From c04546333ae39b93f7531830823122c7eba0c942 Mon Sep 17 00:00:00 2001 From: "lrn@chromium.org" Date: Thu, 16 Oct 2008 11:33:19 +0000 Subject: [PATCH] * Special case for last char in BM-match * Patch from Erik Corry to separate BM-algoritm into special case functions. Also changes condition for bailing out of simple search. * Added simple search with no bailout for very short patterns. git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@513 ce2b1a6d-e550-0410-aec6-3dcde31c8c00 --- src/runtime.cc | 186 +++++++++++++++++++++++++++++++++++++++------------------ 1 file changed, 129 insertions(+), 57 deletions(-) diff --git a/src/runtime.cc b/src/runtime.cc index 8e329b0..bcdd76d 100644 --- a/src/runtime.cc +++ b/src/runtime.cc @@ -956,7 +956,11 @@ static Object* Runtime_CharFromCode(Arguments args) { // Cap on the maximal shift in the Boyer-Moore implementation. By setting a // limit, we can fix the size of tables. static const int kBMMaxShift = 0xff; -static const int kBMAlphabetSize = 0x100; // Reduce alphabet to this size. +// Reduce alphabet to this size. +static const int kBMAlphabetSize = 0x100; +// 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 = 5; // Holds the two buffers used by Boyer-Moore string search's Good Suffix // shift. Only allows the last kBMMaxShift characters of the needle @@ -995,8 +999,6 @@ static int bad_char_occurence[kBMAlphabetSize]; static BMGoodSuffixBuffers bmgs_buffers; // Compute the bad-char table for Boyer-Moore in the static buffer. -// Return false if the pattern contains non-ASCII characters that cannot be -// in the searched string. template static void BoyerMoorePopulateBadCharTable(Vector pattern, int start) { @@ -1006,16 +1008,18 @@ static void BoyerMoorePopulateBadCharTable(Vector pattern, for (int i = 0; i < kBMAlphabetSize; i++) { bad_char_occurence[i] = start - 1; } - for (int i = start; i < pattern.length(); i++) { - bad_char_occurence[pattern[i] % kBMAlphabetSize] = i; + for (int i = start; i < pattern.length() - 1; i++) { + pchar c = pattern[i]; + int bucket = c % kBMAlphabetSize; + bad_char_occurence[bucket] = i; } } template static void BoyerMoorePopulateGoodSuffixTable(Vector pattern, - int start, - int len) { + int start) { int m = pattern.length(); + int len = m - start; // Compute Good Suffix tables. bmgs_buffers.init(m); @@ -1061,31 +1065,44 @@ static void BoyerMoorePopulateGoodSuffixTable(Vector pattern, } } -// Restricted Boyer-Moore string matching. Restricts tables to a + +// Restricted simplified Boyer-Moore string matching. Restricts tables to a // suffix of long pattern strings and handles only equivalence classes // of the full alphabet. This allows us to ensure that tables take only // a fixed amount of space. template -static int BoyerMooreIndexOf(Vector subject, - Vector pattern, - int start_index) { - int m = pattern.length(); +static int BoyerMooreSimplified(Vector subject, + Vector pattern, + int start_index, + bool& complete) { int n = subject.length(); - + int m = pattern.length(); // Only preprocess at most kBMMaxShift last characters of pattern. int start = m < kBMMaxShift ? 0 : m - kBMMaxShift; - int len = m - start; BoyerMoorePopulateBadCharTable(pattern, start); - int badness = 0; // How bad we are doing without a good-suffix table. + int badness = -m; // How bad we are doing without a good-suffix table. int idx; // No matches found prior to this index. + pchar last_char = pattern[m - 1]; // Perform search for (idx = start_index; idx <= n - m;) { int j = m - 1; - schar c; + int c; + while (last_char != (c = subject[idx + j])) { + int bc_occ = bad_char_occurence[c % kBMAlphabetSize]; + int shift = j - bc_occ; + idx += shift; + badness += 1 - shift; // at most zero, so badness cannot increase. + if (idx > n - m) { + complete = true; + return -1; + } + } + j--; while (j >= 0 && pattern[j] == (c = subject[idx + j])) j--; if (j < 0) { + complete = true; return idx; } else { int bc_occ = bad_char_occurence[c % kBMAlphabetSize]; @@ -1096,40 +1113,66 @@ static int BoyerMooreIndexOf(Vector subject, // can skip by shifting. It's a measure of how we are doing // compared to reading each character exactly once. badness += (m - j) - shift; - if (badness > m) break; + if (badness > 0) { + complete = false; + return idx; + } } } + complete = true; + return -1; +} - // If we are not done, we got here because we should build the Good Suffix - // table and continue searching. - if (idx <= n - m) { - BoyerMoorePopulateGoodSuffixTable(pattern, start, len); - // Continue search from i. - do { - int j = m - 1; - schar c; - while (j >= 0 && pattern[j] == (c = subject[idx + j])) j--; - if (j < 0) { - return idx; - } else if (j < start) { - // we have matched more than our tables allow us to be smart about. - idx += 1; - } else { - int gs_shift = bmgs_buffers.shift(j + 1); - int bc_occ = bad_char_occurence[c % kBMAlphabetSize]; - int bc_shift = j - bc_occ; - idx += (gs_shift > bc_shift) ? gs_shift : bc_shift; + +template +static int BoyerMooreIndexOf(Vector subject, + Vector pattern, + int idx) { + int n = subject.length(); + int m = pattern.length(); + // Only preprocess at most kBMMaxShift last characters of pattern. + int start = m < kBMMaxShift ? 