Provided by: libmath-planepath-perl_125-1_all 
NAME
Math::PlanePath::GosperReplicate -- self-similar hexagon replications
SYNOPSIS
use Math::PlanePath::GosperReplicate;
my $path = Math::PlanePath::GosperReplicate->new;
my ($x, $y) = $path->n_to_xy (123);
DESCRIPTION
This is a self-similar hexagonal tiling of the plane. At each level the shape is the
Gosper island.
17----16 4
/ \
24----23 18 14----15 3
/ \ \
25 21----22 19----20 10---- 9 2
\ / \
26----27 3---- 2 11 7---- 8 1
/ \ \
31----30 4 0---- 1 12----13 <- Y=0
/ \ \
32 28----29 5---- 6 45----44 -1
\ / \
33----34 38----37 46 42----43 -2
/ \ \
39 35----36 47----48 -3
\
40----41 -4
^
-7 -6 -5 -4 -3 -2 -1 X=0 1 2 3 4 5 6 7
Points are spread out on every second X coordinate to make a triangular lattice in integer
coordinates (see "Triangular Lattice" in Math::PlanePath).
The base pattern is the inner N=0 to N=6, then six copies of that shape are arranged
around as the blocks N=7,14,21,28,35,42. Then six copies of the resulting N=0 to N=48
shape are replicated around, etc.
Each point can be taken as a little hexagon, so that all points tile the plane with
hexagons. The innermost N=0 to N=6 are for instance,
* *
/ \ / \
/ \ / \
* * *
| 3 | 2 |
* * *
/ \ / \ / \
/ \ / \ / \
* * * *
| 4 | 0 | 1 |
* * * *
\ / \ / \ /
\ / \ / \ /
* * *
| 5 | 6 |
* * *
\ / \ /
\ / \ /
* *
The further replications are the same arrangement, but the sides become ever wigglier and
the centres rotate around. The rotation can be seen N=7 at X=5,Y=1 which is up from the X
axis.
The "FlowsnakeCentres" path is this same replicating shape, but starting from a side
instead of the middle and traversing in such as way as to make each N adjacent. The
"Flowsnake" curve itself is this replication too, but segments across hexagons.
Complex Base
The path corresponds to expressing complex integers X+i*Y in a base
b = 5/2 + i*sqrt(3)/2
with coordinates scaled to put equilateral triangles on a square grid. So for integer X,Y
on the triangular grid (X,Y either both odd or both even),
X/2 + i*Y*sqrt(3)/2 = a[n]*b^n + ... + a[2]*b^2 + a[1]*b + a[0]
where each digit a[i] is either 0 or a sixth root of unity encoded into base-7 digits of
N,
w6 = e^(i*pi/3) sixth root of unity, b = 2 + w6
= 1/2 + i*sqrt(3)/2
N digit a[i] complex number
------- -------------------
0 0
1 w6^0 = 1
2 w6^1 = 1/2 + i*sqrt(3)/2
3 w6^2 = -1/2 + i*sqrt(3)/2
4 w6^3 = -1
5 w6^4 = -1/2 - i*sqrt(3)/2
6 w6^5 = 1/2 - i*sqrt(3)/2
7 digits suffice because
norm(b) = (5/2)^2 + (sqrt(3)/2)^2 = 7
Rotate Numbering
Parameter "numbering_type => 'rotate'" applies a rotation in each sub-part according to
its location around the preceding level.
The effect can be illustrated by writing N in base-7. Part 10-16 is the same as the
middle 0-6. Part 20-26 has a rotation by +60 degrees. Part 30-36 has rotation by +120
degrees, and so on.
22----21
/ / numbering_type => 'rotate'
31 36 23 20 26 N shown in base-7
/ \ \ \ /
32 30 35 24----25 13----12
\ / / \
33----34 3---- 2 14 10----11
/ \ \
46----45 4 0---- 1 15----16
\ \
41----40 44 5---- 6 64----63
\ / / \
42----43 55----54 65 60 62
/ \ \ \ /
56 50 53 66 61
/ /
51----52
Notice this means in each part the 11, 21, 31, etc, points are directed away from the
middle in the same way, relative to the sub-part locations.
Working through the expansions gives the following rule for when an N is on the boundary
of level k,
write N in k many base-7 digits (empty string if k=0)
if any 0 digit then non-boundary
ignore high digit and all 1 digits
if any 4 or 5 digit then non-boundary
if any 32, 33, 66 pair then non-boundary
A 0 digit is the middle of a block, or 4 or 5 digit the inner side of a block, for k>=1,
hence non-boundary. After that the 6,1,2,3 parts variously expand with rotations so that
a 66 is enclosed on the clockwise side and 32 and 33 on the anti-clockwise side.
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
"$path = Math::PlanePath::GosperReplicate->new ()"
"$path = Math::PlanePath::GosperReplicate->new (numbering_type => $str)"
Create and return a new path object. The "numbering_type" parameter can be
"fixed" (default)
"rotate"
"($x,$y) = $path->n_to_xy ($n)"
Return the X,Y coordinates of point number $n on the path. Points begin at 0 and if
"$n < 0" then the return is an empty list.
Level Methods
"($n_lo, $n_hi) = $path->level_to_n_range($level)"
Return "(0, 7**$level - 1)".
