559 lines
20 KiB
C
559 lines
20 KiB
C
/*
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* Copyright (c) Meta Platforms, Inc. and affiliates.
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* All rights reserved.
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*
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* This source code is licensed under both the BSD-style license (found in the
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* LICENSE file in the root directory of this source tree) and the GPLv2 (found
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* in the COPYING file in the root directory of this source tree).
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* You may select, at your option, one of the above-listed licenses.
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*/
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/*-*************************************
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* Dependencies
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***************************************/
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/* Currently relies on qsort when combining contiguous matches. This can probably
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* be avoided but would require changes to the algorithm. The qsort is far from
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* the bottleneck in this algorithm even for medium sized files so it's probably
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* not worth trying to address */
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#include <stdlib.h>
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#include <assert.h>
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#include "zstd_edist.h"
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#include "mem.h"
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/*-*************************************
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* Constants
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***************************************/
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/* Just a sential for the entries of the diagonal matrix */
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#define ZSTD_EDIST_DIAG_MAX (S32)(1 << 30)
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/* How large should a snake be to be considered a 'big' snake.
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* For an explanation of what a 'snake' is with respect to the
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* edit distance matrix, see the linked paper in zstd_edist.h */
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#define ZSTD_EDIST_SNAKE_THRESH 20
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/* After how many iterations should we start to use the heuristic
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* based on 'big' snakes */
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#define ZSTD_EDIST_SNAKE_ITER_THRESH 200
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/* After how many iterations should be just give up and take
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* the best available edit script for this round */
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#define ZSTD_EDIST_EXPENSIVE_THRESH 1024
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/*-*************************************
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* Structures
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***************************************/
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typedef struct {
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U32 dictIdx;
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U32 srcIdx;
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U32 matchLength;
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} ZSTD_eDist_match;
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typedef struct {
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const BYTE* dict;
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const BYTE* src;
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size_t dictSize;
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size_t srcSize;
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S32* forwardDiag; /* Entries of the forward diagonal stored here */
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S32* backwardDiag; /* Entries of the backward diagonal stored here.
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* Note: this buffer and the 'forwardDiag' buffer
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* are contiguous. See the ZSTD_eDist_genSequences */
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ZSTD_eDist_match* matches; /* Accumulate matches of length 1 in this buffer.
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* In a subsequence post-processing step, we combine
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* contiguous matches. */
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U32 nbMatches;
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} ZSTD_eDist_state;
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typedef struct {
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S32 dictMid; /* The mid diagonal for the dictionary */
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S32 srcMid; /* The mid diagonal for the source */
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int lowUseHeuristics; /* Should we use heuristics for the low part */
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int highUseHeuristics; /* Should we use heuristics for the high part */
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} ZSTD_eDist_partition;
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/*-*************************************
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* Internal
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***************************************/
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static void ZSTD_eDist_diag(ZSTD_eDist_state* state,
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ZSTD_eDist_partition* partition,
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S32 dictLow, S32 dictHigh, S32 srcLow,
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S32 srcHigh, int useHeuristics)
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{
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S32* const forwardDiag = state->forwardDiag;
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S32* const backwardDiag = state->backwardDiag;
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const BYTE* const dict = state->dict;
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const BYTE* const src = state->src;
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S32 const diagMin = dictLow - srcHigh;
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S32 const diagMax = dictHigh - srcLow;
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S32 const forwardMid = dictLow - srcLow;
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S32 const backwardMid = dictHigh - srcHigh;
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S32 forwardMin = forwardMid;
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S32 forwardMax = forwardMid;
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S32 backwardMin = backwardMid;
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S32 backwardMax = backwardMid;
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int odd = (forwardMid - backwardMid) & 1;
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U32 iterations;
