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
@ -0,0 +1,397 @@ |
||||
/*
|
||||
* levenshtein.c |
||||
* |
||||
* Functions for "fuzzy" comparison of strings |
||||
* |
||||
* Joe Conway <mail@joeconway.com> |
||||
* |
||||
* contrib/fuzzystrmatch/fuzzystrmatch.c |
||||
* Copyright (c) 2001-2010, PostgreSQL Global Development Group |
||||
* ALL RIGHTS RESERVED; |
||||
* |
||||
* levenshtein() |
||||
* ------------- |
||||
* 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 <volkan.yazici@gmail.com>. |
||||
*/ |
||||
|
||||
/*
|
||||
* External declarations for exported functions |
||||
*/ |
||||
#ifdef LEVENSHTEIN_LESS_EQUAL |
||||
static int levenshtein_less_equal_internal(text *s, text *t, |
||||
int ins_c, int del_c, int sub_c, int max_d); |
||||
#else |
||||
static int levenshtein_internal(text *s, text *t, |
||||
int ins_c, int del_c, int sub_c); |
||||
#endif |
||||
|
||||
#define MAX_LEVENSHTEIN_STRLEN 255 |
||||
|
||||
|
||||
/*
|
||||
* Calculates Levenshtein distance metric between supplied strings. Generally |
||||
* (1, 1, 1) penalty costs suffices for common cases, but your mileage may |
||||
* vary. |
||||
* |
||||
* 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 the bound. 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. |
||||
*/ |
||||
static int |
||||
#ifdef LEVENSHTEIN_LESS_EQUAL |
||||
levenshtein_less_equal_internal(text *s, text *t, |
||||
int ins_c, int del_c, int sub_c, int max_d) |
||||
#else |
||||
levenshtein_internal(text *s, text *t, |
||||
int ins_c, int del_c, int sub_c) |
||||
#endif |
||||
{ |
||||
int m, |
||||
n, |
||||
s_bytes, |
||||
t_bytes; |
||||
int *prev; |
||||
int *curr; |
||||
int *s_char_len = NULL; |
||||
int i, |
||||
j; |
||||
const char *s_data; |
||||
const char *t_data; |
||||
const char *y; |
||||
|
||||
/*
|
||||
* For levenshtein_less_equal_internal, 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 |
||||
|
||||
/* Extract a pointer to the actual character data. */ |
||||
s_data = VARDATA_ANY(s); |
||||
t_data = VARDATA_ANY(t); |
||||
|
||||
/* Determine length of each string in bytes and characters. */ |
||||
s_bytes = VARSIZE_ANY_EXHDR(s); |
||||
t_bytes = VARSIZE_ANY_EXHDR(t); |
||||
m = pg_mbstrlen_with_len(s_data, s_bytes); |
||||
n = pg_mbstrlen_with_len(t_data, t_bytes); |
||||
|
||||
/*
|
||||
* 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 (m > MAX_LEVENSHTEIN_STRLEN || |
||||
n > MAX_LEVENSHTEIN_STRLEN) |
||||
ereport(ERROR, |
||||
(errcode(ERRCODE_INVALID_PARAMETER_VALUE), |
||||
errmsg("argument exceeds the maximum length of %d bytes", |
||||
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 futher |
||||
* 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 != s_bytes || n != t_bytes) |
||||
{ |
||||
int i; |
||||
const char *cp = s_data; |
||||
|
||||
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 = t_data, j = 1; j < n; j++) |
||||
{ |
||||
int *temp; |
||||
const char *x = s_data; |
||||
int y_char_len = n != t_bytes + 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 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) |
||||
s_data += n != t_bytes + 1 ? pg_mblen(s_data) : 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]; |
||||
} |
||||
Loading…
Reference in new issue