1/* File: canny/a.pl 2 Author: Roy Ratcliffe 3 Created: Jun 7 2021 4 Purpose: A* Search 5 6Copyright (c) 2021, Roy Ratcliffe, Northumberland, United Kingdom 7 8Permission is hereby granted, free of charge, to any person obtaining a 9copy of this software and associated documentation files (the 10"Software"), to deal in the Software without restriction, including 11without limitation the rights to use, copy, modify, merge, publish, 12distribute, sublicense, and/or sell copies of the Software, and to 13permit persons to whom the Software is furnished to do so, subject to 14the following conditions: 15 16 The above copyright notice and this permission notice shall be 17 included in all copies or substantial portions of the Software. 18 19THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS 20OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF 21MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. 22IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY 23CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, 24TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE 25SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. 26 27*/ 28 29:- module(canny_a, 30 [ a_star/3 % +Heuristics0,-Heuristics,+Options 31 ]). 32:- predicate_options(a_star/3, 3, 33 [ initially(any), 34 finally(any), 35 reverse(boolean) 36 ]). 37:- autoload(library(option), [option/3, option/2]). 38:- autoload(library(lists), [reverse/2]). 39:- use_module(library(chr/a_star)). 40 41:- public final/1, expand/3.
finalhave semantic significance. You can override these using Options for
finally. For Options see below.
Simplifies the CHR implementation by accepting h/3 terms as a list rather than using predicates to expand nodes. We match heuristic terms using member/2 from the list of heuristics. This interface does not replace a_star/4 since having a pre-loaded list of heuristics is not always possible or feasible, for example when the number of arcs is very large such as when traversing a grid of arcs.
Here is a simple example.
?- a_star([h(a, b, 1)], A, [initially(a), finally(b)]). A = [h(a, b, 1)].
initially(Initial)defines the initial node, defaults to atom
finally(Final)defines the final node, atom
reverse(Boolean)reverses the outgoing selected Heuristics so that the order reflects the forward order of traverse. The underlying expansion pushes path nodes to the head of the list resulting in a final-to-initial traversal by default.
78a_star(Heuristics0, Heuristics, Options) :- 79 option(initially(Initially), Options, initial), 80 option(finally(Finally), Options, final), 81 a_star(a(Initially, Finally, Heuristics0, ), 82 Final^(canny_a:final(Final)), 83 Node^Expand^Cost^(canny_a:expand(Node, Expand, Cost)), 84 a(_, _, _, Heuristics_)), 85 ( option(reverse(true), Options) 86 -> Heuristics_ = Heuristics 87 ; reverse(Heuristics_, Heuristics) 88 ). 89 90final(a(Final, Final, _, _)). 91 92expand(a(A, Final, Heuristics0, Heuristics), 93 a(B, Final, Heuristics0, [h(A, B, Cost)|Heuristics]), 94 Cost) :- 95 member(h(A, B, Cost), Heuristics0), 96 A \== B