Provided by: libmath-planepath-perl_129-1_all 
NAME
Math::PlanePath::MultipleRings -- rings of multiples
SYNOPSIS
use Math::PlanePath::MultipleRings;
my $path = Math::PlanePath::MultipleRings->new (step => 6);
my ($x, $y) = $path->n_to_xy (123);
DESCRIPTION
This path puts points on concentric rings. Each ring has "step" many points more than the previous and
the first is also "step". For example with the default step==6,
24 23 innermost ring 6
25 22 next ring 12
10 next ring 18
26 11 9 21 ... ringnum*step
27 12 3 2 8 20 38
28 13 4 1 7 19 37 <- Y=0
29 14 5 6 18 36
30 15 17 35
16
31 24
32 33
^
X=0
X,Y positions are not integers, except on the axes. The innermost ring like N=1to6 above has points 1
unit apart. Subsequent rings are a unit chord or unit radial, whichever ensures no overlap.
step <= 6 unit spacing radially
step >= 6 unit chords around the rings
For step=6 the two spacings are the same. Unit radial spacing ensures the X axis points N=1,7,19,37,etc
shown above are 1 unit apart. Unit chord spacing ensures adjacent points such as N=7,8,0,etc don't
overlap.
The layout is similar to the various spiral paths of corresponding step. For example step=6 is like the
"HexSpiral", but rounded out to circles instead of a hexagonal grid. Similarly step=4 the
"DiamondSpiral" or step=8 the "SquareSpiral".
The step parameter is also similar to the "PyramidRows" with the rows stretched around circles, but
"PyramidRows" starts from a 1-wide initial row whereas for "MultipleRings" here the first is "step" many.
X Axis
The starting Nring=1,7,19,37 etc on the X axis for the default step=6 is 6*d*(d-1)/2 + 1, counting the
innermost ring as d=1. In general Nring is a multiple of the triangular numbers d*(d-1)/2, plus 1,
Nring = step*d*(d-1)/2 + 1
This is the centred polygonal numbers, being the cumulative count of points making concentric polygons or
rings in the style of this path.
Straight line radials further around arise from adding multiples of d, so for example in step=6 shown
above the line N=3,11,25,etc is Nring + 2*d. Multiples k*d with k>=step give lines which are in between
the base ones from the innermost ring.
Step 1
For step=1 the first ring is 1 point and each subsequent ring has 1 further point.
24
23
18 12 17
25 8
13 5
19 9 3 1 2 4 7 11 16 22 <- Y=0
14 6
26 10
20 15 21
28
27
^
-5 -4 -3 -2-1 X=0 1 2 3 4 5 6
The rings are
polygon radius N values
------------ ------ --------
single point 0 1
two points 1 2, 3
triangle 2 4, 5, 6
square 3 7, 8, 9,10
pentagon 4 11,12,13,14,15
hexagon 5 16,17,18,19,20,21
etc
The X axis as described above is the triangular numbers plus 1, ie. k*(k+1)/2 + 1.
Step 2
For step=2 the arrangement is roughly
34
35 33
24 15 23
36 25 22 32
16 9 4 8 14
37 26 17 10 5 2 1 3 7 13 21 31
18 11 6 12 20
38 27 30 42
28 19 29
39 41
40
The pattern is similar to the "SacksSpiral" (see Math::PlanePath::SacksSpiral). In "SacksSpiral" each
spiral loop is 2 more points than the previous the same as here, but the positioning differs. Here the X
axis is the pronic numbers and the squares are to the left, whereas in "SacksSpiral" rotated around to
squares on X axis and pronics to the left.
Ring Shape
Option "ring_shape => 'polygon'" puts the points on concentric polygons of "step" many sides, so each
concentric polygon has 1 more point on each of its sides than the previous polygon. For example step=4
gives 4-sided polygons, ie. diamonds,
16
/ \ ring_shape=>'polygon', step=>4
17 7 15
/ / \ \
18 8 2 6 14
/ / / \ \ \
19 9 3 1 5 13
\ \ \ / / /
20 10 4 12 24
\ \ / /
21 11 23
\ /
22
The polygons are scaled to keep points 1 unit apart. For step>=6 this means 1 unit apart sideways.
step=6 is in fact a honeycomb grid where each points is 1 away from all six of its neighbours.
