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Math::PlanePath::ArchimedeanChords -- radial spiral chords
use Math::PlanePath::ArchimedeanChords; my $path = Math::PlanePath::ArchimedeanChords->new; my ($x, $y) = $path->n_to_xy (123);
This path puts points at unit chord steps along an Archimedean spiral. The spiral goes outwards by 1 unit each revolution and the points are spaced 1 apart. The result is roughly
31 32 30 ... 3 33 29 14 34 15 13 28 50 2 12 16 3 35 2 27 49 1 4 11 17 36 5 0 1 26 48 <- Y=0 10 18 37 6 25 47 -1 9 19 7 8 24 46 38 -2 20 23 39 21 22 45 -3 40 44 41 42 43 ^ -3 -2 -1 X=0 1 2 3 4
X,Y positions returned are fractional. Each revolution is about 2*pi longer than the previous, so the effect is a kind of 6.28 increment looping.
Because the spacing is by unit chords, adjacent unit circles centred on each N position touch but don't overlap. The spiral spacing of 1 unit per revolution means they don't overlap radially either.
The unit chords here are a little like the TheodorusSpiral. But the TheodorusSpiral goes by unit steps at a fixed right-angle and approximates an Archimedean spiral (of 3.14 radial spacing). Whereas this ArchimedeanChords is an actual Archimedean spiral (of radial spacing 1), with unit steps angling along that.
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
$path = Math::PlanePath::ArchimedeanChords->new ()
Create and return a new path object.
($x,$y) = $path->n_to_xy ($n)
Return the X,Y coordinates of point number
$non the path.
$ncan be any value
$n >= 0and fractions give positions on the chord between the integer points (ie. straight line between the points).
$n==0is the origin 0,0.
$n < 0the return is an empty list, it being considered there are no negative points in the spiral.
$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
$x,$ywithin that circle returns N.
The unit spacing of the spiral means those circles don't overlap, but they also don't cover the plane and if
$x,$yis not within one then the return is
The current implementation is a bit slow.
$n = $path->n_start ()
Return 0, the first
$non the path.
$str = $path->figure ()
The current code keeps a position as a polar angle t and calculates an increment u needed to move along by a unit chord. If dist(u) is the straight-line distance between t and t+u, then squared is the hypotenuse
dist^2(u) = ((t+u)/2pi*cos(t+u) - t/2pi*cos(t))^2 # X + ((t+u)/2pi*sin(t+u) - t/2pi*sin(t))^2 # Y
which simplifies to
dist^2(u) = [ (t+u)^2 + t^2 - 2*t*(t+u)*cos(u) ] / (4*pi^2)
Switch from cos to sin using the half angle cos(u) = 1 - 2*sin^2(u/2) in case if u is small then the cos(u) near 1.0 might lose floating point accuracy, and also as a slight simplification,
dist^2(u) = [ u^2 + 4*t*(t+u)*sin^2(u/2) ] / (4*pi^2)
Then want the u which has dist(u)=1 for a unit chord. The u*sin(u) part probably doesn't have a good closed form inverse, so the current code is a Newton/Raphson iteration on f(u) = dist^2(u)-1, seeking f(u)=0
f(u) = u^2 + 4*t*(t+u)*sin^2(u/2) - 4*pi^2
Derivative f'(u) for the slope from the cos form is
f'(u) = 2*(t+u) - 2*t*[ cos(u) - (t+u)*sin(u) ]
And again switching from cos to sin in case u is small,
f'(u) = 2*[ u + t*[2*sin^2(u/2) + (t+u)*sin(u)] ]
A given x,y point is at a fraction of a revolution
frac = atan2(y,x) / 2pi # -.5 <= frac <= .5 frac += (frac < 0) # 0 <= frac < 1
And the nearest spiral arm measured radially from x,y is then
r = int(hypot(x,y) + .5 - frac) + frac
atan2 is the same as the C library and gives -pi <= angle <= pi, hence allowing for frac<0. It may also be "unspecified" for x=0,y=0, and give +/-pi for x=negzero, which has to be a special case so 0,0 gives r=0. The
int rounds towards zero, so frac>.5 ends up as r=0.
So the N point just before or after that spiral position may cover the x,y, but how many N chords it takes to get around to there is 's not so easily calculated.
The current code looks in saved
n_to_xy() positions for an N below the target, and searches up from there until past the target and thus not covering x,y. With
n_to_xy() points saved 500 apart this means searching somewhere between 1 and 500 points.
One possibility for calculating a lower bound for N, instead of the saved positions, and both for
rect_to_n_range(), would be to add up chords in circles. A circle of radius k fits pi/arcsin(1/2k) many unit chords, so
k=floor(r) pi total = sum ------------ k=0 arcsin(1/2k)
and this is less than the chords along the spiral. Is there a good polynomial over-estimate of arcsin, to become an under-estimate total, without giving away so much?
rect_to_n_range() upper bound, the current code takes the arc length along with spiral with the usual formula
arc = 1/4pi * (theta*sqrt(1+theta^2) + asinh(theta))
Written in terms of the r radius (theta = 2pi*r) as calculated from the biggest of the rectangle x,y corners,
arc = pi*r^2*sqrt(1+1/r^2) + asinh(2pi*r)/4pi
The arc length is longer than chords, so N=ceil(arc) is an upper bound for the N range.
An upper bound can also be calculated simply from the circumferences of circles 1 to r, since a spiral loop from radius k to k+1 is shorter than a circle of radius k.
k=ceil(r) total = sum 2pi*k k=1 = pi*r*(r+1)
This is bigger than the arc length, thus a poorer upper bound, but an easier calculation. (Incidentally, for smallish r have arc length <= pi*(r^2+1) which is a tighter bound and an easy calculation, but it only holds up to somewhere around r=10^7.)
Copyright 2010, 2011, 2012 Kevin Ryde
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