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forwardDiag[forwardMid] = dictLow;
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backwardDiag[backwardMid] = dictHigh;
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/* Main loop for updating diag entries. Unless useHeuristics is
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* set to false, this loop will run until it finds the minimal
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* edit script */
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for (iterations = 1;;iterations++) {
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S32 diag;
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int bigSnake = 0;
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if (forwardMin > diagMin) {
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forwardMin--;
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forwardDiag[forwardMin - 1] = -1;
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} else {
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forwardMin++;
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}
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if (forwardMax < diagMax) {
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forwardMax++;
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forwardDiag[forwardMax + 1] = -1;
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} else {
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forwardMax--;
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}
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for (diag = forwardMax; diag >= forwardMin; diag -= 2) {
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S32 dictIdx;
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S32 srcIdx;
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S32 low = forwardDiag[diag - 1];
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S32 high = forwardDiag[diag + 1];
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S32 dictIdx0 = low < high ? high : low + 1;
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for (dictIdx = dictIdx0, srcIdx = dictIdx0 - diag;
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dictIdx < dictHigh && srcIdx < srcHigh && dict[dictIdx] == src[srcIdx];
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dictIdx++, srcIdx++) continue;
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if (dictIdx - dictIdx0 > ZSTD_EDIST_SNAKE_THRESH)
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bigSnake = 1;
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forwardDiag[diag] = dictIdx;
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if (odd && backwardMin <= diag && diag <= backwardMax && backwardDiag[diag] <= dictIdx) {
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partition->dictMid = dictIdx;
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partition->srcMid = srcIdx;
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partition->lowUseHeuristics = 0;
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partition->highUseHeuristics = 0;
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return;
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}
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}
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if (backwardMin > diagMin) {
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backwardMin--;
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backwardDiag[backwardMin - 1] = ZSTD_EDIST_DIAG_MAX;
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} else {
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backwardMin++;
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}
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if (backwardMax < diagMax) {
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backwardMax++;
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backwardDiag[backwardMax + 1] = ZSTD_EDIST_DIAG_MAX;
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} else {
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backwardMax--;
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}
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for (diag = backwardMax; diag >= backwardMin; diag -= 2) {
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S32 dictIdx;
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S32 srcIdx;
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S32 low = backwardDiag[diag - 1];
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S32 high = backwardDiag[diag + 1];
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S32 dictIdx0 = low < high ? low : high - 1;
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for (dictIdx = dictIdx0, srcIdx = dictIdx0 - diag;
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dictLow < dictIdx && srcLow < srcIdx && dict[dictIdx - 1] == src[srcIdx - 1];
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dictIdx--, srcIdx--) continue;
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if (dictIdx0 - dictIdx > ZSTD_EDIST_SNAKE_THRESH)
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bigSnake = 1;
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backwardDiag[diag] = dictIdx;
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if (!odd && forwardMin <= diag && diag <= forwardMax && dictIdx <= forwardDiag[diag]) {
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partition->dictMid = dictIdx;
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partition->srcMid = srcIdx;
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partition->lowUseHeuristics = 0;
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partition->highUseHeuristics = 0;
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return;
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}
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}
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if (!useHeuristics)
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continue;
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/* Everything under this point is a heuristic. Using these will
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* substantially speed up the match finding. In some cases, taking
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* the total match finding time from several minutes to seconds.
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* Of course, the caveat is that the edit script found may no longer
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* be optimal */
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/* Big snake heuristic */
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if (iterations > ZSTD_EDIST_SNAKE_ITER_THRESH && bigSnake) {
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{
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S32 best = 0;
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for (diag = forwardMax; diag >= forwardMin; diag -= 2) {
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S32 diagDiag = diag - forwardMid;
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S32 dictIdx = forwardDiag[diag];
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S32 srcIdx = dictIdx - diag;
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S32 v = (dictIdx - dictLow) * 2 - diagDiag;
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if (v > 12 * (iterations + (diagDiag < 0 ? -diagDiag : diagDiag))) {
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if (v > best
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&& dictLow + ZSTD_EDIST_SNAKE_THRESH <= dictIdx && dictIdx <= dictHigh
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&& srcLow + ZSTD_EDIST_SNAKE_THRESH <= srcIdx && srcIdx <= srcHigh) {
