NAME
Math::PlanePath::HilbertCurve  2x2 selfsimilar quadrant traversal
SYNOPSIS
use Math::PlanePath::HilbertCurve;
my $path = Math::PlanePath::HilbertCurve>new;
my ($x, $y) = $path>n_to_xy (123);
DESCRIPTION
This path is an integer version of the curve described by Hilbert in 1891 for filling a unit square. It traverses a quadrant of the plane one step at a time in a selfsimilar 2x2 pattern,
David Hilbert, "Ueber die stetige Abbildung einer Linie auf ein Flächenstück", Mathematische Annalen, volume 38, number 3, 1891, pages 459460, DOI 10.1007/BF01199431.
...
 
7  6362 494847 444342
     
6  6061 5051 4645 4041
   
5  59 5655 52 3334 3938
       
4  5857 5453 32 353637
 
3  56 910 31 282726
       
2  4 78 11 3029 2425
   
1  32 1312 1718 2322
     
Y=0  01 141516 192021
+
X=0 1 2 3 4 5 6 7
The start is a sideways U shape N=0 to N=3, then four of those are put together in an upsidedown U as
5,6 9,10
4,7 8,11
 
3,2 13,12
0,1 14,15
The orientation of the sub parts ensure the starts and ends are adjacent, so 3 next to 4, 7 next to 8, and 11 next to 12.
The process repeats, doubling in size each time and alternately sideways or upsidedown U with invert and/or transpose as necessary in the subparts.
The pattern is sometimes drawn with the first step 0>1 upwards instead of to the right. First step right is used here for consistency with other PlanePaths. Swap X and Y for upwards first instead.
See examples/hilbertpath.pl for a sample program printing the path pattern in ascii.
Level Ranges
Within a powerof2 square 2x2, 4x4, 8x8, 16x16 etc (2^k)x(2^k) at the origin, all the N values 0 to 2^(2*k)1 are within the square. The maximum 3, 15, 63, 255 etc 2^(2*k)1 is alternately at the top left or bottom right corner.
Because each step is by 1, the distance along the curve between two X,Y points is the difference in their N values (as from xy_to_n()
).
On the X=Y diagonal N=0,2,8,10,32,etc is the integers using only digits 0 and 2 in base 4, or equivalently have evennumbered bits 0, like x0y0...z0.
Locality
The Hilbert curve is fairly well localized in the sense that a small rectangle (or other shape) is usually a small range of N. This property is used in some database systems to store X,Y coordinates using the resulting Hilbert curve N as an index. A search through a 2D region is then usually a fairly modest linear search through N values. rect_to_n_range()
gives exact N range for a rectangle, or see notes "Rectangle to N Range" below for calculating on any shape.
The N range can be large when crossing subparts. In the sample above it can be seen for instance adjacent points X=0,Y=3 and X=0,Y=4 have rather widely spaced N values 5 and 58.
Fractional X,Y values can be indexed by extending the N calculation down into X,Y binary fractions. The code here doesn't do that, but could be pressed into service by moving the binary point in X and Y an even number of places, the same in each. (An odd number of bits would require swapping X,Y to compensate for the alternating transpose in part 0.) The resulting integer N is then divided down by a corresponding multipleof4 binary places.
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.
$path = Math::PlanePath::HilbertCurve>new ()

Create and return a new path object.
($x,$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.Fractional positions give an X,Y position along a straight line between the integer positions. Integer positions are always just 1 apart either horizontally or vertically, so the effect is that the fraction part is an offset along either
$x
or$y
. $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 unit square and an$x,$y
within that square returns N. ($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.
Level Methods
FORMULAS
N to X,Y
Converting N to X,Y coordinates is reasonably straightforward. The top two bits of N is a configuration
32 12
 or transpose  
01 0 3
according to whether it's an odd or even bitpair position. Then within each of the "3" subparts there's also inverted forms
10 3 0
  
