110 lines
4.0 KiB
Factor
110 lines
4.0 KiB
Factor
! Copyright (C) 2010 Samuel Tardieu.
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! See http://factorcode.org/license.txt for BSD license.
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USING: assocs help.markup help.syntax math sequences ;
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IN: path-finding
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{ <astar> <bfs> <dijkstra> } related-words
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HELP: astar
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{ $description "This tuple must be subclassed and its method " { $link cost } ", "
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{ $link heuristic } ", and " { $link neighbours } " must be implemented. "
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"Alternatively, the " { $link <astar> } " word can be used to build a non-specialized version." } ;
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HELP: cost
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{ $values
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{ "from" "a node" }
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{ "to" "a node" }
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{ "astar" "an instance of a subclassed " { $link astar } " tuple" }
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{ "n" number }
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}
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{ $description "Return the cost to go from " { $snippet "from" } " to " { $snippet "to" } ". "
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{ $snippet "to" } " is necessarily a neighbour of " { $snippet "from" } "."
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} ;
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HELP: heuristic
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{ $values
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{ "from" "a node" }
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{ "to" "a node" }
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{ "astar" "an instance of a subclassed " { $link astar } " tuple" }
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{ "n" number }
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}
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{ $description "Return the estimated (undervalued) cost to go from " { $snippet "from" } " to " { $snippet "to" } ". "
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{ $snippet "from" } " and " { $snippet "to" } " are not necessarily neighbours."
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} ;
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HELP: neighbours
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{ $values
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{ "node" "a node" }
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{ "astar" "an instance of a subclassed " { $link astar } " tuple" }
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{ "seq" "a sequence of nodes" }
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}
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{ $description "Return the list of nodes reachable from " { $snippet "node" } "." } ;
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HELP: <astar>
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{ $values
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{ "neighbours" { $quotation ( node -- seq ) } }
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{ "cost" { $quotation ( from to -- cost ) } }
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{ "heuristic" { $quotation ( pos target -- cost ) } }
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{ "astar" astar }
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}
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{ $description "Build an astar object from the given quotations. The "
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{ $snippet "neighbours" } " one builds the list of neighbours. The "
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{ $snippet "cost" } " and " { $snippet "heuristic" } " ones represent "
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"respectively the cost for transitioning from a node to one of its neighbour, "
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"and the underestimated cost for going from a node to the target. This solution "
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"may not be as efficient as subclassing the " { $link astar } " tuple."
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} ;
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HELP: <bfs>
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{ $values
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{ "neighbours" assoc }
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{ "astar" astar }
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}
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{ $description "Build an astar object from the " { $snippet "neighbours" } " assoc. "
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"When used with " { $link find-path } ", this astar tuple will use the breadth-first search (BFS) "
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"path finding algorithm which is a particular case of the general A* algorithm."
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} ;
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HELP: <dijkstra>
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{ $values
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{ "costs" assoc }
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{ "astar" astar }
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}
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{ $description "Build an astar object from the " { $snippet "costs" } " assoc. "
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"The assoc keys are edges of the graph, while the corresponding values are assocs whose keys are "
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"the edges that can be reached and whose values are the costs to reach those edges. When used with "
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{ $link find-path } ", this astar tuple will use the Dijkstra path finding algorithm which is "
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"a particular case of the general A* algorithm."
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} ;
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HELP: find-path
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{ $values
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{ "start" "a node" }
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{ "target" "a node" }
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{ "astar" astar }
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{ "path/f" "an optimal path from " { $snippet "start" } " to " { $snippet "target" }
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", or f if no such path exists" }
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}
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{ $description "Find a path between " { $snippet "start" } " and " { $snippet "target" }
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" using the A* algorithm."
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} ;
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HELP: considered
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{ $values
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{ "astar" astar }
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{ "considered" sequence }
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}
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{ $description "When called after a call to " { $link find-path } ", return a list of nodes "
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"which have been examined during the A* exploration."
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} ;
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ARTICLE: "path-finding" "Path finding using the A* algorithm"
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"The " { $vocab-link "path-finding" } " vocabulary implements a graph search algorithm for finding the least-cost path from one node to another using the A* algorithm." $nl
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"The " { $link astar } " tuple may be derived from and its " { $link cost } ", " { $link heuristic } ", and " { $link neighbours } " methods overwritten, or the " { $link <astar> } " or " { $link <bfs> } " words can be used to build a new tuple." $nl
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"Make an A* object:"
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{ $subsections <astar> <bfs> }
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"Find a path between nodes:"
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{ $subsections find-path } ;
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ABOUT: "path-finding"
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