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S32 k;
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for (k = 1; dict[dictIdx - k] == src[srcIdx - k]; k++) {
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if (k == ZSTD_EDIST_SNAKE_THRESH) {
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best = v;
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partition->dictMid = dictIdx;
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partition->srcMid = srcIdx;
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break;
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}
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}
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}
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}
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}
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if (best > 0) {
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partition->lowUseHeuristics = 0;
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partition->highUseHeuristics = 1;
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return;
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}
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}
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{
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S32 best = 0;
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for (diag = backwardMax; diag >= backwardMin; diag -= 2) {
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S32 diagDiag = diag - backwardMid;
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S32 dictIdx = backwardDiag[diag];
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S32 srcIdx = dictIdx - diag;
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S32 v = (dictHigh - dictIdx) * 2 + diagDiag;
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if (v > 12 * (iterations + (diagDiag < 0 ? -diagDiag : diagDiag))) {
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if (v > best
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&& dictLow < dictIdx && dictIdx <= dictHigh - ZSTD_EDIST_SNAKE_THRESH
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&& srcLow < srcIdx && srcIdx <= srcHigh - ZSTD_EDIST_SNAKE_THRESH) {
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int k;
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for (k = 0; dict[dictIdx + k] == src[srcIdx + k]; k++) {
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if (k == ZSTD_EDIST_SNAKE_THRESH - 1) {
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best = v;
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partition->dictMid = dictIdx;
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partition->srcMid = srcIdx;
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break;
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}
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}
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}
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}
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}
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if (best > 0) {
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partition->lowUseHeuristics = 1;
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partition->highUseHeuristics = 0;
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return;
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}
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}
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}
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/* More general 'too expensive' heuristic */
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if (iterations >= ZSTD_EDIST_EXPENSIVE_THRESH) {
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S32 forwardDictSrcBest;
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S32 forwardDictBest = 0;
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S32 backwardDictSrcBest;
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S32 backwardDictBest = 0;
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forwardDictSrcBest = -1;
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for (diag = forwardMax; diag >= forwardMin; diag -= 2) {
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S32 dictIdx = MIN(forwardDiag[diag], dictHigh);
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S32 srcIdx = dictIdx - diag;
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if (srcHigh < srcIdx) {
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dictIdx = srcHigh + diag;
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srcIdx = srcHigh;
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}
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if (forwardDictSrcBest < dictIdx + srcIdx) {
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forwardDictSrcBest = dictIdx + srcIdx;
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forwardDictBest = dictIdx;
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}
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}
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backwardDictSrcBest = ZSTD_EDIST_DIAG_MAX;
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for (diag = backwardMax; diag >= backwardMin; diag -= 2) {
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S32 dictIdx = MAX(dictLow, backwardDiag[diag]);
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S32 srcIdx = dictIdx - diag;
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if (srcIdx < srcLow) {
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dictIdx = srcLow + diag;
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srcIdx = srcLow;
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}
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if (dictIdx + srcIdx < backwardDictSrcBest) {
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backwardDictSrcBest = dictIdx + srcIdx;
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backwardDictBest = dictIdx;
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}
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}
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if ((dictHigh + srcHigh) - backwardDictSrcBest < forwardDictSrcBest - (dictLow + srcLow)) {
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partition->dictMid = forwardDictBest;
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partition->srcMid = forwardDictSrcBest - forwardDictBest;
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partition->lowUseHeuristics = 0;
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partition->highUseHeuristics = 1;
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} else {
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partition->dictMid = backwardDictBest;
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partition->srcMid = backwardDictSrcBest - backwardDictBest;
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partition->lowUseHeuristics = 1;
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partition->highUseHeuristics = 0;
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}
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return;
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}
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}
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}
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static void ZSTD_eDist_insertMatch(ZSTD_eDist_state* state,
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S32 const dictIdx, S32 const srcIdx)
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{
