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Math::PlanePath::SquareSpiral -- integer points drawn around a square (or rectangle)
use Math::PlanePath::SquareSpiral; my $path = Math::PlanePath::SquareSpiral->new; my ($x, $y) = $path->n_to_xy (123);
This path makes a square spiral,
37--36--35--34--33--32--31 3 | | 38 17--16--15--14--13 30 2 | | | | 39 18 5---4---3 12 29 1 | | | | | | 40 19 6 1---2 11 28 ... <- Y=0 | | | | | | 41 20 7---8---9--10 27 52 -1 | | | | 42 21--22--23--24--25--26 51 -2 | | 43--44--45--46--47--48--49--50 -3 ^ -3 -2 -1 X=0 1 2 3 4
See examples/square-numbers.pl for a simple program printing these numbers.
This path is well known from Stanislaw Ulam finding interesting straight lines when plotting the prime numbers on it.
Stein, Ulam and Wells, "A Visual Display of Some Properties of the Distribution of Primes", American Mathematical Monthly, volume 71, number 5, May 1964, pages 516-520. http://www.jstor.org/stable/2312588
The cover of Scientific American March 1964 featured this spiral,
See examples/ulam-spiral-xpm.pl for a standalone program, or see math-image using this
SquareSpiral to draw this pattern and more.
Stein, Ulam and Wells above also considered primes on the Math::PlanePath::Corner path, and on a half-plane like two corners.
The perfect squares 1,4,9,16,25 fall on two diagonals with the even perfect squares going to the upper left and the odd squares to the lower right. The pronic numbers 2,6,12,20,30,42 etc k^2+k half way between the squares fall on similar diagonals to the upper right and lower left. The decagonal numbers 10,27,52,85 etc 4*k^2-3*k go horizontally to the right at Y=-1.
In general, straight lines and diagonals are 4*k^2 + b*k + c. b=0 is the even perfect squares up to the left, then incrementing b is an eighth turn anti-clockwise, or clockwise if negative. So b=1 is horizontal West, b=2 diagonally down South-West, b=3 down South, etc.
Honaker's prime-generating polynomial 4*k^2 + 4*k + 59 goes down to the right after the first 30 or so values loop around a bit.
wider parameter makes the path wider, becoming a rectangle spiral instead of a square. For example
wider => 3 29--28--27--26--25--24--23--22 2 | | 30 11--10-- 9-- 8-- 7-- 6 21 1 | | | | 31 12 1-- 2-- 3-- 4-- 5 20 <- Y=0 | | | 32 13--14--15--16--17--18--19 -1 | 33--34--35--36-... -2 ^ -4 -3 -2 -1 X=0 1 2 3
The centre horizontal 1 to 2 is extended by
wider many further places, then the path loops around that shape. The starting point 1 is shifted to the left by ceil(wider/2) places to keep the spiral centred on the origin X=0,Y=0.
Widening doesn't change the nature of the straight lines which arise, it just rotates them around. For example in this wider=3 example the perfect squares are still on diagonals, but the even squares go towards the bottom left (instead of top left when wider=0) and the odd squares to the top right (instead of the bottom right).
Each loop is still 8 longer than the previous, since the widening is a constant amount in each loop.
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. For example to start at 0,
n_start => 0 16-15-14-13-12 ... | | | 17 4--3--2 11 28 | | | | | 18 5 0--1 10 27 | | | | 19 6--7--8--9 26 | | 20-21-22-23-24-25
The only effect is to push the N values around by a constant amount. It might help match coordinates with something else zero-based.
Other spirals can be formed by cutting the corners of the square so as to go around faster. See the following modules,
Corners Cut Class ----------- ----- 1 HeptSpiralSkewed 2 HexSpiralSkewed 3 PentSpiralSkewed 4 DiamondSpiral
PyramidSpiral is a re-shaped
SquareSpiral looping at the same rate. It shifts corners but doesn't cut them.
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
$path = Math::PlanePath::SquareSpiral->new ()
$path = Math::PlanePath::SquareSpiral->new (wider => $integer, n_start => $n)
Create and return a new square spiral object. An optional
widerparameter widens the spiral path, it defaults to 0 which is no widening.
($x,$y) = $path->n_to_xy ($n)
Return the X,Y coordinates of point number
$non the path.
$n < 1the return is an empty list, as the path starts at 1.
$n = $path->xy_to_n ($x,$y)
Return the point number for coordinates
$yare 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.
