// Copyright 2011 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef V8_STRING_SEARCH_H_
#define V8_STRING_SEARCH_H_
#include "src/isolate.h"
#include "src/vector.h"
namespace v8 {
namespace internal {
//---------------------------------------------------------------------
// String Search object.
//---------------------------------------------------------------------
// Class holding constants and methods that apply to all string search variants,
// independently of subject and pattern char size.
class StringSearchBase {
protected:
// Cap on the maximal shift in the Boyer-Moore implementation. By setting a
// limit, we can fix the size of tables. For a needle longer than this limit,
// search will not be optimal, since we only build tables for a suffix
// of the string, but it is a safe approximation.
static const int kBMMaxShift = Isolate::kBMMaxShift;
// Reduce alphabet to this size.
// One of the tables used by Boyer-Moore and Boyer-Moore-Horspool has size
// proportional to the input alphabet. We reduce the alphabet size by
// equating input characters modulo a smaller alphabet size. This gives
// a potentially less efficient searching, but is a safe approximation.
// For needles using only characters in the same Unicode 256-code point page,
// there is no search speed degradation.
static const int kLatin1AlphabetSize = 256;
static const int kUC16AlphabetSize = Isolate::kUC16AlphabetSize;
// Bad-char shift table stored in the state. It's length is the alphabet size.
// For patterns below this length, the skip length of Boyer-Moore is too short
// to compensate for the algorithmic overhead compared to simple brute force.
static const int kBMMinPatternLength = 7;
static inline bool IsOneByteString(Vector<const uint8_t> string) {
return true;
}
static inline bool IsOneByteString(Vector<const uc16> string) {
return String::IsOneByte(string.start(), string.length());
}
friend class Isolate;
};
template <typename PatternChar, typename SubjectChar>
class StringSearch : private StringSearchBase {
public:
StringSearch(Isolate* isolate, Vector<const PatternChar> pattern)
: isolate_(isolate),
pattern_(pattern),
start_(Max(0, pattern.length() - kBMMaxShift)) {
if (sizeof(PatternChar) > sizeof(SubjectChar)) {
if (!IsOneByteString(pattern_)) {
strategy_ = &FailSearch;
return;
}
}
int pattern_length = pattern_.length();
if (pattern_length < kBMMinPatternLength) {
if (pattern_length == 1) {
strategy_ = &SingleCharSearch;
return;
}
strategy_ = &LinearSearch;
return;
}
strategy_ = &InitialSearch;
}
int Search(Vector<const SubjectChar> subject, int index) {
return strategy_(this, subject, index);
}
static inline int AlphabetSize() {
if (sizeof(PatternChar) == 1) {
// Latin1 needle.
return kLatin1AlphabetSize;
} else {
DCHECK(sizeof(PatternChar) == 2);
// UC16 needle.
return kUC16AlphabetSize;
}
}
private:
typedef int (*SearchFunction)( // NOLINT - it's not a cast!
StringSearch<PatternChar, SubjectChar>*,
Vector<const SubjectChar>,
int);
static int FailSearch(StringSearch<PatternChar, SubjectChar>*,
Vector<const SubjectChar>,
int) {
return -1;
}
static int SingleCharSearch(StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int start_index);
static int LinearSearch(StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int start_index);
static int InitialSearch(StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int start_index);
static int BoyerMooreHorspoolSearch(
StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int start_index);
static int BoyerMooreSearch(StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int start_index);
void PopulateBoyerMooreHorspoolTable();
void PopulateBoyerMooreTable();
static inline bool exceedsOneByte(uint8_t c) {
return false;
}
static inline bool exceedsOneByte(uint16_t c) {
return c > String::kMaxOneByteCharCodeU;
}
static inline int CharOccurrence(int* bad_char_occurrence,
SubjectChar char_code) {
if (sizeof(SubjectChar) == 1) {
return bad_char_occurrence[static_cast<int>(char_code)];
}
if (sizeof(PatternChar) == 1) {
if (exceedsOneByte(char_code)) {
return -1;
}
return bad_char_occurrence[static_cast<unsigned int>(char_code)];
}
// Both pattern and subject are UC16. Reduce character to equivalence class.
int equiv_class = char_code % kUC16AlphabetSize;
return bad_char_occurrence[equiv_class];
}
// The following tables are shared by all searches.
