NAME

       Math::PlanePath::KnightSpiral -- integer points around a square, by chess knight moves

SYNOPSIS

        use Math::PlanePath::KnightSpiral;
        my $path = Math::PlanePath::KnightSpiral->new;
        my ($x, $y) = $path->n_to_xy (123);

DESCRIPTION

       This path traverses the plane by an infinite "knight's tour" in the form of a square spiral.

                                   ...
               21   4   9  14  19                 2

               10  15  20   3   8      28         1

                5  22   1  18  13            <- Y=0

               16  11  24   7   2  27             1

               23   6  17  12  25                 2

                                       26

                        ^
               -2  -1  X=0  1   2   3

       Each step is a chess knight's move 1 across and 2 along, or vice versa.  The pattern makes 4 cycles on a
       2-wide path around a square before stepping outwards to do the same again to a now bigger square.  The
       above sample shows the first 4-cycle around the central 1, then stepping out at 26 and beginning to go
       around the outside of the 5x5 square.

       An attractive traced out picture of the path appeared in the past at "www.borderschess.org",

           <http://web.archive.org/web/1id_/http://www.borderschess.org/Infinite.gif>

           <http://web.archive.org/web/1id_/http://www.borderschess.org/KTinfinity.gif>

           <http://web.archive.org/web/20161028114643id_/http://www.borderschess.org/KTart.htm>

       (HTML colours might might make the text invisible.  Try deleting, or browser option to ignore page
       colours, or a text browser.)

       See math-image to draw the path lines too.

FUNCTIONS

       See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.

       "$path = Math::PlanePath::KnightSpiral->new ()"
           Create and return a new knight spiral object.

       "($x,$y) = $path->n_to_xy ($n)"
           Return the X,Y coordinates of point number $n on the path.

           For "$n < 1" the return is an empty list, it being considered the path starts at 1.

       "$n = $path->xy_to_n ($x,$y)"
           Return the point number for coordinates "$x,$y".  $x and $y are each rounded to the nearest integer,
           which has the effect of treating each N in the path as centred in a square of side 1, so the entire
           plane is covered.

OEIS

       This Knight's tour is in Sloane's OEIS following the Knight spiral and giving the resulting X,Y location
       by the "SquareSpiral" numbering.  There's eight forms for 4 rotations and the two spirals same or
       opposite directions.

           <http://oeis.org/A068608> (etc)

           permutations
             A068608   same knight and square spiral directions
             A068609   rotate 90 degrees
             A068610   rotate 180 degrees
             A068611   rotate 270 degrees
             A068612   rotate 180 degrees, spiral opp dir (X negate)
             A068613   rotate 270 degrees, spiral opp dir
             A068614   spiral opposite direction (Y negate)
             A068615   rotate 90 degrees, spiral opp dir (X,Y transpose)

       See examples/knights-oeis.pl for a sample program printing the values of A068608.

SEE ALSO

       Math::PlanePath, Math::PlanePath::SquareSpiral

HOME PAGE

       <http://user42.tuxfamily.org/math-planepath/index.html>

LICENSE

       Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Kevin Ryde

       This file is part of Math-PlanePath.

       Math-PlanePath is free software; you can redistribute it and/or modify it under the terms of the GNU
       General Public License as published by the Free Software Foundation; either version 3, or (at your
       option) any later version.

       Math-PlanePath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
       the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
       License for more details.

       You should have received a copy of the GNU General Public License along with Math-PlanePath.  If not, see
       <http://www.gnu.org/licenses/>.