There's a few ways to break an N into a side and offset into the side. One convenient way is to treat a loop as starting at the bottom right turn, so N=2,10,26,50,etc, If the first loop at N=2 is reckoned loop number d=1 then the loop starts at
Nbase = 4*d^2 - 4*d + 2 = 2,10,26,50,... for d=1,2,3,4,... (A069894 but it going from d=0)
For example d=3 is Nbase=4*3^2-4*3+2=26 at X=3,Y=-2. The biggest d with Nbase <= N can be found by inverting with the usual quadratic formula
d = floor( 1/2 + sqrt(N/4 - 1/4) )
For Perl it's good to keep the sqrt argument an integer (when a UV integer is bigger than an NV float, and for BigRat accuracy), so rearrange to
d = floor( (1+sqrt(N-1))/2 )
So Nbase from this d leaves a remainder which is an offset into the loop
Nrem = N - Nbase = N - (4*d^2 - 4*d + 2)
The loop starts at X=d,Y=d-1 and has sides length 2d, 2d+1, 2d+1 and 2d+2,
2d +------------+ <- Y=d | | 2d | | 2d-1 | . | | | | + X=d,Y=-d+1 | +---------------+ <- Y=-d 2d+1 ^ X=-d
The X,Y for an Nrem is then
side Nrem range X,Y result ---- ---------- ---------- right Nrem <= 2d-1 X = d Y = -d+1+Nrem top 2d-1 <= Nrem <= 4d-1 X = d-(Nrem-(2d-1)) = 3d-1-Nrem Y = d left 4d-1 <= Nrem <= 6d-1 X = -d Y = d-(Nrem-(4d-1)) = 5d-1-Nrem bottom 6d-1 <= Nrem X = -d+(Nrem-(6d-1)) = -7d+1+Nrem Y = -d
The corners Nrem=2d-1, Nrem=4d-1 and Nrem=6d-1 get the same result from the two sides that meet so it doesn't matter if the high comparison is "<" or "<=".
The bottom edge runs through to Nrem < 8d, but there's no need to check that since d=floor(sqrt()) above ensures Nrem is within the loop.
A small simplification can be had by subtracting an extra 4d-1 from Nrem to make negatives for the right and top sides and positives for the left and bottom.
Nsig = N - Nbase - (4d-1) = N - (4*d^2 - 4*d + 2) - (4d-1) = N - (4*d^2 + 1) side Nsig range X,Y result ---- ---------- ---------- right Nsig <= -2d X = d Y = d+(Nsig+2d) = 3d+Nsig top -2d <= Nsig <= 0 X = -d-Nsig Y = d left 0 <= Nsig <= 2d X = -d Y = d-Nsig bottom 2d <= Nsig X = -d+1+(Nsig-(2d+1)) = Nsig-3d Y = -d
This calculation can be found in
Ronald L. Graham, Donald E. Knuth, Oren Patashnik, "Concrete Mathematics", Addison-Wesley, 1989, chapter 3 "Integer Functions", exercise 40 page 99, answer page 498.
They start the spiral from 0, and first step North so their x is -Y here. Their formula for x(n) tests a floor(2*sqrt(N)) to decide whether on a horizontal and so whether to apply the equivalent of Nrem to the result.
wider parameter stretching the spiral loops the formulas above become
Nbase = 4*d^2 + (-4+2w)*d + 2-w d = floor ((2-w + sqrt(4N + w^2 - 4)) / 4)
Notice for Nbase the w is a term 2*w*d, being an extra 2*w for each loop.
The left offset ceil(w/2) described above ("Wider") for the N=1 starting position is written here as wl, and the other half wr arises too,
wl = ceil(w/2) wr = floor(w/2) = w - wl
The horizontal lengths increase by w, and positions shift by wl or wr, but the verticals are unchanged.
2d+w +------------+ <- Y=d | | 2d | | 2d-1 | . | | | | + X=d+wr,Y=-d+1 | +---------------+ <- Y=-d 2d+1+w ^ X=-d-wl
The Nsig formulas then have w, wl or wr variously inserted. In all cases if w=wl=wr=0 then they simplify to the plain versions.
