Provided by: libmath-planepath-perl_122-1_all 
NAME
Math::PlanePath::DekkingCurve -- 5x5 self-similar edge curve
SYNOPSIS
use Math::PlanePath::DekkingCurve;
my $path = Math::PlanePath::DekkingCurve->new;
my ($x, $y) = $path->n_to_xy (123);
DESCRIPTION
This is an integer version of a 5x5 self-similar curve per
F. M. Dekking, "Recurrent Sets", Advances in Mathematics, volume 44, 1982, pages
79-104, section 4.9 "Gosper-Type Curves"
The base pattern is N=0 to N=25. It repeats with rotations or reversals which make the
ends join. For example N=75 to N=100 is the base pattern in reverse, ie. from N=25 down
to N=0. Or N=50 to N=75 is reverse and also rotate by -90.
10 | 123-124-125-... 86--85
| | | |
9 | 115-116-117 122-121 90--89--88--87 84
| | | | | |
8 | 114-113 118-119-120 91--92--93 82--83
| | | |
7 | 112 107-106 103-102 95--94 81 78--77
| | | | | | | | | |
6 | 111 108 105-104 101 96--97 80--79 76
| | | | | |
5 | 110-109 14--15 100--99--98 39--40 75 66--65
| | | | | | | |
4 | 10--11--12--13 16 35--36--37--38 41 74 71--70 67 64
| | | | | | | | | |
3 | 9---8---7 18--17 34--33--32 43--42 73--72 69--68 63
| | | | | |
2 | 5---6 19 22--23 30--31 44 47--48 55--56--57 62--61
| | | | | | | | | | | |
1 | 4---3 20--21 24 29--28 45--46 49 54--53 58--59--60
| | | | | |
Y=0| 0---1---2 25--26--27 50--51--52
+----------------------------------------------------------------
X=0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
The curve segments correspond to edges of squares in a 5x5 arrangement.
+- - -+- - -+- - 14----15 ---+
| | | | v |> |
^ ^ <| |
10----11----12----13- - 16 --+
| v |> |
|> ^ ^ |
9-----8-----7 -- 18----17 --+
v | | |> |
| ^ |> | ^
+- - 5-----6 - 19 22----23
| <| | <|
| <| ^ | <| |
+- - 4-----3 20----21 -- 24
| v <|
^ ^ |> | | |
0-----1-----2 -- + -- -+- 25
The little notch marks show which square each edge represents. This is the side the curve
expands into at the next level. For example N=1 to N=2 has its notch on the left so the
next level N=25 to N=50 expands on the left.
All the directions are made by rotating the base pattern. When the expansion is on the
right the segments go in reverse. For example N=2 to N=3 expands on the right and is made
by rotating the base pattern clockwise 90 degrees. This means that N=2 becomes the 25
end, and following the curve to the 0 start at N=3.
Dekking writes these directions as a sequence of 25 symbols s(i,j) where i=0 for left
plain or i=1 for right reverse and j=0,1,2,3 direction j*90 degrees anti-clockwise so
E,N,W,S.
Arms
The optional "arms" parameter can give up to four copies of the curve, each advancing
successively. Each copy is in a successive quadrant.
arms => 3 |
67-70-73 42-45 5
| | |
43-46-49 64-61 30-33-36-39 48 4
| | | | |
40-37 52-55-58 27-24-21 54-51 3
| | |
34 19-16 7--4 15-18 57 66-69 2
| | | | | | | | |
31 22 13-10 1 12--9 60-63 72 1
| | | |
...--74 28-25 5--2 0--3--6 75-... <-- Y=0
| |
71 62-59 8-11 -1
| | | |
68-65 56 17-14 -2
| |
50-53 20-23-26 -3
| |
47 38-35-32-29 -4
| |
44-41 -5
^
... -5 -4 -3 -2 -1 X=0 1 2 3 4 5 ...
The origin is N=0 and is on the first arm only. The second and subsequent arms begin
1,2,etc. The curves interleave perfectly on the axes where the arms meet. The result is
that arms=4 fills the plane visiting each integer X,Y exactly once and not touching or
crossing.
FUNCTIONS
See "FUNCTIONS" in Math::PlanePath for the behaviour common to all path classes.
"$path = Math::PlanePath::DekkingCurve->new ()"
"$path = Math::PlanePath::DekkingCurve->new (arms => $a)"
Create and return a new path object.
The optional "arms" parameter gives between 1 and 4 copies of the curve successively
advancing.
"($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.
Level Methods
"($n_lo, $n_hi) = $path->level_to_n_range($level)"
Return "(0, 25**$level)", or for multiple arms return "(0, $arms * 25**$level)".
There are 25^level + 1 points in a level, numbered starting from 0. On the second and
third arms the origin is omitted (so as not to repeat that point) and so just 25^level
for them, giving 25^level+1 + (arms-1)*25^level = arms*25^level + 1 many points
starting from 0.
FORMULAS
X Axis Segments
In the sample points above there are some line segments on the X axis. A segment X to X+1
is traversed or not according to
X digits in base 5
traversed if X==0
traversed if low digit 1
not-traversed if low digit 2 or 3 or 4
when low digit == 0
traversed if lowest non-zero 1 or 2
not-traversed if lowest non-zero 3 or 4
In the samples the segments at X=1, X=6 and X=11 segments traversed are low digit 1.