23 21
Working N from high to low with a state variable can record whether there's a transpose, an invert, or both, being four states altogether. A bit pair 0,1,2,3 from N then gives a bit each of X,Y according to the configuration and a new state which is the orientation of that subpart. Bill Gosper's HAKMEM item 115 has this with either bit operations or a table for the state and X,Y bits,
https://dspace.mit.edu/handle/1721.1/6086, http://www.inwap.com/pdp10/hbaker/hakmem/topology.html#item115
And C++ code based on that in Jorg Arndt's book,
http://www.jjj.de/fxt/#fxtbook (section 1.31.1)
It also works to process N from low to high, at each stage applying any transpose (swap X,Y) and/or invert (bitwise NOT) to the low X,Y bits generated so far. This works because there's no "reverse" sections, or equivalently the curve is the same forward and reverse. Low to high saves locating the top bits of N, but if using bignums then the bitwise inverts of the full X,Y values will be much more work.
X,Y to N
X,Y to N can follow the table approach from high to low taking one bit from X and Y each time. The state table of Npair > Xbit,Ybit is reversible, and a new state is based on the Npair thus obtained (or could be based on the X,Y bits if that mapping is combined into the state transition table).
Rectangle to N Range
An easy overestimate of the maximum N in a region can be had by finding the next bigger (2^k)x(2^k) square enclosing the region. This means the biggest X or Y rounded up to the next power of 2, so
find lowest k with 2^k > max(X,Y)
N_max = 2^(2k)  1
Or equivalently rounding down to the next lower power of 2,
find highest k with 2^k <= max(X,Y)
N_max = 2^(2*(k+1))  1
An exact N range can be found by following the high to low N to X,Y procedure above. Start at the 2^(2k) bit pair position in an N bigger than the desired region and choose 2 bits for N to give a bit each of X and Y. The X,Y bits are based on the state table as above and the bits chosen for N are those for which the resulting X,Y subsquare overlaps some of the target region. The smallest N similarly, choosing the smallest bit pair for N which overlaps.
The biggest and smallest N digit for a subpart can be found with a lookup table. The X range might cover one or both subparts, and the Y range similarly, for a total 4 possible configurations. Then a table of state+coverage > digit gives the minimum and maximum N bitpair, and state+digit gives a new state the same as X,Y to N.
Biggest and smallest N must be calculated with separate state and X,Y values since they track down different N bits and thus different states. But they take the same number of steps from an enclosing level down to level 0 and can thus be done in a single loop.
The N range for any shape can be found this way, not just a rectangle like rect_to_n_range()
. At each level the procedure only depends on asking which of the four subparts overlaps some of the target area.
Direction
Each step between successive N values is always 1 up, down, left or right. The next direction can be calculated from N in the hightolow procedure above by watching for the lowest non3 digit and noting the direction from that digit towards digit+1. That can be had from the state+digit > X,Y table looking up digit and digit+1, or alternatively a further table encoding state+digit > direction.
The reason for taking only the lowest non3 digit is that in a 3 subpart the direction it goes is determined by the next higher level. For example at N=11 the direction is down for the invertedU of the next higher level N=0,4,8,12.