// TODO(lrn): Introduce a way for a pattern to keep its tables
// between searches (e.g., for an Atom RegExp).
// Store for the BoyerMoore(Horspool) bad char shift table.
// Return a table covering the last kBMMaxShift+1 positions of
// pattern.
int* bad_char_table() {
return isolate_->bad_char_shift_table();
}
// Store for the BoyerMoore good suffix shift table.
int* good_suffix_shift_table() {
// Return biased pointer that maps the range [start_..pattern_.length()
// to the kGoodSuffixShiftTable array.
return isolate_->good_suffix_shift_table() - start_;
}
// Table used temporarily while building the BoyerMoore good suffix
// shift table.
int* suffix_table() {
// Return biased pointer that maps the range [start_..pattern_.length()
// to the kSuffixTable array.
return isolate_->suffix_table() - start_;
}
Isolate* isolate_;
// The pattern to search for.
Vector<const PatternChar> pattern_;
// Pointer to implementation of the search.
SearchFunction strategy_;
// Cache value of Max(0, pattern_length() - kBMMaxShift)
int start_;
};
template <typename T, typename U>
inline T AlignDown(T value, U alignment) {
return reinterpret_cast<T>(
(reinterpret_cast<uintptr_t>(value) & ~(alignment - 1)));
}
inline uint8_t GetHighestValueByte(uc16 character) {
return Max(static_cast<uint8_t>(character & 0xFF),
static_cast<uint8_t>(character >> 8));
}
inline uint8_t GetHighestValueByte(uint8_t character) { return character; }
template <typename PatternChar, typename SubjectChar>
inline int FindFirstCharacter(Vector<const PatternChar> pattern,
Vector<const SubjectChar> subject, int index) {
const PatternChar pattern_first_char = pattern[0];
const int max_n = (subject.length() - pattern.length() + 1);
const uint8_t search_byte = GetHighestValueByte(pattern_first_char);
const SubjectChar search_char = static_cast<SubjectChar>(pattern_first_char);
int pos = index;
do {
DCHECK_GE(max_n - pos, 0);
const SubjectChar* char_pos = reinterpret_cast<const SubjectChar*>(
memchr(subject.start() + pos, search_byte,
(max_n - pos) * sizeof(SubjectChar)));
if (char_pos == NULL) return -1;
char_pos = AlignDown(char_pos, sizeof(SubjectChar));
pos = static_cast<int>(char_pos - subject.start());
if (subject[pos] == search_char) return pos;
} while (++pos < max_n);
return -1;
}
//---------------------------------------------------------------------
// Single Character Pattern Search Strategy
//---------------------------------------------------------------------
template <typename PatternChar, typename SubjectChar>
int StringSearch<PatternChar, SubjectChar>::SingleCharSearch(
StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int index) {
DCHECK_EQ(1, search->pattern_.length());
PatternChar pattern_first_char = search->pattern_[0];
if (sizeof(PatternChar) > sizeof(SubjectChar)) {
if (exceedsOneByte(pattern_first_char)) {
return -1;
}
}
return FindFirstCharacter(search->pattern_, subject, index);
}
//---------------------------------------------------------------------
// Linear Search Strategy
//---------------------------------------------------------------------
template <typename PatternChar, typename SubjectChar>
inline bool CharCompare(const PatternChar* pattern,
const SubjectChar* subject,
int length) {
DCHECK(length > 0);
int pos = 0;
do {
if (pattern[pos] != subject[pos]) {
return false;
}
pos++;
} while (pos < length);
return true;
}
// Simple linear search for short patterns. Never bails out.
template <typename PatternChar, typename SubjectChar>
int StringSearch<PatternChar, SubjectChar>::LinearSearch(
StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int index) {
Vector<const PatternChar> pattern = search->pattern_;
DCHECK(pattern.length() > 1);
int pattern_length = pattern.length();
int i = index;
int n = subject.length() - pattern_length;
while (i <= n) {
i = FindFirstCharacter(pattern, subject, i);
if (i == -1) return -1;
DCHECK_LE(i, n);
i++;
// Loop extracted to separate function to allow using return to do
// a deeper break.
