in contrib/match_finders/zstd_edist.c [81:323]
static void ZSTD_eDist_diag(ZSTD_eDist_state* state,
ZSTD_eDist_partition* partition,
S32 dictLow, S32 dictHigh, S32 srcLow,
S32 srcHigh, int useHeuristics)
{
S32* const forwardDiag = state->forwardDiag;
S32* const backwardDiag = state->backwardDiag;
const BYTE* const dict = state->dict;
const BYTE* const src = state->src;
S32 const diagMin = dictLow - srcHigh;
S32 const diagMax = dictHigh - srcLow;
S32 const forwardMid = dictLow - srcLow;
S32 const backwardMid = dictHigh - srcHigh;
S32 forwardMin = forwardMid;
S32 forwardMax = forwardMid;
S32 backwardMin = backwardMid;
S32 backwardMax = backwardMid;
int odd = (forwardMid - backwardMid) & 1;
U32 iterations;
forwardDiag[forwardMid] = dictLow;
backwardDiag[backwardMid] = dictHigh;
/* Main loop for updating diag entries. Unless useHeuristics is
* set to false, this loop will run until it finds the minimal
* edit script */
for (iterations = 1;;iterations++) {
S32 diag;
int bigSnake = 0;
if (forwardMin > diagMin) {
forwardMin--;
forwardDiag[forwardMin - 1] = -1;
} else {
forwardMin++;
}
if (forwardMax < diagMax) {
forwardMax++;
forwardDiag[forwardMax + 1] = -1;
} else {
forwardMax--;
}
for (diag = forwardMax; diag >= forwardMin; diag -= 2) {
S32 dictIdx;
S32 srcIdx;
S32 low = forwardDiag[diag - 1];
S32 high = forwardDiag[diag + 1];
S32 dictIdx0 = low < high ? high : low + 1;
for (dictIdx = dictIdx0, srcIdx = dictIdx0 - diag;
dictIdx < dictHigh && srcIdx < srcHigh && dict[dictIdx] == src[srcIdx];
dictIdx++, srcIdx++) continue;
if (dictIdx - dictIdx0 > ZSTD_EDIST_SNAKE_THRESH)
bigSnake = 1;
forwardDiag[diag] = dictIdx;
if (odd && backwardMin <= diag && diag <= backwardMax && backwardDiag[diag] <= dictIdx) {
partition->dictMid = dictIdx;
partition->srcMid = srcIdx;
partition->lowUseHeuristics = 0;
partition->highUseHeuristics = 0;
return;
}
}
if (backwardMin > diagMin) {
backwardMin--;
backwardDiag[backwardMin - 1] = ZSTD_EDIST_DIAG_MAX;
} else {
backwardMin++;
}
if (backwardMax < diagMax) {
backwardMax++;
backwardDiag[backwardMax + 1] = ZSTD_EDIST_DIAG_MAX;
} else {
backwardMax--;
}
for (diag = backwardMax; diag >= backwardMin; diag -= 2) {
S32 dictIdx;
S32 srcIdx;
S32 low = backwardDiag[diag - 1];
S32 high = backwardDiag[diag + 1];
S32 dictIdx0 = low < high ? low : high - 1;
for (dictIdx = dictIdx0, srcIdx = dictIdx0 - diag;
dictLow < dictIdx && srcLow < srcIdx && dict[dictIdx - 1] == src[srcIdx - 1];
dictIdx--, srcIdx--) continue;
if (dictIdx0 - dictIdx > ZSTD_EDIST_SNAKE_THRESH)
bigSnake = 1;
backwardDiag[diag] = dictIdx;
if (!odd && forwardMin <= diag && diag <= forwardMax && dictIdx <= forwardDiag[diag]) {
partition->dictMid = dictIdx;
partition->srcMid = srcIdx;
partition->lowUseHeuristics = 0;
partition->highUseHeuristics = 0;
return;
}
}
if (!useHeuristics)
continue;
/* Everything under this point is a heuristic. Using these will
* substantially speed up the match finding. In some cases, taking
* the total match finding time from several minutes to seconds.
