Provided by: libmath-planepath-perl_129-1_all 
NAME
Math::PlanePath::HilbertCurve -- 2x2 self-similar 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 self-similar 2x2 pattern,
David Hilbert, "Ueber die stetige Abbildung einer Linie auf ein Flächenstück", Mathematische Annalen,
volume 38, number 3, 1891, pages 459-460, DOI 10.1007/BF01199431.
...
| |
7 | 63--62 49--48--47 44--43--42
| | | | | |
6 | 60--61 50--51 46--45 40--41
| | | |
5 | 59 56--55 52 33--34 39--38
| | | | | | | |
4 | 58--57 54--53 32 35--36--37
| |
3 | 5---6 9--10 31 28--27--26
| | | | | | | |
2 | 4 7---8 11 30--29 24--25
| | | |
1 | 3---2 13--12 17--18 23--22
| | | | | |
Y=0 | 0---1 14--15--16 19--20--21
+----------------------------------
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 upside-down 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 upside-down U with invert
and/or transpose as necessary in the sub-parts.
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/hilbert-path.pl for a sample program printing the path pattern in ascii.
Level Ranges
Within a power-of-2 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 even-numbered 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 2-D 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 sub-parts. 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 multiple-of-4 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 $n_hi are the smallest and biggest in the rectangle.
Level Methods
"($n_lo, $n_hi) = $path->level_to_n_range($level)"
Return "(0, 4**$level - 1)".
FORMULAS
N to X,Y
Converting N to X,Y coordinates is reasonably straightforward. The top two bits of N is a configuration
3--2 1--2
| or transpose | |
0--1 0 3
according to whether it's an odd or even bit-pair position. Then within each of the "3" sub-parts
there's also inverted forms
1--0 3 0
| | |
2--3 2--1
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 sub-part. 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 N-pair -> X-bit,Y-bit is reversible, and a new state is based on the N-pair 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 over-estimate 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 sub-square 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 sub-part can be found with a lookup table. The X range might
cover one or both sub-parts, and the Y range similarly, for a total 4 possible configurations. Then a
table of state+coverage -> digit gives the minimum and maximum N bit-pair, 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 sub-parts 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 high-to-low procedure above by watching for the lowest non-3 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 non-3 digit is that in a 3 sub-part the direction it goes is
determined by the next higher level. For example at N=11 the direction is down for the inverted-U of the
next higher level N=0,4,8,12.
This non-3 (or non whatever highest digit) is a general procedure and can be used on any state-based
high-to-low procedure of self-similar 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 transpose-only (no rotate) for the same reason as taking the
lowest non-3 above.
Jorg Arndt in the fxtbook above notes the direction can be obtained just by counting 3s in n and -n (the
twos-complement). 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 twos-complement negation, an even number of base-4 digits of N must be taken. Because -(N+1) =
~N, ie. a ones-complement, 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 439-455. <http://personal.strath.ac.uk/sergey.kitaev/publications.html>,
<http://www.jalc.de/issues/2004/issue_9_4/jalc-2004-439-455.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 top-most U shape fixed and expanding sub-parts. 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^(k-1) = 1,4,16,64,256,1024,4096,...
d(2,k) = 4^(k-1) + 2^(k-1) - 1 = 1,5,19,71,271,1055,4159,...
d(3,k) = 4^(k-1) = 1,4,16,64,256,1024,4096,...
d(4,k) = 4^(k-1) - 2^(k-1) = 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^k-1 segments
c(1,k) = / 0 if k=0
North | 4^(k-1) + 2^(k-1) - 1 if k odd >= 1
\ 4^(k-1) if k even >= 2
= 0, 1, 4, 19, 64, 271, 1024, 4159, 16384, ...
c(2,k) = / 0 if k=0
East | 4^(k-1) if k odd >= 1
\ 4^(k-1) + 2^(k-1) - 1 if k even >= 2
= 0, 1, 5, 16, 71, 256, 1055, 4096, 16511, ...
c(3,k) = / 0 if k=0
South | 4^(k-1) - 2^(k-1) if k odd >= 1
\ 4^(k-1) if k even >= 2
= 0, 0, 4, 12, 64, 240, 1024, 4032, 16384, ...
c(4,k) = / 0 if k=0
West | 4^(k-1) if k odd >= 1
\ 4^(k-1) - 2^(k-1) if k even >= 2
= 0, 1, 2, 16, 56, 256, 992, 4096, 16256, ...
The segment N=4^k-1 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 bit-pair 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 1-bits 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 X-Y
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 2-cycle points
A163903 3-cycle 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 3-cycles 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 n-cycle
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/hilbert-oeis.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/math-planepath/index.html>
LICENSE
Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020, 2021 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/>.
perl v5.32.0 2021-01-23 Math::PlanePath::HilbertCurve(3pm)