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Avoid Reducing Indices to Reserved Values

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Previously, if an index was equal to `reducerValue + 1`, it would get remapped
during index reduction to 1 i.e. `ZSTD_DUBT_UNSORTED_MARK`. This can affect the
parsing of the input slightly, by causing tree nodes to be nullified when they
otherwise wouldn't be. This hardly matters from a correctness or efficiency
perspective, but it does impact determinism.

So this commit changes index reduction to avoid mapping indices to collide with
`ZSTD_DUBT_UNSORTED_MARK`.
This commit is contained in:
W. Felix Handte 2021-11-08 20:03:52 -05:00
parent 793b6cec73
commit 61765cacd0
1 changed files with 7 additions and 5 deletions
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12
lib/compress/zstd_compress.c
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@ -2311,6 +2311,8 @@ ZSTD_reduceTable_internal (U32* const table, U32 const size, U32 const reducerVa
int const nbRows = (int)size / ZSTD_ROWSIZE;
int cellNb = 0;
int rowNb;
/* Protect special index values < ZSTD_WINDOW_START_INDEX. */
U32 const reducerThreshold = reducerValue + ZSTD_WINDOW_START_INDEX;
assert((size & (ZSTD_ROWSIZE-1)) == 0); /* multiple of ZSTD_ROWSIZE */
assert(size < (1U<<31)); /* can be casted to int */
@ -2330,12 +2332,12 @@ ZSTD_reduceTable_internal (U32* const table, U32 const size, U32 const reducerVa
for (rowNb=0 ; rowNb < nbRows ; rowNb++) {
int column;
for (column=0; column<ZSTD_ROWSIZE; column++) {
if (preserveMark) {
U32 const adder = (table[cellNb] == ZSTD_DUBT_UNSORTED_MARK) ? reducerValue : 0;
table[cellNb] += adder;
if (preserveMark && table[cellNb] == ZSTD_DUBT_UNSORTED_MARK) {
} else if (table[cellNb] < reducerThreshold) {
table[cellNb] = 0;
} else {
table[cellNb] -= reducerValue;
}
if (table[cellNb] < reducerValue) table[cellNb] = 0;
else table[cellNb] -= reducerValue;
cellNb++;
} }
}