NAME
Math::PlanePath::AztecDiamondRings  rings around an Aztec diamond shape
SYNOPSIS
use Math::PlanePath::AztecDiamondRings;
my $path = Math::PlanePath::AztecDiamondRings>new;
my ($x, $y) = $path>n_to_xy (123);
DESCRIPTION
This path makes rings around an Aztec diamond shape,
4645 4
/ \
47 2928 44 3
/ / \ \
48 30 1615 27 43 ... 2
/ / / \ \ \ \
49 31 17 76 14 26 42 62 1
/ / / / \ \ \ \ \
50 32 18 8 21 5 13 25 41 61 < Y=0
         
51 33 19 9 34 12 24 40 60 1
\ \ \ \ / / / /
52 34 20 1011 23 39 59 2
\ \ \ / / /
53 35 2122 38 58 3
\ \ / /
54 3637 57 4
\ /
5556 5
^
5 4 3 2 1 X=0 1 2 3 4 5
This is similar to the DiamondSpiral
, but has all four corners flattened to 2 vertical or horizontal, instead of just one in the DiamondSpiral
. This is only a small change to the alignment of numbers in the sides, but is more symmetric.
Y axis N=1,6,15,28,45,66,etc are the hexagonal numbers k*(2k1). The hexagonal numbers of the "second kind" 3,10,21,36,55,78, etc k*(2k+1), are the vertical at X=1 going downwards. Combining those two is the triangular numbers 3,6,10,15,21,etc, k*(k+1)/2, alternately on one line and the other. Those are the positions of all the horizontal steps, ie. where dY=0.
X axis N=1,5,13,25,etc is the "centred square numbers". Those numbers are made by drawing concentric squares with an extra point on each side each time. The path here grows the same way, adding one extra point to each of the four sides.
****
 
 ***  count total "*"s for
    centred square numbers
*  **  *
     
 *  *  * 
     
  **  
*   *
 *** 
 
****
N Start
The default is to number points starting N=1 as shown above. An optional n_start
can give a different start, in the same pattern. For example to start at 0,
n_start => 0
45 44
46 28 27 43
47 29 15 14 26 42
48 30 16 6 5 13 25 41
49 31 17 7 1 0 4 12 24 40
50 32 18 8 2 3 11 23 39 59
51 33 19 9 10 22 38 58
52 34 20 21 37 57
53 35 36 56
54 55
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
$path = Math::PlanePath::AztecDiamondRings>new ()
$path = Math::PlanePath::AztecDiamondRings>new (n_start => $n)

Create and return a new Aztec diamond 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 point in the path as a square of side 1, so the entire plane is covered. ($n_lo, $n_hi) = $path>rect_to_n_range ($x1,$y1, $x2,$y2)

The returned range is exact, meaning
$n_lo
and$n_hi
are the smallest and biggest in the rectangle.
FORMULAS
X,Y to N
The path makes lines in each quadrant. The quadrant is determined by the signs of X and Y, then the line in that quadrant is either d=X+Y or d=XY. A quadratic in d gives a starting N for the line and Y (or X if desired) is an offset from there,
Y>=0 X>=0 d=X+Y N=(2d+2)*d+1 + Y
Y>=0 X<0 d=YX N=2d^2  Y
Y<0 X>=0 d=XY N=(2d+2)*d+1 + Y
Y<0 X<0 d=X+Y N=(2d+4)*d+2  Y
For example
Y=2 X=3 d=2+3=5 N=(2*5+2)*5+1 + 2 = 63
Y=2 X=1 d=2(1)=3 N=2*3*3  2 = 16
Y=1 X=4 d=4(1)=5 N=(2*5+2)*5+1 + 1 = 60
Y=2 X=3 d=3+(2)=5 N=(2*5+4)*5+2  (2) = 34
The two X>=0 cases are the same N formula and can be combined with an abs,
X>=0 d=X+abs(Y) N=(2d+2)*d+1 + Y
This works because at Y=0 the last line of one ring joins up to the start of the next. For example N=11 to N=15,
15 2
\
14 1
\
13 < Y=0
12 1
/
11 2
^
X=0 1 2
Rectangle to N Range
Within each row N increases as X increases away from the Y axis, and within each column similarly N increases as Y increases away from the X axis. So in a rectangle the maximum N is at one of the four corners of the rectangle.

x1,y2 MM x2,y2
  
O
  
  
x1,y1 MM x1,y1

For any two rows y1 and y2, the values in row y2 are all bigger than in y1 if y2>=y1. This is so even when y1 and y2 are on the same side of the origin, ie. both positive or both negative.
For any two columns x1 and x2, the values in the part with Y>=0 are all bigger if x2>=x1, or in the part of the columns with Y<0 it's x2>=x11. So the biggest corner is at
max_y = (y2 >= y1 ? y2 ? y1)
max_x = (x2 >= x1  (max_y<0) ? x2 : x1)
The difference in the X handling for Y positive or negative is due to the quadrant ordering. When Y>=0, at X and X the bigger N is the X negative side, but when Y<0 it's the X positive side.
A similar approach gives the minimum N in a rectangle.
min_y = / y2 if y2 < 0, and set xbase=1
 y1 if y1 > 0, and set xbase=0
\ 0 otherwise, and set xbase=0
min_x = / x2 if x2 < xbase
 x1 if x1 > xbase
\ xbase otherwise
The minimum row is Y=0, but if that's not in the rectangle then the y2 or y1 top or bottom edge is the minimum. Then within any row the minimum N is at xbase=0 if Y<0 or xbase=1 if Y>=0. If that xbase is not in range then the x2 or x1 left or right edge is the minimum.
OEIS
Entries in Sloane's Online Encyclopedia of Integer Sequences related to this path include
http://oeis.org/A001844 (etc)
n_start=1 (the default)
A001844 N on X axis, the centred squares 2k(k+1)+1
n_start=0
A046092 N on X axis, 4*triangular
A139277 N on diagonal X=Y
A023532 abs(dY), being 0 if N=k*(k+3)/2
SEE ALSO
Math::PlanePath, Math::PlanePath::DiamondSpiral
HOME PAGE
http://user42.tuxfamily.org/mathplanepath/index.html
LICENSE
Copyright 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019 Kevin Ryde
This file is part of MathPlanePath.
MathPlanePath 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.
MathPlanePath 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 MathPlanePath. If not, see <http://www.gnu.org/licenses/>.