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Math-PlanePath-129
River stage one • 3 direct dependents • 4 total dependents
2 ++
/ Math::PlanePath::TriangleSpiralSkewed

NAME

Math::PlanePath::TriangleSpiralSkewed -- integer points drawn around a skewed equilateral triangle

SYNOPSIS

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

DESCRIPTION

This path makes an spiral shaped as an equilateral triangle (each side the same length), but skewed to the left to fit on a square grid,

    16                              4
     |\
    17 15                           3
     |   \
    18  4 14                        2
     |  |\  \
    19  5  3 13                     1
     |  |   \  \
    20  6  1--2 12 ...         <- Y=0
     |  |         \  \
    21  7--8--9-10-11 30           -1
     |                  \
    22-23-24-25-26-27-28-29        -2

           ^
    -2 -1 X=0 1  2  3  4  5

The properties are the same as the spread-out TriangleSpiral. The triangle numbers fall on straight lines as they do in the TriangleSpiral but the skew means the top corner goes up at an angle to the vertical and the left and right downwards are different angles plotted (but are symmetric by N count).

Skew Right

Option skew => 'right' directs the skew towards the right, giving

      4                  16      skew="right"
                        / |
      3               17 15
                     /    |
      2            18  4 14
                  /  / |  |
      1        ...  5  3 13
                  /    |  |
    Y=0 ->       6  1--2 12
               /          |
     -1       7--8--9-10-11

                    ^
             -2 -1 X=0 1  2

This is a shear "X -> X+Y" of the default skew="left" shown above. The coordinates are related by

    Xright = Xleft + Yleft         Xleft = Xright - Yright
    Yright = Yleft                 Yleft = Yright

Skew Up

      2       16-15-14-13-12-11      skew="up"
               |            /   
      1       17  4--3--2 10
               |  |   /  /  
    Y=0 ->    18  5  1  9 
               |  |   /  
     -1      ...  6  8 
                  |/  
     -2           7 

                    ^
             -2 -1 X=0 1  2

This is a shear "Y -> X+Y" of the default skew="left" shown above. The coordinates are related by

    Xup = Xleft                 Xleft = Xup
    Yup = Yleft + Xleft         Yleft = Yup - Xup

Skew Down

      2          ..-18-17-16       skew="down"
                           |  
      1        7--6--5--4 15 
                \       |  | 
    Y=0 ->        8  1  3 14 
                   \  \ |  | 
     -1              9  2 13 
                      \    | 
     -2                10 12 
                         \ | 
                          11 

                     ^
              -2 -1 X=0 1  2

This is a rotate by -90 degrees of the skew="up" above. The coordinates are related

    Xdown = Yup          Xup = - Ydown
    Ydown = - Xup        Yup = Xdown

Or related to the default skew="left" by

    Xdown = Yleft + Xleft        Xleft = - Ydown
    Ydown = - Xleft              Yleft = Xdown + Ydown

N Start

The default is to number points starting N=1 as shown above. An optional n_start can give a different start, with the same shape etc. For example to start at 0,

    15        n_start => 0
     |\
    16 14
     |   \
    17  3 13 ...
     |  |\  \  \
    18  4  2 12 31
     |  |   \  \  \
    19  5  0--1 11 30
     |  |         \  \
    20  6--7--8--9-10 29
     |                  \
    21-22-23-24-25-26-27-28

With this adjustment for example the X axis N=0,1,11,30,etc is (9X-7)*X/2, the hendecagonal numbers (11-gonals). And South-East N=0,8,25,etc is the hendecagonals of the second kind, (9Y-7)*Y/2 with Y negative.

FUNCTIONS

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

$path = Math::PlanePath::TriangleSpiralSkewed->new ()
$path = Math::PlanePath::TriangleSpiralSkewed->new (skew => $str, n_start => $n)

Create and return a new skewed triangle spiral object. The skew parameter can be

    "left"    (the default)
    "right"
    "up"
    "down"
$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.

FORMULAS

Rectangle to N Range

Within each row there's a minimum N and the N values then increase monotonically away from that minimum point. Likewise in each column. This means in a rectangle the maximum N is at one of the four corners of the rectangle.

              |
    x1,y2 M---|----M x2,y2        maximum N at one of
          |   |    |              the four corners
       -------O---------          of the rectangle
          |   |    |
          |   |    |
    x1,y1 M---|----M x1,y1
              |

OEIS

Entries in Sloane's Online Encyclopedia of Integer Sequences related to this path include

    n_start=1, skew="left" (the defaults)
      A204439     abs(dX)
      A204437     abs(dY)
      A010054     turn 1=left,0=straight, extra initial 1

      A117625     N on X axis
      A064226     N on Y axis, but without initial value=1
      A006137     N on X negative
      A064225     N on Y negative
      A081589     N on X=Y leading diagonal
      A038764     N on X=Y negative South-West diagonal
      A081267     N on X=-Y negative South-East diagonal
      A060544     N on ESE slope dX=+2,dY=-1
      A081272     N on SSE slope dX=+1,dY=-2

      A217010     permutation N values of points in SquareSpiral order
      A217291      inverse
      A214230     sum of 8 surrounding N
      A214231     sum of 4 surrounding N

    n_start=0
      A051682     N on X axis (11-gonal numbers)
      A081268     N on X=1 vertical (next to Y axis)
      A062708     N on Y axis
      A062725     N on Y negative axis
      A081275     N on X=Y+1 North-East diagonal
      A062728     N on South-East diagonal (11-gonal second kind)
      A081266     N on X=Y negative South-West diagonal
      A081270     N on X=1-Y North-West diagonal, starting N=3
      A081271     N on dX=-1,dY=2 NNW slope up from N=1 at X=1,Y=0

    n_start=-1
      A023531     turn 1=left,0=straight, being 1 at N=k*(k+3)/2
      A023532     turn 1=straight,0=left

    n_start=1, skew="right"
      A204435     abs(dX)
      A204437     abs(dY)
      A217011     permutation N values of points in SquareSpiral order
                    but with 90-degree rotation
      A217292     inverse
      A214251     sum of 8 surrounding N

    n_start=1, skew="up"
      A204439     abs(dX)
      A204435     abs(dY)
      A217012     permutation N values of points in SquareSpiral order
                    but with 90-degree rotation
      A217293     inverse
      A214252     sum of 8 surrounding N

    n_start=1, skew="down"
      A204435     abs(dX)
      A204439     abs(dY)

The square spiral order in A217011,A217012 and their inverses has first step at 90-degrees to the first step of the triangle spiral, hence the rotation by 90 degrees when relating to the SquareSpiral path. A217010 on the other hand has no such rotation since it reckons the square and triangle spirals starting in the same direction.

SEE ALSO

Math::PlanePath, Math::PlanePath::TriangleSpiral, Math::PlanePath::PyramidSpiral, 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/>.