This non3 (or non whatever highest digit) is a general procedure and can be used on any statebased hightolow procedure of selfsimilar curves. In the current code, it's used to apply a fractional part of N in the correct direction but is not otherwise made directly available.
Because the Hilbert curve has no "reverse" sections it also works to build a direction from low to high N digits. 1 and 2 digits make no change to the orientation, 0 digit is a transpose, and a 3 digit is a rotate and transpose, except that low 3s are transposeonly (no rotate) for the same reason as taking the lowest non3 above.
Jorg Arndt in the fxtbook above notes the direction can be obtained just by counting 3s in n and n (the twoscomplement). This numbers segments starting n=1, unlike PlanePath here starting N=0, so it becomes
N+1 count 3s / 0 mod 2 S or E
\ 1 mod 2 N or W
(N+1) count 3s / 0 mod 2 N or E
\ 1 mod 2 S or W
For the twoscomplement negation, an even number of base4 digits of N must be taken. Because (N+1) = ~N, ie. a onescomplement, the second part is also
N count 0s / 0 mod 2 N or E
in even num digits \ 1 mod 2 S or W
Putting the two together then
N count 0s N+1 count 3s direction (0=E,1=N,etc)
in base 4 in base 4
0 mod 2 0 mod 2 0
1 mod 2 0 mod 2 3
0 mod 2 1 mod 2 1
1 mod 2 1 mod 2 2
Segments in Direction
The number of segments in each direction is calculated in
Sergey Kitaev, Toufik Mansour and Patrice Séébold, "Generating the Peano Curve and Counting Occurrences of Some Patterns", Journal of Automata, Languages and Combinatorics, volume 9, number 4, 2004, pages 439455. http://personal.strath.ac.uk/sergey.kitaev/publications.html, http://www.jalc.de/issues/2004/issue_9_4/jalc2004439455.php
(Preprint as Sergey Kitaev and Toufik Mansour, "The Peano Curve and Counting Occurrences of Some Patterns", October 2002. http://arxiv.org/abs/math/0210268/.)
Their form is based on keeping the topmost U shape fixed and expanding subparts. This means the end segments alternate vertical and horizontal in successive expansion levels.
direction k=1 2 2
1 to 4 ** **
2 1 3 1 3
1 ** * ** *
 1 3 1 4 2 4 3
4 2 * * ** **
 1 3 k=2
3 ** **
2 2
count segments in direction, for k >= 1
d(1,k) = 4^(k1) = 1,4,16,64,256,1024,4096,...
d(2,k) = 4^(k1) + 2^(k1)  1 = 1,5,19,71,271,1055,4159,...
d(3,k) = 4^(k1) = 1,4,16,64,256,1024,4096,...
d(4,k) = 4^(k1)  2^(k1) = 0,2,12,56,240, 992,4032,...
(A000302, A099393, A000302, A020522)
total segments d(1,k)+d(2,k)+d(3,k)+d(4,k) = 4^k  1
The path form here keeps the first segment direction fixed. This means a transpose 1<>2 and 3<>4 in odd levels. The result is to take the alternate d values as follows. For k=0 there is a single point N=0 so no line segments at all and so c(dir,0)=0.
first 4^k1 segments
c(1,k) = / 0 if k=0
North  4^(k1) + 2^(k1)  1 if k odd >= 1
\ 4^(k1) if k even >= 2
= 0, 1, 4, 19, 64, 271, 1024, 4159, 16384, ...
c(2,k) = / 0 if k=0
East  4^(k1) if k odd >= 1
\ 4^(k1) + 2^(k1)  1 if k even >= 2
= 0, 1, 5, 16, 71, 256, 1055, 4096, 16511, ...
c(3,k) = / 0 if k=0
South  4^(k1)  2^(k1) if k odd >= 1
\ 4^(k1) if k even >= 2
= 0, 0, 4, 12, 64, 240, 1024, 4032, 16384, ...
c(4,k) = / 0 if k=0
West  4^(k1) if k odd >= 1
\ 4^(k1)  2^(k1) if k even >= 2
= 0, 1, 2, 16, 56, 256, 992, 4096, 16256, ...
The segment N=4^k1 to N=4^k is North (direction 1) when k odd, or East (direction 2) when k even. That could be added to the respective cases in c(1,k) and c(2,k) if desired.
Hamming Distance
The Hamming distance between integers X and Y is the number of bit positions where the two values differ when written in binary. On the Hilbert curve each bitpair of N becomes a bit of X and a bit of Y,
N X Y
  
