Provided by: libmath-planepath-perl_129-1_all bug

NAME

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

SYNOPSIS

        use Math::PlanePath::PeanoCurve;
        my $path = Math::PlanePath::PeanoCurve->new;
        my ($x, $y) = $path->n_to_xy (123);

        # or another radix digits ...
        my $path5 = Math::PlanePath::PeanoCurve->new (radix => 5);

DESCRIPTION

       This path is an integer version of the curve described by Peano for filling a unit square,

           Giuseppe Peano, "Sur Une Courbe, Qui Remplit Toute Une Aire Plane", Mathematische
           Annalen, volume 36, number 1, 1890, pages 157-160.  DOI 10.1007/BF01199438.
           <https://eudml.org/doc/157489>, <https://link.springer.com/article/10.1007/BF01199438>

       It traverses a quadrant of the plane one step at a time in a self-similar 3x3 pattern,

              8    60--61--62--63--64--65  78--79--80--...
                    |                   |   |
              7    59--58--57  68--67--66  77--76--75
                            |   |                   |
              6    54--55--56  69--70--71--72--73--74
                    |
              5    53--52--51  38--37--36--35--34--33
                            |   |                   |
              4    48--49--50  39--40--41  30--31--32
                    |                   |   |
              3    47--46--45--44--43--42  29--28--27
                                                    |
              2     6---7---8---9--10--11  24--25--26
                    |                   |   |
              1     5---4---3  14--13--12  23--22--21
                            |   |                   |
             Y=0    0---1---2  15--16--17--18--19--20

                  X=0   1   2   3   4   5   6   7   8   9 ...

       The start is an S shape of the nine points N=0 to N=8, and then nine of those groups are
       put together in the same S configuration.  The sub-parts are flipped horizontally and/or
       vertically to make the starts and ends adjacent, so 8 is next to 9, 17 next to 18, etc,

           60,61,62 --- 63,64,65     78,79,80
           59,58,57     68,67,55     77,76,75
           54,55,56     69,70,71 --- 72,73,74
            |
            |
           53,52,51     38,37,36 --- 35,34,33
           48,49,50     39,40,41     30,31,32
           47,46,45 --- 44,43,42     29,28,27
                                            |
                                            |
            6,7,8  ----  9,10,11     24,25,26
            3,4,5       12,13,14     23,22,21
            0,1,2       15,16,17 --- 18,19,20

       The process repeats, tripling in size each time.

       Within a power-of-3 square, 3x3, 9x9, 27x27, 81x81 etc (3^k)x(3^k) at the origin, all the
       N values 0 to 3^(2*k)-1 are within the square.  The top right corner 8, 80, 728, etc is
       the 3^(2*k)-1 maximum in each.

       Because each step is by 1, the distance along the curve between two X,Y points is the
       difference in their N values as given by "xy_to_n()".

   Radix
       The "radix" parameter can do the calculation in a base other than 3, using the same kind
       of direction reversals.  For example radix 5 gives 5x5 groups,

            radix => 5

             4  |  20--21--22--23--24--25--26--27--28--29
                |   |                                   |
             3  |  19--18--17--16--15  34--33--32--31--30
                |                   |   |
             2  |  10--11--12--13--14  35--36--37--38--39
                |   |                                   |
             1  |   9-- 8-- 7-- 6-- 5  44--43--42--41--40
                |                   |   |
            Y=0 |   0-- 1-- 2-- 3-- 4  45--46--47--48--49--50-...
                |
                +----------------------------------------------
                  X=0   1   2   3   4   5   6   7   8   9  10

       If the radix is even then the ends of each group don't join up.  For example in radix 4
       N=15 isn't next to N=16, nor N=31 to N=32, etc.

            radix => 4

             3  |  15--14--13--12  16--17--18--19
                |               |               |
             2  |   8-- 9--10--11  23--22--21--20
                |   |               |
             1  |   7-- 6-- 5-- 4  24--25--26--27
                |               |               |
            Y=0 |   0-- 1-- 2-- 3  31--30--29--28  32--33-...
                |
                +------------------------------------------
                  X=0   1   2   4   5   6   7   8   9  10

       Even sizes can be made to join using other patterns, but this module is just Peano's digit
       construction.  For joining up in 2x2 groupings see "HilbertCurve" (which is essentially
       the only way to join up in 2x2).  For bigger groupings there's various ways.