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state->matches[state->nbMatches].dictIdx = dictIdx;
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state->matches[state->nbMatches].srcIdx = srcIdx;
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state->matches[state->nbMatches].matchLength = 1;
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state->nbMatches++;
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}
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static int ZSTD_eDist_compare(ZSTD_eDist_state* state,
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S32 dictLow, S32 dictHigh, S32 srcLow,
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S32 srcHigh, int useHeuristics)
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{
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const BYTE* const dict = state->dict;
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const BYTE* const src = state->src;
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/* Found matches while traversing from the low end */
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while (dictLow < dictHigh && srcLow < srcHigh && dict[dictLow] == src[srcLow]) {
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ZSTD_eDist_insertMatch(state, dictLow, srcLow);
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dictLow++;
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srcLow++;
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}
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/* Found matches while traversing from the high end */
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while (dictLow < dictHigh && srcLow < srcHigh && dict[dictHigh - 1] == src[srcHigh - 1]) {
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ZSTD_eDist_insertMatch(state, dictHigh - 1, srcHigh - 1);
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dictHigh--;
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srcHigh--;
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}
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/* If the low and high end end up touching. If we wanted to make
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* note of the differences like most diffing algorithms do, we would
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* do so here. In our case, we're only concerned with matches
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* Note: if you wanted to find the edit distance of the algorithm,
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* you could just accumulate the cost for an insertion/deletion
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* below. */
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if (dictLow == dictHigh) {
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while (srcLow < srcHigh) {
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/* Reaching this point means inserting src[srcLow] into
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* the current position of dict */
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srcLow++;
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}
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} else if (srcLow == srcHigh) {
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while (dictLow < dictHigh) {
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/* Reaching this point means deleting dict[dictLow] from
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* the current position of dict */
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dictLow++;
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}
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} else {
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ZSTD_eDist_partition partition;
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partition.dictMid = 0;
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partition.srcMid = 0;
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ZSTD_eDist_diag(state, &partition, dictLow, dictHigh,
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srcLow, srcHigh, useHeuristics);
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if (ZSTD_eDist_compare(state, dictLow, partition.dictMid,
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srcLow, partition.srcMid, partition.lowUseHeuristics))
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return 1;
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if (ZSTD_eDist_compare(state, partition.dictMid, dictHigh,
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partition.srcMid, srcHigh, partition.highUseHeuristics))
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return 1;
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}
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return 0;
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}
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static int ZSTD_eDist_matchComp(const void* p, const void* q)
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{
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S32 const l = ((ZSTD_eDist_match*)p)->srcIdx;
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S32 const r = ((ZSTD_eDist_match*)q)->srcIdx;
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return (l - r);
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}
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/* The matches from the approach above will all be of the form
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* (dictIdx, srcIdx, 1). This method combines contiguous matches
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* of length MINMATCH or greater. Matches less than MINMATCH
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* are discarded */
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static void ZSTD_eDist_combineMatches(ZSTD_eDist_state* state)
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{
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/* Create a new buffer to put the combined matches into
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* and memcpy to state->matches after */
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ZSTD_eDist_match* combinedMatches =
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ZSTD_malloc(state->nbMatches * sizeof(ZSTD_eDist_match),
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ZSTD_defaultCMem);
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U32 nbCombinedMatches = 1;
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size_t i;
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/* Make sure that the srcIdx and dictIdx are in sorted order.
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* The combination step won't work otherwise */
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qsort(state->matches, state->nbMatches, sizeof(ZSTD_eDist_match), ZSTD_eDist_matchComp);
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memcpy(combinedMatches, state->matches, sizeof(ZSTD_eDist_match));
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for (i = 1; i < state->nbMatches; i++) {
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ZSTD_eDist_match const match = state->matches[i];
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ZSTD_eDist_match const combinedMatch =
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combinedMatches[nbCombinedMatches - 1];
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if (combinedMatch.srcIdx + combinedMatch.matchLength == match.srcIdx &&
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combinedMatch.dictIdx + combinedMatch.matchLength == match.dictIdx) {
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combinedMatches[nbCombinedMatches - 1].matchLength++;