* Of course, the caveat is that the edit script found may no longer
* be optimal */
/* Big snake heuristic */
if (iterations > ZSTD_EDIST_SNAKE_ITER_THRESH && bigSnake) {
{
S32 best = 0;
for (diag = forwardMax; diag >= forwardMin; diag -= 2) {
S32 diagDiag = diag - forwardMid;
S32 dictIdx = forwardDiag[diag];
S32 srcIdx = dictIdx - diag;
S32 v = (dictIdx - dictLow) * 2 - diagDiag;
if (v > 12 * (iterations + (diagDiag < 0 ? -diagDiag : diagDiag))) {
if (v > best
&& dictLow + ZSTD_EDIST_SNAKE_THRESH <= dictIdx && dictIdx <= dictHigh
&& srcLow + ZSTD_EDIST_SNAKE_THRESH <= srcIdx && srcIdx <= srcHigh) {
S32 k;
for (k = 1; dict[dictIdx - k] == src[srcIdx - k]; k++) {
if (k == ZSTD_EDIST_SNAKE_THRESH) {
best = v;
partition->dictMid = dictIdx;
partition->srcMid = srcIdx;
break;
}
}
}
}
}
if (best > 0) {
partition->lowUseHeuristics = 0;
partition->highUseHeuristics = 1;
return;
}
}
{
S32 best = 0;
for (diag = backwardMax; diag >= backwardMin; diag -= 2) {
S32 diagDiag = diag - backwardMid;
S32 dictIdx = backwardDiag[diag];
S32 srcIdx = dictIdx - diag;
S32 v = (dictHigh - dictIdx) * 2 + diagDiag;
if (v > 12 * (iterations + (diagDiag < 0 ? -diagDiag : diagDiag))) {
if (v > best
&& dictLow < dictIdx && dictIdx <= dictHigh - ZSTD_EDIST_SNAKE_THRESH
&& srcLow < srcIdx && srcIdx <= srcHigh - ZSTD_EDIST_SNAKE_THRESH) {
int k;
for (k = 0; dict[dictIdx + k] == src[srcIdx + k]; k++) {
if (k == ZSTD_EDIST_SNAKE_THRESH - 1) {
best = v;
partition->dictMid = dictIdx;
partition->srcMid = srcIdx;
break;
}
}
}
}
}
if (best > 0) {
partition->lowUseHeuristics = 1;
partition->highUseHeuristics = 0;
return;
}
}
}
/* More general 'too expensive' heuristic */
if (iterations >= ZSTD_EDIST_EXPENSIVE_THRESH) {
S32 forwardDictSrcBest;
S32 forwardDictBest = 0;
S32 backwardDictSrcBest;
S32 backwardDictBest = 0;
forwardDictSrcBest = -1;
for (diag = forwardMax; diag >= forwardMin; diag -= 2) {
S32 dictIdx = MIN(forwardDiag[diag], dictHigh);
S32 srcIdx = dictIdx - diag;
if (srcHigh < srcIdx) {
dictIdx = srcHigh + diag;
srcIdx = srcHigh;
}
if (forwardDictSrcBest < dictIdx + srcIdx) {
forwardDictSrcBest = dictIdx + srcIdx;
forwardDictBest = dictIdx;
}
}
backwardDictSrcBest = ZSTD_EDIST_DIAG_MAX;
for (diag = backwardMax; diag >= backwardMin; diag -= 2) {
S32 dictIdx = MAX(dictLow, backwardDiag[diag]);
S32 srcIdx = dictIdx - diag;
if (srcIdx < srcLow) {
dictIdx = srcLow + diag;
srcIdx = srcLow;
}
if (dictIdx + srcIdx < backwardDictSrcBest) {
backwardDictSrcBest = dictIdx + srcIdx;
backwardDictBest = dictIdx;
}
}
if ((dictHigh + srcHigh) - backwardDictSrcBest < forwardDictSrcBest - (dictLow + srcLow)) {
partition->dictMid = forwardDictBest;
partition->srcMid = forwardDictSrcBest - forwardDictBest;
partition->lowUseHeuristics = 0;
partition->highUseHeuristics = 1;
} else {
partition->dictMid = backwardDictBest;
partition->srcMid = backwardDictSrcBest - backwardDictBest;
partition->lowUseHeuristics = 1;
partition->highUseHeuristics = 0;
}
return;
}
}
}