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Educational decoder: Clarify IO_rewind_bits

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This commit is contained in:
Sean Purcell
2017-03-21 15:03:23 -07:00
parent f643e050dc
commit ea14b73795
1 changed files with 8 additions and 3 deletions
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11
doc/educational_decoder/zstd_decompress.c
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@ -1598,11 +1598,16 @@ static inline void IO_rewind_bits(istream_t *const in, int num) {
ERROR("Attempting to rewind stream by a negative number of bits");
}
/* move the offset back by `num` bits */
const int new_offset = in->bit_offset - num;
const i64 bytes = (new_offset - 7) / 8;
/* determine the number of whole bytes we have to rewind, rounding up to an
* integer number (e.g. if `new_offset == -5`, `bytes == 1`) */
const i64 bytes = -(new_offset - 7) / 8;
in->ptr += bytes;
in->len -= bytes;
in->ptr -= bytes;
in->len += bytes;
/* make sure the resulting `bit_offset` is positive, as mod in C does not
* convert numbers from negative to positive (e.g. -22 % 8 == -6) */
in->bit_offset = ((new_offset % 8) + 8) % 8;
}