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Improve Huffman sorting algorithm

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This commit is contained in:
senhuang42
2021-08-04 12:43:34 -04:00
parent 6ee70bae46
commit aa1957477b
3 changed files with 343 additions and 235 deletions
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lib/common/huf.h
+1 -1
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@@ -90,7 +90,7 @@ HUF_PUBLIC_API size_t HUF_compress2 (void* dst, size_t dstCapacity,
/** HUF_compress4X_wksp() :
* Same as HUF_compress2(), but uses externally allocated `workSpace`.
* `workspace` must be at least as large as HUF_WORKSPACE_SIZE */
#define HUF_WORKSPACE_SIZE ((8 << 10) + 256)
#define HUF_WORKSPACE_SIZE ((8 << 10) + 512 /* sorting scratch space */)
#define HUF_WORKSPACE_SIZE_U64 (HUF_WORKSPACE_SIZE / sizeof(U64))
HUF_PUBLIC_API size_t HUF_compress4X_wksp (void* dst, size_t dstCapacity,
const void* src, size_t srcSize,
lib/compress/huf_compress.c
+129 -21
View File
@@ -428,22 +428,122 @@ static U32 HUF_setMaxHeight(nodeElt* huffNode, U32 lastNonNull, U32 maxNbBits)
}
typedef struct {
U32 base;
U32 curr;
U16 base;
U16 curr;
} rankPos;
typedef nodeElt huffNodeTable[HUF_CTABLE_WORKSPACE_SIZE_U32];
#define RANK_POSITION_TABLE_SIZE 32
/* Number of buckets available for HUF_sort() */
#define RANK_POSITION_TABLE_SIZE 128
typedef struct {
huffNodeTable huffNodeTbl;
rankPos rankPosition[RANK_POSITION_TABLE_SIZE];
} HUF_buildCTable_wksp_tables;
/* RANK_POSITION_DISTINCT_COUNT_CUTOFF == Cutoff point in HUF_sort() buckets for which we use log2 bucketing.
* Strategy is to use as many buckets as possible for representing distinct
* counts while using the remainder to represent all counts up to HUF_BLOCKSIZE_MAX
* using log2 bucketing.
*
* To satisfy this requirement for 128 buckets, we can do the following:
* Let buckets 0-114 represent distinct counts of [0, 114]
* Let buckets 115 to 126 represent counts of [115, HUF_BLOCKSIZE_MAX]. (the final bucket 127 must remain empty)
*
* Note that we don't actually need 17 buckets (assuming 2^17 maxcount) for log2 bucketing since
* the first few buckets in the log2 bucketing representation are already covered by the distinct count bucketing.
*/
#define RANK_POSITION_LOG_BUCKETS_BEGIN (RANK_POSITION_TABLE_SIZE - 1) - BIT_highbit32(HUF_BLOCKSIZE_MAX) - 1
#define RANK_POSITION_DISTINCT_COUNT_CUTOFF RANK_POSITION_LOG_BUCKETS_BEGIN + BIT_highbit32(RANK_POSITION_LOG_BUCKETS_BEGIN)
/* Return the appropriate bucket index for a given count. See definition of
* RANK_POSITION_DISTINCT_COUNT_CUTOFF for explanation of bucketing strategy.
*/
static U32 HUF_getIndex(U32 const count) {
return (count < RANK_POSITION_DISTINCT_COUNT_CUTOFF)
? count
: BIT_highbit32(count) + RANK_POSITION_LOG_BUCKETS_BEGIN;
}
/* Helper swap function for HUF_quickSortPartition() */
static void HUF_swapNodes(nodeElt* a, nodeElt* b) {
nodeElt tmp = *a;
*a = *b;
*b = tmp;
}
/* Returns 0 if the huffNode array is not sorted by descending count */
UNUSED_ATTR
static int HUF_isSorted(nodeElt huffNode[], U32 const maxSymbolValue1) {
U32 i;
for (i = 1; i < maxSymbolValue1; ++i) {
if (huffNode[i].count > huffNode[i-1].count) {
return 0;
}
}
return 1;
}
/* Insertion sort by descending order */
HINT_INLINE void HUF_insertionSort(nodeElt huffNode[], int const low, int const high) {
int i;
int const size = high-low+1;
huffNode += low;
for (i = 1; i < size; ++i) {
nodeElt const key = huffNode[i];
int j = i - 1;
while (j >= 0 && huffNode[j].count < key.count) {
huffNode[j + 1] = huffNode[j];
j--;
}
huffNode[j + 1] = key;
}
}
/* Pivot helper function for quicksort. */
static int HUF_quickSortPartition(nodeElt arr[], int const low, int const high) {
/* Simply select rightmost element as pivot. "Better" selectors like
* median-of-three don't experimentally appear to have any benefit.
