Radix

The default is binary. Option "radix => $r" can select another radix. This radix is used for both the Gray code and the digit splitting. For example "radix => 4",

    radix => 4
      |
    127-126-125-124  99--98--97--96--95--94--93--92  67--66--65--64
                  |   |                           |   |
    120-121-122-123 100-101-102-103  88--89--90--91  68--69--70--71
      |                           |   |                           |
    119-118-117-116 107-106-105-104  87--86--85--84  75--74--73--72
                  |   |                           |   |
    112-113-114-115 108-109-110-111  80--81--82--83  76--77--78--79
     15--14--13--12  19--18--17--16  47--46--45--44  51--50--49--48
                  |   |                           |   |
      8-- 9--10--11  20--21--22--23  40--41--42--43  52--53--54--55
      |                           |   |                           |
      7-- 6-- 5-- 4  27--26--25--24  39--38--37--36  59--58--57--56
                  |   |                           |   |
      0-- 1-- 2-- 3  28--29--30--31--32--33--34--35  60--61--62--63

Apply Type

Option "apply_type => $str" controls how Gray codes are applied to N and X,Y. It can be one of

    "TsF"    to Gray, split, from Gray  (default)
    "Ts"     to Gray, split
    "Fs"     from Gray, split
    "FsT"    from Gray, split, to Gray
     "sT"    split, to Gray
     "sF"    split, from Gray

"T" means integer-to-Gray, "F" means integer-from-Gray, and omitted means no transformation. For example the following is "Ts" which means N to Gray then split, leaving Gray coded values for X,Y.

    apply_type => "Ts"
     7  |  51--50  52--53  44--45  43--42
        |       |       |       |       |
     6  |  48--49  55--54  47--46  40--41
        |
     5  |  60--61  59--58  35--34  36--37  ...-66
        |       |       |       |       |       |
     4  |  63--62  56--57  32--33  39--38  64--65
        |
     3  |  12--13  11--10  19--18  20--21
        |       |       |       |       |
     2  |  15--14   8-- 9  16--17  23--22
        |
     1  |   3-- 2   4-- 5  28--29  27--26
        |       |       |       |       |
    Y=0 |   0-- 1   7-- 6  31--30  24--25
        |
        +---------------------------------
          X=0   1   2   3   4   5   6   7

This "Ts" is quite attractive because a step from N to N+1 changes just one bit in X or Y alternately, giving 2-D single-bit changes. For example N=19 at X=4 followed by N=20 at X=6 is a single bit change in X.

N=0,2,8,10,etc on the leading diagonal X=Y is numbers using only digits 0,2 in base 4. N=0,3,15,12,etc on the Y axis is numbers using only digits 0,3 in base 4, but in a Gray code order.

The "Fs", "FsT" and "sF" forms effectively treat the input N as a Gray code and convert from it to integers, either before or after split. For "Fs" the effect is little Z parts in various orientations.

    apply_type => "sF"
     7  |  32--33  37--36  52--53  49--48
        |    /       \       /       \
     6  |  34--35  39--38  54--55  51--50
        |
     5  |  42--43  47--46  62--63  59--58
        |    \       /       \       /
     4  |  40--41  45--44  60--61  57--56
        |
     3  |   8-- 9  13--12  28--29  25--24
        |    /       \       /       \
     2  |  10--11  15--14  30--31  27--26
        |
     1  |   2-- 3   7-- 6  22--23  19--18
        |    \       /       \       /
    Y=0 |   0-- 1   5-- 4  20--21  17--16
        |
        +---------------------------------
          X=0   1   2   3   4   5   6   7

Gray Type

The "gray_type" option selects the type of Gray code. The choices are

    "reflected"     increment to radix-1 then decrement (default)
    "modular"       increment to radix-1 then cycle back to 0

For example in decimal,

    integer       Gray         Gray
               "reflected"   "modular"
    -------    -----------   ---------
       0            0            0
       1            1            1
       2            2            2
     ...          ...          ...
       8            8            8
       9            9            9
      10           19           19
      11           18           10
      12           17           11
      13           16           12
      14           15           13
     ...          ...          ...
      17           12           16
      18           11           17
      19           10           18

Notice on reaching "19" the reflected type runs the least significant digit back down from 9 to 0, which is a reverse or reflection of the preceding 0 to 9 upwards. The modular form instead continues to increment that least significant digit, wrapping around from 9 to 0.

In binary, the modular and reflected forms are the same (see "Equivalent Combinations" below).

There's various other systematic ways to make a Gray code changing a single digit successively. But many ways are implicitly based on a pre-determined fixed number of bits or digits, which doesn't suit an unlimited path like here.

Equivalent Combinations

Some option combinations are equivalent,

    condition                  equivalent
    ---------                  ----------
    radix=2                    modular==reflected
                               and TsF==Fs, Ts==FsT
    radix>2 odd, reflected     TsF==FsT, Ts==Fs, sT==sF
                               because T==F
    radix>2 even, reflected    TsF==Fs, Ts==FsT

In radix=2 binary, the "modular" and "reflected" Gray codes are the same because there's only digits 0 and 1 so going forward or backward is the same.

For odd radix and reflected Gray code, the "to Gray" and "from Gray" operations are the same. For example the following table is ternary radix=3. Notice how integer value 012 maps to Gray code 010, and in turn integer 010 maps to Gray code 012. All values are either pairs like that or unchanged like 021.

    integer      Gray
              "reflected"       (written in ternary)
      000        000
      001        001
      002        002
      010        012
      011        011
      012        010
      020        020
      021        021
      022        022

For even radix and reflected Gray code, "TsF" is equivalent to "Fs", and also "Ts" equivalent to "FsT". This arises from the way the reversing behaves when split across digits of two X,Y values. (In higher dimensions such as a split to 3-D X,Y,Z it's not the same.)

The net effect for distinct paths is

    condition         distinct combinations
    ---------         ---------------------
    radix=2           four TsF==Fs, Ts==FsT, sT, sF
    radix>2 odd       / three reflected TsF==FsT, Ts==Fs, sT==sF
                      \ six modular TsF, Ts, Fs, FsT, sT, sF
    radix>2 even      / four reflected TsF==Fs, Ts==FsT, sT, sF
                      \ six modular TsF, Ts, Fs, FsT, sT, sF

Peano Curve

In "radix => 3" and other odd radices, the "reflected" Gray type gives the Peano curve (see Math::PlanePath::PeanoCurve). This is since the "reflected" encoding is equivalent to Peano's "xk" and "yk" complementing.

    radix => 3, gray_type => "reflected"
     |
    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
     |                   |   |
     5-- 4-- 3  14--13--12  23--22--21
             |   |                   |
     0-- 1-- 2  15--16--17--18--19--20

Level Methods

"($n_lo, $n_hi) = $path->level_to_n_range($level)"

Return "(0, $radix**(2*$level) - 1)".

Turn

The turns in the default binary TsF curve are either to the left +90 or a reverse 180. For example at N=2 the curve turns left, then at N=3 it reverses back 180 to go to N=4. The turn is given by the low zero bits of (N+1)/2,

    count_low_0_bits(floor((N+1)/2))
      if even then turn 90 left
      if odd  then turn 180 reverse

Or equivalently

    floor((N+1)/2) lowest non-zero digit in base 4,
      1 or 3 = turn 90 left
      2      = turn 180 reverse

The 180 degree reversals are all horizontal. They occur because at those N the three N-1,N,N+1 converted to Gray code have the same bits at odd positions and therefore the same Y coordinate.

See "N to Turn" in Math::PlanePath::KochCurve for similar turns based on low zero bits (but by +60 and -120 degrees).