mirror of https://github.com/postgres/postgres
Alexander Korotkov, heavily revised by me.pull/1/head
Robert Haas
15 years ago
parent
262c1a42dc
commit
604ab08145
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/*
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* levenshtein.c |
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* |
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* Functions for "fuzzy" comparison of strings |
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* |
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* Joe Conway <mail@joeconway.com> |
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* |
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* contrib/fuzzystrmatch/fuzzystrmatch.c |
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* Copyright (c) 2001-2010, PostgreSQL Global Development Group |
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* ALL RIGHTS RESERVED; |
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* |
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* levenshtein() |
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* ------------- |
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* Written based on a description of the algorithm by Michael Gilleland |
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* found at http://www.merriampark.com/ld.htm
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* Also looked at levenshtein.c in the PHP 4.0.6 distribution for |
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* inspiration. |
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* Configurable penalty costs extension is introduced by Volkan |
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* YAZICI <volkan.yazici@gmail.com>. |
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*/ |
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/*
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* External declarations for exported functions |
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*/ |
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#ifdef LEVENSHTEIN_LESS_EQUAL |
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static int levenshtein_less_equal_internal(text *s, text *t, |
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int ins_c, int del_c, int sub_c, int max_d); |
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#else |
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static int levenshtein_internal(text *s, text *t, |
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int ins_c, int del_c, int sub_c); |
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#endif |
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#define MAX_LEVENSHTEIN_STRLEN 255 |
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/*
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* Calculates Levenshtein distance metric between supplied strings. Generally |
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* (1, 1, 1) penalty costs suffices for common cases, but your mileage may |
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* vary. |
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* |
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* One way to compute Levenshtein distance is to incrementally construct |
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* an (m+1)x(n+1) matrix where cell (i, j) represents the minimum number |
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* of operations required to transform the first i characters of s into |
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* the first j characters of t. The last column of the final row is the |
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* answer. |
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* |
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* We use that algorithm here with some modification. In lieu of holding |
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* the entire array in memory at once, we'll just use two arrays of size |
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* m+1 for storing accumulated values. At each step one array represents |
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* the "previous" row and one is the "current" row of the notional large |
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* array. |
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* |
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* If max_d >= 0, we only need to provide an accurate answer when that answer |
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* is less than or equal to the bound. From any cell in the matrix, there is |
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* theoretical "minimum residual distance" from that cell to the last column |
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* of the final row. This minimum residual distance is zero when the |
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* untransformed portions of the strings are of equal length (because we might |
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* get lucky and find all the remaining characters matching) and is otherwise |
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* based on the minimum number of insertions or deletions needed to make them |
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* equal length. The residual distance grows as we move toward the upper |
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* right or lower left corners of the matrix. When the max_d bound is |
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* usefully tight, we can use this property to avoid computing the entirety |
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* of each row; instead, we maintain a start_column and stop_column that |
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* identify the portion of the matrix close to the diagonal which can still |
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* affect the final answer. |
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*/ |
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static int |
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#ifdef LEVENSHTEIN_LESS_EQUAL |
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levenshtein_less_equal_internal(text *s, text *t, |
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int ins_c, int del_c, int sub_c, int max_d) |
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#else |
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levenshtein_internal(text *s, text *t, |
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int ins_c, int del_c, int sub_c) |
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#endif |
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{ |
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int m, |
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n, |
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s_bytes, |
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t_bytes; |
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int *prev; |
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int *curr; |
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int *s_char_len = NULL; |
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int i, |
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j; |
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const char *s_data; |
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const char *t_data; |
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const char *y; |
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/*
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* For levenshtein_less_equal_internal, we have real variables called |
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* start_column and stop_column; otherwise it's just short-hand for 0 |
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* and m. |
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*/ |
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#ifdef LEVENSHTEIN_LESS_EQUAL |
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int start_column, stop_column; |
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#undef START_COLUMN |
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#undef STOP_COLUMN |
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#define START_COLUMN start_column |
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#define STOP_COLUMN stop_column |
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#else |
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#undef START_COLUMN |
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#undef STOP_COLUMN |
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#define START_COLUMN 0 |
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#define STOP_COLUMN m |
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#endif |
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/* Extract a pointer to the actual character data. */ |
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s_data = VARDATA_ANY(s); |
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t_data = VARDATA_ANY(t); |
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/* Determine length of each string in bytes and characters. */ |
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s_bytes = VARSIZE_ANY_EXHDR(s); |
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t_bytes = VARSIZE_ANY_EXHDR(t); |
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m = pg_mbstrlen_with_len(s_data, s_bytes); |
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n = pg_mbstrlen_with_len(t_data, t_bytes); |
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/*
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* We can transform an empty s into t with n insertions, or a non-empty t |
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* into an empty s with m deletions. |
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*/ |
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if (!m) |
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return n * ins_c; |
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if (!n) |
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return m * del_c; |
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/*
