Data.ByteString.Lazy.Search

stringsearch-0.3.6.6: Fast searching, splitting and replacing of ByteStrings

Copyright | Daniel Fischer Chris Kuklewicz
License | BSD3
Maintainer | Daniel Fischer <daniel.is.fischer@googlemail.com>
Stability | Provisional
Portability | non-portable (BangPatterns)
Safe Haskell | None
Language | Haskell98

Data.ByteString.Lazy.Search

Contents

- Overview
  - Performance
  - Caution
  - Complexity
  - Partial application
  - Integer overflow
- Finding substrings
- Breaking on substrings
- Replacing
- Splitting
- Convenience

Description

Fast overlapping Boyer-Moore search of lazy `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).

Synopsis

- indices :: ByteString -> ByteString -> [Int64]
- nonOverlappingIndices :: ByteString -> ByteString -> [Int64]
- breakOn :: ByteString -> ByteString -> (ByteString, ByteString)
- breakAfter :: ByteString -> ByteString -> (ByteString, ByteString)
- breakFindAfter :: ByteString -> ByteString -> ((ByteString, ByteString), Bool)
- replace :: Substitution rep => ByteString -> rep -> ByteString -> ByteString
- split :: ByteString -> ByteString -> [ByteString]
- splitKeepEnd :: ByteString -> ByteString -> [ByteString]
- splitKeepFront :: ByteString -> ByteString -> [ByteString]
- strictify :: ByteString -> ByteString

# Overview

This module provides functions related to searching a substring within a string, using the Boyer-Moore algorithm with minor modifications to improve the overall performance and ameliorate the worst case performance degradation of the original Boyer-Moore algorithm for periodic patterns.

Efficiency demands that the pattern be a strict `ByteString`, to work with a lazy pattern, convert it to a strict `ByteString` first via `strictify` (provided it is not too long). If support for long lazy patterns is needed, mail a feature-request.

When searching a pattern in a UTF-8-encoded `ByteString`, be aware that these functions work on bytes, not characters, so the indices are byte-offsets, not character offsets.

## Performance

In general, the Boyer-Moore algorithm is the most efficient method to search for a pattern inside a string. The advantage over other algorithms (e.g. Naïve, Knuth-Morris-Pratt, Horspool, Sunday) can be made arbitrarily large for specially selected patterns and targets, but usually, it's a factor of 2–3 versus Knuth-Morris-Pratt and of 6–10 versus the naïve algorithm. The Horspool and Sunday algorithms, which are simplified variants of the Boyer-Moore algorithm, typically have performance between Boyer-Moore and Knuth-Morris-Pratt, mostly closer to Boyer-Moore. The advantage of the Boyer-moore variants over other algorithms generally becomes larger for longer patterns. For very short patterns (or patterns with a very short period), other algorithms, e.g. Data.ByteString.Lazy.Search.DFA can be faster (my tests suggest that "very short" means two, maybe three bytes).

In general, searching in a strict `ByteString` is slightly faster than searching in a lazy `ByteString`, but for long targets the smaller memory footprint of lazy `ByteString`s can make searching those (sometimes much) faster. On the other hand, there are cases where searching in a strict target is much faster, even for long targets.

On 32-bit systems, `Int`-arithmetic is much faster than `Int64`-arithmetic, so when there are many matches, that can make a significant difference.

Also, the modification to ameliorate the case of periodic patterns is defeated by chunk-boundaries, so long patterns with a short period and many matches exhibit poor behaviour (consider using `indices` from Data.ByteString.Lazy.Search.DFA or Data.ByteString.Lazy.Search.KMP in those cases, the former for medium-length patterns, the latter for long patterns; none of the functions except `indices` suffer from this problem, though).

## Caution

When working with a lazy target string, the relation between the pattern length and the chunk size can play a big rôle. 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.

## Complexity

Preprocessing the pattern is O(`patternLength` + σ) in time and space (σ is the alphabet size, 256 here) for all functions. The time complexity of the searching phase for `indices` 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 contain a modification which drastically improves the performance for periodic patterns, although less for lazy targets than for strict ones. If I'm not mistaken, the worst case complexity for periodic patterns is O(`targetLength` * (1 + `patternLength` / `chunkSize`)).

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).

## Partial application

All functions can usefully be partially applied. Given only a pattern, the pattern is preprocessed only once, allowing efficient re-use.

## Integer 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 or equal to ``maxBound` :: `Int`` then this will overflow causing an error. We try to detect this and call `error` before a segfault occurs.

