{-# LANGUAGE BangPatterns #-} {-# OPTIONS_HADDOCK hide, prune #-} -- | -- Module : Data.ByteString.Search.Internal.BoyerMoore -- Copyright : Daniel Fischer -- Chris Kuklewicz -- Licence : BSD3 -- Maintainer : Daniel Fischer <daniel.is.fischer@googlemail.com> -- Stability : Provisional -- Portability : non-portable (BangPatterns) -- -- Fast overlapping Boyer-Moore search of both strict and lazy -- 'S.ByteString' values. Breaking, splitting and replacing -- using the Boyer-Moore algorithm. -- -- Descriptions of the algorithm can be found at -- <http://www-igm.univ-mlv.fr/~lecroq/string/node14.html#SECTION00140> -- and -- <http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm> -- -- Original authors: Daniel Fischer (daniel.is.fischer at googlemail.com) and -- Chris Kuklewicz (haskell at list.mightyreason.com). module Data.ByteString.Search.Internal.BoyerMoore ( matchLS , matchSS -- Non-overlapping , matchNOS -- Replacing substrings -- replacing , replaceAllS -- Breaking on substrings -- breaking , breakSubstringS , breakAfterS -- Splitting on substrings -- splitting , splitKeepEndS , splitKeepFrontS , splitDropS ) where import Data.ByteString.Search.Internal.Utils (occurs, suffShifts, strictify) import Data.ByteString.Search.Substitution import qualified Data.ByteString as S import qualified Data.ByteString.Lazy as L import qualified Data.ByteString.Lazy.Internal as LI import Data.ByteString.Unsafe (unsafeIndex) import Data.Array.Base (unsafeAt) import Data.Word (Word8) -- overview -- -- This module exports three search functions for searching in strict -- ByteStrings. One for searching non-overlapping occurrences of a strict -- pattern and one each for possibly overlapping occurrences of a lazy -- resp. strict pattern. The common base name is @match@, the suffix -- indicates the type of search to perform. These functions -- return (for a non-empty pattern) a list of all the indices of the target -- string where an occurrence of the pattern begins, if some occurrences -- overlap, all starting indices are reported. The list is produced lazily, -- so not necessarily the entire target string is searched. -- -- The behaviour of these functions when given an empty pattern has changed. -- Formerly, the @matchXY@ functions returned an empty list then, now it's -- @[0 .. 'length' target]@. -- -- Newly added are functions to replace all (non-overlapping) occurrences -- of a pattern within a string, functions to break ByteStrings at the first -- occurrence of a pattern and functions to split ByteStrings at each -- occurrence of a pattern. None of these functions does copying, so they -- don't introduce large memory overhead. -- -- Internally, a lazy pattern is always converted to a strict ByteString, -- which is necessary for an efficient implementation of the algorithm. -- The limit this imposes on the length of the pattern is probably -- irrelevant in practice, but perhaps it should be mentioned. -- This also means that the @matchL*@ functions are mere convenience wrappers. -- Except for the initial 'strictify'ing, there's no difference between lazy -- and strict patterns, they call the same workers. There is, however, a -- difference between strict and lazy target strings. -- For the new functions, no such wrappers are provided, you have to -- 'strictify' lazy patterns yourself. -- caution -- -- When working with a lazy target string, the relation between the pattern -- length and the chunk size can play a big r&#244;le. -- Crossing chunk boundaries is relatively expensive, so when that becomes -- a frequent occurrence, as may happen when the pattern length is close -- to or larger than the chunk size, performance is likely to degrade. -- If it is needed, steps can be taken to ameliorate that effect, but unless -- entirely separate functions are introduced, that would hurt the -- performance for the more common case of patterns much shorter than -- the default chunk size. -- performance -- -- In general, the Boyer-Moore algorithm is the most efficient method to -- search for a pattern inside a string, so most of the time, you'll want -- to use the functions of this module, hence this is where the most work -- has gone. Very short patterns are an exception to this, for those you -- should consider using a finite automaton -- ("Data.ByteString.Search.DFA.Array"). That is also often the better -- choice for searching longer periodic patterns in a lazy ByteString -- with many matches. -- -- Operating on a strict target string is mostly faster than on a lazy -- target string, but