1Math::PlanePath::PeanoCUusrevre(C3o)ntributed Perl DocumMeanttha:t:iPolnanePath::PeanoCurve(3)
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4

NAME

6       Math::PlanePath::PeanoCurve -- 3x3 self-similar quadrant traversal
7

SYNOPSIS

9        use Math::PlanePath::PeanoCurve;
10        my $path = Math::PlanePath::PeanoCurve->new;
11        my ($x, $y) = $path->n_to_xy (123);
12
13        # or another radix digits ...
14        my $path5 = Math::PlanePath::PeanoCurve->new (radix => 5);
15

DESCRIPTION

17       This path is an integer version of the curve described by Peano for
18       filling a unit square,
19
20           Giuseppe Peano, "Sur Une Courbe, Qui Remplit Toute Une Aire Plane",
21           Mathematische Annalen, volume 36, number 1, 1890, pages 157-160.
22           DOI 10.1007/BF01199438.  <https://eudml.org/doc/157489>,
23           <https://link.springer.com/article/10.1007/BF01199438>
24
25       It traverses a quadrant of the plane one step at a time in a self-
26       similar 3x3 pattern,
27
28              8    60--61--62--63--64--65  78--79--80--...
29                    |                   |   |
30              7    59--58--57  68--67--66  77--76--75
31                            |   |                   |
32              6    54--55--56  69--70--71--72--73--74
33                    |
34              5    53--52--51  38--37--36--35--34--33
35                            |   |                   |
36              4    48--49--50  39--40--41  30--31--32
37                    |                   |   |
38              3    47--46--45--44--43--42  29--28--27
39                                                    |
40              2     6---7---8---9--10--11  24--25--26
41                    |                   |   |
42              1     5---4---3  14--13--12  23--22--21
43                            |   |                   |
44             Y=0    0---1---2  15--16--17--18--19--20
45
46                  X=0   1   2   3   4   5   6   7   8   9 ...
47
48       The start is an S shape of the nine points N=0 to N=8, and then nine of
49       those groups are put together in the same S configuration.  The sub-
50       parts are flipped horizontally and/or vertically to make the starts and
51       ends adjacent, so 8 is next to 9, 17 next to 18, etc,
52
53           60,61,62 --- 63,64,65     78,79,80
54           59,58,57     68,67,55     77,76,75
55           54,55,56     69,70,71 --- 72,73,74
56            |
57            |
58           53,52,51     38,37,36 --- 35,34,33
59           48,49,50     39,40,41     30,31,32
60           47,46,45 --- 44,43,42     29,28,27
61                                            |
62                                            |
63            6,7,8  ----  9,10,11     24,25,26
64            3,4,5       12,13,14     23,22,21
65            0,1,2       15,16,17 --- 18,19,20
66
67       The process repeats, tripling in size each time.
68
69       Within a power-of-3 square, 3x3, 9x9, 27x27, 81x81 etc (3^k)x(3^k) at
70       the origin, all the N values 0 to 3^(2*k)-1 are within the square.  The
71       top right corner 8, 80, 728, etc is the 3^(2*k)-1 maximum in each.
72
73       Because each step is by 1, the distance along the curve between two X,Y
74       points is the difference in their N values as given by "xy_to_n()".
75
76   Radix
77       The "radix" parameter can do the calculation in a base other than 3,
78       using the same kind of direction reversals.  For example radix 5 gives
79       5x5 groups,
80
81            radix => 5
82
83             4  |  20--21--22--23--24--25--26--27--28--29
84                |   |                                   |
85             3  |  19--18--17--16--15  34--33--32--31--30
86                |                   |   |
87             2  |  10--11--12--13--14  35--36--37--38--39
88                |   |                                   |
89             1  |   9-- 8-- 7-- 6-- 5  44--43--42--41--40
90                |                   |   |
91            Y=0 |   0-- 1-- 2-- 3-- 4  45--46--47--48--49--50-...
92                |
93                +----------------------------------------------
94                  X=0   1   2   3   4   5   6   7   8   9  10
95
96       If the radix is even then the ends of each group don't join up.  For
97       example in radix 4 N=15 isn't next to N=16, nor N=31 to N=32, etc.
98
99            radix => 4
100
101             3  |  15--14--13--12  16--17--18--19
102                |               |               |
103             2  |   8-- 9--10--11  23--22--21--20
104                |   |               |
105             1  |   7-- 6-- 5-- 4  24--25--26--27
106                |               |               |
107            Y=0 |   0-- 1-- 2-- 3  31--30--29--28  32--33-...
108                |
109                +------------------------------------------
110                  X=0   1   2   4   5   6   7   8   9  10
111
112       Even sizes can be made to join using other patterns, but this module is
113       just Peano's digit construction.  For joining up in 2x2 groupings see
114       "HilbertCurve" (which is essentially the only way to join up in 2x2).
115       For bigger groupings there's various ways.
116
117   Unit Square
118       Peano's original form was for filling a unit square by mapping a number
119       T in the range 0<T<1 to a pair of X,Y coordinates 0<X<1 and 0<Y<1.  The
120       curve is continuous and every such X,Y is reached by some T, so it
121       fills the unit square.  A unit cube or higher dimension can be filled
122       similarly by developing three or more coordinates X,Y,Z, etc.  Cantor
123       had shown a line is equivalent to the plane, Peano's mapping is a
124       continuous way to do that.
125
126       The code here could be pressed into service for a fractional T to X,Y
127       by multiplying up by a power of 9 to desired precision then dividing X
128       and Y back by the same power of 3 (perhaps swapping X,Y for which one
129       should be the first ternary digit).  Note that if T is a binary
130       floating point then a power of 3 division will round off in general
131       since 1/3 is not exactly representable.  (See "HilbertCurve" or
132       "ZOrderCurve" for binary mappings.)
133
134       Sometimes the curve is drawn with line segments crossing unit squares.
135       See PeanoDiagonals for that sort of path.
136
137   Power of 3 Patterns
138       Plotting sequences of values with some connection to ternary digits or
139       powers of 3 will usually give the most interesting patterns on the
140       Peano curve.  For example the Mephisto waltz sequence
141       (Math::NumSeq::MephistoWaltz) makes diamond shapes,
142
143           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
144           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
145             *** **   *** ** **   *  ***   *  ***   *  *  *** **
146            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
147             *** **   *** ** **   *  ***   *  ***   *  *  *** **
148           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
149             *** **   *** ** **   *  ***   *  ***   *  *  *** **
150            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
151           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
152           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
153           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
154            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
155             *** **   *** ** **   *  ***   *  ***   *  *  *** **
156            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
157           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
158            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
159             *** **   *** ** **   *  ***   *  ***   *  *  *** **
160           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
161             *** **   *** ** **   *  ***   *  ***   *  *  *** **
162            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
163           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
164           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
165           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
166            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
167           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
168           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
169             *** **   *** ** **   *  ***   *  ***   *  *  *** **
170
171       This arises from each 3x3 block in the Mephisto waltz being one of two
172       shapes which are then flipped by the Peano pattern
173
174           * * _                     _ _ *
175           * _ _           or        _ * *    (inverse)
176           _ _ *                     * * _
177
178           0,0,1, 0,0,1, 1,1,0       1,1,0, 1,1,0, 0,0,1
179