   Unit Square
       Peano's original form was for filling a unit square by mapping a number T in the range
       0<T<1 to a pair of X,Y coordinates 0<X<1 and 0<Y<1.  The curve is continuous and every
       such X,Y is reached by some T, so it fills the unit square.  A unit cube or higher
       dimension can be filled similarly by developing three or more coordinates X,Y,Z, etc.
       Cantor had shown a line is equivalent to the plane, Peano's mapping is a continuous way to
       do that.

       The code here could be pressed into service for a fractional T to X,Y by multiplying up by
       a power of 9 to desired precision then dividing X and Y back by the same power of 3
       (perhaps swapping X,Y for which one should be the first ternary digit).  Note that if T is
       a binary floating point then a power of 3 division will round off in general since 1/3 is
       not exactly representable.  (See "HilbertCurve" or "ZOrderCurve" for binary mappings.)

       Sometimes the curve is drawn with line segments crossing unit squares.  See PeanoDiagonals
       for that sort of path.

   Power of 3 Patterns
       Plotting sequences of values with some connection to ternary digits or powers of 3 will
       usually give the most interesting patterns on the Peano curve.  For example the Mephisto
       waltz sequence (Math::NumSeq::MephistoWaltz) makes diamond shapes,

           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
             *** **   *** ** **   *  ***   *  ***   *  *  *** **
            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
             *** **   *** ** **   *  ***   *  ***   *  *  *** **
           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
             *** **   *** ** **   *  ***   *  ***   *  *  *** **
            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
             *** **   *** ** **   *  ***   *  ***   *  *  *** **
            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
             *** **   *** ** **   *  ***   *  ***   *  *  *** **
           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
             *** **   *** ** **   *  ***   *  ***   *  *  *** **
            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
            ** ***  *  *   ***  *   ** ** ***  *  *   ***  *   **
           **   *  ***   *  *  *** **   *** **   *** ** **   *  *
           *  *   ** ** ***   ** ***  *  *   ** ** ***   ** ***
             *** **   *** ** **   *  ***   *  ***   *  *  *** **

       This arises from each 3x3 block in the Mephisto waltz being one of two shapes which are
       then flipped by the Peano pattern

           * * _                     _ _ *
           * _ _           or        _ * *    (inverse)
           _ _ *                     * * _

           0,0,1, 0,0,1, 1,1,0       1,1,0, 1,1,0, 0,0,1

FUNCTIONS

       See "FUNCTIONS" in Math::PlanePath for behaviour common to all path classes.

       "$path = Math::PlanePath::PeanoCurve->new ()"
       "$path = Math::PlanePath::PeanoCurve->new (radix => $integer)"
           Create and return a new path object.

           The optional "radix" parameter gives the base for digit splitting.  The default is
           ternary "radix => 3".

       "($x,$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 $n 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 appears either added to or
           subtracted from X or Y.

       "$n = $path->xy_to_n ($x,$y)"
           Return the integer point number for coordinates "$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)"
           Return the range of N values which occur the a rectangle with corners at $x1,$y1 and
           $x2,$y2.  If the X,Y values are not integers then the curve is treated as unit squares
           centred on each integer point and squares which are partly covered by the given
           rectangle are included.

           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, $radix**(2*$level) - 1)".

FORMULAS

   N to X,Y
       Peano's calculation is based on putting base-3 digits of N alternately to X or Y.  From
       the high end of N, a digit goes to Y then the next goes to X.  Beginning at an even digit
       position in N makes the last digit go to X so the first N=0,1,2 is along the X axis.

       At each stage a "complement" state is maintained for X and for Y.  When complemented, the
       digit is reversed to 2 - digit, so 0,1,2 becomes 2,1,0.  This reverses the direction so
       points like N=12,13,14 shown above go leftwards, or groups like 9,10,11 then 12,13,14 then
       15,16,17 go downwards.

