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#include "bitstream.h"
#include "assert.h"
#define MAX(x, y) (((x) > (y)) ? (x) : (y))
#define MIN(x, y) (((x) < (y)) ? (x) : (y))
void Bitstream_init(Bitstream *bitstream, const uint8_t *data,
size_t data_size) {
ASSERT(bitstream != NULL);
ASSERT(data != NULL);
bitstream->data = data;
bitstream->data_size = data_size;
bitstream->byte_offset = 0;
bitstream->bit_index = 0;
}
void bitstream_advance(Bitstream *bitstream, size_t bit_count) {
ASSERT(bitstream != NULL);
uint8_t current_bit = bitstream->bit_index;
bitstream->byte_offset =
bitstream->byte_offset + (current_bit + bit_count) / 8;
bitstream->bit_index = (current_bit + bit_count) % 8;
}
void Bitstream_skip(Bitstream *bitstream, size_t bit_count) {
ASSERT(bitstream != NULL);
bitstream_advance(bitstream, bit_count);
}
uint16_t Bitstream_next_bits(Bitstream *bitstream, int bit_count) {
ASSERT(bitstream != NULL);
ASSERT(bit_count <= 16);
ASSERT((bitstream->bit_index + bit_count - 1) / 8 + bitstream->byte_offset <
bitstream->data_size);
uint16_t val = 0;
int bits_collected = 0;
while (bits_collected < bit_count) {
int bits_in_current_byte = 8 - bitstream->bit_index;
int bits_to_extract = MIN(bit_count - bits_collected, bits_in_current_byte);
uint16_t extracted_bits =
(bitstream->data[bitstream->byte_offset] >> bitstream->bit_index) &
((1 << bits_to_extract) - 1);
val |= extracted_bits << bits_collected;
bits_collected += bits_to_extract;
bitstream_advance(bitstream, bits_to_extract);
}
return val;
}
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