if (CharCompare(pattern.start() + 1,
subject.start() + i,
pattern_length - 1)) {
return i - 1;
}
}
return -1;
}
//---------------------------------------------------------------------
// Boyer-Moore string search
//---------------------------------------------------------------------
template <typename PatternChar, typename SubjectChar>
int StringSearch<PatternChar, SubjectChar>::BoyerMooreSearch(
StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int start_index) {
Vector<const PatternChar> pattern = search->pattern_;
int subject_length = subject.length();
int pattern_length = pattern.length();
// Only preprocess at most kBMMaxShift last characters of pattern.
int start = search->start_;
int* bad_char_occurence = search->bad_char_table();
int* good_suffix_shift = search->good_suffix_shift_table();
PatternChar last_char = pattern[pattern_length - 1];
int index = start_index;
// Continue search from i.
while (index <= subject_length - pattern_length) {
int j = pattern_length - 1;
int c;
while (last_char != (c = subject[index + j])) {
int shift =
j - CharOccurrence(bad_char_occurence, c);
index += shift;
if (index > subject_length - pattern_length) {
return -1;
}
}
while (j >= 0 && pattern[j] == (c = subject[index + j])) j--;
if (j < 0) {
return index;
} else if (j < start) {
// we have matched more than our tables allow us to be smart about.
// Fall back on BMH shift.
index += pattern_length - 1
- CharOccurrence(bad_char_occurence,
static_cast<SubjectChar>(last_char));
} else {
int gs_shift = good_suffix_shift[j + 1];
int bc_occ =
CharOccurrence(bad_char_occurence, c);
int shift = j - bc_occ;
if (gs_shift > shift) {
shift = gs_shift;
}
index += shift;
}
}
return -1;
}
template <typename PatternChar, typename SubjectChar>
void StringSearch<PatternChar, SubjectChar>::PopulateBoyerMooreTable() {
int pattern_length = pattern_.length();
const PatternChar* pattern = pattern_.start();
// Only look at the last kBMMaxShift characters of pattern (from start_
// to pattern_length).
int start = start_;
int length = pattern_length - start;
// Biased tables so that we can use pattern indices as table indices,
// even if we only cover the part of the pattern from offset start.
int* shift_table = good_suffix_shift_table();
int* suffix_table = this->suffix_table();
// Initialize table.
for (int i = start; i < pattern_length; i++) {
shift_table[i] = length;
}
shift_table[pattern_length] = 1;
suffix_table[pattern_length] = pattern_length + 1;
if (pattern_length <= start) {
return;
}
// Find suffixes.
PatternChar last_char = pattern[pattern_length - 1];
int suffix = pattern_length + 1;
{
int i = pattern_length;
while (i > start) {
PatternChar c = pattern[i - 1];
while (suffix <= pattern_length && c != pattern[suffix - 1]) {
if (shift_table[suffix] == length) {
shift_table[suffix] = suffix - i;
}
suffix = suffix_table[suffix];
}
suffix_table[--i] = --suffix;
if (suffix == pattern_length) {
// No suffix to extend, so we check against last_char only.
while ((i > start) && (pattern[i - 1] != last_char)) {
if (shift_table[pattern_length] == length) {
shift_table[pattern_length] = pattern_length - i;
}
suffix_table[--i] = pattern_length;
}
if (i > start) {
suffix_table[--i] = --suffix;
}
}
}
}
// Build shift table using suffixes.
if (suffix < pattern_length) {
for (int i = start; i <= pattern_length; i++) {
if (shift_table[i] == length) {
shift_table[i] = suffix - start;
}
if (i == suffix) {
suffix = suffix_table[suffix];
}
}
}
}
//---------------------------------------------------------------------
// Boyer-Moore-Horspool string search.
//---------------------------------------------------------------------
template <typename PatternChar, typename SubjectChar>
int StringSearch<PatternChar, SubjectChar>::BoyerMooreHorspoolSearch(
StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int start_index) {
Vector<const PatternChar> pattern = search->pattern_;
int subject_length = subject.length();
int pattern_length = pattern.length();
int* char_occurrences = search->bad_char_table();
int badness = -pattern_length;
// How bad we are doing without a good-suffix table.
