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# NAME

Math::PlanePath::ComplexMinus -- twindragon and other complex number base i-r

# SYNOPSIS

`````` use Math::PlanePath::ComplexMinus;
my \$path = Math::PlanePath::ComplexMinus->new (realpart=>1);
my (\$x, \$y) = \$path->n_to_xy (123);``````

# DESCRIPTION

This path traverses points by complex number base i-r. The default is base i-1 giving an integer version of the "twindragon".

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

^
-5  -4  -3  -2 -1  X=0  1   2   3   4   5   6   7``````

With base b=i-1, a complex integer can be represented

``    X+Yi = a[n]*b^n + ... + a*b^2 + a*b + a``

The digits a[n] to a are all either 0 or 1. N is those a[i] as bits, the X,Y is the resulting complex number. It can be shown that this is a one-to-one transformation so every integer point of the plane is visited.

The shape of a given N=0 to N=2^level-1 range is repeated in the next N=2^level to N=(2*2^level)-1. For example the N=0 to N=7 is repeated as N=8 to N=15, starting at X=2,Y=2 instead of the origin. That 2,2 is because b^3 = 2+2i. There's no rotations or mirroring etc in this replication, just the position.

For i-1 each N=2^level point starts at b^level. The powering of that b means the start rotates around by +135 degrees each time, and outward by a factor sqrt(2) on the radius. So for example b^3 = 2+2i is followed by b^4 = -4, which is 135 degrees around, and radius |b^3|=sqrt(8) becomes |b^4|=sqrt(16).

## Real Part

The `realpart => \$r` option gives a complex base b=i-r for a given r>=1. For example `realpart => 2` is

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

N is broken into digits of base norm=r*r+1, so 0 to r*r inclusive. This makes horizontal runs of r*r+1 many points, for example N=0 to N=4 then N=5 to N=9, etc. In the default r=1 these are 2 long, whereas for r=2 they're 2*2+1=5 long, or r=3 would be 3*3+1=10, etc.

The offset back for each run like N=5 shown is the r in i-r, then the next level is (i-r)^2 = (-2r*i + r^2-1) so N=25 begins at X=-2*2=-4,Y=2*2-1=3.

The successive replications end up tiling the plane for any r, though the N values needed to come around and do so might become large if the norm=r*r+1 is large.

## Fractal

The i-1 twindragon is generally conceived as taking fractional N like 0.abcde in binary and giving fractional complex components X,Y. The twindragon is then all the points of the complex plane reached by such N. Those points can be shown to be connected and cover a certain radius around the origin which is completely covered.

The code here might be pressed into use for that for finite bits of N by taking a suitable power N*256^k to get an integer then use X/16^k, Y/16^k for fractional coordinates. 256 is a good base because b^8=16 so there's no rotations to apply to the X,Y, just a division. b^4=-4 for multiplier 16^k and divisor (-4)^k would be almost as easy too.

# FUNCTIONS

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

`\$path = Math::PlanePath::ComplexMinus->new ()`
`\$path = Math::PlanePath::ComplexMinus->new (realpart => \$r)`

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.

`\$n` should be an integer, it's unspecified yet what will be done for a fraction.

# FORMULAS

## X,Y to N

A given X,Y representing X+Yi can be broken into digits of N by complex division by i-r, with each digit being a remainder 0 to r*r inclusive.

As per the base formula above

``    X+Yi = a[n]*b^n + ... + a*b^2 + a*b + a``

and the target is the a digit 0 to r*r. Taking that digit out and dividing down will be

``````    (X+Yi - digit) / (i-r)
= - (X-digit + Y*i) * (i+r) / norm
= (Y - (X-digit)*r)/norm
+ i * - ((X-digit) + Y*r)/norm``````

ie.

``````    X   <-   Y - (X-digit)*r)/norm
Y   <-   -((X-digit) + Y*r)/norm``````

The digit must make both X and Y real and imaginary parts integers. The easiest to calculate from is the imaginary part,

``    - ((X-digit) + Y*r) == 0 mod norm``

so

``    digit = X + Y*r mod norm``

With that digit value the real part is a multiple of norm too,

``````    Y - (X-digit)*r
= Y - X*r - (X+Y*r)*r
= Y - X*r - X*r + Y*r*r
= Y*(r*r+1)
= Y*norm``````

Notice the new Y is the quotient, rounded towards negative infinity, from (X+Y*r)/norm. Ie. in the division the quotient is the new Y and the remainder is the digit.