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} else {
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/* Discard matches that are less than MINMATCH */
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if (combinedMatches[nbCombinedMatches - 1].matchLength < MINMATCH) {
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nbCombinedMatches--;
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}
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memcpy(combinedMatches + nbCombinedMatches,
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state->matches + i, sizeof(ZSTD_eDist_match));
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nbCombinedMatches++;
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}
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}
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memcpy(state->matches, combinedMatches, nbCombinedMatches * sizeof(ZSTD_eDist_match));
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state->nbMatches = nbCombinedMatches;
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ZSTD_free(combinedMatches, ZSTD_defaultCMem);
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}
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static size_t ZSTD_eDist_convertMatchesToSequences(ZSTD_Sequence* sequences,
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ZSTD_eDist_state* state)
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{
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const ZSTD_eDist_match* matches = state->matches;
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size_t const nbMatches = state->nbMatches;
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size_t const dictSize = state->dictSize;
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size_t nbSequences = 0;
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size_t i;
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for (i = 0; i < nbMatches; i++) {
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ZSTD_eDist_match const match = matches[i];
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U32 const litLength = !i ? match.srcIdx :
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match.srcIdx - (matches[i - 1].srcIdx + matches[i - 1].matchLength);
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U32 const offset = (match.srcIdx + dictSize) - match.dictIdx;
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U32 const matchLength = match.matchLength;
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sequences[nbSequences].offset = offset;
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sequences[nbSequences].litLength = litLength;
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sequences[nbSequences].matchLength = matchLength;
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nbSequences++;
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}
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return nbSequences;
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}
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/*-*************************************
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* Internal utils
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***************************************/
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static size_t ZSTD_eDist_hamingDist(const BYTE* const a,
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const BYTE* const b, size_t n)
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{
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size_t i;
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size_t dist = 0;
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for (i = 0; i < n; i++)
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dist += a[i] != b[i];
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return dist;
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}
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/* This is a pretty naive recursive implementation that should only
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* be used for quick tests obviously. Don't try and run this on a
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* GB file or something. There are faster implementations. Use those
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* if you need to run it for large files. */
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static size_t ZSTD_eDist_levenshteinDist(const BYTE* const s,
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size_t const sn, const BYTE* const t,
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size_t const tn)
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{
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size_t a, b, c;
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if (!sn)
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return tn;
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if (!tn)
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return sn;
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if (s[sn - 1] == t[tn - 1])
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return ZSTD_eDist_levenshteinDist(
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s, sn - 1, t, tn - 1);
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a = ZSTD_eDist_levenshteinDist(s, sn - 1, t, tn - 1);
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b = ZSTD_eDist_levenshteinDist(s, sn, t, tn - 1);
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c = ZSTD_eDist_levenshteinDist(s, sn - 1, t, tn);
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if (a > b)
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a = b;
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if (a > c)
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a = c;
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return a + 1;
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}
|
|
|
|
static void ZSTD_eDist_validateMatches(ZSTD_eDist_match* matches,
|
|
size_t const nbMatches, const BYTE* const dict,
|
|
size_t const dictSize, const BYTE* const src,
|
|
size_t const srcSize)
|
|
{
|
|
size_t i;
|
|
for (i = 0; i < nbMatches; i++) {
|
|
ZSTD_eDist_match match = matches[i];
|
|
U32 const dictIdx = match.dictIdx;
|
|
U32 const srcIdx = match.srcIdx;
|
|
U32 const matchLength = match.matchLength;
|
|
|
|
assert(dictIdx + matchLength < dictSize);
|
|
assert(srcIdx + matchLength < srcSize);
|
|
assert(!memcmp(dict + dictIdx, src + srcIdx, matchLength));
|
|
}
|
|
}
|
|
|
|
/*-*************************************
|
|
* API
|
|
***************************************/
|
|
|
|
size_t ZSTD_eDist_genSequences(ZSTD_Sequence* sequences,
|
|
const void* dict, size_t dictSize,
|
|
const void* src, size_t srcSize,
|
|
int useHeuristics)
|
|
{
|
|
size_t const nbDiags = dictSize + srcSize + 3;
|
|
S32* buffer = ZSTD_malloc(nbDiags * 2 * sizeof(S32), ZSTD_defaultCMem);
|
|
ZSTD_eDist_state state;
|
|
size_t nbSequences = 0;
|
|
|
|
state.dict = (const BYTE*)dict;
|
|
state.src = (const BYTE*)src;
|
|
state.dictSize = dictSize;
|
|
state.srcSize = srcSize;
|
|
state.forwardDiag = buffer;
|
|
state.backwardDiag = buffer + nbDiags;
|
|
state.forwardDiag += srcSize + 1;
|
|
state.backwardDiag += srcSize + 1;
|
|
state.matches = ZSTD_malloc(srcSize * sizeof(ZSTD_eDist_match), ZSTD_defaultCMem);
|
|
state.nbMatches = 0;
|
|
|
|
ZSTD_eDist_compare(&state, 0, dictSize, 0, srcSize, 1);
|
|
ZSTD_eDist_combineMatches(&state);
|
|
nbSequences = ZSTD_eDist_convertMatchesToSequences(sequences, &state);
|
|
|
|
ZSTD_free(buffer, ZSTD_defaultCMem);
|
|
ZSTD_free(state.matches, ZSTD_defaultCMem);
|
|
|
|
return nbSequences;
|
|
}
|