the difference is usually small (according to my -- tests). -- -- The known exceptions to this rule of thumb are -- -- [long targets] Then the smaller memory footprint of a lazy target often -- gives (much) better performance. -- -- [high number of matches] When there are very many matches, strict target -- strings are much faster, especially if the pattern is periodic. -- -- If both conditions hold, either may outweigh the other. -- complexity -- -- Preprocessing the pattern is /O/(@patternLength@ + &#963;) in time and -- space (&#963; is the alphabet size, 256 here) for all functions. -- The time complexity of the searching phase for @matchXY@ -- is /O/(@targetLength@ \/ @patternLength@) in the best case. -- For non-periodic patterns, the worst case complexity is -- /O/(@targetLength@), but for periodic patterns, the worst case complexity -- is /O/(@targetLength@ * @patternLength@) for the original Boyer-Moore -- algorithm. -- -- The searching functions in this module now contain a modification which -- drastically improves the performance for periodic patterns. -- I believe that for strict target strings, the worst case is now -- /O/(@targetLength@) also for periodic patterns and for lazy target strings, -- my semi-educated guess is -- /O/(@targetLength@ * (1 + @patternLength@ \/ @chunkSize@)). -- I may be very wrong, though. -- -- The other functions don't have to deal with possible overlapping -- patterns, hence the worst case complexity for the processing phase -- is /O/(@targetLength@) (respectively /O/(@firstIndex + patternLength@) -- for the breaking functions if the pattern occurs). -- currying -- -- These functions can all be usefully curried. Given only a pattern -- the curried version will compute the supporting lookup tables only -- once, allowing for efficient re-use. Similarly, the curried -- 'matchLL' and 'matchLS' will compute the concatenated pattern only -- once. -- overflow -- -- The current code uses @Int@ to keep track of the locations in the -- target string. If the length of the pattern plus the length of any -- strict chunk of the target string is greater than -- @'maxBound' :: 'Int'@ then this will overflow causing an error. We -- try to detect this and call 'error' before a segfault occurs. ------------------------------------------------------------------------------ -- Wrappers -- ------------------------------------------------------------------------------ -- matching -- -- These functions find the indices of all (possibly overlapping) -- occurrences of a pattern in a target string. -- If the pattern is empty, the result is @[0 .. length target]@. -- If the pattern is much shorter than the target string -- and the pattern does not occur very near the beginning of the target, -- -- > not . null $ matchSS pattern target -- -- is a much more efficient version of 'S.isInfixOf'. -- | @'matchLS'@ finds the starting indices of all possibly overlapping -- occurrences of the pattern in the target string. -- It is a simple wrapper for 'Data.ByteString.Search.indices'. -- If the pattern is empty, the result is @[0 .. 'length' target]@. {-# INLINE matchLS #-} matchLS :: L.ByteString -- ^ Lazy pattern -> S.ByteString -- ^ Strict target string -> [Int] -- ^ Offsets of matches matchLS pat = search where search = strictSearcher True (strictify pat) -- | @'matchSS'@ finds the starting indices of all possibly overlapping -- occurrences of the pattern in the target string. -- It is an alias for 'Data.ByteString.Search.indices'. -- If the pattern is empty, the result is @[0 .. 'length' target]@. {-# INLINE matchSS #-} matchSS :: S.ByteString -- ^ Strict pattern -> S.ByteString -- ^ Strict target string -> [Int] -- ^ Offsets of matches matchSS pat = search where search = strictSearcher True pat -- | @'matchNOS'@ finds the indices of all non-overlapping occurrences -- of the pattern in the Strict target string. {-# INLINE matchNOS #-} matchNOS :: S.ByteString -- ^ Strict pattern -> S.ByteString -- ^ Strict target string -> [Int] -- ^ Offsets of matches matchNOS pat = search where search = strictSearcher False pat -- replacing -- -- These functions replace all (non-overlapping) occurrences of a pattern -- in the target string. If some occurrences overlap, the earliest is -- replaced and replacing continues at the index after the replaced -- occurrence, for example -- -- > replaceAllL \"ana\" \"olog\" \"banana\" == \"bologna\", -- > replaceAllS \"abacab\" \"u\" \"abacabacabacab\" == \"uacu\", -- > replaceAllS \"aa\" \"aaa\" \"aaaa\" == \"aaaaaa\". -- -- Equality of