*/
U32 const pivot = arr[high].count;
int i = low - 1;
int j = low;
for ( ; j < high; j++) {
if (arr[j].count > pivot) {
i++;
HUF_swapNodes(&arr[i], &arr[j]);
}
}
HUF_swapNodes(&arr[i + 1], &arr[high]);
return i + 1;
}
/* Classic quicksort by descending with partially iterative calls
* to reduce worst case callstack size.
*/
static void HUF_simpleQuickSort(nodeElt arr[], int low, int high) {
int const kInsertionSortThreshold = 8;
if (high - low < kInsertionSortThreshold) {
HUF_insertionSort(arr, low, high);
return;
}
while (low < high) {
int const idx = HUF_quickSortPartition(arr, low, high);
if (idx - low < high - idx) {
HUF_simpleQuickSort(arr, low, idx - 1);
low = idx + 1;
} else {
HUF_simpleQuickSort(arr, idx + 1, high);
high = idx - 1;
}
}
}
/**
* HUF_sort():
* Sorts the symbols [0, maxSymbolValue] by count[symbol] in decreasing order.
* This is a typical bucket sorting strategy that uses either quicksort or insertion sort to sort each bucket.
*
* @param[out] huffNode Sorted symbols by decreasing count. Only members `.count` and `.byte` are filled.
* Must have (maxSymbolValue + 1) entries.
@@ -451,44 +551,52 @@ typedef struct {
* @param[in] maxSymbolValue Maximum symbol value.
* @param rankPosition This is a scratch workspace. Must have RANK_POSITION_TABLE_SIZE entries.
*/
static void HUF_sort(nodeElt* huffNode, const unsigned* count, U32 maxSymbolValue, rankPos* rankPosition)
{
int n;
int const maxSymbolValue1 = (int)maxSymbolValue + 1;
static void HUF_sort(nodeElt huffNode[], const unsigned count[], U32 const maxSymbolValue, rankPos rankPosition[]) {
U32 n;
U32 const maxSymbolValue1 = maxSymbolValue+1;
/* Compute base and set curr to base.
* For symbol s let lowerRank = BIT_highbit32(count[n]+1) and rank = lowerRank + 1.
* Then 2^lowerRank <= count[n]+1 <= 2^rank.
* For symbol s let lowerRank = HUF_getIndex(count[n]) and rank = lowerRank + 1.
* See HUF_getIndex to see bucketing strategy.
* We attribute each symbol to lowerRank's base value, because we want to know where
* each rank begins in the output, so for rank R we want to count ranks R+1 and above.
*/
ZSTD_memset(rankPosition, 0, sizeof(*rankPosition) * RANK_POSITION_TABLE_SIZE);
for (n = 0; n < maxSymbolValue1; ++n) {
U32 lowerRank = BIT_highbit32(count[n] + 1);
U32 lowerRank = HUF_getIndex(count[n]);
assert(lowerRank < RANK_POSITION_TABLE_SIZE - 1);
rankPosition[lowerRank].base++;
}
assert(rankPosition[RANK_POSITION_TABLE_SIZE - 1].base == 0);
/* Set up the rankPosition table */
for (n = RANK_POSITION_TABLE_SIZE - 1; n > 0; --n) {
rankPosition[n-1].base += rankPosition[n].base;
rankPosition[n-1].curr = rankPosition[n-1].base;
}
/* Sort */
/* Insert each symbol into their appropriate bucket, setting up rankPosition table. */
for (n = 0; n < maxSymbolValue1; ++n) {
U32 const c = count[n];
U32 const r = BIT_highbit32(c+1) + 1;
U32 pos = rankPosition[r].curr++;
/* Insert into the correct position in the rank.
* We have at most 256 symbols, so this insertion should be fine.
*/
while ((pos > rankPosition[r].base) && (c > huffNode[pos-1].count)) {
huffNode[pos] = huffNode[pos-1];
pos--;
}
U32 const r = HUF_getIndex(c) + 1;
U32 const pos = rankPosition[r].curr++;
assert(pos < maxSymbolValue1);
huffNode[pos].count = c;
huffNode[pos].byte = (BYTE)n;
}
}
/* Sort each bucket. */
for (n = RANK_POSITION_DISTINCT_COUNT_CUTOFF; n < RANK_POSITION_TABLE_SIZE - 1; ++n) {
U32 const bucketSize = rankPosition[n].curr-rankPosition[n].base;
U32 const bucketStartIdx = rankPosition[n].base;
if (bucketSize > 1) {
assert(bucketStartIdx < maxSymbolValue1);
HUF_simpleQuickSort(huffNode + bucketStartIdx, 0, bucketSize-1);
}
}
assert(HUF_isSorted(huffNode, maxSymbolValue1));
}
/** HUF_buildCTable_wksp() :
* Same as HUF_buildCTable(), but using externally allocated scratch buffer.