8 #include "bytesource.h"
11 // About 99% of all Huffman codes are <= 8 bits long (see codelen.txt),
12 // and it's what libjpeg uses. Thus, it seems like a reasonable size.
13 #define DEHUF_TABLE_BITS 8
14 #define DEHUF_TABLE_SIZE (1 << DEHUF_TABLE_BITS)
15 static const int DEHUF_SLOW_PATH = -1;
17 // About 98% of all AC coefficients (control byte + coefficient) are <= 10 bits
18 // long; again, see codelen.txt. This will cost us about 6 kB of data to store
20 #define DEHUF_AC_TABLE_BITS 10
21 #define DEHUF_AC_TABLE_SIZE (1 << DEHUF_AC_TABLE_BITS)
22 static const int AC_DEHUF_SLOW_PATH = 0xf0000000;
24 struct huffman_table {
25 unsigned num_codes[17]; // BITS
26 unsigned char codes[256]; // HUFFVAL
29 unsigned huffsize[256];
30 unsigned huffcode[256];
35 // Lookup table for fast decoding; given eight bits,
36 // return the symbol and length in bits. For longer codes,
37 // DEHUF_SLOW_PATH is returned.
39 // Note that the codes we return are 8-bit, but the type of
40 // the lookup tables is int to avoid extra zero extending.
41 int lookup_table_codes[DEHUF_TABLE_SIZE];
42 int lookup_table_length[DEHUF_TABLE_SIZE];
44 // Further lookup tables for decoding AC coefficients.
45 // (Generated but obviously not used for DC coefficients.)
46 // Maps from 10-bit lookahead values to the signed coeffient (_codes),
47 // number of bits to skip (_length) and the number of zero coefficients
48 // after this one (_skip).
49 int ac_table_codes[DEHUF_AC_TABLE_SIZE];
50 uint8_t ac_table_length[DEHUF_AC_TABLE_SIZE];
51 uint8_t ac_table_skip[DEHUF_AC_TABLE_SIZE];
54 enum coefficient_class {
59 typedef struct huffman_table huffman_tables_t[NUM_COEFF_CLASSES][4];
61 // Read Huffman tables from a stream, and compute the derived values.
62 void read_huffman_tables(huffman_tables_t* dst, input_func_t* input_func, void* userdata);
64 unsigned read_huffman_symbol_slow_path(const struct huffman_table* table,
65 struct bit_source* source);
67 static inline unsigned read_huffman_symbol_no_refill(
68 const struct huffman_table* table,
69 struct bit_source* source)
71 assert(source->bits_available >= DEHUF_TABLE_BITS);
72 unsigned lookup = peek_bits(source, DEHUF_TABLE_BITS);
73 int code = table->lookup_table_codes[lookup];
74 int length = table->lookup_table_length[lookup];
76 if (code == DEHUF_SLOW_PATH) {
77 return read_huffman_symbol_slow_path(table, source);
80 read_bits(source, length);
84 static inline unsigned read_huffman_symbol(const struct huffman_table* table,
85 struct bit_source* source)
87 possibly_refill(source, DEHUF_TABLE_BITS);
88 return read_huffman_symbol_no_refill(table, source);
91 // procedure EXTEND (figure F.12)
93 // Fast lookup table for (1 << (bits - 1)).
94 // The table actually helps, since the load can go in parallel with the shift
96 static const int bit_thresholds[16] = {
97 0, 1 << 0, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10, 1 << 11, 1 << 12, 1 << 13, 1 << 14
100 static inline unsigned extend(int val, unsigned bits)
102 #if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
103 // GCC should ideally be able to figure out that the conditional move is better, but
104 // it doesn't for various reasons, and this is pretty important for speed, so we hardcode.
105 asm("cmp %1, %0 ; cmovl %2, %0"
107 : "g" (bit_thresholds[bits]),
108 "r" (val + (-1 << bits) + 1)
112 if (val < bit_thresholds[bits]) {
113 return val + (-1 << bits) + 1;
120 #endif /* !defined(_DEHUFF_H) */