pattern and substitution is not checked, but -- -- > pat == sub => 'strictify' (replaceAllS pat sub str) == str, -- > pat == sub => replaceAllL pat sub str == str. -- -- The result is a lazily generated lazy ByteString, the first chunks will -- generally be available before the entire target has been scanned. -- If the pattern is empty, but not the substitution, the result is -- equivalent to @'cycle' sub@. {-# INLINE replaceAllS #-} replaceAllS :: Substitution rep => S.ByteString -- ^ Pattern to replace -> rep -- ^ Substitution string -> S.ByteString -- ^ Target string -> L.ByteString -- ^ Lazy result replaceAllS pat | S.null pat = \sub -> prependCycle sub . flip LI.chunk LI.Empty | otherwise = let repl = strictRepl pat in \sub -> L.fromChunks . repl (substitution sub) -- breaking -- -- Break a string on a pattern. The first component of the result -- contains the prefix of the string before the first occurrence of the -- pattern, the second component contains the remainder. -- The following relations hold: -- -- > breakSubstringX \"\" str = (\"\", str) -- > not (pat `isInfixOf` str) == null (snd $ breakSunbstringX pat str) -- > True == case breakSubstringX pat str of -- > (x, y) -> not (pat `isInfixOf` x) -- > && (null y || pat `isPrefixOf` y) -- | This function has the same semantics as 'S.breakSubstring' -- but is generally much faster. {-# INLINE breakSubstringS #-} breakSubstringS :: S.ByteString -- ^ Pattern to break on -> S.ByteString -- ^ String to break up -> (S.ByteString, S.ByteString) -- ^ Prefix and remainder of broken string breakSubstringS = strictBreak breakAfterS :: S.ByteString -> S.ByteString -> (S.ByteString, S.ByteString) breakAfterS pat | S.null pat = \str -> (S.empty, str) breakAfterS pat = breaker where !patLen = S.length pat searcher = strictSearcher False pat breaker str = case searcher str of [] -> (str, S.empty) (i:_) -> S.splitAt (i + patLen) str -- splitting -- -- These functions implement various splitting strategies. -- -- If the pattern to split on is empty, all functions return an -- infinite list of empty ByteStrings. -- Otherwise, the names are rather self-explanatory. -- -- For nonempty patterns, the following relations hold: -- -- > concat (splitKeepXY pat str) == str -- > concat ('Data.List.intersperse' pat (splitDropX pat str)) == str. -- -- All fragments except possibly the last in the result of -- @splitKeepEndX pat@ end with @pat@, none of the fragments contains -- more than one occurrence of @pat@ or is empty. -- -- All fragments except possibly the first in the result of -- @splitKeepFrontX pat@ begin with @pat@, none of the fragments -- contains more than one occurrence of @patq or is empty. -- -- > splitDropX pat str == map dropPat (splitKeepFrontX pat str) -- > where -- > patLen = length pat -- > dropPat frag -- > | pat `isPrefixOf` frag = drop patLen frag -- > | otherwise = frag -- -- but @splitDropX@ is a little more efficient than that. {-# INLINE splitKeepEndS #-} splitKeepEndS :: S.ByteString -- ^ Pattern to split on -> S.ByteString -- ^ String to split -> [S.ByteString] -- ^ List of fragments splitKeepEndS = strictSplitKeepEnd {-# INLINE splitKeepFrontS #-} splitKeepFrontS :: S.ByteString -- ^ Pattern to split on -> S.ByteString -- ^ String to split -> [S.ByteString] -- ^ List of fragments splitKeepFrontS = strictSplitKeepFront {-# INLINE splitDropS #-} splitDropS :: S.ByteString -- ^ Pattern to split on -> S.ByteString -- ^ String to split -> [S.ByteString] -- ^ List of fragments splitDropS = strictSplitDrop ------------------------------------------------------------------------------ -- Search Functions -- ------------------------------------------------------------------------------ strictSearcher :: Bool -> S.ByteString -> S.ByteString -> [Int] strictSearcher _ !pat | S.null pat = enumFromTo 0 . S.length | S.length pat == 1 = let !w = S.head pat in S.elemIndices w strictSearcher !overlap pat = searcher where {-# INLINE patAt #-} patAt :: Int -> Word8 patAt !i = unsafeIndex pat i !patLen = S.length pat !patEnd = patLen - 1 !maxLen = maxBound - patLen !occT = occurs pat -- for bad-character-shift !suffT = suffShifts pat -- for good-suffix-shift !skip = if overlap then unsafeAt suffT 0 else patLen -- shift after a complete match !kept = patLen - skip -- length of known prefix after full match !pe = patAt patEnd -- last pattern byte for fast comparison {-# INLINE occ #-} occ !w = unsafeAt occT (fromIntegral w) {-# INLINE suff #-} suff !i = unsafeAt suffT i searcher str | maxLen < strLen = error "Overflow in