# Finding substrings

indices #

Arguments

:: ByteString | Strict pattern to find
-> ByteString | Lazy string to search
-> [Int64] | Offsets of matches

``indices`` finds the starting indices of all possibly overlapping occurrences of the pattern in the target string. If the pattern is empty, the result is `[0 .. `length` target]`.

nonOverlappingIndices #

Arguments

:: ByteString | Strict pattern to find
-> ByteString | Lazy string to search
-> [Int64] | Offsets of matches

``nonOverlappingIndices`` finds the starting indices of all non-overlapping occurrences of the pattern in the target string. It is more efficient than removing indices from the list produced by `indices`.

# Breaking on substrings

breakOn #

Arguments

:: ByteString | Strict pattern to search for
-> ByteString | Lazy string to search in
-> (ByteString, ByteString) | Head and tail of string broken at substring

``breakOn` pattern target` splits `target` at the first occurrence of `pattern`. If the pattern does not occur in the target, the second component of the result is empty, otherwise it starts with `pattern`. If the pattern is empty, the first component is empty. For a non-empty pattern, the first component is generated lazily, thus the first parts of it can be available before the pattern has been found or determined to be absent.

      uncurry append . breakOn pattern = id

breakAfter #

Arguments

:: ByteString | Strict pattern to search for
-> ByteString | Lazy string to search in
-> (ByteString, ByteString) | Head and tail of string broken after substring

``breakAfter` pattern target` splits `target` behind the first occurrence of `pattern`. An empty second component means that either the pattern does not occur in the target or the first occurrence of pattern is at the very end of target. If you need to discriminate between those cases, use breakFindAfter. If the pattern is empty, the first component is empty. For a non-empty pattern, the first component is generated lazily, thus the first parts of it can be available before the pattern has been found or determined to be absent.

      uncurry append . breakAfter pattern = id

breakFindAfter #

Arguments

:: ByteString | Strict pattern to search for
-> ByteString | Lazy string to search in
-> ((ByteString, ByteString), Bool) | Head and tail of string broken after substring and presence of pattern

``breakFindAfter`` does the same as `breakAfter` but additionally indicates whether the pattern is present in the target.

      fst . breakFindAfter pat = breakAfter pat

# Replacing

replace #

Arguments

:: Substitution rep |
=> ByteString | Strict pattern to replace
-> rep | Replacement string
-> ByteString | Lazy string to modify
-> ByteString | Lazy result

``replace` pat sub text` replaces all (non-overlapping) occurrences of `pat` in `text` with `sub`. If occurrences of `pat` overlap, the first occurrence that does not overlap with a replaced previous occurrence is substituted. Occurrences of `pat` arising from a substitution will not be substituted. For example:

      replace "ana" "olog" "banana" = "bologna"
      replace "ana" "o" "bananana" = "bono"
      replace "aab" "abaa" "aaabb" = "aabaab"

The result is a lazy `ByteString`, which is lazily produced, without copying. Equality of pattern and substitution is not checked, but

      replace pat pat text == text

holds (the internal structure is generally different). If the pattern is empty but not the substitution, the result is equivalent to (were they `String`s) `cycle sub`.

For non-empty `pat` and `sub` a lazy `ByteString`,

      concat . intersperse sub . split pat = replace pat sub

and analogous relations hold for other types of `sub`.

# Splitting

split #

Arguments

:: ByteString | Strict pattern to split on
-> ByteString | Lazy string to split
-> [ByteString] | Fragments of string

``split` pattern target` splits `target` at each (non-overlapping) occurrence of `pattern`, removing `pattern`. If `pattern` is empty, the result is an infinite list of empty `ByteString`s, if `target` is empty but not `pattern`, the result is an empty list, otherwise the following relations hold (where `patL` is the lazy `ByteString` corresponding to `pat`):

      concat . intersperse patL . split pat = id,
      length (split pattern target) ==
                  length (nonOverlappingIndices pattern target) + 1,

no fragment in the result contains an occurrence of `pattern`.

splitKeepEnd #

Arguments

:: ByteString | Strict pattern to split on
-> ByteString | Lazy string to split
-> [ByteString] | Fragments of string

``splitKeepEnd` pattern target` splits `target` after each (non-overlapping) occurrence of `pattern`. If `pattern` is empty, the result is an infinite list of empty `ByteString`s, otherwise the following relations hold:

      concat . splitKeepEnd pattern = id,

all fragments in the result except possibly the last end with `pattern`, no fragment contains more than one occurrence of `pattern`.

splitKeepFront #

Arguments

:: ByteString | Strict pattern to split on
-> ByteString | Lazy string to split
-> [ByteString] | Fragments of string

``splitKeepFront`` is like `splitKeepEnd`, except that `target` is split before each occurrence of `pattern` and hence all fragments with the possible exception of the first begin with `pattern`. No fragment contains more than one non-overlapping occurrence of `pattern`.

# Convenience

strictify :: ByteString -> ByteString #

``strictify`` converts a lazy `ByteString` to a strict `ByteString` to make it a suitable pattern.