FUNCTIONS

181       See "FUNCTIONS" in Math::PlanePath for behaviour common to all path
182       classes.
183
184       "$path = Math::PlanePath::PeanoCurve->new ()"
185       "$path = Math::PlanePath::PeanoCurve->new (radix => $integer)"
186           Create and return a new path object.
187
188           The optional "radix" parameter gives the base for digit splitting.
189           The default is ternary "radix => 3".
190
191       "($x,$y) = $path->n_to_xy ($n)"
192           Return the X,Y coordinates of point number $n on the path.  Points
193           begin at 0 and if "$n < 0" then the return is an empty list.
194
195           Fractional $n give an X,Y position along a straight line between
196           the integer positions.  Integer positions are always just 1 apart
197           either horizontally or vertically, so the effect is that the
198           fraction part appears either added to or subtracted from X or Y.
199
200       "$n = $path->xy_to_n ($x,$y)"
201           Return the integer point number for coordinates "$x,$y".  Each
202           integer N is considered the centre of a unit square and an "$x,$y"
203           within that square returns N.
204
205       "($n_lo, $n_hi) = $path->rect_to_n_range ($x1,$y1, $x2,$y2)"
206           Return the range of N values which occur the a rectangle with
207           corners at $x1,$y1 and $x2,$y2.  If the X,Y values are not integers
208           then the curve is treated as unit squares centred on each integer
209           point and squares which are partly covered by the given rectangle
210           are included.
211
212           The returned range is exact, meaning $n_lo and $n_hi are the
213           smallest and biggest in the rectangle.
214
215   Level Methods
216       "($n_lo, $n_hi) = $path->level_to_n_range($level)"
217           Return "(0, $radix**(2*$level) - 1)".
218

