|Maintainer||Bryan O'Sullivan <firstname.lastname@example.org>|
Fast overlapping Boyer-Moore search of both strict and lazy
Descriptions of the algorithm can be found at
Original authors: Daniel Fischer (daniel.is.fischer at web.de) and
Chris Kuklewicz (haskell at list.mightyreason.com).
This module exports 4 search functions: matchLL, matchLS,
matchSL, and matchSS.
If given an empty pattern, a search will always return an empty
|Parameter and return types
The first parameter is always the pattern string. The second
parameter is always the target string to be searched. The returned
list contains the offsets of all overlapping patterns.
A returned Int or Int64 is an index into the target string
which is aligned to the head of the pattern string. Strict targets
return Int indices and lazy targets return Int64 indices. All
returned lists are computed and returned in a lazy fashion.
matchLL and matchLS take lazy bytestrings as patterns. For
performance, if the pattern is not a single strict chunk then all
the the pattern chunks will copied into a concatenated strict
bytestring. This limits the patterns to a length of (maxBound ::
matchLL and matchSL take lazy bytestrings as targets.
These are written so that while they work they will not retain a
reference to all the earlier parts of the the lazy bytestring.
This means the garbage collector would be able to keep only a small
amount of the target string and free the rest.
|Operating on a strict target string is faster than a lazy target
string. It is unclear why the performance gap is as large as it is
(patches welcome). To slightly ameliorate this, if the lazy string
is a single chunk then a copy of the strict algorithm is used.
|Preprocessing the pattern string is O(patternLength). The search
performance is O(targetLength/patternLength) in the best case,
allowing it to go faster than a Knuth-Morris-Pratt algorithm. With
a non-periodic pattern the worst case uses O(3*targetLength)
comparisons. The periodic pattern worst case is quadratic
O(targetLength*patternLength) complexity. Improvements
(e.g. Turbo-Boyer-Moore) to catch and linearize worst case
performance slow down the loop significantly.
|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
|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.
|Produced by Haddock version 2.3.0|