       The complement is calculated by adding the digits from N which went to the other one of X
       or Y.  So the X complement is the sum of digits which have gone to Y so far.  Conversely
       the Y complement is the sum of digits put to X.  If the complement sum is odd then the
       reversal is done.  A bitwise XOR can be used instead of a sum to accumulate odd/even-ness
       the same way as a sum.

       When forming the complement state, the original digits from N are added, before applying
       any complementing for putting them to X or Y.  If the radix is odd, like the default 3,
       then complementing doesn't change it mod 2 so before or after are the same, but if the
       radix is even then it's not the same.

       It also works to take the base-3 digits of N from low to high, generating low to high
       digits in X and Y.  If an odd digit is put to X then the low digits of Y so far must be
       complemented as 22..22 - Y (the 22..22 value being all 2s in base 3, ie. 3^k-1).
       Conversely if an odd digit is put to Y then X must be complemented.  With this approach,
       the high digit position in N doesn't have to be found, just peel off digits of N from the
       low end.  But the subtract to complement is then more work if using bignums.

   X,Y to N
       The X,Y to N calculation can be done by an inverse of either the high to low or low to
       high methods above.  In both cases digits are put alternately from X and Y into N, with
       complement as necessary.

       For the low to high approach, it's not easy to complement just the X digits in the N
       constructed so far, but it works to build and complement the X and Y digits separately
       then at the end interleave to make the final N.  Complementing is the ternary equivalent
       of an XOR in binary.  On a ternary machine maybe some trit-twiddling would do it.

       For low to high with even radix, the complementing is also tricky since changing the
       accumulated X affects the digits of Y below that, and vice versa.  What's the rule?  Is it
       alternate digits which end up complemented?  In any case the current "xy_to_n()" code goes
       high to low which is easier, but means breaking the X,Y inputs into arrays of digits
       before beginning.

   N on Axes
       N on the X axis is all Y digits 0 in the X,Y to N described above.  This means N is the
       digits of X, and then digit 0 or 2 at each Y position according to odd or even sum of X
       digits above.  The Y digits are at odd positions so the 0 or 2 ternary is 0 or 6 for N in
       base-9.

           N on X axis = 0,1,2, 15,16,17, 18,19,20, 141, ...   (A163480)
                 ternary 0,1,2, 120,121,122, 200,201,202, 12020

       The Y axis is similar but the X digits are at even positions.

           N on Y axis = 0,5,6, 47,48,53, 54,59,60, 425, ...   (A163481)
                 ternary 0,12,20, 1202,1210,1222, 2000,2012,2020, 120202

       N on the X=Y diagonal has the ternary digits of position d go to both X and Y and so both
       complemented according to sum of digits of d above.  That transformation within d is the
       ternary reflected Gray code.

           Gray3(d) = ternary flip 0<->2 when sum of digits above is odd
                    = 0,1,2, 5,4,3, 6,7,8, 17, ...          (A128173)
              ternary 0,1,2, 12,11,10, 20,21,22, 122, ...

           N on X=Y diag = ternary Gray3(d) and 0,1,2 -> 0,4,8 base9,
                                                which is 4*digit
                         = 0,4,8, 44,40,36, 72,76,80, 404, ...  (A163343)
                   ternary 0,11,22, 1122,1111,1100, 2200,2211,2222, 112222,

   N to abs(dX),abs(dY)
       The curve goes horizontally or vertically according to the number of trailing "2" digits
       when N is written in ternary,

           N trailing 2s   direction     abs(dX)     abs(dY)
           -------------   ---------     -------     -------
             even          horizontal       1           0
             odd           vertical         0           1

           abs(dX) = 1,1,0, 1,1,0, 1,1,1, 1,1,0, 1,1,0, 1,1,1, ...  (A014578)
           abs(dY) = 0,0,1, 0,0,1, 0,0,0, 0,0,1, 0,0,1, 0,0,0, ...  (A182581)

       For example N=5 is "12" in ternary has 1 trailing "2" which is odd so the step from N=5 to
       N=6 is vertical.