For step=3, 4 and 5 the polygon sides are 1 apart radially, as measured in the centre of each side. This
makes points a little more than 1 apart along the sides. Squeezing them up to make the closest points
exactly 1 apart is possible, but may require iterating a square root for each ring. step=3 squeezed down
would in fact become a variable spacing with successively four close then one wider.
For step=2 and step=1 in the current code the default circle shape is used. Should that change? Is
there a polygon style with 2 sides or 1 side?
The polygon layout is only a little different from a circle, but it lines up points on the sides and that
might help show a structure for some sets of points plotted on the path.
Step 3 Pentagonals
For step=3 the pentagonal numbers 1,5,12,22,etc, P(k) = (3k-1)*k/2, are a radial going up to the left,
and the second pentagonal numbers 2,7,15,26, S(k) = (3k+1)*k/2 are a radial going down to the left,
respectively 1/3 and 2/3 the way around the circles.
As described in "Step 3 Pentagonals" in Math::PlanePath::PyramidRows, those P(k) and preceding P(k)-1,
P(k)-2, and S(k) and preceding S(k)-1, S(k)-2 are all composites, so plotting the primes on a step=3
"MultipleRings" has two radial gaps where there's no primes.
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
"$path = Math::PlanePath::MultipleRings->new (step => $integer)"
"$path = Math::PlanePath::MultipleRings->new (step => $integer, ring_shape => $str)"
Create and return a new path object.
The "step" parameter controls how many points are added in each circle. It defaults to 6 which is an
arbitrary choice and the suggestion is to always pass in a desired count.
"($x,$y) = $path->n_to_xy ($n)"
Return the X,Y coordinates of point number $n on the path.
$n can be any value "$n >= 1" and fractions give positions on the rings in between the integer
points. For "$n < 1" the return is an empty list since points begin at 1.
Fractional $n currently ends up on the circle arc between the integer points. Would straight line
chords between them be better, reflecting the unit spacing of the points? Neither seems particularly
important.
"$n = $path->xy_to_n ($x,$y)"
Return an integer point number for coordinates "$x,$y". Each integer N is considered the centre of a
circle of diameter 1 and an "$x,$y" within that circle returns N.
The unit spacing of the points means those circles don't overlap, but they also don't cover the plane
and if "$x,$y" is not within one then the return is "undef".
"$str = $path->figure ()"
Return "circle".
FORMULAS
N to X,Y - Circle
As per above, each ring begins at
Nring = step*d*(d-1)/2 + 1
This can be inverted to get the ring number d for a given N, and then subtract Nring for a remainder into
the ring. (N-1)/step in the formula effectively converts into triangular number style.
d = floor((sqrt(8*(N-1)/step + 1) + 1) / 2)
Nrem = N - Nring
Rings are sized so that points are spaced 1 unit apart. There are three cases,
circle, step<=6 unit radially on X axis
polygon, step<=6 unit radially on sides centre
step>=7 unit chord between points
For the circle shape the integer points are on a circle and fractional N is on a straight line between
those integer points. This means it's a polygon too, but one with ever more sides whereas
ring_shape=polygon is a fixed "step" many sides.
circle numsides = d*step
polygon numsides = step
The radial distance to a polygon corner is calculated as
base varying with d
---------------- ---------------------------------------
circle, step<=6 0.5/sin(pi/step) + d-1
polygon, step<=6 0.5/sin(pi/step) + (d-1)/cos(pi/step)
circle, step>=7 0 + 0.5/sin(pi/(d*step))
polygon, step>=7 0 + d * 0.5/sin(pi/step)
The step<=6 cases are an initial polygon of "step" many unit sides, then unit spacing d-1 for circle, or
for polygon (d-1)/cos(pi/step) which is bigger and ensures the middle of the sides have unit spacing
radially.