0 : m - kBMMaxShift; + + // Build the Good Suffix table and continue searching. + BoyerMoorePopulateGoodSuffixTable(pattern, start); + pchar last_char = pattern[m - 1]; + // Continue search from i. + do { + int j = m - 1; + schar c; + while (last_char != (c = subject[idx + j])) { + int shift = j - bad_char_occurence[c % kBMAlphabetSize]; + idx += shift; + if (idx > n - m) { + return -1; } - } while (idx <= n - m); - } + } + while (j >= 0 && pattern[j] == (c = subject[idx + j])) j--; + if (j < 0) { + return idx; + } else if (j < start) { + // we have matched more than our tables allow us to be smart about. + idx += 1; + } else { + int gs_shift = bmgs_buffers.shift(j + 1); // Good suffix shift. + int bc_occ = bad_char_occurence[c % kBMAlphabetSize]; + int shift = j - bc_occ; // Bad-char shift. + shift = (gs_shift > shift) ? gs_shift : shift; + idx += shift; + } + } while (idx <= n - m); return -1; } -template + +template static int SingleCharIndexOf(Vector string, - pchar pattern_char, + uc16 pattern_char, int start_index) { + if (sizeof(schar) == 1 && pattern_char > String::kMaxAsciiCharCode) { + return -1; + } for (int i = start_index, n = string.length(); i < n; i++) { if (pattern_char == string[i]) { return i; @@ -1147,20 +1190,22 @@ static int SingleCharIndexOf(Vector string, template static int SimpleIndexOf(Vector subject, Vector pattern, - int start_index, + int idx, bool &complete) { - int pattern_length = pattern.length(); - int subject_length = subject.length(); - // Badness is a count of how many extra times the same character - // is checked. We compare it to the index counter, so we start - // it at the start_index, and give it a little discount to avoid - // very early bail-outs. - int badness = start_index - 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. pchar pattern_first_char = pattern[0]; - for (int i = start_index, n = subject_length - pattern_length; i <= n; i++) { + for (int i = idx, n = subject.length() - pattern.length(); i <= n; i++) { + badness++; + if (badness > 0) { + complete = false; + return (i); + } if (subject[i] != pattern_first_char) continue; int j = 1; do { @@ -1168,22 +1213,41 @@ static int SimpleIndexOf(Vector subject, break; } j++; - } while (j < pattern_length); - if (j == pattern_length) { + } while (j < pattern.length()); + if (j == pattern.length()) { complete = true; return i; } badness += j; - if (badness > i) { // More than one extra character on average. - complete = false; - return (i + 1); // No matches up to index i+1. - } } complete = true; return -1; } -// Dispatch to different algorithms for different length of pattern/subject +// Simple indexOf that never bails out. For short patterns only. +template +static int SimpleIndexOf(Vector subject, + Vector pattern, + int idx) { + pchar pattern_first_char = pattern[0]; + for (int i = idx, n = subject.length() - pattern.length(); i <= n; i++) { + if (subject[i] != pattern_first_char) continue; + int j = 1; + do { + if (pattern[j] != subject[i+j]) { + break; + } + j++; + } while (j < pattern.length()); + if (j == pattern.length()) { + return i; + } + } + return -1; +} + + +// Dispatch to different algorithms. template static int StringMatchStrategy(Vector sub, Vector pat, @@ -1201,10 +1265,18 @@ static int StringMatchStrategy(Vector sub, } } } - // For small searches, a complex sort is not worth the setup overhead. + if (pat.length() < kBMMinPatternLength) { + // We don't believe fancy searching can ever be more efficient. + // The max shift of Boyer-Moore on a pattern of this length does + // not compensate for the overhead. + return SimpleIndexOf(sub, pat, start_index); + } + // Try algorithms in order of increasing setup cost and expected performance. bool complete; int idx = SimpleIndexOf(sub, pat, start_index, complete); if (complete) return idx; + idx = BoyerMooreSimplified(sub, pat, idx, complete); + if (complete) return idx; return BoyerMooreIndexOf(sub, pat, idx); } -- 2.7.4