       This works because when stepping from N to N+1 a carry propagates through the trailing 2s
       to increment the digit above.  Digits go alternately to X or Y so odd or even trailing 2s
       put that carry into an X digit or Y digit.

                 X Y X Y X
           N   ... 2 2 2 2
           N+1   1 0 0 0 0  carry propagates

   Rectangle to N Range
       An easy over-estimate of the maximum N in a region can be had by going to the next bigger
       (3^k)x(3^k) square enclosing the region.  This means the biggest X or Y rounded up to the
       next power of 3 (perhaps using "log()" if you trust its accuracy), so

           find k with 3^k > max(X,Y)
           N_hi = 3^(2k) - 1

       An exact N range can be found by following the "high to low" N to X,Y procedure above.
       Start with the easy over-estimate to find a 3^(2k) ternary digit position in N bigger than
       the desired region, then choose a digit 0,1,2 for X, the biggest which overlaps some of
       the region.  Or if there's an X complement then the smallest digit is the biggest N, again
       whichever overlaps the region.  Then likewise for a digit of Y, etc.

       Biggest and smallest N must maintain separate complement states as they track down
       different N digits.  A single loop can be used since there's the same "2k" many digits of
       N to consider for both.

       The N range of any shape can be done this way, not just a rectangle like
       "rect_to_n_range()".  The procedure only depends on asking whether a one-third sub-part of
       X or Y overlaps the target region or not.

OEIS

       This path is in Sloane's Online Encyclopedia of Integer Sequences in several forms,

           <http://oeis.org/A163528> (etc)

           A163528    X coordinate
           A163529    Y coordinate
           A163530    X+Y coordinate sum
           A163531    X^2+Y^2 square of distance from origin
           A163532    dX, change in X -1,0,1
           A163533    dY, change in Y -1,0,1
           A014578    abs(dX) from n-1 to n, 1=horiz 0=vertical
           A182581    abs(dY) from n-1 to n, 0=horiz 1=vertical
           A163534    direction mod 4 of each step (ENWS)
           A163535    direction mod 4, transposed X,Y
           A163536    turn 0=straight,1=right,2=left
           A163537    turn, transposed X,Y
           A163342    diagonal sums
           A163479    diagonal sums divided by 6

           A163480    N on X axis
           A163481    N on Y axis
           A163343    N on X=Y diagonal, 0,4,8,44,40,36,etc
           A163344    N on X=Y diagonal divided by 4
           A007417    N+1 of positions of horizontals, ternary even trailing 0s
           A145204    N+1 of positions of verticals, ternary odd trailing 0s

           A163332    Peano N <-> ZOrder radix=3 N mapping (self-inverse)
           A163333    with ternary digit swaps before and after

       And taking X,Y points by the Diagonals sequence, then the value of the following sequences
       is the N of the Peano curve at those positions.

           A163334    numbering by diagonals, from same axis as first step
           A163336    numbering by diagonals, from opposite axis
           A163338    A163334 + 1, Peano starting from N=1
           A163340    A163336 + 1, Peano starting from N=1

       "Math::PlanePath::Diagonals" numbers points from the Y axis down, which is the opposite
       axis to the Peano curve first step along the X axis, so a plain "Diagonals" ->
       "PeanoCurve" is the "opposite axis" form A163336.

       These sequences are permutations of the integers since all X,Y positions of the first
       quadrant are reached eventually.  The inverses are as follows.  They can be thought of
       taking X,Y positions in the Peano curve order and then asking what N the Diagonals would
       put there.

           A163335    inverse of A163334
           A163337    inverse of A163336
           A163339    inverse of A163338
           A163341    inverse of A163340

SEE ALSO

       Math::PlanePath, Math::PlanePath::PeanoDiagonals, Math::PlanePath::HilbertCurve,
       Math::PlanePath::ZOrderCurve, Math::PlanePath::AR2W2Curve, Math::PlanePath::BetaOmega,
       Math::PlanePath::CincoCurve, Math::PlanePath::KochelCurve,
       Math::PlanePath::WunderlichMeander

       Math::PlanePath::KochCurve

HOME PAGE

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

LICENSE

       Copyright 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 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/>.