Comments on: String Search: Boyer-Moore

By: Vikas Tandi

Vikas Tandi — Mon, 09 May 2011 06:28:02 +0000

Here is my code in C [sourcecode lang="cpp"] #include /* Boyer–Moore–Horspool algorithm */ int string_index_3(char *s, int str_size, char *p, int pattern_size) { int i, j; int bad_char_shift[UCHAR_MAX+1]; /* sanity check */ if(!s || !p || pattern_size <= 0) return -1; /* initialize shift table */ for(i = 0; i <= UCHAR_MAX; i++) bad_char_shift[i] = pattern_size; /* build shift table */ for(i = pattern_size-2; i >= 0; i--) if(bad_char_shift[p[i]] == pattern_size) bad_char_shift[p[i]] = pattern_size - i - 1; /* search pattern */ for(i = 0; i < str_size;) { /* check the substring from right to left */ for(j = pattern_size -1; p[j] == s[i+j]; j--) if(j == 0) return i+1; /* move right using skip table */ i += bad_char_shift[s[i+pattern_size-1]]; } return -1; } [/sourcecode]

By: Remco Niemeijer

Remco Niemeijer — Sat, 29 Aug 2009 20:18:42 +0000

My Haskell solution (see http://bonsaicode.wordpress.com/2009/08/29/programming-praxis-string-search-boyer-moore/ for a version with comments):

import Data.Map (findWithDefault, fromList, (!))

horspool :: Ord a => [a] -> Maybe Int -> [a] -> Maybe Int
horspool pat skip xs = f (lp - 1 + maybe 0 id skip) p' where
    (lp, lxs, p') = (length pat, length xs, reverse pat)
    t = fromList $ zip pat [lp - 1, lp - 2..]
    m = fromList $ zip [0..] xs
    f n []     = Just (n + 1)
    f n (p:ps) | n >= lxs   = Nothing
	           | p == m ! n = f (n - 1) ps
               | otherwise  = f (n + findWithDefault lp (m ! n) t) p'

By: Programming Praxis – String Search: Boyer-Moore « Bonsai Code

Programming Praxis – String Search: Boyer-Moore « Bonsai Code — Sat, 29 Aug 2009 20:18:18 +0000

[...] Praxis – String Search: Boyer-Moore By Remco Niemeijer In yesterday’s Programming Praxis problem we have to implement a more efficient string search algorithm than the [...]

By: Connochaetes

Connochaetes — Fri, 28 Aug 2009 13:32:09 +0000

[sourcecode lang='ruby'] Maxchar = 256 def preprocess(pattern) length = pattern.length skip = Array.new(Maxchar, length) index = -1 pattern.each_byte do |b| skip[b] = length - (index += 1) - 1 end skip end def horspool_search(str, pattern) m, n = pattern.length, str.length skip = preprocess(pattern) k, j = m-1, m-1 while k < n i = k while j>=0 and str[i] == pattern[j] i -=1; j -= 1 end return i+1 if j == -1 k += skip[str[k]] end nil end [/sourcecode]