Provided by: libmath-planepath-perl_129-1_all 
NAME
Math::PlanePath::DiamondSpiral -- integer points around a diamond shaped spiral
SYNOPSIS
use Math::PlanePath::DiamondSpiral;
my $path = Math::PlanePath::DiamondSpiral->new;
my ($x, $y) = $path->n_to_xy (123);
DESCRIPTION
This path makes a diamond shaped spiral.
19 3
/ \
20 9 18 2
/ / \ \
21 10 3 8 17 1
/ / / \ \ \
22 11 4 1---2 7 16 <- Y=0
\ \ \ / /
23 12 5---6 15 ... -1
\ \ / /
24 13--14 27 -2
\ /
25--26 -3
^
-3 -2 -1 X=0 1 2 3
This is not simply the "SquareSpiral" rotated, it spirals around faster, with side lengths
following a pattern 1,1,1,1, 2,2,2,2, 3,3,3,3, etc, if the flat kink at the bottom (like
N=13 to N=14) is treated as part of the lower right diagonal.
Going diagonally on the sides as done here is like cutting the corners of the
"SquareSpiral", which is how it gets around in fewer steps than the "SquareSpiral". See
"PentSpiralSkewed", "HexSpiralSkewed" and "HeptSpiralSkewed" for similar cutting just 3, 2
or 1 of the corners.
N=1,5,13,25,etc on the Y negative axis is the "centred square numbers" 2*k*(k+1)+1.
N Start
The default is to number points starting N=1 as shown above. An optional "n_start" can
give a different start, with the same shape etc. For example to start at 0,
n_start => 0 18
/ \
19 8 17
/ / \ \
20 9 2 7 16
/ / / \ \ \
21 10 3 0-- 1 6 15
\ \ \ / /
22 11 4-- 5 14 ...
\ \ / /
23 12--13 26
\ /
24--25
N=0,1,6,15,28,etc on the X axis is the hexagonal numbers k*(2k-1). N=0,3,10,21,36,etc on
the negative X axis is the hexagonal numbers of the "second kind" k*(2k-1) for k<0.
Combining those two is the triangular numbers 0,1,3,6,10,15,21,etc, k*(k+1)/2, on the X
axis alternately positive and negative.
N=0,2,8,18,etc on the Y axis is 2*squares, 2*Y^2. N=0,4,12,24,etc on the negative Y axis
is 2*pronic, 2*Y*(Y+1).
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
"$path = Math::PlanePath::DiamondSpiral->new ()"
"$path = Math::PlanePath::DiamondSpiral->new (n_start => $n)"
Create and return a new diamond spiral object.
"($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
Rectangle to N Range
Within each row N increases as X moves away from the Y axis, and within each column
similarly N increases as Y moves away from the X axis. So in a rectangle the maximum N is
at one of the four corners.
|
x1,y2 M---|----M x2,y2
| | |
-------O---------
| | |
| | |
x1,y1 M---|----M x1,y1
|
For any two columns x1 and x2 with x1<x2, the values in column x2 are all bigger if
x2>-x1. This is so even when x1 and x2 are on the same side of the origin, ie. both
positive or both negative.
For any two rows y1 and y2, the values in the part of the row with X>0 are bigger if
y2>=-y1, and in the part of the row with X<=0 it's y2>-y1, or equivalently y2>=-y1+1. So
the biggest corner is at
max_x = (x2 > -x1 ? x2 : x1)
max_y = (y2 >= -y1+(max_x<=0) ? y2 : y1)
The minimum is similar but a little simpler. In any column the minimum is at Y=0, and in
any row the minimum is at X=0. So at 0,0 if that's in the rectangle, or the edge on the
side nearest the origin when not.
min_x = / if x2 < 0 then x2
| if x1 > 0 then x1
\ else 0
min_y = / if y2 < 0 then y2
| if y1 > 0 then y1
\ else 0
OEIS
Entries in Sloane's Online Encyclopedia of Integer Sequences related to this path include
<http://oeis.org/A010751> (etc)
n_start=1
A130883 N on X axis, 2*n^2-n+1
A058331 N on Y axis, 2*n^2 + 1
A001105 N on column X=1, 2*n^2
A084849 N on X negative axis, 2*n^2+n+1
A001844 N on Y negative axis, centred squares 2*n^2+2n+1
A215471 N with >=5 primes among its 8 neighbours
A215468 sum 8 neighbours N
A217015 N permutation points order SquareSpiral rotate -90,
value DiamondSpiral N at each
A217296 inverse permutation
n_start=0
A010751 X coordinate, runs 1 inc, 2 dec, 3 inc, etc
A305258 Y coordinate
A053616 abs(Y), runs k to 0 to k
A000384 N on X axis, hexagonal numbers
A001105 N on Y axis, 2*n^2 (and cf similar A184636)
A014105 N on X negative axis, second hexagonals
A046092 N on Y negative axis, 2*pronic
A003982 delta(abs(X)+abs(Y)), 1 when N on Y negative axis
which is where move "outward" to next ring
n_start=-1
A188551 N positions of turns, from N=1 up
A188552 and which are primes
SEE ALSO
Math::PlanePath, Math::PlanePath::DiamondArms, Math::PlanePath::AztecDiamondRings,
Math::PlanePath::SquareSpiral, Math::PlanePath::HexSpiralSkewed,
Math::PlanePath::PyramidSides, Math::PlanePath::ToothpickSpiral
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/>.