From 46651ce6fe013220ed397add242004d764fc0153 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 4 May 2024 14:15:05 +0200 Subject: Adding upstream version 14.5. Signed-off-by: Daniel Baumann --- src/backend/utils/adt/levenshtein.c | 401 ++++++++++++++++++++++++++++++++++++ 1 file changed, 401 insertions(+) create mode 100644 src/backend/utils/adt/levenshtein.c (limited to 'src/backend/utils/adt/levenshtein.c') diff --git a/src/backend/utils/adt/levenshtein.c b/src/backend/utils/adt/levenshtein.c new file mode 100644 index 0000000..f897977 --- /dev/null +++ b/src/backend/utils/adt/levenshtein.c @@ -0,0 +1,401 @@ +/*------------------------------------------------------------------------- + * + * levenshtein.c + * Levenshtein distance implementation. + * + * Original author: Joe Conway + * + * This file is included by varlena.c twice, to provide matching code for (1) + * Levenshtein distance with custom costings, and (2) Levenshtein distance with + * custom costings and a "max" value above which exact distances are not + * interesting. Before the inclusion, we rely on the presence of the inline + * function rest_of_char_same(). + * + * Written based on a description of the algorithm by Michael Gilleland found + * at http://www.merriampark.com/ld.htm. Also looked at levenshtein.c in the + * PHP 4.0.6 distribution for inspiration. Configurable penalty costs + * extension is introduced by Volkan YAZICI = 0, maximum distance we care about; see below. + * trusted: caller is trusted and need not obey MAX_LEVENSHTEIN_STRLEN. + * + * One way to compute Levenshtein distance is to incrementally construct + * an (m+1)x(n+1) matrix where cell (i, j) represents the minimum number + * of operations required to transform the first i characters of s into + * the first j characters of t. The last column of the final row is the + * answer. + * + * We use that algorithm here with some modification. In lieu of holding + * the entire array in memory at once, we'll just use two arrays of size + * m+1 for storing accumulated values. At each step one array represents + * the "previous" row and one is the "current" row of the notional large + * array. + * + * If max_d >= 0, we only need to provide an accurate answer when that answer + * is less than or equal to max_d. From any cell in the matrix, there is + * theoretical "minimum residual distance" from that cell to the last column + * of the final row. This minimum residual distance is zero when the + * untransformed portions of the strings are of equal length (because we might + * get lucky and find all the remaining characters matching) and is otherwise + * based on the minimum number of insertions or deletions needed to make them + * equal length. The residual distance grows as we move toward the upper + * right or lower left corners of the matrix. When the max_d bound is + * usefully tight, we can use this property to avoid computing the entirety + * of each row; instead, we maintain a start_column and stop_column that + * identify the portion of the matrix close to the diagonal which can still + * affect the final answer. + */ +int +#ifdef LEVENSHTEIN_LESS_EQUAL +varstr_levenshtein_less_equal(const char *source, int slen, + const char *target, int tlen, + int ins_c, int del_c, int sub_c, + int max_d, bool trusted) +#else +varstr_levenshtein(const char *source, int slen, + const char *target, int tlen, + int ins_c, int del_c, int sub_c, + bool trusted) +#endif +{ + int m, + n; + int *prev; + int *curr; + int *s_char_len = NULL; + int i, + j; + const char *y; + + /* + * For varstr_levenshtein_less_equal, we have real variables called + * start_column and stop_column; otherwise it's just short-hand for 0 and + * m. + */ +#ifdef LEVENSHTEIN_LESS_EQUAL + int start_column, + stop_column; + +#undef START_COLUMN +#undef STOP_COLUMN +#define START_COLUMN start_column +#define STOP_COLUMN stop_column +#else +#undef START_COLUMN +#undef STOP_COLUMN +#define START_COLUMN 0 +#define STOP_COLUMN m +#endif + + /* Convert string lengths (in bytes) to lengths in characters */ + m = pg_mbstrlen_with_len(source, slen); + n = pg_mbstrlen_with_len(target, tlen); + + /* + * We can transform an empty s into t with n insertions, or a non-empty t + * into an empty s with m deletions. + */ + if (!m) + return n * ins_c; + if (!n) + return m * del_c; + + /* + * For security concerns, restrict excessive CPU+RAM usage. (This + * implementation uses O(m) memory and has O(mn) complexity.) If + * "trusted" is true, caller is responsible for not making excessive + * requests, typically by using a small max_d along with strings that are + * bounded, though not necessarily to MAX_LEVENSHTEIN_STRLEN exactly. + */ + if (!trusted && + (m > MAX_LEVENSHTEIN_STRLEN || + n > MAX_LEVENSHTEIN_STRLEN)) + ereport(ERROR, + (errcode(ERRCODE_INVALID_PARAMETER_VALUE), + errmsg("levenshtein argument exceeds maximum length of %d characters", + MAX_LEVENSHTEIN_STRLEN))); + +#ifdef LEVENSHTEIN_LESS_EQUAL + /* Initialize start and stop columns. */ + start_column = 0; + stop_column = m + 1; + + /* + * If max_d >= 0, determine whether the bound is impossibly tight. If so, + * return max_d + 1 immediately. Otherwise, determine whether it's tight + * enough to limit the computation we must perform. If so, figure out + * initial stop column. + */ + if (max_d >= 0) + { + int min_theo_d; /* Theoretical minimum distance. */ + int max_theo_d; /* Theoretical maximum distance. */ + int net_inserts = n - m; + + min_theo_d = net_inserts < 0 ? + -net_inserts * del_c : net_inserts * ins_c; + if (min_theo_d > max_d) + return max_d + 1; + if (ins_c + del_c < sub_c) + sub_c = ins_c + del_c; + max_theo_d = min_theo_d + sub_c * Min(m, n); + if (max_d >= max_theo_d) + max_d = -1; + else if (ins_c + del_c > 0) + { + /* + * Figure out how much of the first row of the notional matrix we + * need to fill in. If the string is growing, the theoretical + * minimum distance already incorporates the cost of deleting the + * number of characters necessary to make the two strings equal in + * length. Each additional deletion forces another insertion, so + * the best-case total cost increases by ins_c + del_c. If the + * string is shrinking, the minimum theoretical cost assumes no + * excess deletions; that is, we're starting no further right than + * column n - m. If we do start further right, the best-case + * total cost increases by ins_c + del_c for each move right. + */ + int slack_d = max_d - min_theo_d; + int best_column = net_inserts < 0 ? -net_inserts : 0; + + stop_column = best_column + (slack_d / (ins_c + del_c)) + 1; + if (stop_column > m) + stop_column = m + 1; + } + } +#endif + + /* + * In order to avoid calling pg_mblen() repeatedly on each character in s, + * we cache all the lengths before starting the main loop -- but if all + * the characters in both strings are single byte, then we skip this and + * use a fast-path in the main loop. If only one string contains + * multi-byte characters, we still build the array, so that the fast-path + * needn't deal with the case where the array hasn't been initialized. + */ + if (m != slen || n != tlen) + { + int i; + const char *cp = source; + + s_char_len = (int *) palloc((m + 1) * sizeof(int)); + for (i = 0; i < m; ++i) + { + s_char_len[i] = pg_mblen(cp); + cp += s_char_len[i]; + } + s_char_len[i] = 0; + } + + /* One more cell for initialization column and row. */ + ++m; + ++n; + + /* Previous and current rows of notional array. */ + prev = (int *) palloc(2 * m * sizeof(int)); + curr = prev + m; + + /* + * To transform the first i characters of s into the first 0 characters of + * t, we must perform i deletions. + */ + for (i = START_COLUMN; i < STOP_COLUMN; i++) + prev[i] = i * del_c; + + /* Loop through rows of the notional array */ + for (y = target, j = 1; j < n; j++) + { + int *temp; + const char *x = source; + int y_char_len = n != tlen + 1 ? pg_mblen(y) : 1; + +#ifdef LEVENSHTEIN_LESS_EQUAL + + /* + * In the best case, values percolate down the diagonal unchanged, so + * we must increment stop_column unless it's already on the right end + * of the array. The inner loop will read prev[stop_column], so we + * have to initialize it even though it shouldn't affect the result. + */ + if (stop_column < m) + { + prev[stop_column] = max_d + 1; + ++stop_column; + } + + /* + * The main loop fills in curr, but curr[0] needs a special case: to + * transform the first 0 characters of s into the first j characters + * of t, we must perform j insertions. However, if start_column > 0, + * this special case does not apply. + */ + if (start_column == 0) + { + curr[0] = j * ins_c; + i = 1; + } + else + i = start_column; +#else + curr[0] = j * ins_c; + i = 1; +#endif + + /* + * This inner loop is critical to performance, so we include a + * fast-path to handle the (fairly common) case where no multibyte + * characters are in the mix. The fast-path is entitled to assume + * that if s_char_len is not initialized then BOTH strings contain + * only single-byte characters. + */ + if (s_char_len != NULL) + { + for (; i < STOP_COLUMN; i++) + { + int ins; + int del; + int sub; + int x_char_len = s_char_len[i - 1]; + + /* + * Calculate costs for insertion, deletion, and substitution. + * + * When calculating cost for substitution, we compare the last + * character of each possibly-multibyte character first, + * because that's enough to rule out most mis-matches. If we + * get past that test, then we compare the lengths and the + * remaining bytes. + */ + ins = prev[i] + ins_c; + del = curr[i - 1] + del_c; + if (x[x_char_len - 1] == y[y_char_len - 1] + && x_char_len == y_char_len && + (x_char_len == 1 || rest_of_char_same(x, y, x_char_len))) + sub = prev[i - 1]; + else + sub = prev[i - 1] + sub_c; + + /* Take the one with minimum cost. */ + curr[i] = Min(ins, del); + curr[i] = Min(curr[i], sub); + + /* Point to next character. */ + x += x_char_len; + } + } + else + { + for (; i < STOP_COLUMN; i++) + { + int ins; + int del; + int sub; + + /* Calculate costs for insertion, deletion, and substitution. */ + ins = prev[i] + ins_c; + del = curr[i - 1] + del_c; + sub = prev[i - 1] + ((*x == *y) ? 0 : sub_c); + + /* Take the one with minimum cost. */ + curr[i] = Min(ins, del); + curr[i] = Min(curr[i], sub); + + /* Point to next character. */ + x++; + } + } + + /* Swap current row with previous row. */ + temp = curr; + curr = prev; + prev = temp; + + /* Point to next character. */ + y += y_char_len; + +#ifdef LEVENSHTEIN_LESS_EQUAL + + /* + * This chunk of code represents a significant performance hit if used + * in the case where there is no max_d bound. This is probably not + * because the max_d >= 0 test itself is expensive, but rather because + * the possibility of needing to execute this code prevents tight + * optimization of the loop as a whole. + */ + if (max_d >= 0) + { + /* + * The "zero point" is the column of the current row where the + * remaining portions of the strings are of equal length. There + * are (n - 1) characters in the target string, of which j have + * been transformed. There are (m - 1) characters in the source + * string, so we want to find the value for zp where (n - 1) - j = + * (m - 1) - zp. + */ + int zp = j - (n - m); + + /* Check whether the stop column can slide left. */ + while (stop_column > 0) + { + int ii = stop_column - 1; + int net_inserts = ii - zp; + + if (prev[ii] + (net_inserts > 0 ? net_inserts * ins_c : + -net_inserts * del_c) <= max_d) + break; + stop_column--; + } + + /* Check whether the start column can slide right. */ + while (start_column < stop_column) + { + int net_inserts = start_column - zp; + + if (prev[start_column] + + (net_inserts > 0 ? net_inserts * ins_c : + -net_inserts * del_c) <= max_d) + break; + + /* + * We'll never again update these values, so we must make sure + * there's nothing here that could confuse any future + * iteration of the outer loop. + */ + prev[start_column] = max_d + 1; + curr[start_column] = max_d + 1; + if (start_column != 0) + source += (s_char_len != NULL) ? s_char_len[start_column - 1] : 1; + start_column++; + } + + /* If they cross, we're going to exceed the bound. */ + if (start_column >= stop_column) + return max_d + 1; + } +#endif + } + + /* + * Because the final value was swapped from the previous row to the + * current row, that's where we'll find it. + */ + return prev[m - 1]; +} -- cgit v1.2.3