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* For security concerns, restrict excessive CPU+RAM usage. (This |
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* implementation uses O(m) memory and has O(mn) complexity.) |
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*/ |
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if (m > MAX_LEVENSHTEIN_STRLEN || |
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n > MAX_LEVENSHTEIN_STRLEN) |
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ereport(ERROR, |
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(errcode(ERRCODE_INVALID_PARAMETER_VALUE), |
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errmsg("argument exceeds the maximum length of %d bytes", |
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MAX_LEVENSHTEIN_STRLEN))); |
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#ifdef LEVENSHTEIN_LESS_EQUAL |
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/* Initialize start and stop columns. */ |
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start_column = 0; |
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stop_column = m + 1; |
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/*
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* If max_d >= 0, determine whether the bound is impossibly tight. If so, |
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* return max_d + 1 immediately. Otherwise, determine whether it's tight |
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* enough to limit the computation we must perform. If so, figure out |
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* initial stop column. |
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*/ |
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if (max_d >= 0) |
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{ |
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int min_theo_d; /* Theoretical minimum distance. */ |
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int max_theo_d; /* Theoretical maximum distance. */ |
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int net_inserts = n - m; |
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min_theo_d = net_inserts < 0 ? |
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-net_inserts * del_c : net_inserts * ins_c; |
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if (min_theo_d > max_d) |
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return max_d + 1; |
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if (ins_c + del_c < sub_c) |
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sub_c = ins_c + del_c; |
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max_theo_d = min_theo_d + sub_c * Min(m, n); |
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if (max_d >= max_theo_d) |
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max_d = -1; |
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else if (ins_c + del_c > 0) |
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{ |
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/*
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* Figure out how much of the first row of the notional matrix |
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* we need to fill in. If the string is growing, the theoretical |
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* minimum distance already incorporates the cost of deleting the |
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* number of characters necessary to make the two strings equal |
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* in length. Each additional deletion forces another insertion, |
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* so the best-case total cost increases by ins_c + del_c. |
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* If the string is shrinking, the minimum theoretical cost |
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* assumes no excess deletions; that is, we're starting no futher |
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* right than column n - m. If we do start further right, the |
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* best-case total cost increases by ins_c + del_c for each move |
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* right. |
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*/ |
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int slack_d = max_d - min_theo_d; |
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int best_column = net_inserts < 0 ? -net_inserts : 0; |
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stop_column = best_column + (slack_d / (ins_c + del_c)) + 1; |
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if (stop_column > m) |
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stop_column = m + 1; |
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} |
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} |
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#endif |
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/*
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* In order to avoid calling pg_mblen() repeatedly on each character in s, |
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* we cache all the lengths before starting the main loop -- but if all the |
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* characters in both strings are single byte, then we skip this and use |
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* a fast-path in the main loop. If only one string contains multi-byte |
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* characters, we still build the array, so that the fast-path needn't |
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* deal with the case where the array hasn't been initialized. |
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*/ |
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if (m != s_bytes || n != t_bytes) |
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{ |
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int i; |
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const char *cp = s_data; |
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s_char_len = (int *) palloc((m + 1) * sizeof(int)); |
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for (i = 0; i < m; ++i) |
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{ |
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s_char_len[i] = pg_mblen(cp); |
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cp += s_char_len[i]; |
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} |
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s_char_len[i] = 0; |
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} |
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/* One more cell for initialization column and row. */ |
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++m; |
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++n; |
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/* Previous and current rows of notional array. */ |
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prev = (int *) palloc(2 * m * sizeof(int)); |
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curr = prev + m; |
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/*
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* To transform the first i characters of s into the first 0 characters |
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* of t, we must perform i deletions. |
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*/ |
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for (i = START_COLUMN; i < STOP_COLUMN; i++) |
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prev[i] = i * del_c; |
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/* Loop through rows of the notional array */ |
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for (y = t_data, j = 1; j < n; j++) |
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{ |
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int *temp; |
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const char *x = s_data; |
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int y_char_len = n != t_bytes + 1 ? pg_mblen(y) : 1; |
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#ifdef LEVENSHTEIN_LESS_EQUAL |
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/*
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* In the best case, values percolate down the diagonal unchanged, so |
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* we must increment stop_column unless it's already on the right end |
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* of the array. The inner loop will read prev[stop_column], so we |
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* have to initialize it even though it shouldn't affect the result. |
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*/ |
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if (stop_column < m) |
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{ |
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prev[stop_column] = max_d + 1; |
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++stop_column; |
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} |
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/*
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* The main loop fills in curr, but curr[0] needs a special case: |
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* to transform the first 0 characters of s into the first j |
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* characters of t, we must perform j insertions. However, if |
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* start_column > 0, this special case does not apply. |
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*/ |
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if (start_column == 0) |
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{ |
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curr[0] = j * ins_c; |
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i = 1; |
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} |
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else |
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i = start_column; |