The 0.5/sin(pi/step) for radius of a unit sided polygon arises from
r ___---*
___--- | 1/2 = half the polygon side
___--- alpha |
o------------------+
alpha = (2pi/numsides) / 2 = pi/numsides
sin(alpha) = (1/2) / base_r
r = 0.5 / sin(pi/numsides)
The angle theta to a polygon vertex is simply a full circle divided by numsides.
side = circle Nrem
polygon floor(Nrem / step)
theta = side * (2pi / numsides)
vertex X = r * cos(theta)
Y = r * sin(theta)
next_theta = (side+1) * (2pi / numsides)
next_vertex X = r * cos(next_theta)
Y = r * sin(next_theta)
frac into side
f = circle frac(Nrem) = Nrem modulo 1
polygon Nrem - side*d = Nrem modulo d
X = vertex_X + f * (next_vertex_X - vertex_X)
Y = vertex_Y + f * (next_vertex_Y - vertex_Y)
If Nrem is an integer for circle, or multiple of d for polygon, then the vertex X,Y is the final X,Y,
otherwise a fractional distance between the vertex X,Y and next vertex X,Y.
For a few cases X or Y are exact integers. Special case code for these cases can ensure floating point
rounding of pi doesn't give small offsets from integers.
For step=6 the base r is r=1 exactly since the innermost ring is a little hexagon. This means for the
circle step=6 case the points on the X axis (positive and negative) are all integers X=1,2,3,etc.
P-----P
/ 1 / \ 1 <-- innermost points 1 apart
/ / \
P o-----P <-- base_r = 1
\ 1 /
\ /
P-----P
If theta=pi, which is when 2*Nrem==d*step, then the point is on the negative X axis. Returning Y=0
exactly for that avoids sin(pi) giving some small non-zero due to rounding.
If theta=pi/2 or theta=3pi/2, which is 4*Nrem==d*step or 4*Nrem==3*d*step, then N is on the positive or
negative Y axis (respectively). Returning X=0 exactly avoids cos(pi/2) or cos(3pi/2) giving some small
non-zero.
Points on the negative X axis points occur when the step is even. Points on the Y axis points occur when
the step is a multiple of 4.
If theta=pi/4, 3*pi/4, 5*pi/4 or 7*pi/4, which is 8*Nrem==d*step, 3*d*step, 5*d*step or 7*d*step then the
points are on the 45-degree lines X=Y or X=-Y. The current code doesn't try to ensure X==Y in these
cases. The values are not integers and floating point rounding might mean sin(pi/4)!=cos(pi/4) resulting
in X!=Y.
N to RSquared - Step 1
For step=1 the rings are point, line, triangle, square, pentagon, etc, with vertices at
radius=numsides-1. For fractional N the triangle, square and hexagon cases are quadratics in the
fraction part, allowing exact values from "n_to_rsquared()".
Ring R^2
--------------------- --------------
triangle 4 <= N < 7 4 - 12*f*(1-f)
square 7 <= N < 11 9 - 18*f*(1-f)
hexagon 16 <= N < 22 25 - 25*f*(1-f)
f = N - int(N) fractional part of N
For example for the square at N=7.5 have f=0.5 and R^2=4.5 exactly. These quadratics arise because sine
of 2pi/3, 2pi/4 and 2pi/6 are square roots, which on squaring up in R^2=X^2+Y^2 become integer factors
for the fraction f along the polygon side.
OEIS
Entries in Sloane's Online Encyclopedia of Integer Sequences related to this path include
<http://oeis.org/A005448> (etc)
A005448 A001844 A005891 A003215 A069099 3 to 7
A016754 A060544 A062786 A069125 A003154 8 to 12
A069126 A069127 A069128 A069129 A069130 13 to 17
A069131 A069132 A069133 18 to 20
N on X axis of step=k, being the centred pentagonals
step=1
A002024 Radius+1, runs of n repeated n times
step=8
A090915 permutation N at X,-Y, mirror across X axis
SEE ALSO
Math::PlanePath, Math::PlanePath::SacksSpiral, Math::PlanePath::TheodorusSpiral,
Math::PlanePath::PixelRings
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/>.