[Arjen Markus] (15 july 2013) A newspaper article this weekend inspired me to this little program. The author used prime
numbers to decide where to go next. That is: take the second turn, then take the third, then the fifth and so. Roundabouts,
culs-de-sac and one-way streets made it more difficult than you might think at first, but that is reality playing cat-and-mouse.

On a canvas, there is no such difficulty. 

Well, I did have a tiny problem: if you let the step size for the next step depend linearly on the prime, then you need a 
very big canvas or you need to scale. So, instead I simply take steps of three pixels and the direction depends on the 
prime number. What scheme you use for turning a prime into a direction determines in an unpredictable (?) way what path 
you get. 

I have preprogrammed four methods, but it is very easy to come up with others. Small reminder: primes larger than 3 are 
all of the form 6n+1 or 6n+5. And all primes larger than 2 are of the form 4n+1 or 4n+3. Two little facts I use in the
program below.

One note: to select the type of path, set the variable type to 0, 1, 2 or 3.

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======
# primetrail.tcl --
#     Use prime numbers to generate a trail in a canvas
#     Inspired by a newspaper article.
#
#     The idea: draw a path in small steps where the direction
#     and perhaps the step size depend on successive primes
#

#
# Four types:
# 0 - Based on mod 6: the path turns left or right, depending on p%6
# 1 - Based on mod 5: the path goes north, west, south or east, based on p%5
# 2 - Based on mod 5: the path turns over 0, 90, 180 or 270 degrees
# 3 - Based on mod 4: as mod 6
#
set type 0

package require math::numtheory

pack [canvas .c -width 800 -height 800]

set x    250
set y    250
set dirx 1
set diry 0

.c create oval 245 245 255 255 -fill red

if { $type == 0 || $type == 3 } {
   set np 5
   set mod [expr {$type == 0? 6 : 4 }]
} else {
   set np 7
}   

set p  $np

for {set i 0} {$i < 50000} {incr i} {
    if { $type == 0 || $type == 3 } {
        if { $p%$mod == 1 } {
            set ndirx [expr {$diry}]
            set ndiry [expr {-$dirx}]
        } else {
            set ndirx [expr {-$diry}]
            set ndiry [expr {$dirx}]
        }
    } elseif { $type == 1 } {
        switch -- [expr {$p%5}] {
            "1" {
                set ndirx  0; set ndiry -1
            }
            "2" {
                set ndirx -1; set ndiry  0
            }
            "3" {
                set ndirx  0; set ndiry  1
            }
            "4" {
                set ndirx  1; set ndiry  0
            }
        }
    } else {
        switch -- [expr {$p%5}] {
            "1" {
                set ndirx $ndirx; set ndiry $ndiry
            }
            "2" {
                set ndirx [expr {-$diry}]; set ndiry $dirx
            }
            "3" {
                set ndirx [expr {-$dirx}]; set ndiry [expr {-$diry}]
            }
            "4" {
                set ndirx $diry; set ndiry [expr {-$dirx}]
            }
        }
    }

    set np [expr {$np%6 == 1? $np+4 : $np+2}]

    set dirx $ndirx
    set diry $ndiry
    set nx   [expr {$x + $dirx * 3}]
    set ny   [expr {$y + $diry * 3}]

    #
    # Note: you could use the prime to scale the step, but then
    # the path quickly runs wild.
    #

    .c create line $x $y $nx $ny

    set x    $nx
    set y    $ny

    while { ! [::math::numtheory::isprime $np] } {
        set np [expr {$np%6 == 1? $np+4 : $np+2}]
    }
    set p $np
}

if { $type == 0 } {
    .c move all 200 400 ;# Centre it, more or less
}
if { $type == 2 } {
    .c move all 200 200 ;# Centre it, more or less
}
======
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[A not-so-random path based on primes screen.png]

[gold] added pix, random? looks like coasts of United Kingdom and Ireland

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