Data.ByteString.Search.DFA

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

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

Data.ByteString.Search.DFA

Contents

- Overview
  - Complexity and performance
  - Partial application
- Finding substrings
- Breaking on substrings
- Replacing
- Splitting

Description

Fast search of strict `ByteString` values. Breaking, splitting and replacing using a deterministic finite automaton.

Synopsis

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

# Overview

This module provides functions related to searching a substring within a string. The searching algorithm uses a deterministic finite automaton based on the Knuth-Morris-Pratt algorithm. The automaton is implemented as an array of `(patternLength + 1) * σ` state transitions, where σ is the alphabet size (256), so it is only suitable for short enough patterns.

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.

## Complexity and performance

The time and space complexity of the preprocessing phase is O(`patternLength * σ`). The searching phase is O(`targetLength`), each target character is inspected only once.

In general the functions in this module are slightly faster than the corresponding functions using the Knuth-Morris-Pratt algorithm but considerably slower than the Boyer-Moore functions. For very short patterns or, in the case of `indices`, patterns with a short period which occur often, however, times are close to or even below the Boyer-Moore times.

## Partial application

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

# Finding substrings

indices

Arguments

:: ByteString | Pattern to find
-> ByteString | String to search
-> [Int] | 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 | Pattern to find
-> ByteString | String to search
-> [Int] | 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 | String to search for
-> ByteString | 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.

      uncurry append . breakOn pattern = id

breakAfter

Arguments

:: ByteString | String to search for
-> ByteString | 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. To discriminate between those cases, use e.g. `isSuffixOf`.

      uncurry append . breakAfter pattern = id

# Replacing

replace

Arguments

:: Substitution rep |
=> ByteString | Substring to replace
-> rep | Replacement string
-> ByteString | 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

      concat . toChunks $ replace pat pat text == text

holds. 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 strict `ByteString`,

      fromChunks . intersperse sub . split pat = replace pat sub

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

# Splitting

split

Arguments

:: ByteString | Pattern to split on
-> ByteString | 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:

      concat . intersperse pat . 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 | Pattern to split on
-> ByteString | 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 | Pattern to split on
-> ByteString | 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`.