```
#!/usr/bin/gnuplot
# Copyright 2012 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/>.
# Usage: gnuplot dragon-curve.gnuplot
#
# Draw the dragon curve by calculating an X,Y position for each
# point n. The plot is in "parametric" mode with t running integers
# 0 to n inclusive.
# Return the position of the highest 1-bit in n.
# The least significant bit is position 0.
# For example n=13 is binary "1101" and the high bit is pos=3.
# If n==0 then the return is 0.
# Arranging the test as n>=2 avoids infinite recursion if n==NaN (any
# comparison involving NaN is always false).
#
high_bit_pos(n) = (n>=2 ? 1+high_bit_pos(int(n/2)) : 0)
# Return 0 or 1 for the bit at position "pos" in n.
# pos==0 is the least significant bit.
#
bit(n,pos) = int(n / 2**pos) & 1
# dragon(n) returns a complex number which is the position of the
# dragon curve at integer point "n". n=0 is the first point and is at
# the origin {0,0}. Then n=1 is at {1,0} which is x=1,y=0, etc. If n
# is not an integer then the point returned is for int(n).
#
# The calculation goes by bits of n from high to low. Gnuplot doesn't
# have iteration in functions, but can go recursively from
# pos=high_bit_pos(n) down to pos=0, inclusive.
#
# mul() rotates by +90 degrees (complex "i") at bit transitions 0->1
# or 1->0. add() is a vector (i+1)**pos for each 1-bit, but turned by
# factor "i" when in a "reversed" section of curve, which is when the
# bit above is also a 1-bit.
#
dragon(n) = dragon_by_bits(n, high_bit_pos(n))
dragon_by_bits(n,pos) \
= (pos>=0 ? add(n,pos) + mul(n,pos)*dragon_by_bits(n,pos-1) : 0)
add(n,pos) = (bit(n,pos) ? (bit(n,pos+1) ? {0,1} * {1,1}**pos \
: {1,1}**pos) \
: 0)
mul(n,pos) = (bit(n,pos) == bit(n,pos+1) ? 1 : {0,1})
# Plot the dragon curve from 0 to "length" with line segments.
# "trange" and "samples" are set so the parameter t runs through
# integers t=0 to t=length inclusive.
#
# Any trange works, it doesn't have to start at 0. But must have
# enough "samples" that all integers t in the range are visited,
# otherwise vertices in the curve would be missed.
#
length=256
set trange [0:length]
set samples length+1
set parametric
set key off
plot real(dragon(t)),imag(dragon(t)) with lines
```