FORMULAS

220   N to X,Y
221       Peano's calculation is based on putting base-3 digits of N alternately
222       to X or Y.  From the high end of N, a digit goes to Y then the next
223       goes to X.  Beginning at an even digit position in N makes the last
224       digit go to X so the first N=0,1,2 is along the X axis.
225
226       At each stage a "complement" state is maintained for X and for Y.  When
227       complemented, the digit is reversed to 2 - digit, so 0,1,2 becomes
228       2,1,0.  This reverses the direction so points like N=12,13,14 shown
229       above go leftwards, or groups like 9,10,11 then 12,13,14 then 15,16,17
230       go downwards.
231
232       The complement is calculated by adding the digits from N which went to
233       the other one of X or Y.  So the X complement is the sum of digits
234       which have gone to Y so far.  Conversely the Y complement is the sum of
235       digits put to X.  If the complement sum is odd then the reversal is
236       done.  A bitwise XOR can be used instead of a sum to accumulate
237       odd/even-ness the same way as a sum.
238
239       When forming the complement state, the original digits from N are
240       added, before applying any complementing for putting them to X or Y.
241       If the radix is odd, like the default 3, then complementing doesn't
242       change it mod 2 so before or after are the same, but if the radix is
243       even then it's not the same.
244
245       It also works to take the base-3 digits of N from low to high,
246       generating low to high digits in X and Y.  If an odd digit is put to X
247       then the low digits of Y so far must be complemented as 22..22 - Y (the
248       22..22 value being all 2s in base 3, ie. 3^k-1).  Conversely if an odd
249       digit is put to Y then X must be complemented.  With this approach, the
250       high digit position in N doesn't have to be found, just peel off digits
251       of N from the low end.  But the subtract to complement is then more
252       work if using bignums.
253
254   X,Y to N
255       The X,Y to N calculation can be done by an inverse of either the high
256       to low or low to high methods above.  In both cases digits are put
257       alternately from X and Y into N, with complement as necessary.
258
259       For the low to high approach, it's not easy to complement just the X
260       digits in the N constructed so far, but it works to build and
261       complement the X and Y digits separately then at the end interleave to
262       make the final N.  Complementing is the ternary equivalent of an XOR in
263       binary.  On a ternary machine maybe some trit-twiddling would do it.
264
265       For low to high with even radix, the complementing is also tricky since
266       changing the accumulated X affects the digits of Y below that, and vice
267       versa.  What's the rule?  Is it alternate digits which end up
268       complemented?  In any case the current "xy_to_n()" code goes high to
269       low which is easier, but means breaking the X,Y inputs into arrays of
270       digits before beginning.
271
272   N on Axes
273       N on the X axis is all Y digits 0 in the X,Y to N described above.
274       This means N is the digits of X, and then digit 0 or 2 at each Y
275       position according to odd or even sum of X digits above.  The Y digits
276       are at odd positions so the 0 or 2 ternary is 0 or 6 for N in base-9.
277
278           N on X axis = 0,1,2, 15,16,17, 18,19,20, 141, ...   (A163480)
279                 ternary 0,1,2, 120,121,122, 200,201,202, 12020
280
281       The Y axis is similar but the X digits are at even positions.
282
283           N on Y axis = 0,5,6, 47,48,53, 54,59,60, 425, ...   (A163481)
284                 ternary 0,12,20, 1202,1210,1222, 2000,2012,2020, 120202
285
286       N on the X=Y diagonal has the ternary digits of position d go to both X
287       and Y and so both complemented according to sum of digits of d above.
288       That transformation within d is the ternary reflected Gray code.
289
290           Gray3(d) = ternary flip 0<->2 when sum of digits above is odd
291                    = 0,1,2, 5,4,3, 6,7,8, 17, ...          (A128173)
292              ternary 0,1,2, 12,11,10, 20,21,22, 122, ...
293
294           N on X=Y diag = ternary Gray3(d) and 0,1,2 -> 0,4,8 base9,
295                                                which is 4*digit
296                         = 0,4,8, 44,40,36, 72,76,80, 404, ...  (A163343)
297                   ternary 0,11,22, 1122,1111,1100, 2200,2211,2222, 112222,
298
299   N to abs(dX),abs(dY)
300       The curve goes horizontally or vertically according to the number of
301       trailing "2" digits when N is written in ternary,
302
303           N trailing 2s   direction     abs(dX)     abs(dY)
304           -------------   ---------     -------     -------
305             even          horizontal       1           0
306             odd           vertical         0           1
307
308           abs(dX) = 1,1,0, 1,1,0, 1,1,1, 1,1,0, 1,1,0, 1,1,1, ...  (A014578)
309           abs(dY) = 0,0,1, 0,0,1, 0,0,0, 0,0,1, 0,0,1, 0,0,0, ...  (A182581)
310
311       For example N=5 is "12" in ternary has 1 trailing "2" which is odd so
312       the step from N=5 to N=6 is vertical.
313
314       This works because when stepping from N to N+1 a carry propagates
315       through the trailing 2s to increment the digit above.  Digits go
316       alternately to X or Y so odd or even trailing 2s put that carry into an
317       X digit or Y digit.
318
319                 X Y X Y X
320           N   ... 2 2 2 2
321           N+1   1 0 0 0 0  carry propagates
322
323   Rectangle to N Range
324       An easy over-estimate of the maximum N in a region can be had by going
325       to the next bigger (3^k)x(3^k) square enclosing the region.  This means
326       the biggest X or Y rounded up to the next power of 3 (perhaps using
327       "log()" if you trust its accuracy), so
328
329           find k with 3^k > max(X,Y)
330           N_hi = 3^(2k) - 1
331
332       An exact N range can be found by following the "high to low" N to X,Y
333       procedure above.  Start with the easy over-estimate to find a 3^(2k)
334       ternary digit position in N bigger than the desired region, then choose
335       a digit 0,1,2 for X, the biggest which overlaps some of the region.  Or
336       if there's an X complement then the smallest digit is the biggest N,
337       again whichever overlaps the region.  Then likewise for a digit of Y,
338       etc.
339
340       Biggest and smallest N must maintain separate complement states as they
341       track down different N digits.  A single loop can be used since there's
342       the same "2k" many digits of N to consider for both.
343
344       The N range of any shape can be done this way, not just a rectangle
345       like "rect_to_n_range()".  The procedure only depends on asking whether
346       a one-third sub-part of X or Y overlaps the target region or not.
347

