12 // Optimize for 64 bits. We might want to replace this for 32-bit machines
14 typedef uint64_t bitreservoir_t;
15 typedef uint32_t bitreservoir_fill_t;
17 static inline bitreservoir_fill_t read_bitreservoir_fill(uint8_t* source)
19 return ntohl(*(bitreservoir_fill_t*)(source));
22 static const unsigned BITRESERVOIR_SIZE = 8 * sizeof(bitreservoir_t);
23 static const unsigned BITRESERVOIR_FILL_SIZE = 8 * sizeof(bitreservoir_fill_t);
24 static const unsigned BYTERESERVOIR_SIZE = 4096;
26 // A data source for efficient reading of bit-level data.
28 // Short-term bit reservoir; holds up to 64 bits. When it's empty,
29 // it needs to get refilled from the medium-term bit reservoir.
31 unsigned bits_available;
33 // Medium-term bit reservoir; holds a few kilobytes of spare data.
34 // When this is empty, it needs to be refilled from the input
37 uint8_t* byte_read_ptr;
38 unsigned bytes_available;
41 input_func_t* input_func;
45 void init_bit_source(struct bit_source* source, input_func_t* input_func, void* userdata);
47 // Internal function. Do not use.
48 void possibly_refill_slow_path(struct bit_source* source, unsigned num_bits);
50 // Make sure there's at least NUM_BITS available in the short-term bit reservoir.
51 // You usually want to call this before read_bits(). The reason it's separate
52 // is that if you want two reads and you know the size of both, it's faster to
53 // refill A+B, read A, read B than refill A, read A, refill B, read B.
54 static inline void possibly_refill(struct bit_source* source, unsigned num_bits)
56 assert(num_bits <= BITRESERVOIR_FILL_SIZE + 1);
58 if (source->bits_available >= num_bits) {
59 // Fast path (~90% of invocations?)
63 // Slower path (~99% of remaining invocations?)
64 assert(source->bits_available + BITRESERVOIR_FILL_SIZE < BITRESERVOIR_SIZE);
65 if (source->bytes_available >= sizeof(bitreservoir_fill_t)) {
66 bitreservoir_fill_t fill = read_bitreservoir_fill(source->byte_read_ptr);
67 source->byte_read_ptr += sizeof(bitreservoir_fill_t);
68 source->bytes_available -= sizeof(bitreservoir_fill_t);
69 source->bits |= (bitreservoir_t)fill << (BITRESERVOIR_SIZE - BITRESERVOIR_FILL_SIZE - source->bits_available);
70 source->bits_available += BITRESERVOIR_FILL_SIZE;
74 // Slow path: Refill from data source.
75 // Should not be inlined, so split into a separate function.
76 possibly_refill_slow_path(source, num_bits);
79 static inline unsigned read_bits(struct bit_source* source, unsigned num_bits)
81 assert(source->bits_available >= num_bits);
82 unsigned ret = (source->bits >> (BITRESERVOIR_SIZE - num_bits));
83 source->bits <<= num_bits;
84 source->bits_available -= num_bits;
88 static inline unsigned peek_bits(struct bit_source* source, unsigned num_bits)
90 assert(source->bits_available >= num_bits);
91 return (source->bits >> (BITRESERVOIR_SIZE - num_bits));
94 #endif /* !defined(_BITSOURCE_H) */