Their preceding X=5 and X=10 segments are low digit==0 and the lowest non-zero 1 or 2
(respectively). At X=15 however the lowest non-zero is 3 and so not-traversed there.
In general in groups of 5 there is always X==1 mod 5 traversed but its preceding X==0 mod
5 is traversed or not according to lowest non-zero 1,2 or 3,4.
This pattern is found by considering how the base pattern expands. The plain base pattern
has its south edge on the X axis. The first two sub-parts of that south edge are the base
pattern unrotated, so the south edge again, but the other parts rotated. In general the
sides are
0 1 2 3 4
S -> S,S,E,N,W
E -> S,S,E,N,N
N -> W,S,E,N,N
W -> W,S,E,N,W
Starting in S and taking digits high to low a segment is traversed when the final state is
S again.
Any digit 1,2,3 goes to S,E,N respectively. If no 1,2,3 at all then S start. At the
lowest 1,2,3 there are only digits 0,4 below. If no such digits then only digit 1 which
is S, or no digits at all for N=0, is traversed. If one or more 0s below then E goes to S
so a lowest non-zero 2 means traversed too. If there is any 4 then it goes to N or W and
in those states both 0,4 stay in N or W so not-traversed.
The transitions from the lowest 1,2,3 can be drawn in a state diagram,
+--+
v |4 base 5 digits of X
North <---+ <-------+ high to low
/ | |
/0 |4 |
/ | |3
+-> v | 2 |
| West East <--- start lowest 1,2,3
+-- ^ | |
0,4 \ | |1
\4 |0 |or no 1,2,3 at all
\ | |
South <---+ <-------+
^ |0
+--+
The full diagram, starting from the top digit, is less clear
+--+
v |3,4
+---> North <---+
3| / | ^ \ |3,4
| /0 1 | 2\ | base 5 digits of X
| / | | \ | high to low
+-> | v | | v | <-+
| West 2---------> East | start in South,
+-- | ^ | | ^ | --+ segment traversed
0,4 | \ | | / | 2 if end in South
| \4 | 3 2/ |
1| \ v | / |0,1
+---> South <---+
^ |0,1
+--+
but allows usual DFA state machine manipulations to reverse to go low to high.
+---------- start ----------+
| 1 0| 2,3,4 | base 5 digits of X
| | | low to high
v 1,2 v 3,4 v
traversed <------- m1 -------> not-traversed
0| ^
+-+
In state m1 a 0 digit loops back to m1 so finds the lowest non-zero. States start and m1
are the same except for the behaviour of digit 2 and so in the rules above the result for
digit 2 differs according to whether there are any low 0s.
Y Axis Segments
The Y axis can be treated similarly
Y digits in base 5 (with a single 0 digit if Y==0)
traversed if lowest digit 3
not-traversed if lowest digit 0 or 1 or 2
when lowest digit == 4
traversed if lowest non-4 is 2 or 3
not-traversed if lowest non-4 is 0 or 1
The Y axis goes around the base square clockwise, so the digits are reversed 0<->4 from
the X axis for the state transitions. The initial state is W.
0 1 2 3 4
S -> W,N,E,S,S
E -> N,N,E,S,S
N -> N,N,E,S,W
W -> W,N,E,S,W
N and W can be merged as equivalent. Their only difference is digit 0 going to N or W and
both of those are final result not-traversed.
Final state S is reached if the lowest digit is 3, or if state S or E are reached by digit
2 or 3 and then only 4s below.
X,Y Axis Interleaving
For arms=2 the second copy of the curve is rotated +90 degrees, and similarly a third or
fourth copy in arms=3 or 4. This means each axis is a Y axis of the quadrant before and
an X axis of the quadrant after. When this happens the segments do not overlap nor does
the curve touch.
This is seen from the digit rules above. The 1 mod 5 segment is always traversed by X and
never by Y. The 2 mod 5 segment is never traversed by either. The 3 mod 5 segment is
always traversed by Y and never by X.
The 0 mod 5 segment is sometimes traversed by X, and never by Y. The 4 mod 5 segment is
sometimes traversed by Y, and never by Y.
0 1 2 3 4
*-------*-------*-------*-------*-------*
X X neither Y Y
maybe maybe
A 4 mod 5 segment has one or more trailing 4s and at +1 for the next segment they become
0s and increment the lowest non-4.
+--------+-----+-------+
| ... | d | 4...4 | N == 4 mod 5 X never
+--------+-----+-------+ Y maybe
+--------+-----+-------+
| ... | d+1 | 0...0 | N+1 == 0 mod 5 X maybe
+--------+-----+-------+ Y never
Per the Y rule, a 4 mod 5 segment is traversed when d=2,3. The following segment is then
d+1=3,4 as lowest non-zero which in the X rule is not-traversed. Conversely in the Y rule
not-traversed when d=0,1 which becomes d+1=1,2 which in the X rule is traversed.
So exactly one of two consecutive 4 mod 5 and 0 mod 5 segments are traversed.
SEE ALSO
Math::PlanePath, Math::PlanePath::DekkingCentres, Math::PlanePath::CincoCurve,
Math::PlanePath::PeanoCurve
HOME PAGE
<http://user42.tuxfamily.org/math-planepath/index.html>
LICENSE
Copyright 2011, 2012, 2013, 2014, 2015 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/>.