Nsig = N - Nbase - (4d-1+w) = N - ((4d + 2w)*d + 1) side Nsig range X,Y result ---- ---------- ---------- right Nsig <= -(2d+w) X = d+wr Y = d+(Nsig+2d+w) = 3d+w+Nsig top -(2d+w) <= Nsig <= 0 X = -d-wl-Nsig Y = d left 0 <= Nsig <= 2d X = -d-wl Y = d-Nsig bottom 2d <= Nsig X = -d+1-wl+(Nsig-(2d+1)) = Nsig-wl-3d Y = -d
Within each row the minimum N is on the X=Y diagonal and N values increases monotonically as X moves away to the left or right. Similarly in each column there's a minimum N on the X=-Y opposite diagonal, or X=-Y+1 diagonal when X negative, and N increases monotonically as Y moves away from there up or down. When wider>0 the location of the minimum changes, but N is still monotonic moving away from the minimum.
On that basis the maximum N in a rectangle is at one of the four corners,
| x1,y2 M---|----M x2,y2 corner candidates | | | for maximum N -------O--------- | | | | | | x1,y1 M---|----M x1,y1 |
This path is in Sloane's Online Encyclopedia of Integer Sequences in various forms. Summary at
And various sequences,
wider=0 (the default) A174344 X coordinate A274923 Y coordinate A268038 negative Y A296030 X,Y pairs A214526 abs(X)+abs(Y) "Manhattan" distance A079813 abs(dY), being k 0s followed by k 1s A055086 direction (total turn) A000267 direction + 1 A063826 direction 1=right,2=up,3=left,4=down A027709 boundary length of N unit squares A078633 grid sticks to make N unit squares A240025 turn 1=left,0=straight (extra initial 1) A033638 N turn positions (extra initial 1, 1) A172979 N turn positions which are primes too A242601 X and Y location of origin then each turn, X=A242601(n+1), Y=A242601(n) A080037 N straight-ahead (except initial 2) A248333 num straight points among the first N A083479 num non-turn points among the first N (straight and the origin) A054552 N values on X axis (East) A054556 N values on Y axis (North) A054567 N values on negative X axis (West) A033951 N values on negative Y axis (South) A054554 N values on X=Y diagonal (NE) A054569 N values on negative X=Y diagonal (SW) A053755 N values on X=-Y opp diagonal X<=0 (NW) A016754 N values on X=-Y opp diagonal X>=0 (SE) A200975 N values on all four diagonals A317186 N on Y axis positive and negative A267682 N on Y axis positive and negative (origin twice) A265400 N inner neighbour A137928 N values on X=-Y+1 opposite diagonal A002061 N values on X=Y diagonal pos and neg A016814 (4k+1)^2, every second N on south-east diagonal A143856 N values on ENE slope dX=2,dY=1 A143861 N values on NNE slope dX=1,dY=2 A215470 N prime and >=4 primes among its 8 neighbours A214664 X coordinate of prime N (Ulam's spiral) A214665 Y coordinate of prime N (Ulam's spiral) A214666 -X \ reckoning spiral starting West A214667 -Y / A053999 prime[N] on X=-Y opp diagonal X>=0 (SE) A054551 prime[N] on the X axis (E) A054553 prime[N] on the X=Y diagonal (NE) A054555 prime[N] on the Y axis (N) A054564 prime[N] on X=-Y opp diagonal X<=0 (NW) A054566 prime[N] on negative X axis (W) A090925 permutation N at rotate +90 A090928 permutation N at rotate +180 A090929 permutation N at rotate +270 A090930 permutation N at clockwise spiralling A020703 permutation N at rotate +90 and go clockwise A090861 permutation N at rotate +180 and go clockwise A090915 permutation N at rotate +270 and go clockwise A185413 permutation N at 1-X,Y being rotate +180, offset X+1, clockwise A068225 permutation N at X+1,Y A121496 run lengths of consecutive N in this permutation A068226 permutation N at X-1,Y A334752 permutation N at X,Y+1 A334751 permutation N at X,Y-1 A020703 permutation N at transpose Y,X (clockwise <-> anti-clockwise) A033952 digits on negative Y axis A033953 digits on negative Y axis, starting 0 A033988 digits on negative X axis, starting 0 A033989 digits on Y axis, starting 0 A033990 digits on X axis, starting 0 A062410 total sum previous row or column wider=1 A069894 N on South-West diagonal
The following have "offset 0" S and therefore start from N=0.
n_start=0 A180714 X+Y coordinate sum A053615 abs(X-Y), runs n to 0 to n, distance to nearest pronic A001107 N on X axis A033991 N on Y axis A033954 N on negative Y axis, second 10-gonals A002939 N on X=Y diagonal North-East A016742 N on North-West diagonal, 4*k^2 A002943 N on South-West diagonal A156859 N on Y axis positive and negative
X11 cursor font "box spiral" cursor which is this style (but going clockwise).
Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 Kevin Ryde
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