0 = 00 0 0
1 = 01 1 0 < difference 1 bit
2 = 10 1 1
3 = 11 0 1 < difference 1 bit
So the Hamming distance for N=0to3 is 1 at N=1 and N=3. At higher levels, these X,Y bits may be transposed (swapped) or rotated by 180 or both. A transpose swapping X<>Y doesn't change the bit difference. A rotate by 180 is a flip 0<>1 of the bit in both X and Y, so that doesn't change the bit difference either.
On that basis, the Hamming distance X,Y is the number of base4 digits of N which are 01 or 11. If bit positions are counted from 0 for the least significant bit then
X,Y coordinates of N
HammingDist(X,Y) = count 1bits at even bit positions in N
= 0,1,0,1, 1,2,1,2, 0,1,0,1, 1,2,1,2, ... (A139351)
See also "Hamming Distance" in Math::PlanePath::CornerReplicate which is the same formula, but arising directly from 01 or 11, no transpose or rotate.
OEIS
This path is in Sloane's OEIS in several forms,
http://oeis.org/A059252 (etc)
A059253 X coord
A059252 Y coord
A059261 X+Y
A059285 XY
A163547 X^2+Y^2 = radius squared
A139351 HammingDist(X,Y)
A059905 X xor Y, being ZOrderCurve X
A163365 sum N on diagonal
A163477 sum N on diagonal, divided by 4
A163482 N values on X axis
A163483 N values on Y axis
A062880 N values on diagonal X=Y (digits 0,2 in base 4)
A163538 dX 1,0,1 change in X
A163539 dY 1,0,1 change in Y
A163540 absolute direction of each step (0=E,1=N,2=W,3=S)
A163541 absolute direction, swapped X,Y
A163542 relative direction (ahead=0,right=1,left=2)
A163543 relative direction, swapped X,Y
A083885 count East segments N=0 to N=4^k (first 4^k segs)
A163900 distance dX^2+dY^2 between Hilbert and ZOrder
A165464 distance dX^2+dY^2 between Hilbert and Peano
A165466 distance dX^2+dY^2 between Hilbert and transposed Peano
A165465 N where Hilbert and Peano have same X,Y
A165467 N where Hilbert and Peano have transposed same X,Y
The following take points of the plane in various orders, each value in the sequence being the N of the Hilbert curve at those positions.
A163355 N by the ZOrderCurve points sequence
A163356 inverse, ZOrderCurve by Hilbert points order
A166041 N by the PeanoCurve points sequence
A166042 inverse, PeanoCurve N by Hilbert points order
A163357 N by diagonals like Math::PlanePath::Diagonals with
first Hilbert step along same axis the diagonals start
A163358 inverse
A163359 N by diagonals, transposed start along the opposite axis
A163360 inverse
A163361 A163357 + 1, numbering the Hilbert N's from N=1
A163362 inverse
A163363 A163355 + 1, numbering the Hilbert N's from N=1
A163364 inverse
The above sequences are permutations of the integers since all X,Y positions of the first quadrant are covered by each path (Hilbert, ZOrder, Peano). The inverse permutations can be thought of taking X,Y positions in the Hilbert order and asking what N the ZOrder, Peano or Diagonals path would put there.
The A163355 permutation by ZOrderCurve can be considered for repeats or cycles,
A163905 ZOrderCurve permutation A163355 applied twice
A163915 ZOrderCurve permutation A163355 applied three times
A163901 fixed points (N where X,Y same in both curves)
A163902 2cycle points
A163903 3cycle points
A163890 cycle lengths, points by N
A163904 cycle lengths, points by diagonals
A163910 count of cycles in 4^k blocks
A163911 max cycle length in 4^k blocks
A163912 LCM of cycle lengths in 4^k blocks
A163914 count of 3cycles in 4^k blocks
A163909 those counts for even k only
A163891 N of previously unseen cycle length
A163893 first differences of those A163891
A163894 smallest value not an ncycle
A163895 position of new high in A163894
A163896 value of new high in A163894
A163907 ZOrderCurve permutation twice, on points by diagonals
A163908 inverse of this
See examples/hilbertoeis.pl for a sample program printing the A163359 permutation values.
SEE ALSO
Math::PlanePath, Math::PlanePath::HilbertSides, Math::PlanePath::HilbertSpiral
Math::PlanePath::PeanoCurve, Math::PlanePath::ZOrderCurve, Math::PlanePath::BetaOmega, Math::PlanePath::KochCurve
Math::Curve::Hilbert, Algorithm::SpatialIndex::Strategy::QuadTree
HOME PAGE
http://user42.tuxfamily.org/mathplanepath/index.html
LICENSE
Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 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/>.