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Diffstat (limited to 'lib/str-kmp.h')
| -rw-r--r-- | lib/str-kmp.h | 153 |
1 files changed, 153 insertions, 0 deletions
diff --git a/lib/str-kmp.h b/lib/str-kmp.h new file mode 100644 index 00000000..06af7c96 --- /dev/null +++ b/lib/str-kmp.h @@ -0,0 +1,153 @@ +/* Substring search in a NUL terminated string of UNIT elements, + using the Knuth-Morris-Pratt algorithm. + Copyright (C) 2005-2015 Free Software Foundation, Inc. + Written by Bruno Haible <bruno@clisp.org>, 2005. + + This program is free software; you can redistribute it and/or modify + it under the terms of the GNU Lesser General Public License as published by + the Free Software Foundation; either version 2, or (at your option) + any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public License + along with this program; if not, see <http://www.gnu.org/licenses/>. */ + +/* Before including this file, you need to define: + UNIT The element type of the needle and haystack. + CANON_ELEMENT(c) A macro that canonicalizes an element right after + it has been fetched from needle or haystack. + The argument is of type UNIT; the result must be + of type UNIT as well. */ + +/* Knuth-Morris-Pratt algorithm. + See http://en.wikipedia.org/wiki/Knuth-Morris-Pratt_algorithm + HAYSTACK is the NUL terminated string in which to search for. + NEEDLE is the string to search for in HAYSTACK, consisting of NEEDLE_LEN + units. + Return a boolean indicating success: + Return true and set *RESULTP if the search was completed. + Return false if it was aborted because not enough memory was available. */ +static bool +knuth_morris_pratt (const UNIT *haystack, + const UNIT *needle, size_t needle_len, + const UNIT **resultp) +{ + size_t m = needle_len; + + /* Allocate the table. */ + size_t *table = (size_t *) nmalloca (m, sizeof (size_t)); + if (table == NULL) + return false; + /* Fill the table. + For 0 < i < m: + 0 < table[i] <= i is defined such that + forall 0 < x < table[i]: needle[x..i-1] != needle[0..i-1-x], + and table[i] is as large as possible with this property. + This implies: + 1) For 0 < i < m: + If table[i] < i, + needle[table[i]..i-1] = needle[0..i-1-table[i]]. + 2) For 0 < i < m: + rhaystack[0..i-1] == needle[0..i-1] + and exists h, i <= h < m: rhaystack[h] != needle[h] + implies + forall 0 <= x < table[i]: rhaystack[x..x+m-1] != needle[0..m-1]. + table[0] remains uninitialized. */ + { + size_t i, j; + + /* i = 1: Nothing to verify for x = 0. */ + table[1] = 1; + j = 0; + + for (i = 2; i < m; i++) + { + /* Here: j = i-1 - table[i-1]. + The inequality needle[x..i-1] != needle[0..i-1-x] is known to hold + for x < table[i-1], by induction. + Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1]. */ + UNIT b = CANON_ELEMENT (needle[i - 1]); + + for (;;) + { + /* Invariants: The inequality needle[x..i-1] != needle[0..i-1-x] + is known to hold for x < i-1-j. + Furthermore, if j>0: needle[i-1-j..i-2] = needle[0..j-1]. */ + if (b == CANON_ELEMENT (needle[j])) + { + /* Set table[i] := i-1-j. */ + table[i] = i - ++j; + break; + } + /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds + for x = i-1-j, because + needle[i-1] != needle[j] = needle[i-1-x]. */ + if (j == 0) + { + /* The inequality holds for all possible x. */ + table[i] = i; + break; + } + /* The inequality needle[x..i-1] != needle[0..i-1-x] also holds + for i-1-j < x < i-1-j+table[j], because for these x: + needle[x..i-2] + = needle[x-(i-1-j)..j-1] + != needle[0..j-1-(x-(i-1-j))] (by definition of table[j]) + = needle[0..i-2-x], + hence needle[x..i-1] != needle[0..i-1-x]. + Furthermore + needle[i-1-j+table[j]..i-2] + = needle[table[j]..j-1] + = needle[0..j-1-table[j]] (by definition of table[j]). */ + j = j - table[j]; + } + /* Here: j = i - table[i]. */ + } + } + + /* Search, using the table to accelerate the processing. */ + { + size_t j; + const UNIT *rhaystack; + const UNIT *phaystack; + + *resultp = NULL; + j = 0; + rhaystack = haystack; + phaystack = haystack; + /* Invariant: phaystack = rhaystack + j. */ + while (*phaystack != 0) + if (CANON_ELEMENT (needle[j]) == CANON_ELEMENT (*phaystack)) + { + j++; + phaystack++; + if (j == m) + { + /* The entire needle has been found. */ + *resultp = rhaystack; + break; + } + } + else if (j > 0) + { + /* Found a match of needle[0..j-1], mismatch at needle[j]. */ + rhaystack += table[j]; + j -= table[j]; + } + else + { + /* Found a mismatch at needle[0] already. */ + rhaystack++; + phaystack++; + } + } + + freea (table); + return true; +} + +#undef CANON_ELEMENT |