code Bits Distance |
-- ---- ---- ---- ---- ---- ------ ---- ---- -------- ---- ---- -------- |
-- 0 0 1 8 3 17-24 16 7 257-384 24 11 4097-6144 |
-- 1 0 2 9 3 25-32 17 7 385-512 25 11 6145-8192 |
-- 2 0 3 10 4 33-48 18 8 513-768 26 12 8193-12288 |
-- 3 0 4 11 4 49-64 19 8 769-1024 27 12 12289-16384 |
-- 4 1 5,6 12 5 65-96 20 9 1025-1536 28 13 16385-24576 |
-- 5 1 7,8 13 5 97-128 21 9 1537-2048 29 13 24577-32768 |
-- 6 2 9-12 14 6 129-192 22 10 2049-3072 |
-- 7 2 13-16 15 6 193-256 23 10 3073-4096 |
-- |
-- |
-- Example: the code # 10 means the LZ distance (# positions back in the circular |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.