OEIS

349       This path is in Sloane's Online Encyclopedia of Integer Sequences in
350       several forms,
351
352           <http://oeis.org/A163528> (etc)
353
354           A163528    X coordinate
355           A163529    Y coordinate
356           A163530    X+Y coordinate sum
357           A163531    X^2+Y^2 square of distance from origin
358           A163532    dX, change in X -1,0,1
359           A163533    dY, change in Y -1,0,1
360           A014578    abs(dX) from n-1 to n, 1=horiz 0=vertical
361           A182581    abs(dY) from n-1 to n, 0=horiz 1=vertical
362           A163534    direction mod 4 of each step (ENWS)
363           A163535    direction mod 4, transposed X,Y
364           A163536    turn 0=straight,1=right,2=left
365           A163537    turn, transposed X,Y
366           A163342    diagonal sums
367           A163479    diagonal sums divided by 6
368
369           A163480    N on X axis
370           A163481    N on Y axis
371           A163343    N on X=Y diagonal, 0,4,8,44,40,36,etc
372           A163344    N on X=Y diagonal divided by 4
373           A007417    N+1 of positions of horizontals, ternary even trailing 0s
374           A145204    N+1 of positions of verticals, ternary odd trailing 0s
375
376           A163332    Peano N <-> ZOrder radix=3 N mapping (self-inverse)
377           A163333    with ternary digit swaps before and after
378
379       And taking X,Y points by the Diagonals sequence, then the value of the
380       following sequences is the N of the Peano curve at those positions.
381
382           A163334    numbering by diagonals, from same axis as first step
383           A163336    numbering by diagonals, from opposite axis
384           A163338    A163334 + 1, Peano starting from N=1
385           A163340    A163336 + 1, Peano starting from N=1
386
387       "Math::PlanePath::Diagonals" numbers points from the Y axis down, which
388       is the opposite axis to the Peano curve first step along the X axis, so
389       a plain "Diagonals" -> "PeanoCurve" is the "opposite axis" form
390       A163336.
391
392       These sequences are permutations of the integers since all X,Y
393       positions of the first quadrant are reached eventually.  The inverses
394       are as follows.  They can be thought of taking X,Y positions in the
395       Peano curve order and then asking what N the Diagonals would put there.
396
397           A163335    inverse of A163334
398           A163337    inverse of A163336
399           A163339    inverse of A163338
400           A163341    inverse of A163340
401

SEE ALSO

403       Math::PlanePath, Math::PlanePath::PeanoDiagonals,
404       Math::PlanePath::HilbertCurve, Math::PlanePath::ZOrderCurve,
405       Math::PlanePath::AR2W2Curve, Math::PlanePath::BetaOmega,
406       Math::PlanePath::CincoCurve, Math::PlanePath::KochelCurve,
407       Math::PlanePath::WunderlichMeander
408
409       Math::PlanePath::KochCurve
410

HOME PAGE

412       <http://user42.tuxfamily.org/math-planepath/index.html>
413

LICENSE

415       Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019,
416       2020 Kevin Ryde
417
418       This file is part of Math-PlanePath.
419
420       Math-PlanePath is free software; you can redistribute it and/or modify
421       it under the terms of the GNU General Public License as published by
422       the Free Software Foundation; either version 3, or (at your option) any
423       later version.
424
425       Math-PlanePath is distributed in the hope that it will be useful, but
426       WITHOUT ANY WARRANTY; without even the implied warranty of
427       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
428       General Public License for more details.
429
430       You should have received a copy of the GNU General Public License along
431       with Math-PlanePath.  If not, see <http://www.gnu.org/licenses/>.
432
433
434
435perl v5.36.0                      2022-07-22    Math::PlanePath::PeanoCurve(3)
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