Ichthyostega
806db414dd
* Lumiera source code always was copyrighted by individual contributors * there is no entity "Lumiera.org" which holds any copyrights * Lumiera source code is provided under the GPL Version 2+ == Explanations == Lumiera as a whole is distributed under Copyleft, GNU General Public License Version 2 or above. For this to become legally effective, the ''File COPYING in the root directory is sufficient.'' The licensing header in each file is not strictly necessary, yet considered good practice; attaching a licence notice increases the likeliness that this information is retained in case someone extracts individual code files. However, it is not by the presence of some text, that legally binding licensing terms become effective; rather the fact matters that a given piece of code was provably copyrighted and published under a license. Even reformatting the code, renaming some variables or deleting parts of the code will not alter this legal situation, but rather creates a derivative work, which is likewise covered by the GPL! The most relevant information in the file header is the notice regarding the time of the first individual copyright claim. By virtue of this initial copyright, the first author is entitled to choose the terms of licensing. All further modifications are permitted and covered by the License. The specific wording or format of the copyright header is not legally relevant, as long as the intention to publish under the GPL remains clear. The extended wording was based on a recommendation by the FSF. It can be shortened, because the full terms of the license are provided alongside the distribution, in the file COPYING.
116 lines
3.5 KiB
C++
116 lines
3.5 KiB
C++
/*
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BINARY-SEARCH.hpp - generic search over continuous domain with a probe predicate
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Copyright (C)
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2024, Hermann Vosseler <Ichthyostega@web.de>
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**Lumiera** is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by the
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Free Software Foundation; either version 2 of the License, or (at your
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option) any later version. See the file COPYING for further details.
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*/
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/** @file binary-search.hpp
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** Textbook implementation of the classical binary search over continuous domain.
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** The domain is given by its lower and upper end points. Within this domain,
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** a _breaking point_ is located, where the result of a _probe predicate_
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** flips from `false` to `true`. For the core search, the _invariant_
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** is assumed, implying that the `predicate(lower) ≡ false` and
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** `predicate(upper) ≡ true`.
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**
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** For good convergence, it is advisable to enter the search with rather tight
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** bounds. For the case that it's not clear if the invariant holds for both ends,
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** two alternative entrance points are provided, which check the condition on the
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** interval ends and possibly shift and expand the search domain in case the
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** assumption is broken.
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**
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** @see stress-test-rig.hpp
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** @see SchedulerStress_test
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*/
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#ifndef LIB_BINARY_SEARCH_H
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#define LIB_BINARY_SEARCH_H
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#include "lib/meta/function.hpp"
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#include <utility>
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namespace lib {
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using std::forward;
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/** binary search: actual search loop
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* - search until (upper-lower) < epsilon
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* - the \a FUN performs the actual test
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* - the goal is to narrow down the breaking point
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* @param fun `bool(PAR)` perform probe and decide criterion.
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* @note `fun(lower)` must be `false` and
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* `fun(upper)` must be `true`
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*/
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template<class FUN, typename PAR>
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inline auto
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binarySearch_inner (FUN&& fun, PAR lower, PAR upper, PAR epsilon)
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{
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ASSERT_VALID_SIGNATURE (FUN, bool(PAR) );
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REQUIRE (lower <= upper);
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while ((upper-lower) >= epsilon)
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{
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PAR div = (lower+upper) / 2;
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bool hit = fun(div);
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if (hit)
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upper = div;
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else
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lower = div;
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}
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return (lower+upper)/2;
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}
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/** entrance point to binary search to ensure the upper point
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* indeed fulfils the test. If this is not the case, the search domain
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* is shifted up, but also expanded so that the given upper point is
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* still located within, but close to the lower end.
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* @note `fun(lower)` must be `false`
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*/
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template<class FUN, typename PAR>
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inline auto
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binarySearch_upper (FUN&& fun, PAR lower, PAR upper, PAR epsilon)
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{
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REQUIRE (lower <= upper);
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while (true)
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{
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bool hit = fun(upper);
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if (hit) break;
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// the upper end breaks contract => search above
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PAR len = (upper-lower);
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lower = upper - len/10;
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upper = lower + 14*len/10;
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}
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return binarySearch_inner (forward<FUN> (fun), lower,upper,epsilon);
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}
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template<class FUN, typename PAR>
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inline auto
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binarySearch (FUN&& fun, PAR lower, PAR upper, PAR epsilon)
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{
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REQUIRE (lower <= upper);
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while (true)
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{
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bool hit = fun(lower);
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if (not hit) break;
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// the lower end breaks contract => search below
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PAR len = (upper-lower);
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upper = lower + len/10;
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lower = upper - 14*len/10;
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}
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return binarySearch_upper (forward<FUN> (fun), lower,upper,epsilon);
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}
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} // namespace lib
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#endif /*LIB_BINARY_SEARCH_H*/
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