FORMULAS
Axis Rotations
In the fixed numbering, digit positions 1,2,3,4,5,6 go around +60deg each, so the N for
rotation of X,Y by +60 degrees is each digit +1.
N = 0, 1, 2, 3, 4, 5, 6, 10, 11, 12
rot+60(N) = 0, 2, 3, 4, 5, 6, 1, 14, 16, 17, ... decimal
= 0, 2, 3, 4, 5, 6, 1, 20, 22, 23, ... base7
rot+120(N) = 0, 3, 4, 5, 6, 1, 2, 21, 24, 25, ... decimal
= 0, 3, 4, 5, 6, 1, 2, 30, 33, 34, ... base7
etc
In the rotate numbering, just adding +1 (etc) at the high digit alone is rotation.
X,Y Extents
The maximum X in a given level N=0 to 7^k-1 can be calculated from the replications. A
given high digit 1 to 6 has sub-parts located at b^k*w6^(d-1). Those sub-parts are all
the same, so the one with maximum real(b^k*w6^(d-1)) contains the maximum X.
N_xmax_digit(j) = d=1to6 where real(w6^(d-1) * b^j) is maximum
= 1,1,6,6,6,5,5,5,4,4,4,3,3,3,3,2,2, ...
k-1
N_xmax(k) = digits N_xmax_digit(j) low digit j=0
j=0
= 0, 1, 8, 302, 2360, 16766, 100801, ... decimal
= 0, 1, 11, 611, 6611, 66611, 566611, ... base7
k-1
z_xmax(k) = sum w6^d[j] * b^j
j=0 each d[j] with real(w6^d[j] * b^j) maximum
= 0, 1, 7/2+1/2*sqrt3*i, 10-sqrt3*i, 57/2-3/2*sqrt3*i,...
xmax(k) = 2*real(z_xmax(k))
= 0, 2, 7, 20, 57, 151, 387, 1070, 2833, 7106, ...
For computer calculation these maximums can be calculated from the powers. The parts
resulting can also be written in terms of the angle
arg(b) = atan(sqrt(3)/5) = 19.106... degrees
For successive k, if adding this pushes the b^k angle past +30deg then the preceding digit
goes past -30deg and becomes the new maximum X. Write the angle as a fraction of 60deg
(pi/3),
F = atan(sqrt(3)/5) / (pi/3) = 0.318443 ...
This is irrational since b^k is never on the X or Y axes. That can be seen since
2/sqrt3*imag(b^k) mod 7 goes in a repeating pattern 1,5,4,6,2,3. Similarly 2*real(b^k)
mod 7 so not on the Y axis, and also anything on the Y axis would have 3*k fall on the X
axis.
Digits low to high are successive steps back cyclically 6,5,4,3,2,1 so that (with mod
giving 0 to 5),
N_xmax_digit(j) = (-floor(F*j+1/2) mod 6) + 1
The +1/2 is since initial direction b^0=1 is angle 0 which is half way between -30 and +30
deg.
Similarly for the location, using conj(w6) for rotation back
z_xmax_exp(j) = floor(F*j+1/2)
= 0,0,1,1,1,2,2,2,3,3,3,4,4,4,4,5,5,5, ...
z_xmax(k) = sum(j=0,k-1, conj(w6)^z_xmax_exp(j) * b^j)
By symmetry the maximum extent is the same in 60deg, 120deg, etc directions, suitably
rotated. The N in those cases has the digits 1,2,3,4,5,6 cycled around for the rotation.
In PlanePath triangular X,Y coordinates direction 60deg means when sum X+3*Y is a maximum,
etc.
If the +1/2 in the floor is omitted then the effect is to find the maximum point in
direction +30deg. In the PlanePath coordinates this means maximum sum S = X+Y.
N_smax_digit(j) = (-floor(F*j) mod 6) + 1
= 1,1,1,1,6,6,6,5,5,5,4,4,4,3,3, ...
k-1
N_smax(k) = digits N_smax_digit(j) low digit j=0
j=0
= 0, 1, 8, 57, 400, 14806, 115648, ... decimal
= 0, 1, 11, 111, 1111, 61111, 661111, ... base7
and also N_smax() + 1
z_smax_exp(j) = floor(F*j)
= 0,0,0,0,1,1,1,2,2,2,3,3,3,4,4,4,5,5,5,6, ...
z_smax(k) = sum(j=0,k-1, conj(w6)^z_smax_exp(j) * b^j)
= 0, 1, 7/2+1/2*sqrt3*i, 9+3*sqrt3*i, 19+12*sqrt3*i, ...
and also z_smax() + w6^2
smax(k) = 2*real(z_smax(k)) + imag(z_smax(k))*2/sqrt3
= 0, 2, 8, 24, 62, 172, 470, 1190, 3202, 8740, ...
coordinate sum X+Y max
In the base figure, points 1 and 2 have the same X+Y=2 and this remains so in subsequent
levels, so that for k>=1 N_smax(k) and N_smax(k)+1 are equal maximums.
SEE ALSO
Math::PlanePath, Math::PlanePath::GosperIslands, Math::PlanePath::Flowsnake,
Math::PlanePath::FlowsnakeCentres, Math::PlanePath::QuintetReplicate,
Math::PlanePath::ComplexPlus
HOME PAGE
<http://user42.tuxfamily.org/math-planepath/index.html>
LICENSE
Copyright 2011, 2012, 2013, 2014, 2015, 2016, 2017 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/>.