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gh-119879: str.find(): Utilize last character gap for two-way periodic needles (#119880)

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d.grigonis 2024-06-04 10:44:49 +03:00 committed by GitHub
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commit a8f1152b70
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2 changed files with 36 additions and 28 deletions
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1
Misc/NEWS.d/next/Core and Builtins/2024-06-02-06-12-35.gh-issue-119879.Jit951.rst Normal file
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@ -0,0 +1 @@
String search is now slightly faster for certain cases. It now utilizes last character gap (good suffix rule) for two-way periodic needles.

63
Objects/stringlib/fastsearch.h
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@ -256,7 +256,7 @@ STRINGLIB(_factorize)(const STRINGLIB_CHAR *needle,
The local period of the cut is the minimal length of a string w
such that (left endswith w or w endswith left)
and (right startswith w or w startswith left).
and (right startswith w or w startswith right).
The Critical Factorization Theorem says that this maximal local
period is the global period of the string.
@ -337,21 +337,20 @@ STRINGLIB(_preprocess)(const STRINGLIB_CHAR *needle, Py_ssize_t len_needle,
if (p->is_periodic) {
assert(p->cut <= len_needle/2);
assert(p->cut < p->period);
p->gap = 0; // unused
}
else {
// A lower bound on the period
p->period = Py_MAX(p->cut, len_needle - p->cut) + 1;
// The gap between the last character and the previous
// occurrence of an equivalent character (modulo TABLE_SIZE)
p->gap = len_needle;
STRINGLIB_CHAR last = needle[len_needle - 1] & TABLE_MASK;
for (Py_ssize_t i = len_needle - 2; i >= 0; i--) {
STRINGLIB_CHAR x = needle[i] & TABLE_MASK;
if (x == last) {
p->gap = len_needle - 1 - i;
break;
}
}
// The gap between the last character and the previous
// occurrence of an equivalent character (modulo TABLE_SIZE)
p->gap = len_needle;
STRINGLIB_CHAR last = needle[len_needle - 1] & TABLE_MASK;
for (Py_ssize_t i = len_needle - 2; i >= 0; i--) {
STRINGLIB_CHAR x = needle[i] & TABLE_MASK;
if (x == last) {
p->gap = len_needle - 1 - i;
break;
}
}
// Fill up a compressed Boyer-Moore "Bad Character" table
@ -383,6 +382,8 @@ STRINGLIB(_two_way)(const STRINGLIB_CHAR *haystack, Py_ssize_t len_haystack,
const STRINGLIB_CHAR *window;
LOG("===== Two-way: \"%s\" in \"%s\". =====\n", needle, haystack);
Py_ssize_t gap = p->gap;
Py_ssize_t gap_jump_end = Py_MIN(len_needle, cut + gap);
if (p->is_periodic) {
LOG("Needle is periodic.\n");
Py_ssize_t memory = 0;
@ -408,8 +409,16 @@ STRINGLIB(_two_way)(const STRINGLIB_CHAR *haystack, Py_ssize_t len_haystack,
Py_ssize_t i = Py_MAX(cut, memory);
for (; i < len_needle; i++) {
if (needle[i] != window[i]) {
LOG("Right half does not match.\n");
window_last += i - cut + 1;
if (i < gap_jump_end) {
LOG("Early right half mismatch: jump by gap.\n");
assert(gap >= i - cut + 1);
window_last += gap;
}
else {
LOG("Late right half mismatch: jump by n (>gap)\n");
assert(i - cut + 1 > gap);
window_last += i - cut + 1;
}
memory = 0;
goto periodicwindowloop;
}
@ -442,10 +451,8 @@ STRINGLIB(_two_way)(const STRINGLIB_CHAR *haystack, Py_ssize_t len_haystack,
}
}
else {
Py_ssize_t gap = p->gap;
period = Py_MAX(gap, period);
LOG("Needle is not periodic.\n");
Py_ssize_t gap_jump_end = Py_MIN(len_needle, cut + gap);
windowloop:
while (window_last < haystack_end) {
for (;;) {
@ -463,19 +470,19 @@ STRINGLIB(_two_way)(const STRINGLIB_CHAR *haystack, Py_ssize_t len_haystack,
window = window_last - len_needle + 1;
assert((window[len_needle - 1] & TABLE_MASK) ==
(needle[len_needle - 1] & TABLE_MASK));
for (Py_ssize_t i = cut; i < gap_jump_end; i++) {
Py_ssize_t i = cut;
for (; i < len_needle; i++) {
if (needle[i] != window[i]) {
LOG("Early right half mismatch: jump by gap.\n");
assert(gap >= i - cut + 1);
window_last += gap;
goto windowloop;
}
}
for (Py_ssize_t i = gap_jump_end; i < len_needle; i++) {
if (needle[i] != window[i]) {
LOG("Late right half mismatch.\n");
assert(i - cut + 1 > gap);
window_last += i - cut + 1;
if (i < gap_jump_end) {
LOG("Early right half mismatch: jump by gap.\n");
assert(gap >= i - cut + 1);
window_last += gap;
}
else {
LOG("Late right half mismatch: jump by n (>gap)\n");
assert(i - cut + 1 > gap);
window_last += i - cut + 1;
}
goto windowloop;
}
}