PatternChar last_char = pattern[pattern_length - 1];
int last_char_shift = pattern_length - 1 -
CharOccurrence(char_occurrences, static_cast<SubjectChar>(last_char));
// Perform search
int index = start_index; // No matches found prior to this index.
while (index <= subject_length - pattern_length) {
int j = pattern_length - 1;
int subject_char;
while (last_char != (subject_char = subject[index + j])) {
int bc_occ = CharOccurrence(char_occurrences, subject_char);
int shift = j - bc_occ;
index += shift;
badness += 1 - shift; // at most zero, so badness cannot increase.
if (index > subject_length - pattern_length) {
return -1;
}
}
j--;
while (j >= 0 && pattern[j] == (subject[index + j])) j--;
if (j < 0) {
return index;
} else {
index += last_char_shift;
// Badness increases by the number of characters we have
// checked, and decreases by the number of characters we
// can skip by shifting. It's a measure of how we are doing
// compared to reading each character exactly once.
badness += (pattern_length - j) - last_char_shift;
if (badness > 0) {
search->PopulateBoyerMooreTable();
search->strategy_ = &BoyerMooreSearch;
return BoyerMooreSearch(search, subject, index);
}
}
}
return -1;
}
template <typename PatternChar, typename SubjectChar>
void StringSearch<PatternChar, SubjectChar>::PopulateBoyerMooreHorspoolTable() {
int pattern_length = pattern_.length();
int* bad_char_occurrence = bad_char_table();
// Only preprocess at most kBMMaxShift last characters of pattern.
int start = start_;
// Run forwards to populate bad_char_table, so that *last* instance
// of character equivalence class is the one registered.
// Notice: Doesn't include the last character.
int table_size = AlphabetSize();
if (start == 0) { // All patterns less than kBMMaxShift in length.
memset(bad_char_occurrence,
-1,
table_size * sizeof(*bad_char_occurrence));
} else {
for (int i = 0; i < table_size; i++) {
bad_char_occurrence[i] = start - 1;
}
}
for (int i = start; i < pattern_length - 1; i++) {
PatternChar c = pattern_[i];
int bucket = (sizeof(PatternChar) == 1) ? c : c % AlphabetSize();
bad_char_occurrence[bucket] = i;
}
}
//---------------------------------------------------------------------
// Linear string search with bailout to BMH.
//---------------------------------------------------------------------
// Simple linear search for short patterns, which bails out if the string
// isn't found very early in the subject. Upgrades to BoyerMooreHorspool.
template <typename PatternChar, typename SubjectChar>
int StringSearch<PatternChar, SubjectChar>::InitialSearch(
StringSearch<PatternChar, SubjectChar>* search,
Vector<const SubjectChar> subject,
int index) {
Vector<const PatternChar> pattern = search->pattern_;
int pattern_length = pattern.length();
// Badness is a count of how much work we have done. When we have
// done enough work we decide it's probably worth switching to a better
// algorithm.
int badness = -10 - (pattern_length << 2);
// We know our pattern is at least 2 characters, we cache the first so
// the common case of the first character not matching is faster.
for (int i = index, n = subject.length() - pattern_length; i <= n; i++) {
badness++;
if (badness <= 0) {
i = FindFirstCharacter(pattern, subject, i);
if (i == -1) return -1;
DCHECK_LE(i, n);
int j = 1;
do {
if (pattern[j] != subject[i + j]) {
break;
}
j++;
} while (j < pattern_length);
if (j == pattern_length) {
return i;
}
badness += j;
} else {
search->PopulateBoyerMooreHorspoolTable();
search->strategy_ = &BoyerMooreHorspoolSearch;
return BoyerMooreHorspoolSearch(search, subject, i);
}
}
return -1;
}
// Perform a a single stand-alone search.
// If searching multiple times for the same pattern, a search
// object should be constructed once and the Search function then called
// for each search.
template <typename SubjectChar, typename PatternChar>
int SearchString(Isolate* isolate,
Vector<const SubjectChar> subject,
Vector<const PatternChar> pattern,
int start_index) {
StringSearch<PatternChar, SubjectChar> search(isolate, pattern);
return search.Search(subject, start_index);
}
} // namespace internal
} // namespace v8
#endif // V8_STRING_SEARCH_H_