BoyerMoore.strictSearcher" | maxDiff < 0 = [] | otherwise = checkEnd patEnd where !strLen = S.length str !strEnd = strLen - 1 !maxDiff = strLen - patLen {-# INLINE strAt #-} strAt !i = unsafeIndex str i -- After a full match, we know how long a prefix of the pattern -- still matches. Do not re-compare the prefix to prevent O(m*n) -- behaviour for periodic patterns. afterMatch !diff !patI = case strAt (diff + patI) of !c | c == patAt patI -> if patI == kept then diff : let !diff' = diff + skip in if maxDiff < diff' then [] else afterMatch diff' patEnd else afterMatch diff (patI - 1) | patI == patEnd -> checkEnd (diff + 2*patEnd + occ c) | otherwise -> let {-# INLINE badShift #-} badShift = patI + occ c {-# INLINE goodShift #-} goodShift = suff patI !diff' = diff + max badShift goodShift in if maxDiff < diff' then [] else checkEnd (diff + patEnd) -- While comparing the last byte of the pattern, the bad- -- character-shift is always at least as large as the good- -- suffix-shift. Eliminating the unnecessary memory reads and -- comparison speeds things up noticeably. checkEnd !sI -- index in string to compare to last of pattern | strEnd < sI = [] | otherwise = case strAt sI of !c | c == pe -> findMatch (sI - patEnd) (patEnd - 1) | otherwise -> checkEnd (sI + patEnd + occ c) -- Once the last byte has matched, we enter the full matcher -- diff is the offset of the window, patI the index of the -- pattern byte to compare next. findMatch !diff !patI = case strAt (diff + patI) of !c | c == patAt patI -> if patI == 0 -- full match, report then diff : let !diff' = diff + skip in if maxDiff < diff' then [] else if skip == patLen then checkEnd (diff' + patEnd) else afterMatch diff' patEnd else findMatch diff (patI - 1) | otherwise -> let !diff' = diff + max (patI + occ c) (suff patI) in if maxDiff < diff' then [] else checkEnd (diff' + patEnd) ------------------------------------------------------------------------------ -- Breaking Functions -- ------------------------------------------------------------------------------ strictBreak :: S.ByteString -> S.ByteString -> (S.ByteString, S.ByteString) strictBreak pat | S.null pat = \str -> (S.empty, str) | otherwise = breaker where searcher = strictSearcher False pat breaker str = case searcher str of [] -> (str, S.empty) (i:_) -> S.splitAt i str ------------------------------------------------------------------------------ -- Splitting Functions -- ------------------------------------------------------------------------------ strictSplitKeepFront :: S.ByteString -> S.ByteString -> [S.ByteString] strictSplitKeepFront pat | S.null pat = const (repeat S.empty) strictSplitKeepFront pat = splitter where !patLen = S.length pat searcher = strictSearcher False pat splitter str | S.null str = [] | otherwise = case searcher str of [] -> [str] (i:_) | i == 0 -> psplitter str | otherwise -> S.take i str : psplitter (S.drop i str) psplitter !str | S.null str = [] | otherwise = case searcher (S.drop patLen str) of [] -> [str] (i:_) -> S.take (i + patLen) str : psplitter (S.drop (i + patLen) str) strictSplitKeepEnd :: S.ByteString -> S.ByteString -> [S.ByteString] strictSplitKeepEnd pat | S.null pat = const (repeat S.empty) strictSplitKeepEnd pat = splitter where !patLen = S.length pat searcher = strictSearcher False pat splitter str | S.null str = [] | otherwise = case searcher str of [] -> [str] (i:_) -> S.take (i + patLen) str : splitter (S.drop (i + patLen) str) strictSplitDrop :: S.ByteString -> S.ByteString -> [S.ByteString] strictSplitDrop pat | S.null pat = const (repeat S.empty) strictSplitDrop pat = splitter' where !patLen = S.length pat searcher = strictSearcher False pat splitter' str | S.null str = [] | otherwise = splitter str splitter str | S.null str = [S.empty] | otherwise = case searcher str of [] -> [str] (i:_) -> S.take i str : splitter (S.drop (i + patLen) str) ------------------------------------------------------------------------------ -- Replacing Functions -- ------------------------------------------------------------------------------ -- replacing loop for strict ByteStrings, called only for -- non-empty patterns and substitutions strictRepl :: S.ByteString -> ([S.ByteString] -> [S.ByteString]) -> S.ByteString -> [S.ByteString] strictRepl pat = repl where !patLen = S.length pat searcher = strictSearcher False pat repl sub = replacer where replacer str | S.null str = [] | otherwise = case searcher str of [] -> [str] (i:_) | i == 0 -> sub $ replacer (S.drop patLen str) | otherwise -> S.take i str : sub (replacer (S.drop (i + patLen) str))