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#else |
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curr[0] = j * ins_c; |
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i = 1; |
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#endif |
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/*
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* This inner loop is critical to performance, so we include a |
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* fast-path to handle the (fairly common) case where no multibyte |
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* characters are in the mix. The fast-path is entitled to assume |
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* that if s_char_len is not initialized then BOTH strings contain |
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* only single-byte characters. |
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*/ |
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if (s_char_len != NULL) |
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{ |
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for (; i < STOP_COLUMN; i++) |
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{ |
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int ins; |
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int del; |
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int sub; |
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int x_char_len = s_char_len[i - 1]; |
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/*
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* Calculate costs for insertion, deletion, and substitution. |
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* |
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* When calculating cost for substitution, we compare the last |
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* character of each possibly-multibyte character first, |
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* because that's enough to rule out most mis-matches. If we |
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* get past that test, then we compare the lengths and the |
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* remaining bytes. |
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*/ |
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ins = prev[i] + ins_c; |
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del = curr[i - 1] + del_c; |
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if (x[x_char_len-1] == y[y_char_len-1] |
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&& x_char_len == y_char_len && |
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(x_char_len == 1 || rest_of_char_same(x, y, x_char_len))) |
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sub = prev[i - 1]; |
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else |
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sub = prev[i - 1] + sub_c; |
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/* Take the one with minimum cost. */ |
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curr[i] = Min(ins, del); |
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curr[i] = Min(curr[i], sub); |
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/* Point to next character. */ |
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x += x_char_len; |
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} |
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} |
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else |
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{ |
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for (; i < STOP_COLUMN; i++) |
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{ |
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int ins; |
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int del; |
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int sub; |
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/* Calculate costs for insertion, deletion, and substitution. */ |
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ins = prev[i] + ins_c; |
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del = curr[i - 1] + del_c; |
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sub = prev[i - 1] + ((*x == *y) ? 0 : sub_c); |
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/* Take the one with minimum cost. */ |
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curr[i] = Min(ins, del); |
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curr[i] = Min(curr[i], sub); |
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/* Point to next character. */ |
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x++; |
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} |
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} |
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/* Swap current row with previous row. */ |
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temp = curr; |
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curr = prev; |
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prev = temp; |
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/* Point to next character. */ |
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y += y_char_len; |
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#ifdef LEVENSHTEIN_LESS_EQUAL |
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/*
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* This chunk of code represents a significant performance hit if used |
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* in the case where there is no max_d bound. This is probably not |
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* because the max_d >= 0 test itself is expensive, but rather because |
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* the possibility of needing to execute this code prevents tight |
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* optimization of the loop as a whole. |
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*/ |
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if (max_d >= 0) |
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{ |
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/*
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* The "zero point" is the column of the current row where the |
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* remaining portions of the strings are of equal length. There |
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* are (n - 1) characters in the target string, of which j have |
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* been transformed. There are (m - 1) characters in the source |
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* string, so we want to find the value for zp where where (n - 1) |
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* - j = (m - 1) - zp. |
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*/ |
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int zp = j - (n - m); |
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/* Check whether the stop column can slide left. */ |
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while (stop_column > 0) |
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{ |
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int ii = stop_column - 1; |
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int net_inserts = ii - zp; |
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if (prev[ii] + (net_inserts > 0 ? net_inserts * ins_c : |
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-net_inserts * del_c) <= max_d) |
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break; |
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stop_column--; |
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} |
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/* Check whether the start column can slide right. */ |
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while (start_column < stop_column) |
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{ |
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int net_inserts = start_column - zp; |
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if (prev[start_column] + |
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(net_inserts > 0 ? net_inserts * ins_c : |
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-net_inserts * del_c) <= max_d) |
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break; |
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/*
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* We'll never again update these values, so we must make |
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* sure there's nothing here that could confuse any future |
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* iteration of the outer loop. |
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*/ |
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prev[start_column] = max_d + 1; |
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curr[start_column] = max_d + 1; |
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if (start_column != 0) |
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s_data += n != t_bytes + 1 ? pg_mblen(s_data) : 1; |
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start_column++; |
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} |
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/* If they cross, we're going to exceed the bound. */ |
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if (start_column >= stop_column) |
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return max_d + 1; |
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} |
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#endif |
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} |
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/*
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* Because the final value was swapped from the previous row to the |
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* current row, that's where we'll find it. |
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*/ |
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return prev[m - 1]; |
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} |
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