10 // Optimize for 64 bits. We might want to replace this for 32-bit machines
12 typedef uint64_t bitreservoir_t;
13 typedef uint32_t bitreservoir_fill_t;
15 static inline bitreservoir_fill_t read_bitreservoir_fill(uint8_t* source)
17 return ntohl(*(bitreservoir_fill_t*)(source));
20 static const unsigned BITRESERVOIR_SIZE = 8 * sizeof(bitreservoir_t);
21 static const unsigned BITRESERVOIR_FILL_SIZE = 8 * sizeof(bitreservoir_fill_t);
22 static const unsigned BYTERESERVOIR_SIZE = 4096;
24 // A function to read bytes from some input source. The bytes should be
25 // already unstuffed (and thus without markers).
26 // A return value of -1 indicates error, a return value of 0 indicates EOF.
27 typedef ssize_t (input_func_t)(void*, uint8_t*, size_t);
29 // A data source for efficient reading of bit-level data.
31 // Short-term bit reservoir; holds up to 64 bits. When it's empty,
32 // it needs to get refilled from the medium-term bit reservoir.
34 unsigned bits_available;
36 // Medium-term bit reservoir; holds a few kilobytes of spare data.
37 // When this is empty, it needs to be refilled from the input
40 unsigned bytes_available;
43 input_func_t* input_func;
47 void init_bit_source(struct bit_source* source, input_func_t* input_func, void* userdata);
49 // Internal function. Do not use.
50 void possibly_refill_slow_path(struct bit_source* source, unsigned num_bits);
52 // Make sure there's at least NUM_BITS available in the short-term bit reservoir.
53 // You usually want to call this before read_bits(). The reason it's separate
54 // is that if you want two reads and you know the size of both, it's faster to
55 // refill A+B, read A, read B than refill A, read A, refill B, read B.
56 static inline void possibly_refill(struct bit_source* source, unsigned num_bits)
58 assert(num_bits <= BITRESERVOIR_FILL_SIZE + 1);
60 if (source->bits_available >= num_bits) {
61 // Fast path (~90% of invocations?)
65 // Slower path (~99% of remaining invocations?)
66 assert(source->bits_available + BITRESERVOIR_FILL_SIZE < BITRESERVOIR_SIZE);
67 if (source->bytes_available >= sizeof(bitreservoir_fill_t)) {
68 bitreservoir_fill_t fill = read_bitreservoir_fill(source->bytes);
69 source->bytes += sizeof(bitreservoir_fill_t);
70 source->bytes_available -= sizeof(bitreservoir_fill_t);
71 source->bits |= (bitreservoir_t)fill << (BITRESERVOIR_SIZE - BITRESERVOIR_FILL_SIZE - source->bits_available);
72 source->bits_available += BITRESERVOIR_FILL_SIZE;
76 // Slow path: Refill from data source.
77 // Should not be inlined, so split into a separate function.
78 possibly_refill_slow_path(source, num_bits);
81 static inline unsigned read_bits(struct bit_source* source, unsigned num_bits)
83 assert(source->bits_available >= num_bits);
84 unsigned ret = (source->bits >> (BITRESERVOIR_SIZE - num_bits));
85 source->bits <<= num_bits;
86 source->bits_available -= num_bits;
90 static inline unsigned peek_bits(struct bit_source* source, unsigned num_bits)
92 assert(source->bits_available >= num_bits);
93 return (source->bits >> (BITRESERVOIR_SIZE - num_bits));
96 #endif /* !defined(_INPUT_H) */